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Page 1: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Aliphatic cyclic carbonates and spiroorthocarbonates as monomers

G. Rokicki*

Division of Polymer Synthesis and Processing, Faculty of Chemistry, Warsaw University of Technology,ul. Noakowskiego 3, 00-664 Warsaw, Poland

Received 15 November 1999; accepted 4 January 2000

Abstract

In this review the synthesis and polymerization of aliphatic cyclic carbonates of different size and type arepresented. The mechanisms of cationic, anionic, coordination and enzymatic polymerization of cyclic carbonatesare discussed for obtaining polymers with well-defined structures and oligomers with reactive pendant and endgroups. The reactions of cyclic carbonates with different nucleophilic reagents leading to products with CO2

retention or with decarboxylation are reported. Special attention has been paid to the synthesis and polymerizationof spiroorthocarbonates, bicyclic acetals, which are the intermediates in the reaction of cyclic carbonates withcyclic ethers. The influence of the presence of neighboring groups such as methylene or epoxy groups on thepolymerization mechanism of spiroorthocarbonates, as well as the volume expansion during polymerization isdiscussed. As a result of the above synthetic findings, a variety of novel materials have been developed withversatile applications in different fields such as biomedicine and electronics.q 2000 Elsevier Science Ltd. Allrights reserved.

Keywords: Aliphatic cyclic carbonates; Spiroorthocarbonates; Ring-opening polymerization; Aliphatic polycarbonates;Biodegradable polymers; Expandable polymers

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2602. Synthesis of cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

2.1. Synthesis of aliphatic cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2652.1.1. Synthesis of five-membered cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2682.1.2. Synthesis of six-membered cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2712.1.3. Synthesis of seven-membered and of larger ring size cyclic carbonates. . . . . . . . . . . . . 274

2.2. Synthesis of aromatic cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2753. Polymerization of cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

3.1. Polymerization of five-membered cyclic carbonates (1,3-dioxolan-2-one)s. . . . . . . . . . . . . . . . . 277

Prog. Polym. Sci. 25 (2000) 259–342

0079-6700/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0079-6700(00)00006-X

* Corresponding author. Tel.:122-660-7317; fax:122-628-2741.E-mail address:[email protected] (G. Rokicki).

Page 2: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

3.1.1. Polymerization of vinylene carbonate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2793.2. Polymerization of six-membered cyclic carbonates (1,3-dioxan-2-ones). . . . . . . . . . . . . . . . . . . 279

3.2.1. Cationic polymerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2803.2.2. Anionic polymerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2853.2.3. Coordination polymerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2923.2.4. Enzymatic polymerization . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

3.3. Polymerization of seven-membered and of larger ring size cyclic carbonates. . . . . . . . . . . . . . . 2993.3.1. Polymerization of seven-membered cyclic carbonates. . . . . . . . . . . . . . . . . . . . . . . . . . 2993.3.2. Polymerization of cyclobis(alkylene carbonate)s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

4. Copolymerization of cyclic carbonates with other heterocyclic monomers. . . . . . . . . . . . . . . . . . . . . 3034.1. Copolymerization of 1,3-dioxolan-2-ones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3034.2. Copolymerization of 1,3-dioxan-2-ones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

4.2.1. Copolymerization of 1,3-dioxan-2-ones with cyclic carbonates. . . . . . . . . . . . . . . . . . . 3064.2.2. Copolymerization of 1,3-dioxan-2-ones with cyclic esters. . . . . . . . . . . . . . . . . . . . . . . 3064.2.3. Copolymerization of 1,3-dioxan-2-ones with N- and P-containing heterocyclic monomers 3114.2.4. Block copolycarbonates from hydroxytelechelic-based initiators. . . . . . . . . . . . . . . . . . 312

5. Polymerization of cyclic thiocarbonates . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3136. Cyclic carbonates as reagents and modifying agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

6.1. Cyclic carbonates in reactions with phenols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3156.2. Cyclic carbonates in reactions with amines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3176.3. Transformation of cyclic carbonates into other cyclic functional groups. . . . . . . . . . . . . . . . . . . 3196.4. Cyclic carbonates used in modification of epoxy resins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

6.4.1. Cyclic carbonates used for the modification of epoxy resins cured with amines . . .. . . . 3206.4.2. Cyclic carbonates used for modification of epoxy resins cured with Lewis acids . .. . . . 321

6.5. Cyclic carbonates in polymer foams formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3216.6. Five-membered cyclic carbonates as a source of linear carbonate groups. . . . . . . . . . . . . . . . . . 322

7. Spiroorthocarbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3237.1. Synthesis of spiroorthocarbonates . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3247.2. Polymerization of spiroorthocarbonates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

7.2.1. Cationic polymerization of methylene substituted spiroorthocarbonates. . . . . . . . . . . . . 3297.2.2. Radical polymerization of methylene substituted spiroorthocarbonates. . . . . . . . . . . . . . 3307.2.3. Cationic ring-opening polymerization of spiroorthocarbonates containing epoxy groups . 3337.2.4. Spiroorthocarbonates used for modification of acrylic and epoxy resins. . . . . . . . . . . . . 334

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

1. Introduction

Among the various polymerization modes, ring-opening polymerization is one of the most important(Scheme 1).

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342260

X

catalystX[ ]n n

Scheme 1.

Page 3: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Many commercially important polymers such as polyamides, silicones, polyesters, polyethers, poly-alkylenes and cured epoxy resins are produced according to the general reaction pathway outlined inScheme 1. Besides chain growth polymerization, bifunctional cyclic monomers can also be subjected tosimple step growth polyaddition with monomers containing at least two reactive functional groups

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 261

Nomenclature

e -CL e -caprolactoneAIBN azobis(isobutyronitrile)AMP activated monomer polymerizationAMTC 2-acetoxymethyl-2-methyltrimethylene carbonateCHTC 2,2-(2-pentene-1,5-diyl)trimethylene carbonateDABCO 1,4-diazabicyclo[2.2.2]octaneDBU 1,8-diazabicyclo[5.4.0]undec-7-eneDDS drug delivery systemDMAc dimethyl acetamideDMAP 4-(dimethylamino)pyridineDMF dimethyl formamideDMSO dimethyl sufoxideDON 1,4-dioxan-2-oneDOXTC 2,4,8,10-tetraoxaspiro[5,5]undecane-3-oneDSC Differential scanning calorimetryDTC 2,2-dimethyltrimethylene carbonateGPC gel permeation chromatographyGTP group transfer polymerizationHDI 1,6-hexamethylene diisocyanateIBAO iso-butyl aluminoxaneMAO methyl aluminoxaneMDI diphenylmethane diisocyanateMMTC 2-methoxycarbonyl-2-methyltrimethylene carbonateNBC 5,5-(bicyclo[2.2.1]hept-2-en-5,5-ylidene)-1,3-dioxan-2-onePPL porcine pancreatic lipaseROP ring-opening polymerizationDROP double ring opening polymerizationSnOct2 tin 2-ethylhexanoateSOC spiroorthocarbonateTETA triethylene tetraamineTeU tetramethylene ureaTFA trifluoroacetic acidTHF tetrahydrofuranTMC trimethylene carbonateTMP 2-hydro-2-oxo-1,3,2-dioxaphosphorinaneWAXS wide angle X-ray scattering

Page 4: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

(Scheme 2). In contrast to chain growth polymerization, in this reaction cyclic groups usually aremonofunctional and after ring opening no further reaction takes place.

The latter process is less common, but still many types of epoxy resins are produced and crosslinked usingpolyfunctional reagents such as bisphenols, polyamines or polythiols. Cyclic carbonates can react similarlyto oxiranes via ring opening according to two modes: by chain growth and step growth.

The chemistry of cyclic carbonates, which has been explored since the 1930s, has come to be a richarea of research within the past 20 years. Two main approaches for the use of cyclic carbonates havebeen investigated. Brunelle and the research group from General Electric have focussed on the synthesisof aromatic cyclic carbonate oligomers and their applications in the preparation of bisphenol A poly-carbonates, copolymers, and composites [1]. At the same time Kricheldorf, Ho¨cker and Heitz groups[2,60] from Germany and Endo [3] from Japan have been exploring aliphatic cyclic carbonates as usefulmonomers for the preparation of polycarbonates as well as copolymers with other heterocyclic monomers[4].

Synthetic biodegradable polymers have become interesting materials for a variety of biomedicalapplications. In those fields homopolymers and copolymers of five- (1,3-dioxolan-2-ones) and six-membered carbonates (1,3-dioxan-2-ones) with cyclic esters (lactones and lactides) have been foundto be good materials because of their biocompatibility, low toxicity and biodegradability.

Much of the interest in ring-opening polymerizations stems from the fact that the polymers formedmay have lower densities than the monomers from which they are derived (volume expansion mayaccompany polymerization) [3,5–7]. This is in marked contrast with conventional polymerizations,which invariably involve a net volume contraction. Such polymerizations are therefore of particularinterest in adhesive, mold filling, and other applications where volume contraction is undesirable. Inrelation to this, besides expandable spiroorthocarbonates known since the 1970s [8,9], cyclic carbonates(six- and seven-membered) also polymerize with volume expansion in which the degree of densityreduction may reach 10% as was recently found by Endo et al. [10,11,32].

The ability of cyclic carbonate monomers to undergo ring-opening polymerization depends on boththermodynamic and kinetic factors. The size of a ring and its strain as well as the kind and number ofsubstituents determine the reaction enthalpy and entropy. Medium size rings (six- and seven-membered), because of relatively small ring strain, have low enthalpies of ring opening (polymerizewith moderate exotherms) and their polymerization carried out under equilibrating conditions does notproceed for 100% conversion and a relatively significant amount of cyclic monomer as well as oligo-meric cycles are usually present in the post-reaction mixture [12–14]. The polymerization of oligomericaromatic cyclic carbonates of larger size, not discussed here, is known to be entropy driven [1].

A wide variety of cyclic carbonate monomers have been successfully used for ring-opening poly-merization carried out in the presence of different kinds of initiators and catalysts according to cationic,anionic, coordination and enzymatic mechanisms. This includes monomers with both small and largersize molecules. Representative examples of the monomers and the polymerization initiators andcatalysts are shown in Tables 1 and 2, respectively.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342262

X X+ HY YH Y Y

n

HX XH

n ncatalyst

Scheme 2.

Page 5: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 263

Table 1Polymerization of different size of cyclic carbonates

Monomer Polymer References

Five-membered

O O

O

RO O O

O

R

y

n

R

R� H,R� CH3

[15–25]

O O

O O On

O [26–28]

Six-membered

O O

O

R1

R2

R2

R1

O O

On

or

R2

R1

OR

2

R1

O O

On

m

R1� R2� H [29–32]R1� H; R2� CH3 [33]R1� H; R2� C4H9 [32]R1� R2� CH3 [29,32,34]R1� R2� C2H5 [29]R1� Ph; R2� CH3 [11,29]R1� Ph; R2� C2H5 [29]R1� R2� Ph [11]R1� C2H5; R2� CH2OH [35]R1� C2H5; [35]R2� CH2Osi(CH3)3

R1� C2H5; [35]R2� CH2OC(O)OCH2Ph

Page 6: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342264

Table 1 (continued)

Monomer Polymer References

R1� C2H5; [35]R2� CH2OC(O)NHPhR1� C2H5; [36]R2� CH2OCH2CH� CH2

R1� CH3; R2� CN [37]R1� CH3; R2� COOCH3 [38]R1� C2H5; R2� CH2OCOCH3 [38]R1� CH3; R2� CH2OCOCH3 38R1� R2� CH2 [39,40]R1� R2� CH2OCH2OCH2 [41]R1� R2� CH2OC(CH3)2OCH2 [42]

R1-R2 = CH2

[43–45]

O O

O

R2

R1

R2

R1

O O

On

R1� H; R2� CH3 [46]

R1

O O

O

R2

R1

R2

O O

On

R1� NHCO2CH2Ph; R2� CH3 [47]

Seven-membered

O O

O

O O

On

[30,48–50]

Page 7: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

2. Synthesis of cyclic carbonates

2.1. Synthesis of aliphatic cyclic carbonates

Carbon dioxide, aliphatic or aromatic esters of carbonic acid and phosgene derivatives are usuallyused as a source of carbonate groups in the synthesis of cyclic carbonates. The carbon dioxide addition toa cyclic ether ring seems to be the most convenient synthetic method when taking into consideration thereaction yield and selectivity. In the case of oxiranes, the addition proceeds selectively with a high yield, butwhen cyclic ethers with larger ring sizes (oxetanes, pyranes) are used, the CO2 addition is not so effective.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 265

Table 1 (continued)

Monomer Polymer References

Cyclobis(polymethylenecarbonate)s

OCO

O

CH2n

OCO

O

CH2 n

O O (CH2)

O

m

n

n� 4 [51]n� 6 [52,53]n� 10 [54]

O

OO

OCO

O

CO

O

O O

OO n

[55]

(CF2)4

OC

OO

O

OCO

(CF2)4

O O (CF2)4

On

[53]

Page 8: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342266

Table 2Catalysts in polymerization of cyclic carbonates

Catalyst Cyclic carbonate References

Cationic polymerizationCH3OSO2CF3 Six- and seven-

membered[32,56,57][32,48]

HOSO2CF3

Lewis acids [4,31,40,58,59]BF3·OEt2 [31,32,58]SnCl4 Six- and seven-

membered[44,48][60]

SbCl5TiCl4 [44]CH3I Six-membered [32,44]C6H5CH2Br [32]CH2� CHCH2I [32]I2 [32]Bu42nSnCln [61]Anionic polymerizationK2CO3 [31,62,65]t-BuOK [29,66]

CH 3

CH 3

O Si( C H3)3

O C H3

(C 4H 9) 4N 3 H 2O.+

[67]

s-BuLi [38,53,68,69]Bu2Mg [53,70]NaH [31]CH3COOK1 18-crown-6ether

[31]

4-(dimethylamino)pyridine(DMAP)

[43]

Quinuclidine [43]1,4-diazabicyclo[2.2.2]octane(DABCO)

[43]

1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)

[43]

Samarium complex [71]Rare earth chloride (Y, La,Pr, Nd, Dy)

[72–75]

Coordination polymerizationBu2SnO [31]Bu2Sn(OCH3)2 [53,76]SnOct2 [31,59,76–80]BuSnCl3 [77]Al(OsBu)3 [38,81]

Page 9: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The first information about the formation of cyclic carbonates was given by Carothers et al. [90–92] inthe early 1930s. Cyclic carbonates were obtained by the depolymerization of respective linear poly-carbonates at high temperature and in the presence of different catalysts (Scheme 3, Eq. (1)). A mixtureof volatile cycles (mostly monomeric and dimeric cyclic carbonates) was distilled off from the equi-librating reaction in yields of 40–80% (Eq. (2)) [17,93–95]. The most effective catalysts in this processare: Sn(II), Mn(II), Fe(II) and Mg(II) chlorides, carbonates and oxides. Carothers also reported thepolymerization of aliphatic cyclic carbonates, but very few details were given [91].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 267

Table 2 (continued)

Catalyst Cyclic carbonate References

Al(OiPr)3 [31,81]Al(Et)(32x)(OR)x [82]Al[O(CH2PhNO2]x(Et)32x [82]Aluminoxanes (methyl andisobutyl) (MAO,IBAO)

[41,42]

ZnEt2 [65,76]zinc stearate [78,81]di(n-butyl)stannic diiodide-triphenylphosphine system

[83,84]

Porphinatoaluminumcompounds (TPP)AlOR

[66]

[Cp2ZrMe]1[B(C6F5)4]2 [50]

Enzymatic polymerizationHematin [85]Lipase Six- and seven-

membered[86–89,192]

HOCH2 (CH2) CH2OHn

+ RO O R

O

n

OH(CH2)

O O

O

(CH2)OH

m

n

+ 2m ROH

mcatalyst

n

OH(CH2)

O O

O

(CH2)OH

m

n

O

(CH2)

O

O

n

HOCH2 (CH2) CH2OHnm + 1

+

n = 1 - 2; R = CH3 or C2H5

m catalyst

(2)

(1)

Scheme 3.

Page 10: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Nowadays, the process of depolymerization of aliphatic oligocarbonates is still applied in thesynthesis of six-, seven-membered and even of larger size cyclic carbonates [i.e. cyclobis(alkylenecarbonate)s]. The process of depolymerization of aliphatic polycarbonates is also utilized on a technicalscale in ceramic technology [96] and for nanofoam preparation [97].

2.1.1. Synthesis of five-membered cyclic carbonatesThe alkylene carbonates are easily available through transesterification of appropriate glycols with

dialkyl carbonates (usually diethyl or dimethyl carbonate) in the presence of alkaline catalysts [62].Phosgene or its derivatives can also be used for cyclic carbonate synthesis instead of dialkyl esters ofcarbonic acid.

Five-membered cyclic carbonates are easily available as a result of the insertion of gaseous carbondioxide into an oxirane ring (mostly ethylene and propylene oxide) (Scheme 4) [98].

It is also known that alternating copolymers of carbon dioxide and oxiranes, poly(alkylenecarbonate)s, can be synthesized by the use of organozinc [99], organoaluminum [100,101] compoundsas well as cobalt acetates [102,103] under mild conditions. The copolymerization is generally accom-panied by the formation of five-membered cyclic carbonates [104,105], Alkylene carbonate, however, isnot the precursor for the copolymer, because of its lack of polymerizability under such conditions.

The literature reports on the synthesis of alkylene carbonates from oxiranes and CO2 catalyzed byvarious kinds of catalysts such as Lewis acids, Lewis acid–base systems, ammonium or phosphoniumsalts. Also transition-metal complexes are effective in the fixation of CO2 [106]. Organotin compoundssuch as methyltin tribromide and butanestannoic acid have a good catalytic effect for the reaction of CO2

with oxirane to form ethylene carbonate under mild conditions [107]. It was found that pentavalentorganoantimony compounds such as PhnSbX52n more effectively catalyze the reaction than tincompounds [108,109]. Additionally, the pentavalent organoantimony compounds, in contrast to tincompounds, do not initiate the oxirane polymerization. In the case of nickel(0) phosphine complexescyclic carbonates are produced in high yields. The rate of carbonate formation is dependent on thestructure of the epoxide and decreases in the order: epichlorohydrin. ethylene oxide. propyleneoxide. 2,3-epoxybutane [106].

Monomeric quaternary ammonium salts, as well as an anion exchange resin containing those groups,are used in the industrial scale preparation of five-membered cyclic carbonates [110,111]. The patentliterature on the preparation of five-membered cyclic carbonates is fairly extensive [112–120].

Although alkali metal salts appeared to be rather poorly active catalysts for the CO2-epoxide reaction,the introduction of crown ethers or phase-transfer agents to the system enhances their catalytic activities[121]. In the presence of a crown ether, the respective nucleophilic anion (“naked” anion) is activated tosuch an extent that the cyclic carbonate is produced in a high yield under mild conditions. In the system

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342268

O O

O

R

O

R

+ CO2

catalyst

Scheme 4.

Page 11: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

studied the intermediate product of the reaction of the halide anion with the oxirane ring—alcoholateion, does not undergo further reaction with oxirane and no poly(alkylene oxide) is formed. This resultsfrom the greater stability of the alkyl carbonate anion than that of the initial alcoholate one. Under theseconditions, no poly(alkylene carbonate) was formed. Due to anchimeric assistance (kinetic factor) and arelatively high thermodynamic stability, the five-membered 1,3-dioxolan-2-one ring is more easilyformed from the cyclization of that anion with respect to linear poly(alkylene carbonate) when nocoordination catalyst is present. Relatively high pressures of CO2 (.8 bar) and temperatures above1008C are needed for an acceptable reaction rate.

Nevertheless, it was found that oxiranes with ammonium groups at theb-position to the oxirane ring,due to the lowering of the activation energy of nucleophilic substitution, are able to fix carbon dioxide atroom temperature and low CO2 pressures [122–124]. The presence of an equimolar amount of reactivebenzyl bromide or chloride in the reaction of a quaternary ammonium salt ofN,N-diglycidylamine withCO2 at 908C yields products containing linear carbonate groups apart from the cyclic carbonates(Scheme 5). The reaction proceeding in the system can be explained by the mechanism involving halideanion attack on thea-carbon atom of the oxirane ring resulting in the formation of the alcoholate ion. Itshould be emphasized that the intermediates1 and2 are zwitterions. When reactive benzyl bromide orchloride is present in the reaction system, the stable carbonate anion, due to its interaction with ammo-nium gegenion in the zwitterion molecule, can attack the benzylic carbon atom and the linear carbonatederivative is formed besides the cyclic carbonate.

Functionalized cyclic carbonates can also be obtained in good yields under mild conditions fromepoxides and carbon dioxide by an electrochemical procedure [125]. The cyclic carbonate formation iscatalyzed by Ni(cyclam)Br and is carried out in single-compartment cells fitted with a magnesiumanode. The presence of functional groups such as chlorine, bromine, ether, ester or olefins is compatiblewith the reaction conditions.

It may be of interest that alkali metal carbonates activated in the presence of crown ethers can be

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 269

N

O

++ X N

O

X+

-

+ CO2

N

O

X

O

O

+ N

O

O

O

+X

-

+ C6H5CH2X

NO O C6H5

CH2XO

+

X = Cl or Br

X

1

2

(1) (2)

(3)

(4)

3

4

Scheme 5.

Page 12: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

applied as an alternative source of carbonate linkages in the reaction with dihalo compounds [126,127].When epihalohydrin is used instead of the dihalo compound in the reaction with K2CO3, correspondingfive-membered cyclic carbonates containing an epoxy group are formed (Scheme 6) [121,128,129].

In contrast, potassium bicarbonate reacts with epihalohydrin yielding glycerol carbonate (4-hydro-xymethyl-1,3-dioxolan-2-one) (Scheme 7) [128].

A new approach for the cyclic carbonate synthesis was given by Leboisselier et al. [130] Cycliccarbonates were obtained in good yields from terminal epoxides and DMF in a BiBr3-catalyzed reactionwith molecular oxygen used as the oxidant.

The O2-radical-anion/CO2 system, originating from the electrochemical one-electron reduction ofdioxygen in dipolar aprotic solvents and in the presence of CO2, converts primary and secondaryalcohols bearing a leaving group ata- or b-position into the corresponding cyclic carbonates in high

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342270

XO

+ K2CO3

18-crown-6

O

O

O

O

XK+ -

- K+

OO

O

OK

+ -

-K

+

XO

O O

O

-K

+

-K

+

O

O

O

O

XO

+

-K

+

O

O

O

OX

O

O

O

O

OO

Scheme 6.

XO

+ KHCO3

18-crown-6

OX

O

OH

O

- K+

XOH

O O

O

-K

+

O

OH

O

O

Scheme 7.

Page 13: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

yields. Unsubstituted alcohols are also converted, but in unsatisfactory yields, into the correspondingalkyl ethyl carbonates after completion of the reaction by addition of EtI. Tertiary alcohols and phenolsare stable to the reagent, thus allowing selective carboxylation of polyhydroxy derivatives. CH-acidcontaining compounds undergo different [131] reactions, if any, with the reagent but in the cases understudy the formation of carboxylation products has never been observed [132].

The carbonylation of cyclic alkynyl carbonates in the presence of a palladium(0) catalyst givesfunctional 5-hydroxyalka-2,3-dienoates in one step via zwitterionic allenylpalladium intermediates.From 3-en-1-ynyl cyclic carbonates, double vicinal carbonylation takes place to selectively affordoxolenones or 4-oxabicyclo[4.3.0]-nona-5,9-dien-2,7-diones [133].

Pd(0)-catalyzed arylation of the aryl-substituteda-allenic alcohols with hypervalent iodonium saltsafforded substitutedtrans-epoxides [134]. Alternatively, arylation of the alkyl-substituteda-allenicalcohols in the presence of K2CO3 affordedsyn-diol cyclic carbonates andtrans-epoxides in the presenceof Cs2CO3 (Scheme 8).

The treatment of carbonyl compounds with SmI2 and methyl chloroformate in the presence ofmolecular sieves provides the cyclic carbonates or biscarbonates of pinacols. This one-pot reactionproceeds rapidly even with aliphatic ketones. The stereochemistry of the reaction run by this procedureis different from that of conventional pinacolic couplings [135].

2.1.2. Synthesis of six-membered cyclic carbonatesThe synthesis of six-membered cyclic carbonates by transesterification of propane-1,3-diols with

diethyl carbonate catalyzed with sodium ethanolate described by Carothers and Van Natta [62] givesa yield of 40%. Also Pohoryles and Sarel [64,136,137] reported on the synthesis of various six-membered cyclic carbonates from propane-1,3-diols with different substituents. Thus, when 2,2-dimethyl-, 2-methyl-2-n-propyl- and 2-methyl-2-iso-amylpropane-1,3-diol were treated with diethylcarbonate in the presence of catalytic amounts of sodium methoxide, only polycarbonates were producedin high yield. On the other hand, six-membered cyclic carbonates are exclusively produced when 2,2-diethyl-, 2-ethyl-2-phenylpropane-1,3-diol, pentane-2,4-diol, 2-methylpentane-2,4-diol [33] and butane-1,3-diol were subjected to the transesterification reaction (Scheme 9).

A similar method reported by Albertsson et al. [31] in which equimolar amounts of propane-1,3-dioland diethyl carbonate with stannous 2-ethylhexanoate as the transesterification catalyst were used,afforded a yield of 53%. In this method reactants were refluxed for 8 h before ethanol was removed.It is postulated that stannous 2-ethylhexanoate can act both as a polymerization (at lower temperature)and as a depolymerization agent (at higher temperature) [138–140].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 271

R

OHPd(OAc)2 Ph3P,

K2CO3 , DMF

60oC, 3 h

Ph2I BF4-+

O O

O

Ph

R = alkyl

Scheme 8.

Page 14: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Zhuo et al. [141] reported that the method afforded 70% yield of trimethylene carbonate (TMC). Forthe transesterification reaction two catalysts: sodium ethanolate and dibutyltin dilaurate were used andthe reaction temperature was controlled with refluxing of xylene. In contrast to the earlier reportedmethods, the prepolymer depolymerization was carried out using tin powder as a catalyst after neutral-ization of the basic catalyst byp-toluenesulfonic acid.

Recently, Endo et al. [29] have revealed a universal method of cyclic carbonate synthesis frompropane-1,3-diols and ethyl chloroformate in the presence of a stoichiometric amount of triethylamine.A specific ammonium salt (the absorption band of carbonyl group was shifted to 1810 cm21 [142]) as anintermediate seems to be the driving force for the reaction proceeding at low temperatures favoring six-membered cycles (yields up to 60%) (Scheme 10).

Other phosgene derivatives (di- and triphosgene) also give cyclic products with good yields. Thereaction of 2,2-disubstituted propane-1,3-diols carried out with phosgene dimer affords the cycliccarbonate quantitatively in the reaction of 2-ethyl-2-phenylpropane-1,3-diol and 2,2-diphenylpropane-1,3-diol, while the corresponding oligocarbonates were formed in the reaction of 2,2-diethylpropane-1,3-diol in 24% yield, apart from the corresponding cyclic carbonate [29].

2.1.2.1. Synthesis of six-membered cyclic carbonates bearing reactive groups.Although extensiveefforts have been devoted to the synthesis of functionalized monomers, which is very often complexand tedious, only a few example of the synthesis of polycarbonates with hydroxyl pendant groups havebeen reported so far. Poly(2-ethyl-2-hydroxymethyltrimethylene carbonate) is one of them, which isreadily synthesized from cyclic carbonate with a protected hydroxyl group. Such a procedure should beapplied to avoid the reaction of OH groups with an initiator or growing chain end, leading to branchedor crosslinked structure of the resultant polycarbonates. 5-Ethyl-5-hydroxymethyl-1,3-dioxan-2-onecan be obtained from trimethylolpropane and alkyl ester of carbonic acid used in the molar ratio1:1 [143]. Trimethylsilyl, benzyloxycarbonyl and phenylcarbamoyl groups were applied for OH

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342272

OH OH

R2

R1

R3

R4 R

5

+ EtO O

Et

O

catalyst O O

O

R2

R1

n

R3 R

4

R5

catalyst

O O

O

R1 R2

R3 R4

R5

catalyst

a

b

Scheme 9.

Page 15: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

group protection [35]. Thus, cyclic carbonate with a hydroxyl group was reacted with trimethylsilylchloride, benzyl chloroformate and phenyl isocyanate, respectively, in the presence of pyridine ortriethylamine.

Cyclic acetal formation can also be utilized for OH group protection. The new spiro-monomerconsisting of cyclic carbonate and cyclic acetal—2,4,8,10-tetraoxaspiro[5,5]undecane-3-one [41] wasprepared in high yield by the reaction of 5,5-bis(hydroxymethyl)-1,3-dioxane with ethyl chloroformatein tetrahydrofuran.

With PdCl2(PhCN)2 and SnCl2 catalysts, 4-pentene-1,3-diol caused regioselective carbonyl allylationat 3-position in DMF producing 2-substituted 3-vinyltetrahydrofurans and/or 1-substituted 2-vinylbu-tane-1,4-diols, and the cyclic carbonate of 4-pentene-1,3-diol caused regioselective carbonyl allylationat the terminal 5-position in THF producing 1-substituted 3-hexene-1,6-diols [144].

Bruneau et al. [145] presented a new method of synthesis of a six-membered cyclic carbonatecontaining an unsaturated bond (4H-[1,3]dioxin-2-one). It was easily prepared from propargylic alco-hols and carbon dioxide without phosgene derivatives (Scheme 11). Their activation with suitablepalladium catalyst precursors leads to reactive zwitterionic allenyl palladium species and offers straight-forward routes to functionala-allenol derivatives upon hydrogenolysis, by coupling with terminalalkynes, and monocarbonylation.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 273

N

Et Et

Et

O

O

Et

+

Cl

EtN

Et

Et

+

O

Cl OEt

N

Et Et

Et

O

O

Et

+

Cl

+ N

Et Et

Et

O

O OH

+

Cl

OH OH

O O

O

+ Et3N HCl.

Scheme 10.

CHC

CH2

OH + O C OPd-catalyst

O O

O

Scheme 11.

Page 16: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The specificity of these cyclic carbonates as compared to acyclic propargylic derivatives, due to thepresence of the reactive homopropargylic oxygen atom, is evidenced by the synthesis of oxygenatedheterocycles resulting from double carbonylation, and dihydrofurans formed by coupling with electrondeficient olefins.

2.1.2.2. Synthesis of polycyclic six-membered cyclic carbonates.Polycyclic six-membered carbonatescan be obtained by radical cyclopolymerization of divinyl carbonate (Scheme 12) [146,147].

Poly(divinyl carbonate) was utilized to prepare poly(vinyl alcohol) by alkaline hydrolysis. The struc-ture of the hydrolyzed product was like that from the poly(vinyl acetate) hydrolysis. Kikukawa et al.have found that the hydrolysis product contained 1,2-glycol structures due to the formation, but in muchlower amount, of five-membered carbonate rings [148]. Divinyl carbonate is available from the reactionof vinyl chloroformate with silver acetate [149].

As was shown by Endo et al. [150], polycyclic six-membered carbonates can also be prepared byradical polymerization of acrylic monomers with pendant cyclic carbonate groups (Scheme 13).

2.1.3. Synthesis of seven-membered and of larger ring size cyclic carbonatesSeven-membered and larger ring size cyclic carbonates can be obtained according to the same

transesterification reaction pathways as that leading to five- and six-membered rings [48,49]. However,due to the low stability of seven-membered cyclic carbonate, the polymerization of its cyclic dimers is abetter way of synthesis of the corresponding polycarbonate [51,52]. The same concerns eight-, nine- and13-membered cyclic carbonates [52–55]. Moreover, since 1,5-, 1,6- and 1,10-diols have high boiling

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342274

O O

O

R.

+ R

O O

O

.R

O O

O

.

O O

O

n

OH OH

n

O O

O

R.

+O O

R

O

.R

O O

O

.

O O

O

n

hydrolysis

hydrolysis

OH OH

n

Scheme 12.

Page 17: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

points, a more reactive diphenyl carbonate can be used instead of dimethyl- or diethyl carbonate for thetransesterification and higher yields of the respective intermediate polymers can be obtained. Thetransesterification is carried out with dibutyltin dimethoxide as catalyst at 1308C under reduced pressures(0.07–11 mbar). The resultant polycarbonate subjected to ring-closing depolymerization at 260–3008Caffords cyclic products collected after distillation over a heated column [53,54].

The 18-membered cyclic carbonates: cyclobis(hexamethylene carbonate) and its fluorinated analogrepresent the smallest ring carbonates on the hexane-1,6-diol basis and are obtained as the major productupon cyclization under pseudo-dilution conditions or upon depolymerization of the correspondingoligomers [53].

2.2. Synthesis of aromatic cyclic carbonates

Besides the large size cyclic carbonates obtained in the reaction of bisphenol A with phosgene [1],other aromatic cyclic carbonates of a small ring size are known.

For the synthesis of benzo-1,3-dioxolan-2-one (o-phenylene carbonate)1, the smallest aromatic cyclic

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 275

OHOH

OHCH3

O

O

OH

CH3

O

O

O

CH3

O

OH

OH

O

CH3

O

O

O

O

CH3

OO

O O

O

O

n

CH3

O

acetone, BF3

CH2Cl2

methacroyl chloride

Et3N, benzene

HCl conc.

MeOH

AIBNC2H5OCOCl

Et3N, THF

Scheme 13.

Page 18: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

carbonate, three methods are available:

O

O

O

1

• the reaction of catechol with phosgene or its dimer or trimer [151];• the reaction of catechol with chloroformate [152];• transesterification of catechol with diphenyl carbonate [153].

For the synthesis of aromatic cyclic carbonates from 2,20-biphenol and 1,10-bi(2-naphthol) as diols,p-nitrophenyl chloroformate [154] and a tertiary amine as a base afforded 80% yield (Scheme 14).

As was shown by Kricheldorf et al. [155] also disproportionation of 2,20-bis(methoxycarbonyloxy)-biphenyl catalyzed by Sn(II) octoate can be utilized for obtaining this aromatic seven-membered cycliccarbonate.

A new possibility of obtaining aromatic polycyclic carbonates was also reported by Prochaska [156]from o,o0-bisphenols and Hay et al. [157] from novolac-resin. A novolac-type phenol-formaldehyderesin (ortho-coupled through methylene group 4-tert-butylphenol resins) was reacted with an excess oftriphosgene under high-dilution conditions (Scheme 15).

Because of the relatively low steric hindrance in these systems, the formation of cyclic carbonates waspossible and oligomeric polycyclic carbonates were obtained. The polycyclic carbonates were used forthe modification of commercial aromatic polycarbonates. They were mixed with the linear poly-carbonate in different proportions and cured at different temperatures by using lithium stearate as

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342276

OH OH

+ OO

O

NO2OC

O

Cl

Scheme 14.

OH OH

n

+ Cl3CO O

CCl3

O

OO

O

n

Scheme 15.

Page 19: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

catalyst. The curing of a mixture which contained 10 wt.% of the polycyclic carbonates at 3508C for30 min resulted in a highly crosslinked system.

3. Polymerization of cyclic carbonates

The ring-opening polymerization of cyclic carbonates in the presence of alkaline, Lewisacids, enzymes and coordination catalysts concerns monomers containing a five-membered ring likeethylene carbonate (1,3-dioxolan-2-one) [15,17,18,158], six-membered ring like trimethylene carbonate(1,3-dioxan-2-one) [66,78,159], seven-membered ring like tetramethylene carbonate (1,3-dioxepan-2-one) [30,48,49], as well as 14- [51], 16- [55], 18- [52,53], and 26-membered [54] cyclic carbonatedimers.

3.1. Polymerization of five-membered cyclic carbonates (1,3-dioxolan-2-one)s

Cyclic carbonates of the smallest ring size (five-membered) hardly undergo ring opening. Theirpolymerization, however, has been reported to proceed in the presence of metal alkoxides, metalacetylacetonates as well as metal alkyls as catalysts. It is characteristic that no polymerization wasfound at below 1008C for ethylene carbonate [15], but in the case of propylene carbonate the polymer-ization does not take place significantly even at 1408C within a few days [17]. The reaction temperatureof 1708C for ethylene carbonate and of 1808C for propylene carbonate in the presence of transesterifi-cation catalysts [17] was found to be the most suitable. The polymerization involved partial decarboxy-lation and the loss of carbon dioxide during the polymerization and exceeds 50 mol%, irrespective of thepolymerization conditions. Thus, the polymerization of five-membered ring alkylene carbonates fails toproduce thermodynamically disfavored poly(alkylene carbonate)s but leads to poly(alkylene ether-carbonate)s with a contents of carbonate units lower than 50 mol% (Scheme 16). The reason is notthe low ceiling temperature, but the positive enthalpy of polymerization [15–18,95]. Vogdanis and Heitzemployed a variety of catalysts for the polymerization of ethylene carbonate, ranging from dibutyltindimethoxide to butyllithium, and found that the retention of CO2 decreased as the alkalinity of thecatalyst increased [18]. Under optimum conditions the maximum carbonate units in the copolymer ofethylene oxide and ethylene carbonate do not exceed 50%.

One of the mechanisms having been taken into consideration for the propylene carbonate polymer-ization was that this polymerization proceeded via spiroorthocarbonate species. The plausibility of sucha mechanism might be ascertained by the polymerization of spiroorthocarbonate with diethylzinc as thecatalyst, which was found to yield poly(propylene ether-carbonate) [17]. It seems that the alkylenecarbonate polymerization proceeds via the monomer decarboxylation in the first reaction step. The

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 277

O O

R

O

O OO

O

R

R

n

m + mCO2

catalystn + m

Scheme 16.

Page 20: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

decarboxylation involves most probably metal carbonate species formation due to the 1,3-dioxolan-2-one ring opening via the C–O bond cleavage [158]. The loss of CO2 depends on the initiator. Detailedinvestigations on the mechanism of metal alkoxides of tin and zirconium are given by Kricheldorf[19,160]. Standard enthalpies of formation and the enthalpies of polymerization were calculated(125.6 kJ/mol at 258C) from the enthalpies of combustion and the heat capacity data of poly(ethylenecarbonate). Since three- to seven-membered ring monomers show an entropy loss during ring openingpolymerization [161], the process of formation of poly(ethylene ether-carbonate) is possible only fornegative enthalpy.DHp for ethylene carbonate becomes negative at 1708C. The reaction is additionallyfavored by the loss of small CO2 molecules, which partly compensate the entropy loss.

Harris has reported the use of sodium stannate trihydrate as a heterogeneous catalyst in ethylenecarbonate polymerization [20,21]. Diols used as starters limited the molecular weight of theethylene oxide and ethylene carbonate copolymer. Harris and McDonald [22] have found that 2-hydro-xyethyl carbonate and 2-hydroxyethyl ether end groups are present at the beginning in the polymeriza-tion mixture. However, only 2-hydroxyethyl ether end groups were present during the latter stages ofpolymerization. When poly(ethylene ether-carbonate) diols are heated to elevated temperatures(.1808C) at reduced pressure, volatile impurities are removed, followed by molecular weight advance-ment. As diethylene glycol (DEG) is removed as a distillate, molecular weight builds up in a controllablemanner. This is thought to be a transesterification process in which –OC(O)OCH2CH2OCH2CH2OH endgroups on one molecule react with carbonate moieties on a second molecule with loss of DEG(Scheme 17).

These advanced polyols with high CO2 retention have relatively low polydispersity [23]. Sodiumstannate is a preferred catalyst for the preparation of poly(ethylene ether-carbonate) polyols. The rate ofadvancement to 3000 Da molecular weight products increased with good CO2 retention (95%) when thetin concentration level is 100–500 ppm. At higher catalyst concentration levels, the product decompo-sition to 1,4-dioxane becomes increasingly important [24,25].

On the other hand, the high molecular weight aliphatic polycarbonate with an alternating sequence ofthe monomeric units can be obtained by copolymerization of oxiranes with CO2 using zinc-basedcoordination catalysts and under appropriate reaction conditions [100,162–164].

Attempts of anionic polymerization of five-membered aromatic cyclic carbonate (benzo-1,3-dioxo-lan-2-one) usingsec-BuLi and potassium dihydronaphthylide as initiators, similarly to aliphatic five-membered cyclic carbonates, failed and only the initiation took place.

As far as concerns cationic polymerization, it was shown by Kricheldorf et al. [56] that five-memberedcyclic carbonate (ethylene carbonate) does not react with a cationic catalyst such as methyl triflate. After48 h of reaction, carried out in nitrobenzene at 1008C, no polymer was found on precipitation withmethanol or diethyl ether.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342278

O O

O

OH

O

OO

O

OH

O

+

+OO

O

O O

OO

OH

O

OH

Scheme 17.

Page 21: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

3.1.1. Polymerization of vinylene carbonateVinylene carbonate—a five-membered cyclic carbonate with a C–C double bond in the ring—

exhibits interesting properties. Such a cyclic carbonate can be easily obtained by ethylene carbonatechlorination followed by dehydrochlorination (Scheme 18) [165,166].

Vinylene carbonate subjected to radical polymerization forms a colorless solid polymer, which onhydrolysis yields a water-soluble product containing –[CH(OH)]n– repeating units. The copolymer ofvinylene carbonate with styrene [26] after hydrolysis affords poly(styrene-co-vinylene glycol), whichcan be cleaved with periodic acid to yield polystyrene terminated by aldehyde groups (Scheme 19)[26,27], or alternatively cleaved by periodic acid and potassium permanganate to polystyrene withcarboxylic end-groups [28].

3.2. Polymerization of six-membered cyclic carbonates (1,3-dioxan-2-ones)

In contrast to 1,3-dioxolan-2-ones, six-membered cyclic carbonates (1,3-dioxan-2-ones) easily poly-merize and copolymerize with various heterocyclic monomers. The homopolymer of the six-memberedcyclic carbonate, first reported in the 1930s by Carothers et al. [62,63], was obtained by heating

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 279

O O

O

Cl2O O

O

Cl

O O

O

- HCl

Scheme 18.

+ O O

O

R.

+ CH

O O

CHCH

O O

CH CH2 CH

O O

n

CH

OH OHCHCH

OH OH

CH CH2 CH n

CO

C CH2 CH n

H O

HC

O

OH

O

C CH2CH n

OH

hydrolysis

HIO4

HIO4, KMnO4

Scheme 19.

Page 22: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

1,3-dioxan-2-one with a small amount of K2CO3, used as an initiator. The molecular weight of thepolycarbonates was rather low, not exceeding 4000 Da.

The stereochemistry and mechanism of reversible polymerization of 2,2-disubstituted trimethylenecarbonates was discussed by Sarel and Pohoryles [64]. Thermal degradation of poly(trimethylenecarbonate) leads to cyclic carbonate and carbonate oligomers as the main degradation products [167].

In the 1980s Kricheldorf et al. [56,57] reported the cationic polymerization of 1,3-dioxan-2-ones byring-opening and the anionic mechanism was discussed by Ho¨cker et al. [36,68].

The homopolymerization of six-membered cyclic carbonates and copolymerization with cyclic estersusing initiators comprising Li, K, Mg, Al, Zn and Sn were studied. The systems with alkali metals [69],Mg [70] and Sn [76] were found to afford cyclic oligomers during polymerization. This shows that forthis group of initiators the rate of back-biting is of the same order of magnitude as the rate of poly-merization. For Al- [66] and Zn-based [76] catalysts no oligomers were detected, so the back-bitingreaction is slower by orders of magnitude than the chain growth reaction. For former mentionedinitiators the intramolecular and intermolecular transesterification reaction also take place. High mole-cular weight polymer is formed in the regime of kinetic control. In the regime of thermodynamic controlring-chain equilibrium takes place [69].

It is characteristic that besides spiroorthocarbonates also some cyclic carbonates, as was recentlyfound by Endo et al. [10], exhibit an unusual feature: their ring-opening polymerization is accompaniedby volume expansion. The degree of volume expansion depends on the substituents and their position inthe carbonate ring and ranges from 1.8 to 10.8%. For trimethylene and 2,2-dimethyltrimethylenecarbonate polymerization a volume expansion of ca. 3.5% was observed, while 1.8% for 2-n-butyl-trimethylene carbonate [32]. The highest volume expansion (10.8%) of cyclic carbonate polymerizationwas observed for 5-methyl-5-phenyl-1,3-dioxan-2-one anionic polymerization [29]. The volume expan-sion was examined by assuming a change in molecular interaction such as dipole moment between themonomer and polymer states [10]. Cyclic carbonates were found as expandable monomers upon not onlyanionic [3,10,168] but also cationic polymerization [32].

3.2.1. Cationic polymerizationThe mechanism of cationic polymerization of six-membered cyclic carbonates (1,3-dioxan-2-ones)

was satisfactorily explained, in the 1980s, by Kricheldorf et al. [57]. Using IR and1H NMR spectro-scopies it was shown that methyl triflate initiated the polymerization of six-membered cyclic carbonatesby alkylation of theexo-cyclic oxygen atom of the carbonate linkage, generating a trioxocarbenium ion(Scheme 20, Eq. (1)). The equilibrium with triflate is established after ring opening by the counter-ion(Eq. (2)). Another molecule of cyclic carbonate can be attacked by the trioxocarbenium ion leading tothe alkyl-oxygen bond cleavage and theexo-cyclic oxygen atom of the nucleophile is alkylated (Eq. (3)).The second possible propagation reaction is between the covalent triflate and monomer (Eq. (4)).

Using a twofold excess of methyl triflate it was possible to establish that the covalent initiator isslightly more reactive than the trioxocarbenium ion [56]. For the cationic polymerization of six-membered cyclic carbonates the decarboxylation is a side reaction resulting in the formation of ethergroups. The mole concentration of the ether groups, depending on the chemical structure of the cyclicmonomer, cationic initiator and temperature, is in the range of 3–10% relative to carbonate groups.

The intramolecular migration of an alkyl group is proposed to explain the formation of ether linkages(Scheme 21, Eqs. (1)–(4)). The equilibrium 2 is shifted to the trioxocarbenium ion due to its betterstabilization. But still intramolecular migration is possible and the initiator can be regenerated in Eq. (4).

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342280

Page 23: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The catalytic decarboxylation of carbonates was confirmed by the reaction of diethyl carbonate with5 mol% of methyl triflate carried out under reflux. Slow CO2 evolution and ether formation wereobserved. After 16 h 30% conversion of carbonate linkages was found.

The 1H NMR spectra of the reaction mixture also revealed the formation of methyl carbonate groupsand ethyl triflate that supports the reaction mechanism illustrated by Eqs. (1)–(4) (Scheme 21). These

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 281

CF3SO3CH3 +O

O

OR

R O

O

OR

RCH3 +

-+ CF3SO3

O C

O

OCH3 CH2CR2CH2OSO2CF3

(1)

(2)

+O

O

OR

R

O C

O

OCH3 CH2CR2CH2OSO2CF3

(3)

(4)

O

O

OR

RCH3 +

O

O

OR

R+

O

O

OR

R+O C

O

OCH3 CH2CR2CH2

Scheme 20.

CF3SO3CH3+O C

O

OCH2 CH2(1)

(2)

O C

O

OCH2 CH2

CH3

+

O C

O

OCH2 CH2

CH3

+

-CF3SO3

-CF3SO3

+

CO CH3

OCH2 CH2

O-CF3SO3

+

CO CH3

OCH2 CH2

O-CF3SO3

CH3OCOSO2CF3

O

+ OCH2 CH2 (3)

CH3OCOSO2CF3

OCF3SO3CH3 CO2

+ (4)

Scheme 21.

Page 24: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

results indicate a higher reactivity of unsubstituted 1,3-dioxan-2-one than substituted 5,5-dimethyl-1,3-dioxan-2-one and moreover, in the case of 1,3-dioxan-2-one cationic polymerization can be conducted atlower temperatures. Also Endo et al. reported that using methyl triflate or boron trifluoride etherate,decarboxylation during the polymerization of cyclic carbonates (5,5-dimethyl-1,3-dioxan-2-one, 1,3-dioxan-2-one, 5-n-butyl-1,3-dioxan-2-one, and 1,3-dioxaspiro[5.5]undecan-2-one) occurred, yieldingthe corresponding polycarbonate with ether units (5–10%) in the main chain. Prolonged reactiontime and higher temperature accelerate the decarboxylation [32,44].

Albertsson et al. [31] have reported that bulk polymerization of 1,3-dioxan-2-one, especially athigher temperatures (80–1008C), and preferably when using BF3·OEt2 as initiator, affordedpoly(trimethylene carbonate) with a molecular weight in excess of 100 000 Da. At lower temperature,besides a linear polymer, also cyclic oligomers are formed. Higher reaction temperatures gave ahigher molecular weight and a faster polymerization rate, but caused thermal degradation ofpoly(trimethylene carbonate). The high molecular weight poly(trimethylene carbonate) had a rubberycharacter at room temperature with a glass transition temperature of2178C. The polymer obtainedwith a cationic initiator contained 2.6% ether linkages formed by decarboxylation during polymer-ization at high temperatures. Other cationic initiators such as AlCl3 gave a much lower molecularweight polymer. Polymerization in solution gives mostly oligomers. In less polar solvents, such astoluene, higher molecular weights were obtained, due to the limited solubility of the poly(trimethy-lene carbonate) leading to phase separation. Using dichloromethane and 1,2-dichloroethane assolvents no phase separation was observed but a polymer of molecular weight less than 10 000 Dawas formed.

Recently, Endo et al. [32,44] have found that when alkyl halides were used as cationic initiators forcyclic carbonates the ring-opening polymerization proceeded without decarboxylation and the corre-sponding polycarbonates, but of rather low molecular weights, were formed. The reactions ofpoly(trimethylene carbonate) with several cationic initiators including methyl iodide were monitoredby 1H NMR and gel permeation chromatography. Both, a decrease in the polymer molecular weight andan increase in the ratio of ether units strongly depend on the kind of initiator. PM3 molecular orbitalcalculations using model compounds confirmed that the decarboxylation occurs in the propagation step(Scheme 22).

The direct reaction of 1,3-dioxan-2-one with an excess of alkyl halide such as methyl iodide andbenzyl bromide at 1208C afforded the corresponding 1:1 adducts. The yield of the adduct from methyliodide was only 10%, while the yield of that from benzyl bromide was 78%. The cationic polymeriza-tions of different cyclic carbonates with a few alkyl halides such as methyl iodide (RYyCH3I on Scheme22), benzyl bromide (RYyC6H5CH2Br), and allyl iodide (RYyCH2yCHCH2I) carried out under variousconditions led to the corresponding polycarbonates without any ether units (route a on Scheme 22). Thepolymerization with ethyl 3-iodopropyl carbonate as a model compound was examined to prove thepropagation structure of the active site.

The decarboxylation seems to occur by the attack of the monomer at the propagating end morepreferably than the attack of the polymer at the propagating end. This is because of the higher nucleo-philic properties of a monomer than those of a polymer, as was postulated from PM3 molecular orbitalcalculation. The comparison of the HOMO level of cyclic carbonate (trimethylene carbonate) and linearcarbonate (dimethyl carbonate) indicates that a cyclic carbonate is more nucleophilic than a linear one.On the other hand, the heat of the formation of cationic intermediates suggests that the stabilizationenergy of the formation of the cyclic oxonium cation is 7.4 kcal/mol larger than that of the linear

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342282

Page 25: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

oxonium cation. Therefore, decarboxylation from the monomer should be thermodynamically andkinetically predominant over that from the linear carbonate (polycarbonate).

Taking into consideration that decarboxylation is a competitive process with the propagation one, tosuppress the decarboxylation Endo et al. [32] proposed to use propagating species of lower reactivity,according to the selectivity-reactivity rule governing in organic chemistry. They selected a less reactivehalide ion instead of the triflate one. Alkyl halides, such as methyl iodide and benzyl bromide used ascationic initiators of TMC afforded at 1208C a polycarbonate�Mn � 3000–5000� having no etherlinkages. The lack of polymerizability of 5,5-dialkyl-1,3-dioxan-2-ones is explained by unfavorablenucleophilic substitution at the sterically hindered neopentyl position. In contrast to methyl triflate-initiated polymerization�RY � CH3OSO2CF3 in Scheme 22), in which the resultant polymer hadhydroxyl end groups, the polycarbonate molecules obtained using alkyl halides as a cationic initiatorwere terminated by haloalkyl groups.

A haloalkyl carbonate structure is formed by the initial ring-opening reaction of the monomer withalkyl halide, which was indicated by using ethyl 3-iodopropyl carbonate as the initiator for cycliccarbonate polymerization. The polymer obtained showed both ethyl and iodopropyl end groups.Alkyl halopropylcarbonate is attacked, according this mechanism, by cyclic carbonate at the carbonatom connected with the carbonate linkage to afford biscarbonate via ring opening and a polycarbonatewith alkyl and iodopropyl end groups is formed�Y � I; Scheme 22).

Since the polymerization initiated by ethyl 3-iodopropyl carbonate is faster than that initiated bymethyl iodide, it is suggested that ethyl 3-iodopropyl carbonate is more reactive than methyl iodide andthe halopropyl carbonates are the actual propagating species.

The polymerization of cyclic carbonate with alkyl halide proceeds according to routea (Scheme 22)and owing to the higher nucleophilicity of the halide anion, the covalent macrohalide is more favoredthan the carbenium ion. However, when a solvent of higher polarity is used as the reaction medium,partial decarboxylation takes place due to the shifting of the balance to the carbenium ion species, and anucleophilic attack along routed (Scheme 22), causing decarboxylation, is more probable. High

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 283

O

OO

R-Y

R O O

O

YO

OO

OO

O

R O O

O

O O

OR n

Y

aa

b

+

Y-

b

OO

O

R O OO

+

Y-

O

OOR

Oy O O

Ox

monomer

- CO2

O

OO

c

d O O

On c

d- CO2

Y-

+

Scheme 22.

Page 26: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

molecular weight and ether-unit free polycarbonate by ring-opening polymerization with butyltinchlorides was also reported by Kricheldorf et al. [61].

Information concerning the reaction of cyclic carbonates with alcohols catalyzed by acids, althoughrather scant, has also been given in the literature [169]. Ring-opening reactions of 1,3-dioxan-2-one withalcohols in the presence of trifluoroacetic acid (TFA) proceeded smoothly to afford the ring-openedadducts and corresponding polycarbonates�Mn � 25002 6800� [30]. The molecular weights increasedwith an increase in the conversions of the monomer. The observed polymerization rate was determinedas 0:8 × 1026 s21

: Endo et al. have shown that the signals ofa- and b-methylene protons to thecarbonate moieties were shifted to lower fields by 0.06–0.11 ppm in the1H NMR spectra upon theaddition of TFA. A downfield shift of the carbonyl carbon signals was observed by 4.15 ppm in the13CNMR spectra. These results strongly suggest that the cyclic carbonates are activated by TFA. The chaingrowth seems to proceed by a nucleophilic attack of the hydroxyl end group on the carbonyl group of themonomer activated by TFA (Scheme 23).

The mechanism of cationic polymerization of cyclic carbonate in the presence of alcohol (Scheme 23)is similar to that of oxirane called activated monomer polymerization (AMP) by Penczek and Kubisa[170,171].

3.2.1.1. Cationic polymerization of cyclic carbonates bearing reactive substituents.The presence ofvarious substituents at the carbonate ring may change the reaction mechanism or disturb the polymer-ization pathway of six-membered cyclic carbonates. Endo et al. [44] reported that the cationic poly-merization of a six-membered cyclic carbonate with a norbornene group (1) (Scheme 24) leads to apolycarbonate. However, due to the presence of an olefinic group the polymer structure was not finallysolved, which was a result of the monomer bifunctional structure and participation, besides ring-open-ing, of also an unsaturated group in the cationic polymerization.

In contrast, 5-methylene-1,3-dioxan-2-one (2), obtained from1 in the retro Diels-Alder reaction also

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342284

OO

O

H+

OO

OH

+

ROH

RO O OH

O

H+

-+

Scheme 23.

O O

O

O O

O

1

2

400-450oC

- C5H6

BF3OEt2 O O

O

n

3

crosslinked productBF3OEt2

Scheme 24.

Page 27: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

having two sensitive sites towards cationic polymerization, subjected to polymerization in bulk initiatedwith BF3·OEt2 leads at 608C to linear polycarbonate (3), which has anexo-methylene function in themain chain [40]. The selective ring-opening polymerization of 5-methylene-1,3-dioxan-2-one to thelinear polycarbonate can be explained by assuming that the competitive vinyl polymerization issuppressed by the neighboring electron-withdrawing carbonate group.

3.2.2. Anionic polymerizationIn contrast to polycarbonates obtained with cationic initiators, high molecular weight polymers

produced from six-membered cyclic carbonates with anionic initiators do not contain ether units.This is especially important for oligocarbonate diols, which can be used as polyols for the polyurethanesynthesis. The presence of ether linkages makes the polyurethanes susceptible for oxidation and thatmaterial cannot be utilized in biomedical practice, where besides good mechanical properties, compat-ibility with blood is very significant e.g. for the production of implanted medical devices [172].However, anionic ring-opening polymerization shows an equilibrium character [69], similar to that ofcommon heterocyclic compounds [173], and according to Daiton’s equation, equilibrium monomerconcentration in the post-reaction mixture is present. Carbonate monomers which undergo equilibriumpolymerization can be applied to thermodynamic recycling of polymeric materials [174].

As was earlier mentioned (Section 3.2), in the polymerization of six-membered cyclic carbonatesaccording to the anionic mechanism [62–64], alkali metal alcoholates and K2CO3 can be used asinitiators. Other initiators based on alkali metals aresec-butyllithium as well as sodium- and potassiumnaphthalide [69].

Polymeric Li, Na and K alcoholates were also used for the initiation of the polymerization of six-membered cyclic carbonates. Thus, besides living vinyl polymers [175], hydroxyl group terminatedpolymers of poly(oxyethylene) [176], poly(tetrafuran) [177], and poly(dimethylsiloxane) [178] weretransformed to alcoholates by treatment withsec-BuLi or K-naphthalide and used as initiators.

Polymerization of DTC initiated by poly(THF) alcoholates with different counterions such as Li1,K1, Bu4N

1 proceeds slower than that initiated by poly(EO) alcoholates, what can be explained by thereduced solvation ability of poly(THF).

It was shown that the nucleophilicity of the active species is significantly reduced by exchanging K1

counterion versus Li1. As a result, using Li1 as a counterion, high polymer yields were obtained afterseveral minutes. The lower nucleophilicity of the lithium alcoholate was ascribed to the more covalentcharacter of the lithium-oxygen bond, as compared to the potassium-oxygen one. Moreover, Li1 exhibitsonly a low tendency for complexation with poly(EO); self association prevails, as compared to thecorresponding potassium alcoholates [179].

Commonly used alkoxide and alkyllithium initiators [10,35,36,68] in anionic ring-opening polymer-ization of cyclic carbonates seem not to be useful for industrial application because of their instabilitiesand high reactivities. Recently, Murayama et al. [43] have found that amine initiators such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4-(dimethylami-no)pyridine (DMAP) could initiate the anionic ring-opening polymerization of cyclic carbonates toafford the corresponding polycarbonate (Scheme 27). At 1208C in short time (1 h) a cyclic carbonatehaving a norbornene structure (5,5-(bicyclo[2.2.1]hept-2-en-5,5-ylidene-1,3-dioxan-2-one) (NBC)initiated by DBU yielded a polycarbonate of 6400 Da. No polymer was obtained when triethylamine,aniline,N,N-dimethylaniline or pyridine were used for the cyclic carbonate polymerization. The muchlower activity of triethylamine than that of DABCO might be caused by the steric hindrance around the

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 285

Page 28: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

nitrogen atom, while aromatic amines showed low activities due to the aromatic resonance effectdecreasing their nucleophilicities.

A plausible zwitterionic polymerization mechanism (Scheme 25) was confirmed by FD-MS spectrumanalysis of the polymerization products, where besides cyclic oligomers a linear polymer with DBU endgroup was found. Thus, the reaction between DBU and cyclic carbonate to form an alkoxide anion is theinitiating step of the polymerization. In the propagation reaction the alkoxide anion attacks the carbonylgroup of cyclic carbonate to yield the corresponding polycarbonate.

It is worth mentioning that pure six-membered cyclic carbonate such as TMC (R� H) undergoesspontaneous polymerization in bulk at above 1008C [85]. In the plausible formulation of a TMCcleavage, a zwitterion intermediate with the trioxocarbenium ion, well stabilized by delocalization,and an alkoxide ion is formed (Scheme 26). The highly reactive alkoxide ion can initiate chain growthaccording to the anionic mechanism.

In contrast, despite high purity, DTC does not undergo spontaneous polymerization up to 1258C. Twogeminal methyl groups considerably stabilize the cyclic structure and the equilibrium in Scheme 26�R� CH3� is shifted to the cyclic carbonate.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342286

N N

DBU

+ O O

ON N O

O

O+-

N N O

O

O+-

+O O

ON N O

O

O O

O

On +

-

Scheme 25.

O O

O

+CH3

CH3

OSi(CH3)3

O CH3

O O

On (C4H9)4NF 3H2O

.

Scheme 27.

O

O

OR

R

O

O

OR

R+

> 125oCO

O

OOO

OR

R

R

R- +

R = H or CH3

Scheme 26.

Page 29: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The presence of alkylating agents such as CCl4 (a residue of solvent used in recrystallization) inhibitsthe spontaneous polymerization. An inhibitor effect can only be rationalized in the case of anionicpolymerization. The possibility of obtaining high molecular weight polycarbonates without toxic initia-tors, such as heavy metal ions, is of interest as a preparative method, because it permits the synthesis ofbiodegradable or biocompatible materials without toxic impurities.

It was suggested that the polymerization of six-membered cyclic carbonates might proceed by thegroup transfer polymerization mechanism (GTP) (Scheme 27) [180]. Ho¨cker et al. [67] reported that thepolymerization of DTC took place under GTP conditions: 1-methoxy-2-methyl-1-trimethylsiloxy-propene used as initiator and tetrabutylammonium fluoride as catalyst. However, the probable explana-tion for the polymerization data consistent with the experimental data is for a metal-free anionicmechanism [181], a quaternary ammonium alcoholate seems to be the likely propagating species.

For the synthesis of block copolymer, poly(MMA)-b-poly(DTC) [182] the combination of the grouptransfer polymerization of MMA [183] with anionic metal free ring-opening polymerization of DTC wasapplied [67].

Anionic metal-free initiation based on quaternary ammonium alcoholate was successfully applied tocyclic carbonates [67,184]. Ammonium alcoholate was formed in the reaction of a silyl ether withammonium fluoride, Bu4NF (Scheme 28).

An interesting example of the anionic polymerization of a special cyclic carbonate initiated with azo-containing radical initiators such as azobis(isobutyronitrile) (AIBN) was reported by Endo et al. [39] Thecyclic carbonate bearing anexo-methylene group (5-methylene-1,3-dioxan-2-one,1) in polymerizationwith AIBN in chlorobenzene afforded a polymer in 43% yield�Mn � 8900; Mw=Mn � 2:76�. A 1H NMRanalysis indicated that the polymer formed with AIBN consists of two kinds of units: the ring openingpolymerization unit (a) and the vinyl polymerization unit (b) (Scheme 29).

The vinyl polymerization takes place initially, but the ring-opening polymerization gradually predo-minates over the vinyl one. The presence ofa unit in the polymer was observed, when besides AIBN an

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 287

OCH2CH2O Si(CH3)3

Bu4NF+

- FSi(CH3)3

OCH2CH2O- Bu4N

+

OCH2CH2O- Bu4N

+ OCH2CH2O C

O

O O

O O

O - Bu4N+

Scheme 28.

O

O

OAIBN

O O

O O

On

O

m

a

b

1

Scheme 29.

Page 30: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

amine was added. In contrast, a polymer mainly having the vinyl polymerization unitb was formed inthe presence of acetic acid under the same conditions. These results indicate the occurrence of anionicpolymerization in addition to the radical one.

The effect of alkyl substituents positioned at carbon atoms 4 and 5 of 1,3-dioxan-2-one on theirtendencies to undergo reversible polymerization was described. Sarel et al. suggested that alkyl substi-tuents at the 2-position in propane-1,3-diol enhanced the formation of cyclic carbonates, because thesteric repulsion of the substituent was larger in the polymer than that in the cyclic monomer. A tentativespiral structure (a-helix) having a six-membered ring fold was proposed for the polymer chain. Such anarrangement would be maintained by the dipole-dipole interactions between O-atoms of one O–C(O)–Ogroup with the CO of such a vicinal group along the chain.

In the 1990s Endo et al. [29] have also examined anionic equilibrium polymerization behavior ofseveral six-membered cyclic carbonates containing two substituents at the 5-position of 1,3-dioxan-2-one. The conversions of the monomers reached a constant below 100%, and the final conversiondecreased in the order of 1,3-dioxan-2-one. 5,5-dimethyl-1,3-dioxan-2-one. 5,5-diethyl-1,3-dioxan-2-one greater than or equal to 5-methyl-5-phenyl-1,3-dioxan-2-one. 5-ethyl-5-phenyl-1,3-dioxan-2-one. Thermodynamic parameters were estimated in the anionic ring-opening polymerizationsof cyclic carbonates by Dainton’s equation. The obtainedDHp value in the ring-opening polymerizationof each cyclic carbonate reflected their polymerizability. Molecular orbital calculations of the modelcompounds of the polymers were carried out to find whether the polymerizabilities of the cycliccarbonates correlated with the stabilities of the corresponding polymer structures. The steric repulsionof substituents on the polymer chain is responsible for the thermodynamic stability of the polycarbonate.The concentrations of the monomers formed in the depolymerization of different polycarbonates usingt-BuOK in THF well agreed with the equilibrium monomer concentrations in the anionic ring-openingpolymerizations. The cyclic carbonate with more bulky substituents showed a higher equilibrium

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342288

O O O O

O

R1

R2

O

OHR

1

R2

t-BuO-

- t-BuOH

O O O O

O

R1

R2

O

OR

1

R2

route a

O O O O

O

R1

R2

O

R1

R2

-

t-BuO-

route b

O O

O

OR

1

R2

+ O O

O

R1

R2t-Bu

Scheme 30.

Page 31: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

monomer concentration. Two reaction routes are proposed as plausible in the depolymerization(Scheme 30).

In routea the alkoxide end group of the polymer attacks the carbonate linkage in the main chain, and acyclic carbonate is formed as a result of back-biting process. Routeb, in which the initiator attacksdirectly the polymer carbonate linkages, seems to be more probable, because besides the cyclic carbo-nate, large amounts of oligomers are usually observed in the post-reaction mixture.

In the anionic ring-opening polymerization of six-membered cyclic carbonate having two aromaticsubstituents, 5,5-diphenyl-1,3-dioxan-2-one showed only slight conversion [11]. This is due to a rapidback-biting reaction of the propagating polymer end leading to the formation of the starting monomer.The conformational restriction of the adduct of the cyclic carbonate with an alkoxide, originating from theelectrostatic repulsion between the alkoxide anion and thep-electrons of the aromatic rings, might causethe rapid back-biting reaction. However, the anionic copolymerization of 5,5-diphenyl-1,3-dioxan-2-onewith 5,5-dimethyl-1,3-dioxan-2-one proceeds to afford the copolymer. The presence of a phenyl substi-tuent at the monomer ring caused a relatively high volume expansion (10.8%) during the cyclic carbonatepolymerization [11]. The presence of different substituents at 2-position of trimethylene carbonatedetermines also the properties of the corresponding polycarbonates. The thermal properties (Tg andTm) of the polymers obtained from 2,2-disubstituted trimethylene carbonates are collected in Table 3.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 289

Table 3Thermal properties of poly(2,2-disubstituted trimethylene carbonate)s:

O O

O

R1

R2

R1 R2 Tg (8C) Tm (8C) References

H H 2 17.2 – [29–32]CH3 CH3 27 86/121; 123 [29,32,34]C2H5 CH2OH 27 117 [35]C2H5 CH2OC(O)NHPh 40 71 [35]C2H5 CH2OCH2CH� CH2 2 29.8 – [36]CH3 CN 68.8 118.1/131.3; 132.9 [37]CH3 COOCH3 8.7 85.8 [38]C2H5 CH2OCOCH3 2 7.3; 21.2 47.7 [38]CH3 CH2OCOCH3 2 9.3; 25.5 – [38]CH2OH CH2OH – 163/179; 147 [185]

–CH2OCH2OCH2– 36 202 [41]–CH2OC(CH3)2OCH2– 99; 68 199 [185]

CH2

60, 51 117 [43–45]

Page 32: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Although there are many reports on the polymerization of six-membered cyclic carbonates containingvarious substituents, the polymerization of optically active cyclic carbonates has been scarcely reported.Recently, Endo et al. [47] revealed that the cyclic carbonate obtained by the reduction of Z-protectedl-threonine and subsequent reaction with triphosgene when subjected to the polymerization initiated withtert-BuLi or tert-BuOK affords an optically active and chemically functional polycarbonate (Scheme 31).

In the case of six-membered aliphatic-aromatic cyclic carbonate (benzo-1,3-dioxan-2-one) (1), theanionic ring-opening polymerization yields a polymer with a lower concentration of carbonyl groupsthan in the starting monomer, and since of insolubility of a not thoroughly characterized structure [153].

O

O

O1

3.2.2.1. Anionic polymerization of cyclic carbonates bearing reactive substituents.5-Allyloxymethyl-5-ethyl-1,3-dioxan-2-one, a cyclic carbonate with functionality in the substituent, can be subjected toanionic ring opening polymerization, since in contrast to cationic polymerization, the allyl group doesnot interfere with the polymerization process [36]. The homo- and copolymers of 5-allyloxymethyl-5-ethyl-1,3-dioxan-2-one are suitable for selective crosslinking via the pendant allyl system. 5-Allyloxy-methyl-5-ethyl-1,3-dioxan-2-one was polymerized in toluene at 08C with sec-butyllithium as initiator.The resultant product exhibits a bimodal molecular weight distribution. The low molecular weightfraction consists mainly of cyclic oligomers and the higher one consists of acyclic macromolecules.The polycarbonate with allyloxymethylene pendant groups can be crosslinked using typical free radicalinitiators such as di-tert-butyl peroxide. It was shown that crosslinking leads to a rise of the glasstransition temperature, from229.88C of the linear polymer to 138C of the crosslinked one. Due tothe presence of the allyl groups in the side chain, storage of the polymer in the air causes unintentionalcrosslinking upon initiation by atmospheric oxygen. The lack of crystallinity of the polycarbonate can beexplained by the atactic arrangement of the two different substituents.

To obtain a polycarbonate with hydroxymethyl pendant groups, six-membered cyclic carbonates withdifferent protected hydroxyl groups (trimethylsilyl, benzyloxycarbonyl and phenylcarbamoyl) were usedas monomers in the anionic polymerization initiated by lithium alkoxide (Scheme 32). The resultantpolymers subjected to hydrolysis or hydrogenation, by which the protective groups were removed,afforded the polycarbonate with pendant hydroxymethyl groups [35].

It should be mentioned that when a more nucleophilic initiator, such assec-butyllithium wasused in the polymerization of 5-benzyloxycarbonyloxymethyl-5-ethyltrimethylene carbonate

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342290

O O

O

CH3

NH

Z Z = CO2CH2Ph

NH

O O

O CH3

n

Z

tert-BuOLi

- 78oC, 1 h

Scheme 31.

Page 33: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

�R� C�O�OCH2Ph�; a crosslinked product was formed. Mild reaction conditions (ROLi,2308C)enabled to suppress the branching and crosslinking (of which a trifunctional monomer potentiallyundergoes) resultant from the transesterification reaction between the main-chain and side-chaincarbonate groups.

Rokicki et al. have found that a linear polycarbonate with methoxycarbonyloxymethyl pendant groupscan be formed in situ during the reaction between trifunctional trimethylolpropane and dimethyl carbo-nate catalyzed by K2CO3 [33]. A cyclic carbonate with a pendant carbonate group, 5-ethyl-5-methoxy-carbonyloxymethyl-1,3-dioxan-2-one, subjected to anionic polymerization initiated by K2CO3, alsoyielded a non-crosslinked product.

As concerns the anionic polymerization of other six-membered cyclic carbonates having functionalgroups such as ester functional groups, the kinetics of their polymerization revealed that the esterside chain enhances the rate of propagation, as compared to the polymerization of DTC [38]. Thepolymerization of 2-acetoxymethyl-2-alkyl-trimethylene carbonates�alkyl � methyl : AMTC,alkyl � ethyl : AETC) and of 2-methoxycarbonyl-2-methyltrimethylene carbonate (MMTC) (Scheme33) in toluene withsec-BuLi as initiator results in the respective polymer with yields between 78 and88%.

In contrast, when aluminum-based [Al(Os-Bu)3] catalyst was used no polymerization took place,probably because of intramolecular complexation of the active species by the adjacent ester group:

CH3

OO

OO

O Al

CH3

OiBu(Os-Bu)2

CH3

OO

OO

O Al

OCH3

iBu(Os-Bu)2

AMTC + MMTC +Al(Os-Bu)3Al(Os-Bu)3

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 291

O O

OH

O

O O

OR

O

O O

O

OR

n

O O

O

OR

n

O O

O

OH

n

OH group protection polymerization

ROLi , -20 - -50oC

deprotection:

hydrolysis or hydrogenation

R = Si(CH3)3, CH2Ph orOCO

CH2PhNHCO

Scheme 32.

Page 34: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

However, the proposed structure of the non-active species seems to be in contradiction to the resultsconcerning aluminum alkoxides derived from donor-functionalized alcohols. For example, dialkylalu-minum derivatives of hydroxycarbonyl compounds form the five-coordinated aluminum dimeric speciesas was recently reported by Lewinski et al. [186]; simple trialkoxide aluminum compounds are evenmore complex [187].

The analysis of the polymer microstructure by means of NMR spectroscopy reveals linear chainswithout branching, which indicated no attack of the active chain end at the ester moiety of the repeatingunits. Poly(MMTC) and poly(AETC) afford crystalline materials upon precipitation from solution, whilepoly(AMTC) is amorphous.

3.2.3. Coordination polymerizationThe coordination polymerization of six-membered cyclic carbonates carried out in the presence of

metal carboxylates, e.g. zinc stearate [78], tin-based catalysts such as the di-n-butylstannic diio-dide—triphenylphosphine system [83,84] or porphinatoaluminum compounds like (TPP)AlOR [66]is not accompanied by the decarboxylation and yields respective polycarbonates (Table 2). The ringcleavage during the polymerization of TMC and DTC in the presence of the above catalysts wasfound [66,78,188] to occur at the C(O)–O bond, resulting in the formation of metal alcoholatepropagating species. The living character of the polymerization the (TPP)AlOR catalyst is of interest[66].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342292

OH OH

CH3

OH1.Et2CO3, CH3ONa

2. Bu2Sn(OCH3)2

O O

O

CH3

OH

CH3COCl/CH2Cl2/Et3N

O O

O

CH3

OCOCH3

AMTC

OH OH

CH3COOH

CH3OH/H+

OH OH

CH3COOCH3

O O

CH3COOCH3

O

C6H5OCOCl/THF/Et3N

MMTC

Scheme 33.

Page 35: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

NN

NN

Ph Ph

Ph

Ph

Al

OR

(TPP)AlOR

With Bu2Mg as a catalyst the polymerization of DTC yields a product with bimodal molecular weightdistribution [70], i.e. a high molecular weight polymer and a low molecular fraction consisting of cyclicoligomers.

The polymerization of DTC with tri-sec-butoxyaluminum [Al(Osec-Bu)3], diethyl zinc (ZnEt2), anddibutyldimethoxytin [Bu2Sn(OMe)2] in toluene as a solvent reveals a clear distinction between thealuminum- and zinc-based initiators on the one hand and the tin-based initiator on the other. For tinthe active site, the rates of back-biting reactions (intramolecular transesterification) and intermoleculartransesterification reactions are as high as the rate of the propagation reaction, while for aluminum- andzinc-based initiators, the rate of the propagation reaction is much higher than that of transesterification[76]. Hence, within the scope of a kinetic treatment of the polymerization of DTC with Al(Osec-Bu)3 asa catalyst only propagation reaction must be considered [81].

Two different mechanisms have been proposed for ring-opening polymerization of six-membered cycliccarbonates depending on the nature of the organometallic derivatives. Metal halides, oxides and carboxylatesact as Lewis acid catalysts in a ROP actually initiated with water or alcohol. Polymerization is assumed toproceed through an insertion mechanism (Eq. (1) in Scheme 34). On the other hand, when metal alkoxidescontaining free p-, d-, or f-orbitals of a favorable energy are used as initiators, a two-step “coordination-insertion” mechanism takes place, which consists of the cyclic carbonate complexation onto the propagationspecies, followed by rearrangement of covalent bonds leading to the cleavage of a metal-oxygen bond of thepropagating species and the acyl-oxygen bond of the cyclic monomer (Eq. (2) in Scheme 34).

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 293

O O

O

H

SnXOR

RO O O

O

O

O

OH SnX2 (1)

O O

O

MX

OR

n

RO O O

O

O

O

O MX n

O O

O

O O

O(2)

2

Scheme 34.

Page 36: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

SnOct2 is known to be a highly efficient initiator of cyclic esters (lactides) polymerization [59,189].Moreover, tin octoate is also the efficient initiator in polymerization of cyclic carbonates as was found byKricheldorf et al. [78] The molecular weights of SnOct2-initiated polycarbonates correlate with theM/Iratios more or less. This finding and the1H NMR spectra suggest that stoichiometric reactions betweenmonomer and SnOct2 take place, yielding polymers with covalently bound octoate and OH end groups.2,2-Dimethyltrimethylene carbonate recrystallized from CCl4 polymerizes more rapidly and giveshigher molecular weights than DTC recrystallized from THF. This result can be explained by assump-tion that the small amounts of THF complex SnOct2, which is a strong Lewis acid [77]. Taking intoconsideration the strong influence of the monomer/initiator ratio on the molecular weight one cansuggest the stoichiometric reaction between SnOct2 and monomer that limits the molecular weight.Two reaction pathways are proposed for ring-opening polymerization of six-membered cyclic carbo-nates with SnOct2: one in the presence of small amount of water or free 1,3-diol (routea), and secondwithout such impurities (routeb) (Scheme 35).

BuSnCl3 seems to be an even more attractive initiator. The polymerization of DTC in bulk at 1208C

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342294

Oct2Sn...OH2

OO

O...

Oct2Sn... OH

O OHCH3

CH3 O

- CO2

Oct2SnOH

OH

CH3

CH3

.. .

.

Oct2SnOH

OH

CH3

CH3

....

OO

O OH

O

CH3

CH3

OH

OCH3

CH3

OOct2Sn....

aroute

OO

O...

OctSnO O OCH3

CH3 O

R

O

- CO2

OO

O

broute

OctSn O C RO

OctSnO O RCH3

CH3 O

OctSnO OCH3

CH3 O

OctSnO

CH3

CH3O

O OCH3

CH3

O

O

R

Scheme 35.

Page 37: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

initiated with BuSnCl3 was more rapid and yielded higher molecular weights (Mn up to 150 000 andMw

up to 270 000). The polymer obtained had up to 50% of crystallinity degree. Furthermore, the molecularweights were almost independent of theM/I ratio. The melting point (Tm) of 1278C was the highestreported so far for this polycarbonate.

A salt of rare earth metals can be used as an initiator with good results for the polymerization of cycliccarbonates. Thus, bulk polymerization of TMC conducted at 808C using rare earth chloride as catalystafforded poly(TMC). The polymerization reaction showed an induction period. The polymerizationactivity of various rare earth chlorides was in the order: LaCl3 . YCl3, PrCl3 . NdCl3 much greaterthan DyCl3 [72].

An interesting example of using an Al-based catalyst in ring-opening polymerization was reported byCarter et al. [82] Initiators of the formula AlEt32x(OR)x, wherex� 1–2; R� �CH2�2PhNO2; are efficientinitiators for the solution polymerization of 1,3-dioxan-2-one, and it is possible to obtain polymers witheach chain terminated by a single nitrophenyl group�Mn � 9500� (Scheme 36). These nitro groups afterreduction under mild conditions afforded amino-terminated polycarbonates. In these polymerizations,molecular weight is proportional to the monomer/initiator ratio, with each polymer chain containing onenitrophenyl or aminophenyl functionality.

This amino-functionalized polycarbonate is used in the copoly(imide-carbonate), ABA-type blockpolymer formation, which can be utilized in nanofoam production. Aliphatic polycarbonates degradequantitatively in an inert atmosphere into cyclic monomers and other products of low molecular weightleaving pores, the size and shape of which depend on the initial block copolymer morphology.

3.2.3.1. Coordination polymerization of cyclic carbonates bearing spiroconnected groups.The cycliccarbonate monomer 2,4,8,10-tetraoxaspiro[5,5]undecane-3-one (DOXTC) homopolymerization andcopolymerizations with TMC were carried out in bulk using aluminoxanes [methyl- (MAO) andiso-butyl-aluminoxane (IBAO)] as catalysts (Scheme 37) [41].

The copolymer yields normally exceeded 90%. When the DOXTC content in the monomer feed was# 3:7 the polymerization at 908C for 2 h resulted in 96% yield, andMn � 40 000 Da. The polymeriza-tion with MAO and IBAO gives polycarbonates and copolycarbonates showing no evidence ofdecarboxylation. Copolymers in which the DOXTC content is greater than 70 mol% are insoluble in

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 295

O2N

O AlEt2+

O O

O

ROP

H3O+

O2N

O O O

O

Hn

NH2

O O O

O

Hn

H2

Pd/C

n

Scheme 36.

Page 38: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

a majority of organic solvents. Studies by DSC showed that the DOXTC homopolymer, as well as thecopolymers with high DOXTC content, were semicrystalline. By increasing the molar content of 1,3-dioxane side groups in DOXTC-TMC copolymers from 0 to 50%, the water uptake by the correspondingfilms was increased from 5 to 19% (w/w), which is important in the case of polycarbonate biodegrada-tion.

Vandenberg and Tian [42] used Zn- and Al-based organometallic catalysts for the polymerization ofsimilar cyclic carbonate bearing a spiroconnected 1,3-dioxane group: 9,9-dimethyl-2,4,8,10-tetraoxas-piro[5,5]undecan-3-one. The cyclic carbonate obtained from the monoacetal diol of pentaerythritol waspolymerized in CHCl3 at 608C in 4 h to a quantitative yield of high molecular weight crystallinepolymer, melt point of 1998C andTg of 998C (Scheme 38).

The resultant polycarbonate is readily hydrolyzed with 80% acetic acid to the water-insoluble butwater-swollen poly[2,2-bis(hydroxymethyl)trimethylene carbonate] withMw � 310 000: This newfunctional aliphatic polycarbonate is soluble in polar solvents such as DMSO, DMF, and DMAC, andcompletely dissolved in water only at higher temperature (.708C) with complete degradation. Apolycarbonate bearing hydroxymethyl pendant groups (HPC), seems to be an attractive material forbiomedical area application. Hydroxy functional polymers can be used to bind drugs, proteins, or

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342296

O O

OH OHEtOC(O)Cl

Et3N ,in THF, 0oC

O O

O O

OOO

O O

On MAO

or IBAO

DOXTC

Scheme 37.

OH

OHOH

OH

+ CH3 CH3

O

O

CH3

CH3

p-TSA

OHOH

O O

EtOC(O)Cl

Et3N,THF,0oC

O O

O O

O

O O

O O

O

ZnEt2

CHCl3, 60oC

OO

O O

On

OH

OH

O O

On

80% HAc

HPC

Scheme 38.

Page 39: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

carbohydrate polymers chemically or via hydrogen bonding to facilitate drug delivery and utility withsubsequent biodegradability to acceptable byproducts. HPC degrades completely in vitro in,16 h inPBS-1X buffer�pH� 7:4; 378C� to pentaerythritol and presumably CO2. This rapid degradation rate isdecreased with random copolymers of HPC with trimethylene carbonate,e-caprolactone, orl-lactide.

3.2.4. Enzymatic polymerizationMany bacteria synthesize accumulate and deposit aliphatic polyesters in the cells. The high stereo-

selectivity of the enzymatic synthesis produces as a rule polyesters with high crystallinity which haveattracted a great deal of attention during the last few years [190,191]. The enzymatic ring-openingpolymerization ofd -valerolactone ande -caprolactone was first conducted using lipase as a catalyst[192].

On the other hand, it is known that the introduction of carbonate groups into the polymer chain leadsto improving the mechanical properties of biodegradable polyesters [193]. The carbonate groups in thepolymer chain are enzymatically hydrolyzable but more resistant towards hydrolysis than ester groups.The copolymerization of cyclic lactones and carbonates needs pure monomers and anhydrous conditionsas well as organometallic catalysts, which must be completely removed before use in medical applica-tions. In contrast to chemical methods, enzyme-catalyzed polymerization of six-membered cyclic estersand carbonates seems to be the practicable method avoiding the above mentioned difficulties. Kobayashiet al. [194] for the first have shown that lipase not only catalyzes the hydrolysis of esters and carbonates[86] but can initiate ring-opening polymerization. Enzymatic ring-opening polymerization of a six-membered cyclic carbonate, 1,3-dioxan-2-one, was investigated by using lipase as catalyst in bulk.Similarly, supported lipase derived fromCandida antarcticalipase catalyzes the polymerization togive the corresponding aliphatic polycarbonate.

Bisht et al. [87] also directed at extending the use of lipase-catalyzed ring-opening polymerizations tocyclic carbonate monomers. From several lipases screened for bulk TMC polymerization (708C, 120 h),Novozym-435 (triacylglycerol hydrolase1 carboxylesterase) fromCandida antarcticagave almostquantitative monomer conversion (97%) and poly(TMC) with aMn � 15 000 �Mw=Mn � 2:2� andwith no decarboxylation during propagation. The lipases fromPseudomonasspecies (AK and PS-30)andporcine pancreas(PPL) also exhibited high monomer conversions (.80%, 120 h) but gave lowermolecular weight polymers with broad polydispersity. Analyses by1H NMR spectroscopy suggestedthat poly(TMC) prepared by Novozym-435-catalyzed polymerization had terminal –CH2OH function-alities at both chain ends. Novozym-435-catalyzed TMC bulk polymerization at 708C has chain-typepropagation kinetics. The highest poly(TMC) molecular weight�Mn � 24 400� was obtained byconducting the polymerization at 558C. The reaction temperature increase leads to lower molecularweight (at 858C, 6000 Da). Increasing the water content resulted in enhanced polymerization rates anddecreased molecular weights. It is proposed that the polymerization rates increase due to an increase inthe number of propagating chain ends [195].

Separation of the oligomeric products from the polymerizations of TMC in dried dioxane and toluenecatalyzed byporcine pancreatic lipaseled to the isolation of di- and tri-adducts of trimethylene carbo-nate. Bisht et al. [87] proposed a mechanism for chain initiation and propagation for lipase-catalyzedTMC polymerization, based on the symmetrical structure of these products and the end-group structureof high molecular weight chains (Scheme 39).

In contrast to the results obtained by Bisht and co-workers, Matsumura et al. [88] asserted that nopolymerization of TMC took place when lipase Novozym-435 was used at 1008C, while porcine

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 297

Page 40: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

pancreatic lipase(PPL) showed the best results with respect to the monomer conversion and themolecular weight of the polycarbonate. TMC readily polymerized in bulk in the presence of PPLyielding a polycarbonate withMw of up to 170 000 at 1008C after 24 h. The enzymatic polymerizationof TMC was significantly enhanced by the immobilization of PPL on Celite.

This apparent contradiction is probably caused by the different reaction temperatures applied [194]and different water content. At 1008C the lipase Novozym-435 seems to be inactive, and at lowertemperatures (55–608C) exhibits the highest activity in the ring-opening polymerization of TMCamong the tested enzymes.

The higher activity of lipase Novozym-435 over PPL was also confirmed by Deng et al. [89] in thestudy of lipase-catalyzed ring-opening copolymerization ofe -caprolactone (e-CL) and TMC. Anincrease in the lipase concentration ine-CL and TMC polymerization leads to higher initial rates ofconversion. In contrast, theMn decreased at the same level of conversion with increasing catalystconcentration.

It is suggested that for higher activity some amount of water should be present in lipase [87,195,196],however, higher concentration of water and suitable pH lead to enantioselective hydrolysis of substitutedcyclic carbonate. The reaction of a six-membered cyclic carbonate, 4-(2-benzyloxyethyl)-1,3-dioxan-2-one, with porcine pancreas lipasein phosphate buffer containing 50% ofi-Pr2O at 08C proceedsenantioselectively to afford optically active (S)-4-(2-benzyloxyethyl)-1,3-dioxan-2-one and (S)-5-benzyloxypropane-1,3-diol (Scheme 40) [86].

Taking into consideration that aliphatic polycarbonates and poly(ester-carbonate)s seem to be

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342298

E OH + O O

O

E O O OH

O

E O O OH

O

+ OH2 OH O OH

O

E OH+

- CO2

OH OH

E O O OH

O

OH OH+

OH O O OH

O

E OH+

E O O OH

O

OH O O OH

O

n +

OH O O OH

O

n E OH+

+1

Initiation

Propagation

Scheme 39.

Page 41: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

potential biodegradable or bioabsorbable materials, special interest has been devoted to the so calledbiocompatible initiators, which are part of the human metabolism. Hematin, an insoluble pigmentformed from the breakdown of hemoglobin, is an example of an enzyme-type initiator used in thepolymerization of heterocyclic monomers. In 1993 Kricheldorf and Boettcher used hematin in thepolymerization of cyclic esters:l,l- andl,d-dilactides [197]. Recently, hematin was also utilized inthe polymerization of TMC and DTC [85]. The polymerization was conducted in bulk at 1008C, but highyield and high molecular weights (up to 75 000 Da) were obtained only for unsubstituted 1,3-dioxan-2-one. No polymerization was observed for the reaction carried out in toluene, even at 1008C.

The hematin molecule possesses two types of functional groups: the Fe–OH group and three carboxylgroups, which can react with cyclic carbonate. The postulated insertion mechanism involves initiationby the Fe–OH group and subsequent alkoxide formation with CO2 evolution (Scheme 41).

Ring-opening polymerization of cyclic carbonates initiated by enzyme or enzyme like species inwhich polycarbonates terminated by hydroxyl groups are obtained seems very attractive, especiallyfor the production of polyurethanes for medical application.

3.3. Polymerization of seven-membered and of larger ring size cyclic carbonates

3.3.1. Polymerization of seven-membered cyclic carbonatesThe ring-opening polymerization of seven-membered cyclic carbonate (1,3-dioxepan-2-one), due to

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 299

O O

O

R PPLO RO

O

OH ROH+

dl (S) (S) R = OCH2Ph

50 % i-Pr2O,0oC

(R) R = (CH2)8CH3R = OCH2Ph or (CH2)8CH3

Scheme 40.

Fe-OH

OO

O

...Fe O O OH

O

Fe O OH

- CO2

Fe O OH

OO

O...

Fe O O O

O

OH

hematin

Scheme 41.

Page 42: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

ring strain, seems to proceed easier than that of the six-membered one. However, the number of reportsconcerning the polymerization of seven-membered cyclic carbonate is rather limited, mainly because ofdifficulty in the synthesis of the monomer. In contrast to 1,3-dioxan-2-one, seven-membered tetramethy-lene carbonate is thermally unstable and difficult to isolate and purify [51].

The anionic ring-opening polymerization of tetramethylene carbonate initiated withsec-BuLi andcarried out in THF affords the corresponding polycarbonate in a relatively high yield in a short time(Scheme 42) [49].

It was shown that the polymerization carried out at higher temperature and for the lower initialmonomer concentration led to a lower yield, lower molecular weight of the polymer and higher contentof cyclic oligomers. These results can be explained by the formation of cyclic oligomers via back-bitingreaction proceeding besides the propagation reaction, which is characteristic for equilibrium polymer-izations. However, the relative polymerization rate of tetramethylene carbonate is about 35 times fasterthan that of TMC. The drastic difference in the polymerization rates is caused by the larger ring strain.The negatively largerDHp for tetramethylene carbonate in comparison with that of trimethylene wasestimated by the MO calculations [51].

As concerns the cationic ring-opening polymerization of seven-membered cyclic carbonate, it wasfound [48] that in contrast to that of six-membered cyclic carbonate, the corresponding polymer�Mn �10 000–57 000� had no polyether units. The polymerization carried out in the presence of typical cationicinitiators: CH3OSO2CF3, C2H5OSO2CF3, HOSO2CF3 and SnCl4 proceeds at 208C about 100 times fasterthan that of TMC and is not accompanied by elimination of carbon dioxide. The decarboxylation, in thecase of the polymerization of seven-membered cyclic carbonate may be suppressed since the propaga-tion reaction proceeds predominantly to back-biting degradation (compare Schemes 20 and 21). Theactivation energies in the polymerization of tetramethylene and trimethylene carbonates were estimatedto be 6.27 and 8.52 kcal/mol, respectively. Tetramethylene carbonate, similarly to trimethylene carbo-nate, can be polymerized according to a cationic mechanism using an alcohol-acid catalyst to afford thecorresponding polycarbonates�Mn � 2500–6800�: [30] Also similarly to 1,3-dioxan-2-ones, 1,3-dioxe-pan-2-one undergoes spontaneous polymerization in bulk at above 1008C affording the correspondingpolycarbonate [85].

It is worthwhile to note that living ring-opening polymerization of 1,3-dioxepan-2-one can beperformed using the cationic zirconocene complex [Cp2ZrMe]1 [B(C6F5)4]

2 as a catalyst at roomtemperature [50]. A linear relation between conversion and molecular weight of the obtained polymerwas observed. Furthermore, block copolymerization of the cyclic carbonate ande -caprolactone wassuccessfully performed. The molecular weight distribution is maintained narrow during polymerization,and that of the resultant block copolymer was determined to be 1.17.

The polymerization is assumed to proceed according to a cationic mechanism initiated by the attack of

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342300

OO

O

O O

On sec-BuLi O O

O

m

+

Scheme 42.

Page 43: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

carbonyl oxygen to the zirconium cation and the polymer chain grew via O-alkyl bond cleavage(Scheme 43).

Also a novel catalyst, 2,20-methylene-bis(6-tert-butyl-4-methylphenolate) titanium dichloride (1)initiates the living polymerization of 1,3-dioxepan-2-one to give polymers with narrow molecularweight distributions. The molecular weight of the polycarbonate obtained can be controlled by changingthe initial mole ratio of cyclic carbonate and catalyst1. It is characteristic that the polycarbonate formedin the presence of this catalyst has hydroxymethylene terminal units at both ends [198].

Ti

O O

Cl Cl

1

3.3.2. Polymerization of cyclobis(alkylene carbonate)sAs was shown by Kricheldorf and Mahler, to obtain poly(tetramethylene carbonate) it is more

convenient to use cyclobis(tetramethylene carbonate) instead of its monomeric form: 1,3-dioxepan-2-one [51]. Cyclobis(tetramethylene carbonate) is easy to isolate and purify by recrystallization. Thepolymerization withn-BuSnCl3 or SnOct2 as initiator was carried out in bulk, because the polymeriza-tion in solution (CH2Cl2) at lower temperatures is very slow. The monomer’s high melting point (mp172–1748C) impels that the polymerization of cyclobis(tetramethylene carbonate) in bulk requires ahigh temperature (.1808C). However, despite high polymerization temperature no ether groupsformation was observed, which is common for the cationic polymerization of cyclic carbonates.

The 1H NMR studies revealed in the case of SnOct2-initiated samples that these polycarbonatespossess covalently bonded octoate end groups and OH end groups. These end groups suggest that thepolymerizations mainly follow the reaction sequences outlined in Scheme 44.

It is particularly interesting that poly(tetramethylene carbonate) crystallizes upon annealing, whereaspoly(trimethylene carbonate) does not crystallize despite a higher concentration of polar carbonategroups. The susceptibility to degradation at relatively low temperatures is a second interesting featureof this polycarbonate. The degradation to CO2 and tetrahydrofuran is complete at approximately 3408Cwithout leaving any residue. The material exhibiting such a complete degradation at below 3508C maybe applied for the production of solid foams, when heated together with a thermostable polymer [97].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 301

OO

O

Cp2ZrCH3+B(C6F5)4

-

OO

O

OO

O

Cp2ZrCH3

+

B(C6F5)4- O

O

OO O

OCp2ZrCH3+

B(C6F5)4-

Scheme 43.

Page 44: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Similarly, cyclobis(hexamethylene carbonate), a dimer of nine-membered cyclic carbonate, wassubjected to polymerization in bulk at 1408C using BuSnCl3 or SnOct2 as initiator [52]. The best resultswere obtained with BuSnCl3 as a catalyst affording polymer yields up to 89% and weight averagemolecular weights ofMw � 200 000: In all cases theMw/Mn ratio was of the order of 2. DSC measure-ments showed that the polycarbonate rapidly crystallizes even at a cooling rate of 408C min21.Thermogravimetrical analyses showed that the degradation of the polymer begins at around 2508Cand reaches its maximum around 3508C. Under nitrogen, CO2, H2O and tetrahydropyran were foundas low molar mass degradation products, whereas formaldehyde and the monomeric cyclic carbonatewere not formed. Under an O2 atmosphere, oxidation is the main degradation process above 3008C.

Also Hocker et al. [53] have reported the synthesis, polymerization and copolymerization of cyclo-bis(hexamethylene carbonate) and its fluorinated analog using pseudo-anionic initiators such assec-butyllithium (in toluene), dibutylmagnesium (in THF) dibutyltin dimethoxide (in melt). The resultantpoly(hexamethylene carbonate) with molecular weight ofMn � 48 000�Mw=Mn � 1:7� was a semicrys-talline polymer with a melting point of 548C and a glass transition temperature of251.38C, whilepoly(2,2,3,3,4,4,5,5-octafluorohexamethylene carbonate) was obtained either as a semicrystallinematerial with molecular weight ofMn � 12 800�Mw=Mn � 1:94� and a melting point of 40.88C or anamorphous material with a glass transition temperature of239.88C.

It was found that the other cyclic dimmer, cyclobis(diethylene glycol carbonate), is more stable andeasier to isolate than the corresponding cyclic monomer, similarly as in the case of cyclic carbonatesderived froma,v-dihydroxyalkanes with 6, 7 and 8 methylene groups. A comparison of the polymer-izations of cyclobis(diethylene glycol carbonate) in bulk conducted at 1458C initiated by BuSnCl3,SnOct2 and Bu2SnO under similar conditions suggests that Bu2SnO appeared to be the most reactivecatalyst, but SnOct2 is more attractive for preparative purposes because of higher molecular weights ofthe resultant polymers. Both1H and13C NMR spectra proved that all poly(diethylene glycol carbonate)sprepared possess an alternating sequence of ether and carbonate groups. Despite a regular sequence, theamorphous character of poly(ethylene ether-carbonate) was proven by DSC and WAXS measurements.The highest yields and molecular weights were obtained in the shortest time, whereas prolonged timescaused rapid depolymerization. The thermal degradation of poly(ethylene ether-carbonate) beginsslowly above 2008C, reaches its maximum rate at 3208C and yields CO2 and 1,4-dioxane as the maindegradation products (Scheme 45) [55].

Polymerization of the largest (26-membered) cyclobiscarbonate, cyclobis(decamethylene carbonate),was conducted in bulk at 1208C with BuSnCl3 and SnOct2 as initiators. Both catalysts showed almostequal reactivities, but the highest yields (up to 98%) and the highest molecular weights (Mw up to88 000) were obtained with BuSnCl3. Both catalysts yielded poly(decamethylene carbonate)

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342302

OctO Sn OOct

O

(CH2)

O

O

n

...OctO SnO C

OO CH2(CH2) CH2n

O CO

O Oct

- CO2

OctO SnO CO

O CH2(CH2) CH2 OOctn

Scheme 44.

Page 45: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

(polyDMC) free of ether groups. DSC measurements revealed that polyDMC is a rapidly crystallizingmaterial with aTm of 678C. A comparison with poly(alkylene carbonate)s having fewer CH2 groups inthe repeating unit demonstrated that the rate of crystallization increases with an increasing number ofCH2 groups in a molecule.

4. Copolymerization of cyclic carbonates with other heterocyclic monomers

4.1. Copolymerization of 1,3-dioxolan-2-ones

It has been found that hardly polymerizable cyclic carbonates with a five-membered 1,3-dioxolan-2-one ring could undergo copolymerization with epoxides in the presence of catalysts formed in thediethylzinc phenol and/or polyhydric phenol system [158,199]. This copolymerization yieldedrespective low-molecular-weight poly(ether-carbonate)s with prevailing contents of ether linkages.Predominant head-to-tail structure with almost equal proportion of isotactic and syndiotactic diadsinvolving carbonate groups has been proved, by using of the13C NMR spectroscopy, for copolymersof propylene oxide and propylene carbonate [199].

It seems that the initiation step of the copolymerization involves most likely the epoxide reaction.Zinc alcoholate species formed in this reaction can easily propagate the copolymer chain, coordinatingand enchaining the both epoxide and cyclic carbonate comonomers. However, in the case of the cycliccarbonate its ring opening may also proceed according to reaction outlined in Scheme 46 leading todecarboxylation. Thus, the poly(ether-carbonate)s obtained are characterized by a lower content ofcarbonate units with respect to ether units [158,200].

A fairly good confirmation for the postulated mechanism of cyclic carbonate polymerization accord-ing to the mechanism outlined in Scheme 47 may be obtaining of poly(oxypropylene-co-oxycarbonyl-oxypropylene) of a predominant head-to-tail regioregularity from bispropylene spiroorthocarbonatewith a zinc-based coordination catalyst [199].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 303

OHO

O OO

O

O

O

O OO

HO

O

O

+ O OO

OH

OHO

CO2 +

Scheme 45.

Page 46: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The inert in “true” homopolymerization five-membered cyclic carbonates subjected to the copoly-merization with oxiranes in the presence of cationic initiators such as BF3·OEt2 afford poly(ether-carbonate)s, but with rather low content of carbonate linkages [201]. The copolymerization proceedsthrough a trioxocarbenium cation stabilized by adjacent three oxygen atoms, being thermodynamicallyfavored. A negative charge located on the oxygen atom of the carbonyl group of the carbonate is twice aslarge as that located on the oxygen atom of the oxirane, so the reaction with the carbonate monomer ismore probable than with the oxirane one. Trioxocarbenium ion 1 can react according to three possiblereaction pathways (Scheme 48):

• exo-cyclic carbon atom of1 is attacked by oxirane and an ether-carbonate group2 is formed (pathwaya);

• endo-cyclic carbon atom of1 is attacked by oxirane leading to the ether group3 and cyclic carbonateformation (pathwayb);

• in intramolecular reaction the carbocation is attacked by an oxygen atom of linear ether and spir-oorthocarbonate4 is formed (pathwayc).

The reaction proceeds partially according to the pathwaya in case of using cyclic carbonates withphenyloxymethyl and chloromethyl substituents, but for other five-membered cyclic carbonates reactionpathwayb predominates. Due to higher stability of the trioxocarbenium cation the polymerization rate isslower than that of oxirane homopolymerization.

Similarly to the copolymerization with oxiranes, only a small amount of five-membered cycliccarbonate is inserted into the polycarbonate backbone when polymerization of six-membered

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342304

Scheme 46.

Scheme 47.

Page 47: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

cyclic carbonate, TMC initiated by lipase or Al-based catalyst is conducted in the presence of ethylenecarbonate [202].

Recently, Ho¨cker et al. reported that ethylene carbonate (EC) copolymerized with cyclic tetramethy-lene urea (TeU) in the presence of dibutylmagnesium resulting in a polyurethane with an approximatenumber-average molecular weight of 20 000 and a polydispersity index of 2. Polyurethane from ethy-lene carbonate is a semicrystalline material with a melting point of 2088C [203]. Since five-memberedethylene carbonate does not homopolymerize, the butylmagnesium salt of TeU reacts with EC. Theresultant butylmagnesium alkoxide reacts with TeU leading to an intermediate (2/3, Scheme 49) andregeneration of the butylmagnesium salt of TeU. The intermolecular addition of either2 or 3 results in apolyurethane. The intermediate adduct of EC and TeU represents an A-B type monomer suitable for apolyaddition reaction.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 305

O

O

O

O

R

+ + O

1

a

b

c

OR

OO O

O +

ORO +

+ O O

O

O

O

O

O

+ OR+

2

3

4

Scheme 48.

+NHN

O

NHN

O

OBuMgO

OBuMg

1

O O

O

EC

NHNH

O

NHN

O

OOH

O

NH

OOH

O

NCO

NH

O

O

NH

O

On

polyurethane

2 3

Scheme 49.

Page 48: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

4.2. Copolymerization of 1,3-dioxan-2-ones

Six-membered cyclic carbonates easily copolymerize with different cyclic carbonates (five-, six- andseven-membered) as well as with various other heterocyclic monomers. The majority of copolymeriza-tions proceed according to the coordination or anionic mechanisms and most of the copolymers wereobtained with the participation of TMC or DTC.

4.2.1. Copolymerization of 1,3-dioxan-2-ones with cyclic carbonatesAs was mentioned in Section 4.1, six-membered cyclic carbonates copolymerize with five-membered

cyclic carbonates, but still the content of ethylene carbonate units in the resulting copolymer was rathersmall, not exceeding few mol%.

TMC and DTC can be used for the copolymerization with a less reactive six-membered cycliccarbonate such as 2,2-diphenyltrimethylene carbonate [11]. The copolymerization proceeded accordingto an anionic mechanism to furnish a polymer containing 2,2-diphenyltrimethylene carbonate units, butin lower ratio than that in the monomer feed.

DTC was also copolymerized with other six-membered cyclic carbonates to afford random as well asblock copolymers upon addition of the initiator (sec-butyllithium) to a mixture of the monomers or uponconsecutive addition of the monomers to the initiator, respectively [36]. A polycarbonate with lowerconcentration of reactive groups in the backbone can be accomplished according to this procedure.

4.2.2. Copolymerization of 1,3-dioxan-2-ones with cyclic estersBiodegradable polymers have attracted much attention during the last few years [204]. This is

important research effort taking into consideration the need for materials of specific application in thebiomedical field and by the search for biodegradable substitutes of conventional commodity thermo-plastics. Aliphatic polyesters have a leading position among the various biodegradable polymers, due tothe hydrolytic chain cleavage catalyzed by enzyme yielding hydroxyacids, which are in many casesmetabolized. The modification of the properties of the brittle biodegradable polyesters such aspoly(hydroxyalkanoate), polylactide, and polyglycolide has been intensively investigated [205,206].Usually a biodegradable or biocompatible elastomers were introduced to toughen the brittle polyester.Maxonw, a bioabsorbable suture material, is produced by the copolymerization of glycolide withapproximately 32.5 mol% of the softer TMC [207]. The addition of TMC decreases the brittleness ofpure polyglycolide. The incorporation of carbonate linkages into the polyester constitutes an additionalroute for improvement of the performance of polyesters. Because the carbonate linkages are more stableto hydrolysis in vitro (no autocatalytic effect of acid groups) the material has prolonged shelf life.However, the hydrolysis of the carbonate proceeds faster in vivo and the copolymer can be used as abioabsorbable material [79,208]. In relation to this, the copolymerizations of cyclic carbonates withlactones and lactides have been explored.

Hocker and co-workers usede -caprolactone for the copolymerization with DTC. The reaction wascarried out usings-BuLi as an anionic initiator in toluene resulting in the formation of a copolymercontaining carbonate block and caprolactone block fractions (Scheme 50) [34].

The reactivity of the cyclic carbonate is higher than that of lactone. The copolymers showed low glasstransition temperature (2658C) and melting points around 60 and 898C [209]. The copolymerization ofDTC ande-caprolactone was also initiated by polystyryllithium and lithium polystyrylethoxide leadingto a linear polymer and a cyclic oligomer mixture [68].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342306

Page 49: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

It is worth mentioning that a blend of polycaprolactone (PCL) and poly(2,2-dimethyltrimethylenecarbonate) (PDTC) containing catalytic amounts of dibutyltin dimethoxide subjected to melt-mixing at2008C revealed after 70 min lack of crystallizability and the system became completely amorphous.13CNMR data suggest that the loss of crystallizability of the blend is accompanied by an abrupt decrease inPCL and PDTC sequence lengths, reaching after 70 min at 2008C an average length of about tworepeating units each. These effects are explained by transesterification reactions between the lactoneand carbonate units [210].

The random copolymer of TMC with (R)-b-butyrolactone [(R)-b-BL] exhibits an interesting property(Scheme 51) [211]. The presence ofb-hydroxyester segments susceptible to enzymatic hydrolysis causethat the poly(ester-carbonate) containing even 80% of TMC easily undergoes biodegradation, while purepoly(TMC) does not degrade under the same conditions.

The ring-opening copolymerization was carried out using distannoxane complexes:

Sn

O

O

Sn

Sn

O Sn

OBu

Bu

Bu Bu

BuBu

Bu

Bu

Cl

Cl

Et

Et

The copolymerization of DTC with another four-membered cyclic ester, pivalolactone, initiated bypotassium dihydronaphthalide at2108C in toluene afforded block copolymers in a yield of about 90%(Scheme 52) [212].

The softening points of the copolymers increased from 1528C for a molar ratio of 7:3 (DTC:pivalolactone) to 2368C for pure polypivalolactone.

The ring-opening polymerization of mixtures of DTC andl,l-lactide (l-LA) in bulk with diethyl zincor dibutyltin dimethoxide as initiator, results in the formation of random copolymers ofMw � 20 000275 000 in a yield of 76–79% (Scheme 53, R� R0 � CH3) [213].l-LA polymerizes first in a fast reaction,followed by slow polymerization of DTC. Investigations of the polymerization mechanism reveal thatDTC is randomly inserted intol-LA-l-LA diads. The microstructure of the copolymers was determinedby triad analysis of the13C NMR spectra. A comparison of the triad distribution in polymers prepared

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 307

O O

O

+O

O

s-BuLi O O O

On

Om

Scheme 50.

OO

CH3

+O O

O

R R

O O

CH3 O

OR R

O

n

m

Bu2SnO

(R)-β-BL R = H or CH3poly[(R)-3HB-co-carbonate]

Scheme 51.

Page 50: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

with Sn- and Zn-based catalysts confirms the analysis of the carbonyl region; a high concentration ofDTC-l-LA-DTC triads in the polymers prepared with Bu2Sn(OMe)2 and a relatively high concentrationof DTC-DTC-DTC triads in the polymers prepared with ZnEt2 are an argument for the higher transes-terification activity of the Sn-based initiator as compared to the Zn-based initiator. It should be noted thatfor the DTC contents in the copolymer higher than 50 mol%, the glass transition temperature is belowbody temperature (378C), which may be of importance for biomedical applications. Mechanical proper-ties and the rate of hydrolysis change drastically aboveTg.

Similarly, the biodegradable poly(l-lactide-co-TMC) was synthesized by ring-opening copolymer-ization in bulk (Scheme 53, R� H, R0 � CH3). The glass transition temperature (Tg), the melting point(Tm) and the crystallinity of the copolymer decreased with increasing TMC content. The elongation ofthe copolymer significantly increased with increasing TMC content while the toughness passed througha maximum [214].

Surface biodegradable copolymers, poly(d,l-lactide-co-1-methyltrimethylene carbonate) andpoly(d,l-lactide-co-DTC), have been synthesized by ring opening polymerization with SnOct2 ascatalyst. Samples of the copolymer were implanted in rats to observe the degradation characteristics.It was found that, in both copolymer systems, the surface biodegradation characteristics in vivo wererelated to polymer hydrophobicities, which mainly depended on the copolymer compositions. Thedegradation of copolymers having a smaller ester fraction became a typical surface reaction. Thesecopolymers may be useful in protein delivery systems [46]. Also the copolymerizations ofl-LA andTMC can be carried out using SnOct2 as the catalyst [215–217] leading to a biodegradable polymer.

A rare earth metal chloride can be used at 80–1208C as catalyst for bulk copolymerization of TMCandd,l-lactide to afford a copolymer ofMw � 6000–28 000 in a 84–95% yield. Thus, LaCl3, showingthe highest activity seems to be a useful catalyst, because it is cheap, stable and easy to prepare [72].

A cyclohexene containing six-membered cyclic carbonate, 2,2-(2-pentene-1,5-diyl)trimethylenecarbonate (CHTC) has been copolymerized withl-lactide to introduce CyC functional groups in thepolymer chains (Scheme 54) [218].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342308

O O

O

+O

O

O O O

O On

-K+

.

Scheme 52.

O O

RR

O

O

O

O

O

R'

R'+

Zn(C2H5)2

R = H, CH3

R'= H, CH3

Bu2Sn(OMe)2

O O OO

O

R R

On

R' O

R'm or

Scheme 53.

Page 51: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

Ring-opening copolymerizations ofl-LA with CHTC were successfully conducted in bulk by usingAlR3-H2O (R� ethyl, isobutyl), Al(OPri)3, ZnEt2-H2O, and SnOct2 as catalysts. The comparison of thesecopolymerizations showed that the SnOct2 catalyst system gave copolymers of relatively higher mole-cular weight (Mn up to 109 000). However, regardless of the catalyst used, the mole percent of CHTCincorporated into the copolymer was lower than that used in the monomer feed. Determination of thecomonomer reactivity ratios for SnOct2 catalyzed copolymerizations gave values of 8.8 and 0.52 forl-LA and CHTC, respectively. A similar observation was reported by Grijpma and Pennings for theTMC-l-LA copolymerization catalyzed by SnOct2 [215]. All gel permeation chromatography (GPC)traces showed unimodal molecular weight distributions. On the basis of13C NMR studies of copolymersprepared by SnOct2 catalysis it was concluded that the copolymer repeat unit sequence distribution andaverage block lengths generally were closed to that expected for a statistically random distribution ofl-LA and CHTC repeat units. This behavior may result from the insertion of CHTC occurring randomlyalong the chain or/and CHTC polymerization by propagation at chain ends which are subsequentlyredistributed by intramolecular exchange reaction, similarly to that reported by Kricheldorf et al.[210,219] for copolymerization of lactone with cyclic carbonate. The introduction of increasingCHTC units into copolymers leads to decreasing glass transition temperature, melting transitiontemperature, and enthalpy of fusion. Copolymers with CHTC content of.52 mol% haveTg valuesthat are below normal body temperature.

The presence of CyC groups in the copolymer enables its valuable modification by introduction ofepoxy groups or by using cyclohexene side groups for subsequent free radical crosslinking [218].

Storey et al. [220] utilized copolymerization of TMC withd,l-lactide (dl-LA) for obtaining a seriesof three-arm, methacrylate end-capped prepolymers. Trimethylolpropane initiated ring-openingcopolymerization of TMC withdl-LA was carried out in the presence of SnOct2 to afford low molecularcopolymer triols, which were subsequently end-capped with methacryoyl chloride in the presence oftriethylamine to yield reactive prepolymers (Scheme 55).

The prepolymers were free-radically crosslinked to give amorphous, bioabsorbable networks. Tensilemodulus, ultimate strength, andTg increased with increasingd,l-lactide content. Networks with highercontents ofd,l-lactide were strong and fairly rigid, but failed catastrophically at the yield point;networks with lower contents ofd,l-lactide showed a higher elongation to break, failing catastrophi-cally, however, at the yield point. Hydrolytic degradation experiments revealed that the network basedon poly(d,l-lactide) homopolymer degraded fastest owing to its hydrophilicity. Hydrolytic degradationin the copolymer networks was controlled by two opposing effects, which occurred as the TMC wasincreased:Tg depression, which increased water uptake, and increased hydrophobicity, which decreasedwater uptake. An increase in the TMC content in the copolymer networks caused a decrease in the water

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 309

O O

O

O

OCH3

CH3

O

O

+

CHTCL-LA

O O OO

O

CH3 O

CH3

n

m

poly(L-LA-co-CHTC)

Scheme 54.

Page 52: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

uptake and the degradation rate, since these networks are both glassy at the degradation temperature of378C.

Blends from poly(trimethylene carbonate) and poly(adipic anhydride) with different compositionsand different molecular weights were prepared by a solution blending method to make easier thedegradation of aliphatic polycarbonates. When comparing blends with copolymers generally, the origi-nal degradation properties of the components used were changed through physical blending (or filling anoligomer as a plasticizer into a polymer), which represents a more convenient, lower cost and composi-tion-controlled method to combine two components with different degradation properties. The degrada-tion rate and pH of the degradation media change depending on the composition of the blends [221].

Poly(1,4-dioxan-2-one-co-trimethylene carbonate) can also be included to the group of syntheticbiodegradable carbonate copolymers. Copolymers with different compositions were synthesized bycopolymerizations of 1,4-dioxan-2-one (DON) with TMC at 1208C in the presence of SnOct2

(Scheme 56) [222].The intrinsic viscosity of copolymers increased with an increase in the TMC fraction in the feed. The

DSC traces of copolymers showed that theTg values of the copolymers are lower than those of the

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342310

OH

OHOH

+O

O

O

O

O O

O

+

O O OO

CH3 O

Hn

m

O

O

CH3

O O OO

CH3 O

Hn

m OOO

O

CH3O

Hn

m

O

+ Cl CH2

O

CH3

CH2

O

CH3 O O OO

CH3 On

m

O

O

CH3

O O OO

CH3 On m

OOOO

CH3On

m

OCH2

O

CH3

CH2

O

CH3

TEA

Scheme 55.

O

O

O

O O

O

+120oC

O O OO

O On

m

TMC DONpoly(TMC-co-DON)

SnOct2

Scheme 56.

Page 53: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

homopolymers. Most copolymers are amorphous except for one with a high DON content. Thecopolymer can be used as a matrix for a sustained drug delivery system (DDS). DDS based on thosecopolymers show that the accumulative release amount of drug is nearly in linear proportion with timefor a 1 month period, which is much better than DDS of poly(caprolactone-co-lactide) and homopoly-mers of DON and TMC.

4.2.3. Copolymerization of 1,3-dioxan-2-ones with N- and P-containing heterocyclic monomersIn the 1980s it was reported in the patent literature [223] that lactams can be anionically copolymer-

ized with cyclic carbonates to give a polymer containing urethane and ester groups beside carbonategroups. Similar groups were identified when polycarbonates were treated withe-caprolactam in thepresence of sodium lactamate [224].

In the early 1990s Wurm, Keul and Ho¨cker [225] found that the copolymerization of DTC withe -caprolactam afforded a copolymer with alternating ester and urethane groups. From mechanisticinvestigation it was revealed that the polymer is formed in two reaction steps: first the polycarbonateis formed, then insertion of the ring-opened lactam moiety into the carbonate group occurs, which leadsto ester and urethane groups (Scheme 57).

Similarly, the copolymerization of equimolar amounts of DTC with tetramethylene urea (TeU) in thepresence ofsec-butyllithium, dibutylmagnesium and diethylzinc as a catalyst in melt and in solutionwith N,N0-dimethylpropylene urea as a solvent at 1208C results in an almost alternating copolymer, i.e.polyurethane (Scheme 58) [226]. In contrast to the copolymerization with ethylene carbonate [203], itwas revealed that sequences of the polycarbonate are formed first. Later, when TeU is consumed, theconcentration of carbonate groups in the polymer decreases and that of urethane groups increases. After24 h both monomers are consumed. The results suggest that the copolymerization of DTC with TeUproceeds according to a mechanism known for the copolymerization of cyclic carbonates withe -caprolactam (Scheme 57).

Bu2Mg initiates the polymerization of DTC, followed by a transfer of the active species to cyclic urea.This nucleophilic TeU species (1 in Scheme 58) reacts with a carbonate moiety of the polycarbonatechain, resulting in chain cleavage and formation of two active chain ends. The intermolecular reaction of

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 311

O O O O

O O

O +N

ONa+ -

O O O

O O

N

OO O-Na++

O O O N

O OO

O

O-

Na+

Scheme 57.

Page 54: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

these two fragments generates a new molecule in which a TeU is inserted formally into a DTC-DTC diadwith the formation of two urethane groups. At the same time a new TeU active species (1) is generatedby the deprotonation of TeU. In contrast, the copolymerization of DTC with TeU in the presence ofdibutyldimethoxytin, tris(sec-butoxy)aluminum, and tetrakis(iso-propoxy)titanium as a catalyst leads tocopolymers with carbonate, urea, and urethane groups [226]. The polyurethane obtained in such a way isan amorphous material withTg � 23:38C:

It is known that poly(phosphoester)s have a great potential as a class of biomedical polymers becauseof their promising applications in tissue engineering and controlled drug release [227]. The ring-openingpolymerization is a versatile method to get high molecular weight poly(phosphoester)s. The synthesis ofpoly(2-hydro-2-oxo-1,3,2-dioxaphosphorinane) and its application as a drug carrier was reported, in the1970s, by Penczek et al. [228]. Recently, a new type of synthetic biodegradable copolymer wassynthesized by ring-opening copolymerization of TMC with 2-hydro-2-oxo-1,3,2-dioxaphosphorinane(TMP) in the presence of Al(i-Bu)3 as a catalyst (Scheme 59) [141]. The highest molecular weight of therandom copolymer was up to 16 000 Da.

Conversion studies showed that the composition of the copolymer is in agreement with the feedcomposition at high conversions, since TMC is more reactive than TMP.

4.2.4. Block copolycarbonates from hydroxytelechelic-based initiatorsAn interesting method of synthesis of block copolymers from cyclic carbonates was proposed by Keul

et al. [38,174] Block copolymers of AB and ABA types were obtained using initiators based on hydroxy-telechelic poly(ethylene oxide) [176], poly(tetrahydrofuran) [177] and poly(dimethylsiloxane) [178,229]

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342312

O O O O

O O

O +NHN

O

O O O

O O

NHN

OO O+

O O O N

O ONH

O

O

O

BuMg

BuMg

Mg u

1poly(DTC)

Scheme 58.

O O

O

O OPO H

+

Al( i-Bu)3

r.t.

O O OP

O

O O

H

n m

TMC TMP poly(TMC-co-TMP)

Scheme 59.

Page 55: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

in anionic ring-opening polymerization of DTC. The preparation of the initiators was performed bytreatment of the hydroxy-ended oligomers withsec-BuLi or potassium dihydronaphthalide. It is worthmentioning that the polymerization of DTC with the potassium salt of hydroxy-ended poly(ethyleneoxide) resulted in a product with bimodal distribution (37 wt.% of polymeric fraction and 63 wt.% ofoligomers). This enhancement of the rate of back-biting has its origin in an interaction between thepotassium counterion and poly(ethylene oxide) moiety. The self-solvation effect (Scheme 60) withparticipation of the active chain leads to the formation of higher amounts of DTC-cycles [230].

The block copolymers obtained from typical cyclic carbonates above ceiling temperature and in thepresence of some catalysts are able to produce respective cyclic carbonates due to an equilibriumpolymerization. It was found that the initiators and catalysts comprising Li, K, Mg and Sn afford cyclicoligomers during polymerization. This shows that for this group of initiators and catalysts the rate ofbackbiting is of the same order of magnitude as that of polymerization. Therefore, initiators based onalkali metals and catalysts based on Sn were used for the depolymerization of copolymers containingcarbonate blocks. The optimum temperature was 240–2608C. At higher temperature decarboxylationleads to the formation 3,3-dimethyloxetane and other volatile products. For THF-blocks the samecatalysts (methyltriflate, triflic anhydride) were used for the depolymerization as were used for thecopolymer synthesis. Thus, the triblock copolymer poly(DTC-b-THF-b-DTC) can be completelydepolymerized step by step to form two fractions containing the pure monomers (.99%). This ring-closing depolymerization seems to be a reasonable method for the recycling of high value polymerswhich is called by Ho¨cker et al. a thermodynamical recycling [174].

Similarly, living vinyl or diene polymers with alkali metal counterions may serve as initiator of cycliccarbonate polymerization leading mainly to an AB-block copolymer [175]. Using the reagent loweringthe nucleophilicity of the active species such as ethylene oxide transforms polystyryl carbanion toalcoholate one (PSt-CH2CH2O

2Li 1) and AB-block-copolymer is formed in a high yield without sidereactions.

5. Polymerization of cyclic thiocarbonates

The polymerization of ethylene monothiocarbonate (1,3-oxathiolan-2-one) was carried out in thepresence of metal alkyls such as ZnEt2 or CdEt2 and metal alkoxides like Mg(OCH3)2, Al(OnBu)3 orTi(OnBu)4. The polymerization with these catalysts was accompanied by carbon dioxide elimination andthe polymers obtained appeared to be poly(ethylene sulfide-monothiocarbonate)s. The content of ethy-lene monothiocarbonate (oxycarbonylthioethylene) units and ethylene sulfide (thioethylene) units in thepolymers produced was dependent upon the catalysts used but all these polymers yielded from the

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 313

O

OO

O

O

O

OO

K+

-

Scheme 60.

Page 56: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

ethylene monothiocarbonate polymerization at 808C contained less than 50 mol% of monothiocarbonateunits [83].

As compared with the ethylene carbonate polymerization, the ethylene monothiocarbonate polymer-ization with coordination catalysts proceeds easier and also may involve the decarboxylation to a lesserextent. The decarboxylation during the ethylene monothiocarbonate polymerization seems to involve themetal monothiocarbonate species that are analogous to those formed in the system with ethylenecarbonate (Scheme 46). But the ethylene monothiocarbonate decarboxylation leads to metal thiolatespecies, which may be outlined schematically as in Scheme 61.

The metal thiolate species are more reactive towards coordinating the monothiocarbonate monomerduring the polymerization (Scheme 61), than the corresponding metal alcoholate species operating in theethylene carbonate polymerization (Scheme 46).

The relatively highest efficiency of the diethylcadmium catalyst in the ethylene monothiocarbonatepolymerization (at 808C), as regards the high content of ethylene monothiocarbonate units in the poly-mer obtained, results probably from the softness of both cadmium and sulfur atoms fitting one to theother, to participate in the covalent bonding. Thus, the decarboxylation occurs to the lowest extent.Moreover, the propagation according to Scheme 62, involving the formation of the cadmium-sulfur bondleading to the metal thiolate species of relatively high activity, occurs more likely than that which mightinvolve the formation of the less reactive metal alkoxide species.

In contrast, five-membered cyclic dithiocarbonates undergo cationic ring-opening polymerization toafford corresponding poly(dithiocarbonate)s without evolution gaseous compounds [231].

A five-membered cyclic dithiocarbonate containing a benzyloxymethyl group exhibits interestingfeature [232]. The monomer, 5-benzoxymethyl-1,3-oxathiolane-2-thione subjected to cationicpolymerization initiated by CF3SO3H and CF3SO3CH3 at 608C afforded corresponding poly(dithio-carbonate) withMn � 9000–25 000 in 60–100% yields (Scheme 63). The narrow polydispersity of

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342314

O

S

O Mt X XS O

Mt

O

XS

Mt + CO2

Scheme 61.

XS

Cd

SO

O

XS O

Cd

SO

XS O

O

SCd

Scheme 62.

Page 57: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

the poly(dithiocarbonate)s (1.09–1.29) and theMn increase in direct proportion to monomer conversionindicates living cationic polymerization based on neighboring group participation.

The formation of a carbenium cation stabilized by neighboring benzyloxymethyl group participation,as observed by NMR spectroscopy, might result in the living polymerization [232].

6. Cyclic carbonates as reagents and modifying agents

The reaction of five-membered cyclic carbonates with nucleophilic reagents proceeds in two manners.Depending on the nucleophilicity of the reagent, the cyclic carbonate structure and the reaction tempera-ture, a carbon atom neighboring an oxygen atom (pathwaya or a0) or a carbonyl group (pathwayb) offive-membered cyclic carbonate is attacked (Scheme 64).

6.1. Cyclic carbonates in reactions with phenols

Phenols in the presence of a basic catalyst (i.e. K2CO3, NaH) react with five-membered cycliccarbonates yieldingb-hydroxy-aliphatic-aromatic ethers accompanied by CO2 evolution [233–235].However, when the reaction of the dipotassium salt of bisphenol A with ethylene or propylene carbonateis carried out in the presence of 18-crown-ether under CO2 atmosphere (10 bar pressure) and subse-quently witha,v-dibromo compounds such asa,v-dibromo-p-xylene, used in stoichiometric amounts,poly(ether-carbonate)s are formed (Scheme 65) [236].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 315

O

O

O

PhO

O

Ph

OS

SCS2 (1.2 equiv.)LiBr (5 mol %)

THF, rt, 24 h

95 %

E+

O

O

Ph

OS

S

E

+ ES S

O

O

O

Ph

monomer

+

O O

S S

O

Ph

n

Scheme 63.

Page 58: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342316

O O

O

R

OOHPh

R

PhOH

OOHR'

R

O OR' OH

O

R

R'OHand

NH

OR' OH

O

R

NH

OHPh

R

PhNH2

R'NH2

R'COOH

R' OOH

O

R+ CO2+ CO2

+ CO2

+ CO2

R'NCO

O N

R

O

R'+ CO2

KCSN

S

R

+ KOCN

O

O

OOO

O O

R

n

O O

O

RCl

O O

O

R

Cl2

+ CO2

aromatic-aliphatic ether

aliphatic ether

linear carbonate

aromatic-aliphatic amine

carbamate (urethane)

ester

polyester

tiirane

oxazolidone

vinylene carbonate

a

b

a'

Scheme 64.

KO

CH3

CH3

OK + 2 O O

R

O

O

CH3

CH3

OO

R

KO

O

O OK

R O

O

CH3

CH3

OO

R

KO

O

O OK

R O+ BrCH2 CH2Br2nn

CH2 CH2O

CH3

CH3

O

R

O

O

O O

R On

poly(ether-carbonate)

18-crown-6

CO2 atmosphere

CO2 atmosphere

R = H or CH3

O

Scheme 65.

Page 59: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The concentration of carbonate groups in the polymer depends on the reaction stoichiometry andtemperature. When the reaction is carried out at 100–1208C the contribution of cyclic carbonate ringopening is high; at lower temperature (,708C) only polyether is formed without insertion of cycliccarbonate. At a temperature higher than 140–1508C decarboxylation takes place and a polyethercontaining ethylene or propylene oxide units is formed.

Poly(hydroxy-ether)s (phenoxy resins), high molecular weight analogs of epoxy resins usually areobtained in the reaction of bisphenols with diglycidyl ethers of bisphenols [237–239]. The main draw-back of this method is the presence of branches in the reaction product. In contrast, as was found byRokicki [240], when instead of diglycidyl ethers bifunctional five-membered cyclic carbonates are used,poly(hydroxy-ether)s with a minimized amount of branches are produced (Scheme 66). The reaction iscarried out in bulk at 130–1508C in the presence of potassium carbonate as a catalyst affordingpoly(hydroxy-ether)s of 60 000 Da. It should be underlined that the bifunctional cyclic carbonates areeasily synthesized from corresponding diepoxy compounds and CO2 [124,240].

Entirely alternating copolymers can be produced in this method when the bisphenol used in thereaction is different from that used for the synthesis of bifunctional cyclic carbonate. It was shownthat the molecular weights of copoly(hydroxy-ether)s were diminished when bisphenols of low pKa

values were used in the reaction with bifunctional cyclic carbonates.

6.2. Cyclic carbonates in reactions with amines

In the case of reaction with aliphatic amines the carbonyl group of five-membered cyclic carbonates isattacked to form the product with ab-hydroxy-urethane (b-hydroxy-carbamate) group and no CO2 iseliminated [241–244]. The reaction of mono-substituted 1,3-dioxolan-2-one with primary aliphaticamines leads tob-hydroxy- urethanes with two isomeric structures in the position of the hydroxylgroup (Scheme 67).

Studies on the distribution of isomers showed that the product consists of 70–75% of isomer1 and25–30% of isomer2 [243,245,246]. At temperature higher than 1008C theb-hydroxy-urethanes reactwith another molecule of the amine to form disubstituted urea. The investigations of the model reaction

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 317

+ n

K2CO3

R'OO

O

OO

OO

O

ROH OH

OHOO

OHOR O R'

n + CO22n

n

Scheme 66.

O O

R

O

+ NH2R'R

O NHR'OH

O

+ O NHR'OH

OR

1 2

Scheme 67.

Page 60: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

of 4-phenoxymethyl-1,3-dioxolan-2-one show that in the formation ofb-hydroxy-urethane linkagesmainly primary aliphatic amines take part [247].

In contrast, aromatic amines react with five-membered cyclic carbonates in a similar to phenolreaction pathway yielding aliphatic-aromatic amines accompanied by CO2 elimination (Scheme 64)[243].

Bifunctional five-membered aliphatic cyclic carbonates in reaction with diamines afford reactivepolyurethanes (Scheme 68) [122,124]. In this method, in contrast to the production of typical commoditypolyurethanes, no toxic isocyanates are used.

A network of nonisocyanate polyurethanes can be obtained when multifunctional cyclic carbonateoligomers (e.g.1, 2, Scheme 69) are used in the reaction with multifunctional primary amines [248].

Intramolecular hydrogen bonds are formed within the structure of poly(hydroxy-urethane)s via thehydroxy group at theb-carbon atom and carbonyl group of the polyurethane chain:

O

OH

O

NO

RH

Quantum-mechanical calculations as well as IR and NMR spectroscopic investigations haveconfirmed the stability of such a ring [249]. The hindrance of the carbonyl oxygen atom considerablylowers the susceptibility of the urethane group to hydrolysis. Moreover, polyurethanes exhibiting

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342318

+ n R'OO

O

OO

OO

O

OHOO

OHO R'ON

HRN

H

O

n

n NH2 R NH2

O

Scheme 68.

O O

CH2

CH

O O

CH3

n

OO

O OO

CH3

n

OO

O OO

CH3

n

O

CH2

OO

O

O

OO

O

O

OO

O

O

n

1

2

Scheme 69.

Page 61: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

intramolecular hydrogen bonds display chemical resistance up to twice greater in comparison withpolyurethanes of similar chemical structure without such bonds. The tensile strength of nonisocyanatepolyurethanes are similar to those of conventional isocyanate polyurethanes, but display chemicalresistance of 30–50% higher and have significantly reduced permeability.

Linear polyurethanes bearing side hydroxyl groups undergo crosslinking reaction by treatment withdiisocyanate or aluminum alkoxide [124].

When a highly functionalized diamine such asl-lysine is used for the polyurethane synthesis, theresulting polymer contains additionally carboxyl groups besides primary and secondary hydroxyl groups(Scheme 70) [123].

The polyaddition ofl-lysine hydrochloride and bifunctional five-membered cyclic carbonate1(Scheme 70) was carried out in DMAc in the presence of 1 equiv. of DBU or 2 equiv. of sodium hydridefor 24 h to afford an optically active polyurethane bearing hydroxy and carboxyl groups. The polyaddi-tion of l-lysinol, a simple derivative ofl-lysine, where the nucleophilicity of thea-amino group is notreduced by the neighboring carboxyl group, and1 carried out in NMP affords a higher molecular weightof optically active polyurethane bearing a hydroxy group in quantitative yield. The polyurethanesreacted with cupric acetate, sodium tetrahydroborate, and titanium tetra-iso-propoxide to afford thecorresponding crosslinked gels immediately. These metallic gels are expected to be novel opticallyactive catalysts.

When a bifunctional cyclic carbonate containing a quaternary ammonium group was used in thereaction with diamines, a polyurethane-ionomer exhibiting potential bacteriostatic properties wasobtained [122].

6.3. Transformation of cyclic carbonates into other cyclic functional groups

A cyclic carbonate group can also be transformed, with relatively high yield, into other cyclicfunctional groups such as oxirane (by thermal decarboxylation), tiirane [250] and oxazolidone [251]groups in reaction with KSCN and isocyanates, respectively (Scheme 64).

6.4. Cyclic carbonates used in modification of epoxy resins

Epoxy resins are one of the most important classes of thermosetting resins, but when cured they arehighly crosslinked and brittle. A number of approaches have been used to modify the resins in order toimprove their impact resistance, elasticity and other mechanical properties. Polyurethanes, for example,

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 319

NH2 H2N HCl

COOH

. +O

OOO CH3

CH3

O O

O O

base

1

O

OHO OH

OCH3

CH3

O O

O

NH

COOH

NH

Scheme 70.

Page 62: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

have been used to improve the flexibility of epoxy resins for lacquers and adhesives. Taking intoconsideration that nonisocyanate urethanes can be obtained in the reaction of cyclic carbonates withaliphatic amines (Scheme 68), this reaction carried out in situ, was used in the modification of epoxyresins.

6.4.1. Cyclic carbonates used for the modification of epoxy resins cured with aminesRokicki et al. [252] have reported the use of five-membered cyclic carbonates for the modification of

epoxy resins hardened with amines. Two methods of modification are available: by reaction of some ofthe epoxy groups with CO2 to convert them into five-membered cyclic carbonate groups [252] and byintroduction of aliphatic oligomers terminated by cyclic carbonate groups (1,3-dioxolan-2-ones) into anepoxy resin [253]. As concerns the first method, due to a drastic increase in the viscosity of the epoxyresin, the reaction with CO2 was carried out till the conversion did not exceeded 15%. Such modifiedepoxy resins were next hardened with a typical amine curing agent, triethylene tetraamine (TETA) toproduce a network containing amine andb-hydroxy-urethane linkages. Curing epoxy resins containingcarbonate groups with polyamines provides interesting effects:

• gel time decreases with raising cyclic carbonate concentration;• reaction rate of the carbonate has a lower energy of activation than that of the corresponding oxirane;• reaction of these modified resins is less exothermic than the reaction of unmodified epoxy resins.

It should be underlined that the modified epoxy resins cured with TETA exhibit higher impactresistance and hardness in comparison with those of unmodified resins. The deflection temperatureswere also higher than those of the unmodified epoxy resin. This resulted from the presence of polarcarbonate and urethane groups and hydrogen bond formation.

Similar effects were observed when ethylene oxide oligomers with terminal 1,3-dioxolan-2-one ringswere used for epoxy resin modification. The low viscosity of these oligomers, lower than that ofbisphenol A epoxy resin, provides that a higher amount of bifunctional cyclic carbonates can beintroduced to the modified resin. A twice higher impact resistance of the epoxy-carbonate compositionin connection with unchanged other mechanical properties in comparison with those of the unmodifiedepoxy resin is due to the formation of hydrogen bonds and the presence of aliphatic ether linkages in theresultant resin network [254].

It was also shown that instead of epoxy resin, polyamine curing agents can be modified. Thus,polyamines (e.g. TETA) subjected to the reaction with cyclic carbonates afford correspondingpoly(amino-urethane)s containing additionally hydroxyl groups (Scheme 71) [255]. This leads to alowering of volatility and toxicity of amine curing agents and enables to maintain lower viscosity of

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342320

NH2

NH

NH

NH2 + O O

R

O O

O O

2

NH2

NH

NH

NH

O

O

OH

RNH2

NH

NH

NH

O

O

OH

Scheme 71.

Page 63: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

the epoxy resin composition during its processing. In contrast to the epoxy resin modification, shorteningof gel times was observed for the resin hardened with the poly(amino-urethane)s. The higher curing ratemay be explained by the presence of a higher amount of hydroxyl groups at the beginning of the process,catalyzing the resin curing.

6.4.2. Cyclic carbonates used for modification of epoxy resins cured with Lewis acidsThe epoxy-carbonate compositions were also hardened using Lewis acids as initiators in cationic ring-

opening polymerization. The very reactive BF3·OEt2 can be used as a curing agent for these composi-tions. As it was discussed in Section 3.1, five-membered cyclic carbonates do not homopolymerize undernormal conditions and thus, the Lewis acid can be mixed with bifunctional cyclic carbonates and thenintroduced into the epoxy resin. The presence of five-membered cyclic carbonates reduces the rate of theepoxy groups polymerization. The ring-opening polymerization proceeds according to the mechanism inwhich the oxygen atom of the carbonate group attacks the oxonium ion, resulting in the formation of atrioxocarbenium cation, less stressed and stabilized by neighboring three oxygen atoms (Scheme 48Section 4.1). This cation can react according to three reaction pathways [201].

It should be emphasized that the presence of cyclic carbonates confines the termination reactionconsisting in chain transfer on the linear polyether. On the basis of spectrophotometric and calorimetricstudies it was found that the conversion of epoxy groups is higher than that of unmodified resin and thecured epoxy resin exhibits enhanced mechanical properties [256]. The impact resistance of the epoxy-carbonate compositions cured with BF3·OEt2 was a few times higher than that of similar compositionscured with polyamine TETA [253].

6.5. Cyclic carbonates in polymer foams formation

Dicarboxylic acids as well as their cyclic anhydrides react with cyclic carbonates at elevated tempera-tures in the presence of alkaline catalysts to yield polyesters with the elimination of CO2 [257]. It wasshown that in the presence of Ti- and Sn-based catalysts the reaction proceeds through the formation of acarboxylic-carbonic anhydride group as an intermediate [258–261], which by the elimination of CO2,leads to the formation of the other half of the quantity of ester (Scheme 72).

In the presence of 4-(dimethylamino)pyridine, carboxylic-carbonic anhydrides decompose to giveesters in very good yields [262,263].

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 321

O

O

O

+O O

O

O O

O O OO

OO

O O

+ CO2

carboxylic-carbonic anhydride

X-

Scheme 72.

Page 64: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

The reaction of cyclic carbonates with cyclic anhydrides consisting in CO2 evolution was adapted byRokicki et al. [264] for obtaining epoxy resin foams. Bisphenol A epoxy resin containing epoxy groupspartly transformed into cyclic carbonate groups crosslinks with CO2 evolution when cured with cyclicanhydrides at elevated temperature (140–2008C) in the presence of alkali metal carboxylates as acatalyst. The epoxy resin foam density can be adjusted by changing the concentration of cyclic carbonategroups in the modified resin and the reaction temperature.

Hashida et al. [265] has proposed an opposite method in foam technology, which consists in CO2

transformation into cyclic carbonate. The thermal conductivity reduction of the polyurethane foam wasachieved by the conversion of CO2 in the foam into five-membered cyclic carbonates. Carbon dioxidewas chemically fixed in the reaction with epoxy compounds. In this method the thermal insulation ofpolyurethane rigid foam blown by a non-ozone-depleting agent was improved without using chloro-fluorocarbons. A reduction by about 10% in the thermal conductivity in comparison with that of theconventional foam including carbon dioxide was achieved.

Cyclic carbonates were also applied for obtaining polyimide nanofoams, used in microelectronics,which exhibit high thermal resistance and significantly low dielectric constant value [97]. The foams areprepared from block copolymers consisting of thermally stable and thermally labile blocks (Scheme 36,Section 3.2.3.1), the latter being the dispersed phase. The foam formation is effected by thermolysisof the thermally labile block leaving pores of a size corresponding to that of the initial copolymermorphology.

Owing to thermal degradability, polycarbonates can be used in powder metallurgy and as a binder andlubricant in ceramics [93]. It was found that thermal decomposition of poly(propylene carbonate) as aneat polymer and in the presence of AlN powder followed a depolymerization mechanism and left nodetectable residues. In the presence of AlN powder, the polycarbonate binder left primarily “gas phasemediated char” on the powder surface [96].

6.6. Five-membered cyclic carbonates as a source of linear carbonate groups

Five-membered cyclic carbonates, a substitute of phosgene derivatives, can be used as a cheap andnontoxic source of linear carbonate groups.

Taking into consideration that five-membered cyclic carbonates are easy to obtain, Watanabe andTatsumi proposed to use ethylene carbonate and methanol in the presence of hydrotalcite-type materialsas base catalysts in the synthesis of dimethyl carbonate [266]. Especially, Mg–Al hydrotalcite-typematerials with low Al concentration in brucite-like layers and high OH-proportion in the intercalatedanions give high yield of dimethyl carbonate. An ester exchange reaction can also be catalyzed by atitanium silicate molecular sieve, exchanged with an aqueous solution of K2CO3 [267].

A series of polycarbonate and copolycarbonate macrodiols was prepared by using an ester interchangereaction with ethylene carbonate and diols such as 1,6-hexanediol, 10-decanediol, 2,2-diethyl-1,3-propanediol, 1,4-cyclohexanedimethanol, and 1,3-bis(4-hydroxybutyl)-1,1,3,3-tetramethyldisiloxane[268]. The diols were chosen for preparing a series of macrodiols with different structural featuresincluding linear, branched, rigid, and flexible ones. The commercial macrodiol based on 1,6-hexanediolexhibited a high level of crystallinity, while with the exception of 1,10-decanediol-based copoly-carbonates all the others were completely amorphous. 1,10-Decanediol-based materials showed partialcrystallinity under ambient conditions. A series of polyurethane elastomers with a constant hard segmentpercentage (40 wt.%) was prepared using 4,40-methylenediphenyl diisocyanate and 1,4-butanediol as

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342322

Page 65: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

the hard segment. Tensile test results and Shore hardness measurements demonstrated that polyur-ethanes based on polycarbonate macrodiols prepared from 1,3-bis(4-hydroxybutyl)-1,1,3,3-tetramethyl-disiloxane had the lowest modulus and hardness of the series of polyurethanes.

Carbanions and anionoradicals react with five-membered cyclic carbonates used in a stoichiometricamount to yield corresponding derivatives terminated with carbonate anions (Scheme 731 [269].

An oligomeric dicarbanion such as “living” polystyrene can also be used for the reaction with cycliccarbonates. The resulting oligomers with carbonate anion end-groups subjected to reaction witha,v-dibromo-p-xylene formed a polycarbonate containing styrene blocks (Scheme 74) [269].

7. Spiroorthocarbonates

Spiroorthocarbonates (SOCs) are two-ring compounds with the central carbon atom connected withfour oxygen atoms. These double cyclic acetals are stable under basic conditions but easily undergo ringopening catalyzed by cationic catalysts [7,9,270–272].

O

O

O

O(CH2)n (CH2)n

n ≥ 2

Some SOCs exhibit a valuable property, when polymerizing, unlike common monomers, their volumeexpands. Bailey, was the first, to find their abnormal properties [270]. SOCs generally polymerize in the

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 323

.

+ O O

OO O

O

H

H

O O

O

-

-

2 2 +K+

K+

K+

O O

O

H

H

O O

O

-

+

K+

K+

BrCH2 CH2Brn n

O

OO O O

On + 2n KBr

18-crown-6

Scheme 73.

1 Second reaction pathway consisting in 1,2-dihydronaphthalene is not shown for better clarity.

Page 66: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

presence of Lewis acids such as BF3·OEt2, but some of them, containing double bonds, similarly tocyclic carbonate with anexo-methylene group can polymerize with radical initiators.

7.1. Synthesis of spiroorthocarbonates

Several methods are available for the syntheses of spiroorthocarbonates (Scheme 75).In the first method transesterification of orthocarbonate with respective diol in the presence of an acid

catalyst is applied (Eq. (1)) [273–275]. The second method consists of the reaction of organotincompounds with diols (Eq. (2)). The preparation of a cyclic tin compound from diol and an alkyltincompound is followed by the preparation of thionocarbonate in the reaction with carbon disulfide. Thereaction of thionocarbonate with the same or other cyclic tin compound affords symmetrical or unsym-metrical SOC [276–279].

Thionocarbonate can be prepared in one step using thiophosgene in reaction with a diol (Eq. (3))[280]. It is worth mentioning that the latter two methods are not recommended because of the usage oftoxic reagents.

The versatile method of synthesis of both symmetrical and unsymmetrical spiroorthocarbonates havebeen recently reported by Endo et al. [281]. Spiroorthocarbonates according this method are formed inthe reaction of dichlorodiphenoxymethane with diols (Scheme 75, Eq. (4)) [282]. Dichlorodiphenox-ymethane can be easily obtained from diphenyl carbonate and phosphorus pentachloride in a yield of80%.

Five-membered spiroorthocarbonates can be easily synthesized by the one stage reaction of oxiraneswith 1,3-dioxolan-2-ones catalyzed with Lewis acid with yields up to 70% (Scheme 76) [283].

7.2. Polymerization of spiroorthocarbonates

In 1973, Sakai et al. [284] reported the cationic ring-opening polymerization of three spiroorthocar-bonates (SOCs), of different size, catalyzed by BF3·OEt2. Analyzing the reaction products they proposedthree pathways of ring-opening polymerization of the SOCs in terms of the attacking sites of themonomer toward cationic propagating species (Scheme 77).

Five-membered spiro-monomer: 1,4,6,9-tetraoxaspiro[4,4]nonane (1) polymerizes in the presence of

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342324

Ph Ph Ph Ph

x

y

- -K+ K+

"living" polystyrene

+ O O

O

2

Ph Ph Ph Ph

x

y OO O O

O O

- -K+ K+

BrCH2 CH2Br+

Ph Ph Ph Ph

x

y OO O O

O On

18-crown-6

Scheme 74.

Page 67: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

BF3·OEt2 (1 mol%) at 308C in CH2Cl2 to afford poly(ethylene oxide) containing some amounts ofcarbonate linkages and cyclic ethylene carbonate as a side product (pathwaysa andb). According tothe proposed mechanism an oxygen atom of the spiro-monomer attacks a linear methylene carbon atomadjacent to an oxygen atom (pathwaya) or a ring methylene carbon atom adjacent to an oxygen atom

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 325

OR

OR

RO

RO2 OH R OH

O

O

O

OR' R'' (1)+ 4 ROH

R'OH

OH

SnOR

RR' Sn

O R

RO

CS2

R'O

OS

R'O

OS

R'' SnO R

RO

O

O

O

OR' R'' (2)

R'OH

OH

SCl

ClR'

O

OS

R'' SnO R

RO

O

O

O

OR' R'' (3)

O

OO

Ph

Ph

PCl5 O

O

ClPh

Ph Cl

R'OH

OHR'

O

O

OPh

OPh

R'O

O

OPh

OPh

R''OH

OH

O

O

O

OR' R'' (4)

+

+

+

+ +

++

+

+

H+

Scheme 75.

O O

O

R1

O

R2

+

R2

R1

O

OO

OH+

Scheme 76.

Page 68: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

(pathwayb) of the trioxocarbenium cation, resulting in the formation of an ether unit or carbonate unit,respectively. The amounts of in-built carbonate units depend on the substituent in the spiro-monomerrings. Thus, spiroorthocarbonates with phenoxymethyl and chloromethyl groups polymerize yielding apolyether with some carbonate units (up to 10 mol%), while spiroorthocarbonate with a methylsubstituent gave almost pure poly(propylene oxide) [201].

Six-membered spiroorthocarbonate: 1,5,7,11-tetraoxaspiro[5,5]undecane (2) was polymerized analo-gously to afford the respective poly(ether-carbonate) (pathwayb). Seven-membered spiro-monomer:1,6,8,13-tetraoxaspiro[6,6]tridecane (3) yielded almost quantitatively poly(tetrametylene carbonate) andtetrahydrofuran as a side product. In this case the spiro-monomer attacks the carbon atom of thetrioxocarbenium ion (pathwayc).

Bailey and Endo studied the syntheses and polymerization of various derivatives of six-memberedspiroorthocarbonates promising no shrinkage monomers [280,285–289].

According to Bailey’s explanation, in the case of the polymerization of six-membered spiroortho-carbonates leading to poly(ether-carbonate), no shrinkage or even volume expansion originates from thedouble-ring-opening process of spirocyclic compounds. One van der Waals distance plus two covalentdistances are changed to two near van der Waals distances plus one covalent distance in that double-ring-polymerization.

However, SOCs with norbornene groups, as has been reported [290–292], polymerize by an evidentlydifferent mechanism involving initial decomposition of a SOC skeleton to a cyclic carbonate and anoxetane ring (Scheme 78).

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342326

O

O

O

O(CH2)n (CH2)n

E+

O

O

OCH2(CH2)n-1OE +

CH2

(CH2)n-1

ab

c

monomer

O CH2CH2 m

a: n = 2

O

OO-

b: n =2, 3

c: n = 4

n(CH2)O(CH2)O CO

O m

n

O-(CH2)O C

OO m

4

1 n = 22 n = 33 n = 4

Scheme 77.

O

O

O

OE+

O

O

O + O polymer

Scheme 78.

Page 69: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

It was found that similarly to cyclic carbonates, a seven-membered spiro-monomer is more reactive incationic polymerization than that of a six-membered one. Unfortunately, because of the elimination ofTHF it is rather difficult to use seven-membered SOCs as expanding materials. However, Takata andEndo showed that amongst two seven-membered-SOC monomers bearing fused-ring systems based onspiro[6,6]tridecane skeletons, the SOC having aromatic substituents (3a in Scheme 79) polymerizes withvarious cationic initiators (BF3·OEt2, Ph3C

1 BF42) yielding the poly(ether-carbonate) without the elim-

ination of low molecular side products [291].In the case of SOC having aliphatic cyclohexane rings (3b) the polymerization proceeds according to

pathwayc in Scheme 77 (as was discussed by Sakai et al. [284] to afford polycarbonate and a THF-derivative (2-oxabicyclo[3.4.0]nonane) as a side product. This seems to suggest that the polymerizationcourse is determined by the electrophilicity of the reaction site of the propagating cationic species. Thebenzo-derivative of SOC (3a) is more electrophilic than the cyclohexane derivative and the attackproceeds along pathwayb (Scheme 77). Also the leaving ability of the eliminated side product seemsto play an important role, dihydrobenzofuran as less stable than furan or its aliphatic derivatives, due tothe antiaromatic 4p system, is not eliminated.

Five-eight-membered spiroorthocarbonate1 polymerize with BF3·OEt2 (5 mol%) at 08C affording apolymer consisting mainly of monocyclic orthocarbonate unitsa (Scheme 80) [293]. Monocyclic ortho-carbonate unita seems to be formed by opening only one (the aliphatic) ring of1, while unit b is thetypical unit formed by double ring-opening polymerization. Other units observed might result bysecondary reactions including elimination of phenylene carbonate. The highest ratio of the polymerwith unit a (86%) was obtained in bulk polymerization with pyridiniump-toluenesulfonate as a catalystat high temperature (1208C).

In contrast to the polymerization behavior of1, analogous benzo-substituted SOCs having aliphaticfive- (3) and six-membered (4) rings selectively polymerize to give corresponding poly(ether-carbo-nate)s via double ring-opening polymerization (Scheme 81) [294].

The polymer consisting mostly of unita was obtained by cationic ring-opening polymerization of a16-membered-ring-containing bis(spiroorthocarbonate) (2) in bulk with pyridinium hydrochloride(Scheme 80). This remarkably selective formation of poly(monocyclic orthocarbonate) was attainedonly in the presence of a weak cationic catalyst. The measurements of density of polymers and mono-mers suggested the volume expansion on polymerization to a polymer consisting mainly of monocyclicorthocarbonate unit.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 327

O

O

O

O

O

O

O

OH H

O OO

On

OO

On + OH

H

E+

E+

3a

3b

poly(ether-carbonate)

polycarbonate

Scheme 79.

Page 70: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342328

O

O O

OO

O O

O

E

O

O O

O

E

+ ++

OO

O On + O O

n O

O

On O

OO

O

n O

n

O

O

O

O

O

O

O

O

2

1

a b c

d eE+

E+

Scheme 80.

O

OO

O

O

O

O

O

E+

E+

O O

O

O n

O O

O

On

3

4

Scheme 81.

Page 71: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

7.2.1. Cationic polymerization of methylene substituted spiroorthocarbonatesMost spiroorthocarbonate monomers are crystalline under reaction conditions. When crystalline

monomers are polymerized in melt, volume shrinkage occurs [295–298]. Therefore, the spiroorthocar-bonate system is actually pseudoexpanding. The expansion in volume is mainly the result of the higherdensity of the monomer in comparison to polymer density, common for all known crystalline monomers.In the “real expansion” process volume expansion should take place during polymerization, both at hightemperature and at room temperature.

Bolln, Frey and Mu¨lhaupt [299] used a liquid spiroorthocarbonate: 2-methylene-7-phenyl-1,4,6,9-tetraoxaspiro[4.4]nonane for photoinitiated cationic double-ring-opening polymerization to eliminatethe phase transition that accounts for the apparent volume expansion (Scheme 82). However, despitequantitative double-ring-opening, no volume expansion occurred during polymerization. In contrast,volume shrinkage of 4.1% was found.

In relation to this, it seems difficult to compare the above-presented results of five-membered spiro-monomers with those obtained by Bailey and Endo for expandable six-membered spiroorthocarbonates.Typical five-membered spiroorthocarbonates polymerize with cyclic carbonate elimination, so partiallydouble ring-opening polymerization takes place (Scheme 77) [300]. In the case of other liquid spiro-cyclic monomers volume shrinkage was not measured [301,302]. Furthermore, the unsymmetrical five-membered spiro-monomer bearing anexo-methylene group (1a, Scheme 82) undergoes cationic poly-merization proceeding in a different manner.

The mechanism of the first ring opening is similar to that of the ring opening earlier reported for 4-methylene-1,3-dioxolane derivatives [303]. The cationic initiator attacks theexo-cyclic double bond ofthe monomer and the resulting energetically favored trioxocarbenium cation with carbonyl group1c isformed. The opening of the second ring results in the isomerization to the intermediate with an

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 329

O

OO

OPhE+ O

OO

OPh

E+

O

OO

OPh

E+

O

OO

OPh

E

+ O

OO

OPh

E+

O

OO

OPh

E

+

+ monomer

O O

O

OPh

n

O O

O

O

n

Ph

+ monomer

1a

2a 2b 2c

3 4

1b 1c

Scheme 82.

Page 72: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

energetically favored carbonate group. Opening of the second ring leads to a stabilized benzyl cation2aor a methylene cation2b, which transfers to the more stable intermediate tertiary carbenium cation2c.Polymerization leads to a polymer with carbonate and ketone groups.

The NMR analysis proved that the stable intermediate benzylic cation2aand poly(ketone-carbonate)3 are obtained.

An interesting example of a spiro-monomer was given by Millich et al. [304]. The N,S-analogs ofunsymmetrical spiroorthocarbonates are able to polymerize without elimination of low molecularproducts. These unsymmetrical spiro-monomers containing a nitrogen atom in the five-memberedring adjoining the aromatic group polymerize according to the mechanism presented in Scheme 83.An electrophile attack on the nitrogen atom is accompanied by double ring opening leading to theformation of a tiocarbonate group and carbenium ion originating from the cumenyl group.

7.2.2. Radical polymerization of methylene substituted spiroorthocarbonatesThe ring-opening polymerization of ketene acetals (1 in Scheme 84) proceeding according to a radical

mechanism is a novel route to polyesters and many examples have been reported [295–298]. The maindriving force for ring opening in polymerizations of these compounds is the formation of a strongcarbon–oxygen double bond. The sulfur analog undergoes ring-opening polymerization with selective

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342330

O

O

N

S

CH3

CH3

CH3

O

O

N

S

CH3

CH3

CH3E

+

O

O

N

S

CH3CH3

CH3

E

+

E+

C

Scheme 83.

O (CH2)

X

n O (CH2)

X

n

. kb

X (CH2)

O

n.

X (CH2)

O

m

n

kp

1

X = O or S

Scheme 84.

Page 73: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

cleavage of the C–O bond to give polythioester (X� S) (Scheme 84). The specificity is most likely areflection of the greater bond strength of CyO vs. CyS double bonds. The corresponding dithianes donot give ring-opening, even though ring-opening would involve cleavage of a weaker C–S bond [305].

A special type of SOCs, with a methylene group can also polymerize with radical initiators.Symmetrical five-membered SOC (1 in Scheme 85) [300], subjected to polymerization initiated bydi-tert-butyl peroxide at 1308C led to an insoluble polymer, where only vinylene groups are engagedin the polymerization. In contrast, the polymerization of six-membered SOCs (2) [270] gave both apolymer with ether-carbonate linkages (double ring-opening polymerization) and a vinylene polymerwith spiroorthocarbonate rings (vinylene polymerization) [266].

Thus, the radical polymerization of symmetrical spiroorthocarbonates bearing twoexo-methylenegroups in which two vinylene groups are engaged leads to a crosslinked product and causes substantialvolume shrinkage.

Due to a convenient method of synthesis of unsymmetrical spiroorthocarbonates (Scheme 75, Eq. (4))it was possible to obtain unsymmetrical SOCs with oneexo-methylene group. These SOCs undergoradical ring-opening polymerization according to the mechanism presented in Scheme 86 [266,270].

The conversion of monomers increases as the polymerization temperature rises. The GPC of thesepolymers showed single modal curves. Ring opening in the polymerizations occurred together in anycase. The degree of ring opening in the polymerization depends upon the ring size. The order of ease ofring opening was as follows: spiroorthocarbonate consisting of two six-membered rings, six- andseven-membered rings, two seven-membered rings. This result appears to correspond to the orderof ring strain. Enthalpies of polymerization of cyclic acetals indicate that the ring strain of five- and six-membered cycles have similar values24.0 and23.6 kcal/mol, respectively). Eight-membered cyclicacetals possesses the highest enthalpy of polymerization:212.7 kcal/mol that means that SOCs consist-ing of eight-membered rings should exhibit large ring-opening polarizability. The authors proposed thatbesides the mechanism leading to poly(ether-carbonate)s (pathwayc), another one, in which onlyvinylene groups polymerize, is possible (pathwaya). No evidence of a single opening polymerizationunit in the products means that the double ring-opened intermediate (pathwayc) is more stable than the

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 331

O

O

O

O

O

O

O

O

O

O

O

On m

w

R

1

O

OO

O

OO

OO

O O O

On

m

2

Scheme 85.

Page 74: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

single ring opened intermediate (pathwayb) and after the first ring opening the following second ringopening occurs more easily.

Moszner et al. [306] have reported radical polymerization of liquid five-membered spiroortho-carbonates bearing one or two methylene groups (Scheme 87). According to the polymerizationmechanism of SOCs, the volume shrinkage during polymerization of the most reactive monomer1bis about 12.5%, similar to that of typical vinyl monomers.

Spiroorthocarbonates containingexo-methylene groups (1 in Scheme 88) in the presence of thiols canbe subjected to radical polymerization with the preservation of the spiroorthocarbonate skeleton in thepolymer backbone [307]. This polymer easily hydrolyzes at room temperature to afford oligomers withtwo hydroxyl terminal groups (Scheme 88). The mechanism of radical polymerization is similar to thatproposed by Marvel et al. [308] for olefin polymerization with thiols, in which a thiyl radical is formed asa result of hydrogen atom abstraction. This radical undergoes addition to methylene groups and a carbonradical is formed.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342332

O

O O (CH2)

O

R

O

O O (CH2)

O R'R

.

+ R'

O

O O (CH2)

O R'R

.

O O (CH2)

O R'R

.O

O

OO

(CH2)O

m

R

O

OO

O (CH2) m

R

R

O O O

O

(CH2)m

.

a b c

n n n

nn

3

n

R= (CH2)2, n=1; (CH2)3, n=1; (CH2)4, n=1; CH2C(CH3)2CH2, n=1;

R= , n=1; , n=2CH2CH2 CH2CH2

Scheme 86.

O

O

O

O

O

O

O

O

1b 1c

Scheme 87.

Page 75: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

7.2.3. Cationic ring-opening polymerization of spiroorthocarbonates containing epoxy groupsRecently, Endo et al. [309] have reported a novel cationic ring-opening mechanism of the six-

membered SOCs assisted by the neighboring epoxy group (Scheme 89). In contrast to the cationicpolymerization of SOCs, which proceeds at above 308C, spiroorthocarbonate with an epoxy groupseparated by a methylene group can be polymerized at lower temperature (278 to 2208C). Cationicpolymerization was carried out with BF3·OEt2 as a initiator in CH2Cl2. On the other hand, there was nocopolymerization of SOC with propylene oxide at2158C and only a linear polyether was formed.

According to the proposed mechanism, at first, a initiator cation (E1) attacks the more nucleophilicepoxide oxygen, followed by ring opening which induces the ring-opening of one SOC ring resultingin the formation of tetrahydrofuran (pathwaya) and/or tetrahydropyran (pathwayb) rings. Once thetrioxocarbenium cationic species is formed, it should rapidly isomerize to form a carbonate group. Thus,as the result of intramolecular copolymerization, poly(ether-carbonate)s containing tetrahydrofuran

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 333

R + SH SH RH + SH S

O

O

O

O

SH S +

O

O

O

OS

SH.

O

O

O

OS

SH. + SH SH

O

O

O

OS

SH

1

SH S +

Scheme 88.

O

OO

O

OE

+

a

b

O

O

O

OOE

+

O

O

OOE

O+

monomer

monomer

O

O

O

OO n

O

O

OOn

O

pathway a

pathway b

Scheme 89.

Page 76: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

and/or tetrahydropyran rings in the main chain are produced. In contrast, in the case of a spiroortho-carbonate having a styrene oxide group, the polymerization proceeds only via the epoxide ring openingunder the same conditions.

Spiro-monomers having epoxy groups can be polymerized in two manners: firstly using anionicinitiators, such as DBU; selectively only epoxy groups are reacted to obtain soluble polymers. Next,cationic polymerization with BF3·OEt2 proceeds with double ring opening of unreacted SOC moietiesand a crosslinked product was obtained. The volume changes during the polymerization and crosslinkingof the monomer and polymers were evaluated to find whether the monomer and polymers showedexpansion in volume [309].

7.2.4. Spiroorthocarbonates used for modification of acrylic and epoxy resinsThe double ring-opening polymerization of spiroorthocarbonates combined with volume expansion

was applied for improving properties of various polymeric materials especially those of potential usagein dental applications. Thus, a spiro-monomer with twoexo-methylene groups (3,9-dimethylene-1,5,7,11-tetraoxaspiro[5,5]undecane) (1 in Scheme 90)) was used in dental materials to increase theadhesive fracture energy [310]. A six-membered spiroorthocarbonate having bicyclo[2.2.1]hept-2-ene(norbornene) groups (2) was utilized in epoxy resin composites with carbon fibers [311,312].

The addition of a SOC to acrylic dental resin composites (i.e. bis-GMA) resulted in improved physicaland bonding properties [310]. The presence of such dilutant comonomers reduces the polymerizationshrinkage of the dental resins. Also expandable SOCs monomers, among them such as those given inScheme 91, were added to the epoxy resins containing polyols, to reduce the resin shrinkage and the heatflow during hardening [8,310,313–315].

The compositions containing SOCs can be hardened by photoinitiated cationic polymerization [316].Spiroorthocarbonates were also used to prepare prepolymers for the modification of the curing process

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342334

O

OO

O

1

O

OO

O

2

Scheme 90.

CH3CH2

HOCH2 CH2OH

CH2CH3

O

O

O

O CH3CH2

CH3COOCH2 CH2COOCH3

CH2CH3

O

O

O

O

CH3CH2

C2H5COOCH2 CH2COOC2H5

CH2CH3

O

O

O

O

O

O

O

O

Scheme 91.

Page 77: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

and properties of crosslinked epoxy resin. The prepolymers were obtained in the reaction of 3,9-bis(hy-droxyethyl)-3,9-dibenzyl-1,5,7,11-tetraoxaspiro[5,5]undecane with 4,40-diphenylmethane diisocyanate(MDI) and 1,6-hexamethylene diisocyanate (HDI) (Scheme 92) [317].

An epoxy resin modified by the prepolymers was cured with BF3·EtNH2 at 1208C. The gel timesbecame prolonged and the activation energy increased as more prepolymers were added into the curingsystem. It was observed that less epoxy groups were left in the network containing the spiroorthocarbo-nate groups in comparison with those of the unmodified resin. These results suggested that the unreactedprepolymer remained capable of reacting with epoxy groups embedded in the network. The copolymer-ization would decrease the free volume in the network and reduced the internal stress of the matrix.

8. Conclusions

Polycarbonates are usually synthesized by a step-growth process, i.e., polycondensation, from phos-gene or its derivatives and dihydroxy compounds. The ring-opening polymerization of cyclic carbonatesis an alternative method of the synthesis of aliphatic and aromatic polycarbonates. A comparison of thesetwo methods is in favor of chain-growth process. The polycondensation affords rather limited molecularweight polymers, while high molecular weight polycarbonates can be prepared in the ring-openingpolymerization of cyclic monomers.

The chemistry of cyclic carbonates is really in a reach area of research taking into consideration thecopolymerization with a variety of heterocycles and reaction with different difunctional monomers.Additionally, aliphatic polycarbonates are of interest due to their new valuable applications in such areasas biodegradable and biocompatible materials as well as solid rocket propellant binders. The process ofdepolymerization of aliphatic polycarbonates is utilized on a technical scale in ceramic technology andnanofoams preparation. The unique property of six- and seven-membered cyclic carbonates concerningvolume expansion during polymerization additionally makes this class of compounds valuableprospective monomers.

Acknowledgements

I wish to thank my PhD student, Mr. Tomasz Kowalczyk, for his efforts in the investigations of thesynthesis of mono- and bifunctional cyclic carbonates as well as their polymerizations.

G. Rokicki / Prog. Polym. Sci. 25 (2000) 259–342 335

CH2OHO

O

O

OPh Ph

HOCH2

+ ROCN NCO

CH2O

O

O

OPh Ph

CH2 O CO

NH R NH CO

On

R = CH2 (CH2)6or

Scheme 92.

Page 78: Aliphatic Cyclic Carbonates and Spiroorthocarbonates as Monomers

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