-
Green polycarbonates from orange oil :
synthesis,functionalization, coating applications and
recyclabilityLi, C.
Published: 15/02/2017
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Citation for published version (APA):Li, C. (2017). Green
polycarbonates from orange oil : synthesis, functionalization,
coating applications andrecyclability Eindhoven: Technische
Universiteit Eindhoven
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Green Polycarbonates from Orange Oil: synthesis,
functionalization, coating applications
and recyclability
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische
Universiteit Eindhoven, op
gezag van de rector magnificus prof.dr.ir. F.P.T. Baaijens,
voor een commissie aangewezen door het College voor Promoties,
in het openbaar te
verdedigen op woensdag 15 februari 2017 om 16:00 uur
door
Chunliang Li
geboren te Anhui, China
-
Dit proefschrift is goedgekeurd door de promotoren en de
samenstelling van de
promotiecommissie is als volgt:
voorzitter: prof.dr.ir. R. Tuinier
1e promotor: prof.dr.C.E. Koning
copromotor(en): dr. R.J. Sablong leden: prof.dr. C.K. Williams
(University of Oxford)
prof.dr. M. Johansson (KTH Royal Institute of Technology)
prof.dr. ir. E.J.M. Hensen
prof.dr. R.A.T.M. van Benthem
prof.dr. G. de With
Het onderzoek of ontwerp dat in dit proefschrift wordt
beschreven is
uitgevoerd in overeenstemming met de TU/e Gedragscode
Wetenschapsbeoefening.
-
Nature does not hurry, yet everything is accomplished.
Lao Tzu
-
Chunliang Li
Green Polycarbonates from Orange Oil: synthesis,
functionalization,
coating applications and recyclability
Eindhoven University of Technology, 2017
This research has received funding from the European Unions
Seventh
Framework Program for research, technological development
and
demonstration under grant agreement no.289253 (REFINE). This
research
forms part of the research programme of the Dutch Polymer
Institute (DPI),
project #796p.
A catalogue record is available from the Eindhoven University of
Technology
Library
ISBN: 978-90-386-4214-7
Copyright 2017 by Chunliang Li
Cover design by Huiying Ma and Chunliang Li
Printed by Ipskamp, Enchede, The Netherland (www.
ipskampprinting.nl)
-
Table of Contents
Chapter 1. Introduction
1.1 Polycarbonates
......................................................................................................
1
1.2 Conventional APCs
..............................................................................................
3
1.2.1 Poly(propylene
carbonate).............................................................................
3
1.2.2 Poly(cyclohexene carbonate)
.........................................................................
4
1.3 Functionalized APCs
............................................................................................
5
1.3.1 Functionalized epoxide monomers
................................................................
6
1.3.2 Properties and Applications of Functionalized APCs
................................... 7
1.4 Aim of this study
................................................................................................
21
1.5 Outline of this thesis
...........................................................................................
21
Chapter 2. Hydroxy-functional poly(limonene carbonate)s
2.1 Introduction
........................................................................................................
28
2.2 Experimental section
..........................................................................................
30
2.2.1 Materials and general considerations
.......................................................... 30
2.2.2
Characterization...........................................................................................
31
2.2.3. Copolymerizations of limonene oxide with CO2
........................................ 32
2.2.4. Transcarbonation reactions of PLCs with polyols
...................................... 32
2.2.5 Thiol-ene modification
................................................................................
32
2.2.6. Solvent casting and curing of hydroxyl-functional
polycarbonates............ 32
2.2.7. Evaluation of the cured coatings by the acetone double
rub test and the
reverse impact test
................................................................................................
33
2.3 Results and discussion
........................................................................................
34
2.3.1 Synthesis of ,-dihydroxyl-terminated PLC and the initial
coating
evaluation
.............................................................................................................
34
2.3.2 Post-modification of PLCs with mercaptoalcohol: curing and
coating
properties
..............................................................................................................
43
2.4 Conclusions
........................................................................................................
53
-
Chapter 3. High glass transition temperature thiol-ene
networks
based on poly(limonene carbonate)s
3.1 Introduction
........................................................................................................
60
3.2 Experimental section
..........................................................................................
61
3.2.1 Materials and general equipment used
........................................................ 61
3.2.2 Methods
.......................................................................................................
62
3.2.3 Preparation of PLCs
....................................................................................
63
3.2.4 Solvent casting and curing of PLCs
............................................................ 63
3.2.5 Coating evaluation
.......................................................................................
64
3.3 Results and discussion
........................................................................................
64
3.3.1 Preparation of the PLC prepolymers
........................................................... 64
3.3.2 Thermal curing
............................................................................................
65
3.3.3 UV curing
....................................................................................................
80
3.4 Conclusions
........................................................................................................
85
Chapter 4. Novel poly(limonene-8,9-oxide carbonate)s:
synthesis,
post-modification and application in alkyd paints
4.1 Introduction
........................................................................................................
90
4.2 Experimental section
..........................................................................................
91
4.2.1 Materials
......................................................................................................
91
4.2.2
Characterization...........................................................................................
92
4.2.3 Copolymerization of R-limonene dioxide and CO2
..................................... 93
4.2.4. Typical post-modification of polymers
...................................................... 93
4.2.5 Synthesis of the PLOC-derived alkyd resins
............................................... 94
4.2.6 Film preparation.
.........................................................................................
95
4.2.7 Evaluation of the films
................................................................................
95
4.3 Results and discussion
........................................................................................
96
4.3.1. Synthesis of PLOCs
....................................................................................
96
4.3.2. Modification of PLOC with thiols, carboxylic acids and
amines ............. 104
4.3.3. CO2 insertion into PLOC
..........................................................................
107
4.4. Fatty acid-modified PLOC as alternative alkyd resin
...................................... 109
4.4.1. Introduction
..............................................................................................
109
4.4.2. Results and discussion.
.............................................................................
110
4.5 Conclusions
......................................................................................................
113
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Chapter 5. Network formation based on
poly(limonene-8,9-oxide
carbonate)
5.1 Introduction
......................................................................................................
120
5.2 Experimental section
........................................................................................
121
5.2.1 Materials and general equipment
...............................................................
121
5.2.2 Methods
.....................................................................................................
122
5.2.3 Preparation of PLOC
.................................................................................
122
5.2.4 Solvent casting and curing of PLOC
......................................................... 123
5.2.5 Coating evaluation
.....................................................................................
124
5.3 Results and discussion
......................................................................................
124
5.3.1 Preparation of the PLOC prepolymer
........................................................ 124
5.3.2 Network formation
....................................................................................
125
5.3.3 Thermomechanical properties of the networks using DMTA
................... 133
5.3.4 Formulation and coating properties
........................................................... 136
5.4 Conclusions
......................................................................................................
138
Chapter 6. Application of limonene-based polycarbonates as
powder
coating resins
6.1 Introduction
......................................................................................................
144
6.2 Experimental section
........................................................................................
146
6.2.1 Materials and general considerations
........................................................ 146
6.2.2 Methods
.....................................................................................................
146
6.2.3 Preparation of the polymer resins
..............................................................
147
6.2.4 Synthesis of
1,5,7-triazabicyclo[4.4.0]dec-5-enyltetraphenylborate
(TBDHBPh4)
.....................................................................................................
147
6.2.5 Powder coating preparation and curing
..................................................... 148
6.2.6 Coating evaluations
...................................................................................
149
6.3 Results and discussion
......................................................................................
150
6.3.1 Thiol-ene system
.......................................................................................
151
6.3.2 Thiol-epoxy system
...................................................................................
153
6.3.3 Carboxylic acid-epoxy system
..................................................................
156
6.4 Conclusions
......................................................................................................
157
-
Chapter 7. Depolymerization of limonene-based polycarbonates
7.1 Introduction
......................................................................................................
162
7.2 Experimental section
........................................................................................
163
7.2.1 Materials
....................................................................................................
163
7.2.2
Characterization.........................................................................................
164
7.2.3. Depolymerization-related experiments
..................................................... 164
7.3 Results and discussion
......................................................................................
165
7.3.1 TBD-initiated depolymerization of PLC and PLOC
................................. 165
7.3.2 Inorganic base-initiated depolymerizations of PLC
.................................. 169
7.3.3 Metal-assisted depolymerization of
PLC................................................... 172
7.3.4 Metal-assisted depolymerization of PLOC
................................................ 179
7.4 Conclusions
......................................................................................................
180
Chapter 8. Epilogue
8.1 Highlights
.........................................................................................................
183
8.2 Technology assessment
....................................................................................
185
8.3 Outlook
.............................................................................................................
188
Glossary
...............................................................................................
191
Summary
.............................................................................................
193
Acknowledgement
..............................................................................
196
-
1
1
Introduction
1.1 Polycarbonates
Poly(bisphenol-A carbonate) (BA-PC), the most common aromatic
polycarbonate, is an
important engineering polymer with superior properties such as
optical clarity,
excellent thermal and flame resistance and high impact
strength.1, 2
It has been widely
used in electronic devices, household appliance parts,
automotive applications and
containers. The consumption of BA-PC was estimated to be 3.3 Mt
in 2008 and to
increase by 7% per year.3 Currently, there are two main routes
for the synthesis of high
molecular weight BA-PC, i.e. the interfacial reaction between
phosgene and the sodium
salt of bisphenol A in a heterogeneous system and a melt-phase
transesterification
between bisphenol-A and diphenyl carbonate (Scheme 1.1).
However, the production
of BA-PC requires hazardous dichloromethane and phosgene, which
is
environmentally undesirable, or very high temperatures to remove
phenol, which
consumes high amounts of energy.4 Recently, an phosgene-free
production process
from CO2 has been reported.5
Scheme 1.1. Synthetic routes to poly(bisphenol-A carbonate).
Aliphatic polycarbonates (APCs) have gained increasing attention
due to their good
biodegradability and biocompatibility.6-8
Many efforts have also been dedicated to
improve their properties as potential alternatives to BA-PC.
Generally three (catalytic)
methods are employed to prepare APCs (Scheme 1.2), namely
ring-open
polymerization (ROP) of cyclic carbonates, polycondensation
between dimethyl
carbonate and diols and catalyzed copolymerization of epoxides
and carbon dioxide
(CO2). The first approach is mainly used to prepare
functionalized APCs for
biomedical applications, during which the reaction proceeds
under mild conditions and
gives no side product. However, the functional monomers, mostly
six-membered cyclic
-
2
carbonates like, e.g. trimethylene carbonate, need to be
specially designed and prepared prior to the ROP reaction.
6, 8-12 In comparison, more monomers are available for
condensation polymerizations and thus APCs with various
structures and properties can
be readily obtained.13
Nonetheless, several drawbacks are limiting the use of this
method. First, phosgene is used in the production of dimethyl
carbonate. Then, many
factors are essential to achieve high molecular weight (MW)
polymers, such as strict
stoichiometry control, high conversion of the monomers and high
reaction temperatures
or vacuum for efficient removal of the small molecules generated
during the
polycondensation. Besides, high temperatures are also required
to maintain a
homogeneous mixture in the presence of diols with high melting
points, in particular
for the production of high MW polymers.
Scheme 1.2. Preparation of APCs: a) ROP of cyclic carbonate
monomers. b)
Polycondensation of polyols and dimethyl carbonate. c)
Copolymerization of carbon
dioxide and epoxides.
The epoxide/CO2 copolymerization approach was first reported by
Inoue et al. in 1969
and has been widely investigated in the following decades.7 This
approach presents
several advantages over the previous two. First, the
chain-growth mechanism of the
reaction allows a good molecular weight control and high
molecular weights can be
reached at low monomer conversion at low catalyst loading.
Another advantage lies in
the use of less harmful monomers which avoids the safety hazards
related to processes
using phosgene. Furthermore, the comonomer CO2 is readily
available, cheap, non-
flammable, non-toxic and renewable. In the light of those
features, the work described
in this thesis is exclusively based on the APCs prepared via
this strategy. Catalysts for
epoxide/CO2 copolymerizations have been systematically described
in recent reviews.11,
13-16 This chapter will focus on the current state-of-the-art
related to the properties and
applications of the PCs produced by catalyzed epoxide/ CO2
copolymerizations, paying
special attention to functionalized APCs.12, 14-17
-
3
1.2 Conventional APCs
Scheme 1.3. Synthesis of polypropylene carbonate (PPC) and
polycyclohexane
carbonate (PCHC) by epoxide/CO2 copolymerization.
Propylene oxide (PO) and cyclohexene oxide (CHO) are the most
commonly used
terminal and alicyclic epoxides, respectively, for APCs
synthesis in academia and
industry (Scheme 1.3).
1.2.1 Poly(propylene carbonate)
Poly(propylene carbonate) (PPC) has been the most successful APC
so far. A general
introduction to its properties and applications is given below.
Detailed information on
PPC can be found in several reviews.18-22
1.2.1.1 Properties
PPCs are biodegradable polycarbonates with glass transition
temperatures (Tgs) ranging
from 25 to 45 oC, depending on molecular weight and
microstructure
(regioregularity).24-25
They are typically thermally stable until 180 oC.
18 The fracture
toughness of PPC increases with molecular weight (9.1 kJ/m2 at
Mn = 29 kDa and 12.6
kJ/m2 at Mn = 141 kDa). The Youngs modulus was determined to be
830 MPa, with an
elongation at break of 330% and a tensile strength of 21.5 MPa
for a purified PPC (Mn
= 50 kDa). A tensile modulus of about 680 MPa was reported for a
thermally stable
PPC (Mn = 260 kDa, PDI 5).23-25
1.2.1.2 Applications
In spite of the low rigidity and Tg, PPC has been commercialized
in different countries.
The industrial production of PPC has recently reached 3,000
tons/year. New PPC
plants with a total capacity of 10,000 ton/year were
commissioned recently by China
BlueChemical Ltd and Inner Mengxi High-Tech.19
-
4
As mentioned above, one favorable property of high molecular
weight PPC is the large elongation at break, which allows the
enhancement of the elastic modulus by using
(inorganic) fillers or blending with other polymers without
severe loss of toughness. By
blending polyhydroxybutyrate with PPC, Siemens and BASF have
developed an
alternative material to styrene-based
acrylonitrile-butadiene-styrene (ABS) plastic.
Empower Materials is producing PPC on a pilot scale, which is
used as sacrificial
binder for high quality products in advanced ceramics, powder
metals and sealing
glasses.7
PPCs can also be utilized as functional polymers via
terpolymerizations of PO,
functional epoxides and CO2 or end-functionalization.26
In polyurethane (PU) industry,
low MW PPCs have been successfully used as polyol components,
acting as soft blocks
in PUs, particularly for rigid and flexible foams
production.29-30
Other potential applications are packaging material and
adhesives,27
organic fillers in
containers due to the good compatibility with other polymers,
flame-retarding materials
via incorporation of phosphorous groups as a green alternative
to halogenated flame-
retarding polymers,28
passive electronic components,29
and medical dressings and
tailored tissue scaffold materials due to good
biocompatibility.30
1.2.2 Poly(cyclohexene carbonate)
CHO is frequently used in academia as it can be readily
copolymerized with CO2 under
mild conditions. However, in spite of many efforts made to
improve the polymer
properties, the applications of PCHC are still limited because
of its brittle character.
1.2.2.1 Properties
PCHCs usually have Tgs varying from 65 to 115 oC, depending on
the content of ether
linkages and the molecular weight. They are also thermally more
stable than PPC and
unzipping of PCHC starts only at 250 oC. PCHC is a brittle
polymer with an elongation
at break of 1-2%. The tensile modulus of PCHC (3,600 MPa) is
much higher than the
corresponding value for BA-PC (2,400 MPa). The brittle behavior
of PCHC can be
explained by the relatively low plateau modulus in the melt,
implying a low
entanglement density.31
A comparison of the general properties of BA-PC, PPC, PCHC
is shown in Table 1.1.
-
5
Table 1.1. General properties of BA-PC, PPCand PCHC and PLC.
Property BA PC PPC PCHC
Tg, oC 145 30-41 112-115
Modulus (MPa) 2,400 993 3,600
Tensile strength (MPa) 65 33.2 43
Density, g/cm3 1.20 1.275 -
Dielectric constant (103 Hz) 2.96 3.0 -
Elongation at break (%) 80 330 2
Refractive index, n 1.586 1.463 -
Burning heat, 103 kJ/kg - 18.5 -
Water up-take, % (23 oC) 0.12 0.397 -
Td-5%, oC 458 218 240-280
1.2.2.2 Applications
So far there is no report on the scale-up of PCHC. Koning and
coworkers described the
synthesis and subsequent curing of ,-dihydroxyl-terminated
PCHCs. The resulting
coatings showed promising properties such as high transparency
and high scratch
resistance.32
1.3 Functionalized APCs
As described above, PCHC and PPC as such have limited commercial
applications
because of their unsatisfactory physical and mechanical
properties, like low rigidity (in
case of PPC) and low elongation at break (in case of PCHC).
Moreover, the lack of
additional functionality in the corresponding epoxides makes it
rather difficult to
enhance the properties by chemical modification. On the other
hand, the selective
polymerization of functional epoxides, namely epoxy monomers
with an extra
functionality like an alkenyl, carbonate, or hydrophilic group,
leads to functional
polymers of interest for many applications such as reactive
substrates, coating resins,
polymeric nanoparticles, electronic and biomedical materials. A
review related to
functional epoxide monomers, functional APCs, their properties
and potential
applications is presented in this section.
-
6
1.3.1 Functionalized epoxide monomers
Figure 1.1. Structures of functional epoxide monomers.
Functionalized epoxide monomers are epoxy monomers bearing
either a functional
group that is stable under the polymerization conditions
(oxirane, ester, carbonate,
alkenyl) or that is protected with a protective group, allowing
the introduction of
various functionalities such as hydroxyl groups, vicinal diols
and carboxylic acids after
deprotection. The post-modification of the resulting APCs can be
further realized via
various chemistries to produce new materials for specific
applications. Figure 1.1 gives
an overview of different functional epoxides discussed in this
chapter.
-
7
1.3.2 Properties and Applications of Functionalized APCs This
section is dedicated to the synthesis, properties and potential
applications of the
functional APCs. This discussion is divided into six parts,
based on the nature of the
monomer functionalities, viz.: (i) alkenyl/alkynyl, (ii)
ester/carbonate, (iii)
hydroxyl/carboxylic acid, (iv) hydrophilic/ hydrophobic, (v)
epoxy/cyclic carbonate
and (vi). side-chain liquid crystalline (SCLC).
1.3.2.1 APCs with pendant alkenyl/alkynyl groups
The functionalized epoxides bearing an extra alkenyl or alkynyl
group have been
studied for various purposes, e.g. for improving the
thermo-mechanical properties of
the polycarbonate or for enhancing the hydrophilicity. First,
the extra alkenyl/alkynyl
groups typically show high stability during the copolymerization
with CO2, so the extra
functionality is retained in the copolymers. Moreover, the
versatility of ene/yne groups
permits the post-modification of the resulting polycarbonate via
different chemistries
such as radical polymerization, thiol-ene coupling, epoxidation,
azide-alkyne or Diels-
Alder addtion. This strategy favors the modulation of the
properties of the APCs and
thus is able to extend their potential application fields.
ukaszczyk et al. described the synthesis of the first
functionalized APCs bearing
pendant allyl groups in 2000.33
The copolymerization of allyl glycidyl ether (AGE) and
CO2 was catalyzed by a ZnEt2/pyrogallol (PG) system. The
resulting poly(AGE
carbonate) (PAGEC) was further oxidized with m-chloroperbenzoic
acid to give an
epoxy polycarbonate (see Scheme 1.4, Route a), which degraded
much slower than the
parent polycarbonate while exposed to an aqueous buffer. A
tendency of gelation of the
parent and epoxy polycarbonates during storage illustrated the
high reactivity of the
pendant functional groups, leading to the
homopolymerization.34
Tan and coworkers
also prepared PAGECs with high carbonate content using
Y(CF3COO)3 as catalyst.35
The pendant allylic groups reacted with
3-(trimethoxysilyl)propyl methacrylate in the
presence of benzoyl peroxide, generating alkoxysilane-containing
copolymer
precursors for a subsequent sol-gel process, which effectively
produced PAGEC-SiO2
nanocomposites. (Scheme 1.4, Route b) The thermo-mechanical
properties of the
nanocomposites including Tg, the thermal stability, the tensile
strength and the
elongation at break were much better than that of the parent
PAGEC, without
sacrificing the transparency. Similarly, Tao et al. synthesized
UV-crosslinkable PPCs
by terpolymerization of PO, CO2 and a low level of AGE, which
has limited effect on
the yield of the copolymerization. However, the crosslinking
significantly enhanced the
thermo-mechanical properties of PPC and extended the application
temperature
window of the polymer accordingly (Scheme 1.4, Route c).36
-
8
Scheme 1.4. Several examples for synthesis and post-modification
of AGE-based PCs .
Darensbourg and coworkers also contributed significantly to the
preparation of alkenyl
functionalized APCs and their post-modification, aiming in
particular at surface-
functionalized films and hydrophilic/amphiphilic polycarbonates
for biomedical
applications, Random terpolymers were prepared from PO/AGE/CO2
under ambient
conditions in the presence of a Salen cobalt catalyst.37
The terpolymers were then
partially crosslinked via thiol-ene chemistry using polythiols
and modified further with
N-acetyl-L-cysteine and 2-(Boc-amino)ethanethiol to produce
films with carboxyl or
amine-functionalized surfaces (Scheme 1.5, Route a). In spite of
some observed
degradation of the films during the deprotonation process, this
strategy leads to
materials for potential biomedical applications such as coatings
on biomedical devices
with good biocompatibility and mechanical properties.
Amphiphilic polycarbonates
could also be prepared from these monomers. First, ABA triblock
polycarbonates were
obtained through a one pot ,two-step strategy, in which PPC
diols were produced as
macro-initiators and used in subsequent AGE/CO2 coupling
reactions. Secondly, the
triblock precursors were modified by thiols with different
functional groups, which
were then converted into anionic or cationic groups, affording
amphiphilic
polycarbonates (Scheme 1.5, Route a). Nanoparticles with high
uniformity were
formed by self-assembly of these polycarbonates in water, whose
size could be tuned
by controlling the length of the blocks. These charged
nanoparticles could be
potentially used in drug delivery systems.
Similarly, poly(2-vinyloxirane carbonate)s (PVICs) with more
than 99% carbonate
linkages were prepared using the same catalyst system (Scheme
1.5, Route b). The
subsequent quantitative thiol-ene click modification afforded
the amphiphilic
polycarbonates (PVIC-OH and PVIC-COOH) bearing pendant hydroxyl
and carboxyl
-
9
groups, respectively.38 Further modifications, e.g. by
ring-opening of the hydrochloride salt of L-aspartic acid anhydride
for PVIC-OH and deprotonation by aqueous
ammonium hydroxide for PVIC-COOH, yielded two water-soluble
polycarbonates,
showing an average hydrodynamic diameter of 32 nm while
dispersed in water. These
amphiphilic/water-soluble materials were expected to form a
powerful platform for
bioconjugation and being ideal candidates for polymer
therapeutics.
Scheme 1.5. Synthesis and post-modification of AGE-based
PCs.37-38
Geschwind et al. reported the terpolymerization of PO,
1,2-epoxy-5-hexene (EH) and
CO2 catalyzed by a cobalt-based catalyst system under mild
reaction conditions (25 bar
CO2, 30 C, 2 h).39
The resulting polycarbonates were highly alternating terpolymers
(>
99% carbonate linkage) with random structure with Mns of up to
34 kDa and Tgs of up
to 29 oC. The pending double bonds were then quantitatively
converted into
functionalities such as thiols, alcohols and carboxylic acids
via thiol-ene reactions
(Scheme 1.6). The hydroxyl-functional polycarbonates were used
further to synthesize
graft polymers with well-defined structures and variable
grafting densities via ring-
opening polymerization of L-lactide through the grafting-from
strategy. These
polymers may be useful for low temperature processing of
poly(lactic acid) (PLA) due
to their lower melting temperature compared to that of linear
PLA. The full
degradability also makes them promising materials for biomedical
use.
-
10
Scheme 1.6. Synthesis and post-modification of EH-based
APCs.39
Vinyl cyclohexene oxide (VCHO) is also an interesting monomer,
exhibiting similar
reactivity as CHO. Hsu and coworker synthesized block copolymers
based on a living
PPC macro-initiator by the subsequent copolymerization of VCHO
with CO2, catalyzed
by a Y(CF3COO)3ZnEt2m-hydroxybenzoic acid coordination system.
The resulting
block copolymers showed moderate thermal and mechanical
properties, lying between
PPC, PCHC and PVCHC. The PVCHC block could be further modified
to expand their
applications.40
Cherian et al. reported the transformation of a linear
CHO/VCHO/CO2
terpolymer into nanoparticles of controlled dimensions via
cross-metathesis (Scheme
1.7, Route a). The intramolecular crosslinking of the
vinyl-containing polycarbonate
under dilute conditions caused an increase of Tg from 114 to 194
oC and a decrease of
the hydrodynamic volume. The resulting nanoparticles are
potential materials for
electronics applications.41
Recently this terpolymer has also been evaluated in our
laboratory as a potential new generation coating resin. This
polycarbonate was
successfully cured with a trithiol by UV or by thermal curing,
generating coatings
showing promising properties for (powder) coating applications
(Scheme 1.7, Route
b).32
Similarly, Taherimehr et al. reported the crosslinking of PVCHC
obtained from
VCHO/CO2 copolymerization, catalyzed by an iron
pyridylamino-bis(phenolate)
catalyst. The reaction with 1,3-propanethiol resulted in
crosslinked polycarbonates with
high Tgs and good chemical resistance, which may be suitable for
high performance
applications.42
In another study, poly(vinyl cyclohexene carbonate) (PVCHC)
was
quantitatively modified with 2-mercaptoethanol via thiol-ene
chemistry by Zhang and
coworkers.43
The resulting hydroxyl-functionalized PVCHC-OH was used as
the
initiator in the ring-opening polymerization of -caprolactone,
affording brush
copolymers with well-defined structures and nearly 100% of
grafting density (Scheme
1.7, Route c). The full degradability and large space between
the side chains, favorable
for the accommodation of small molecules, make the brush
copolymers potentially
useful materials for biomedical applications.
-
11
Scheme 1.7. Synthesis and post-modification of VCHO-based
APCs.40-43
All summarized reports described potential applications for each
type of polycarbonate.
The great dependence on the petroleum sources might however
limit their production
and use in the future, when the non-renewable fossil feedstock
will dry up. Thus, the
development of new PCs based on bio-renewable resources is not
only promising but
even necessary as a solution to this problem.
Scheme 1.8. Synthesis and post-modification of
1,4-cyclohexadiene-1,2-oxide
(CHDO)-based APCs.44-47
-
12
1,4-cyclohexadiene-1,2-oxide (CHDO) is a potentially biobased
functional epoxide since its precursor, 1,4-cyclohexadiene, is a
waste by-product generated during the self-
metathesis of polyunsaturated fatty acid methyl esters.44
Honda et al. described the first
synthesis of poly(cyclohexadiene carbonate) (PCHDC) through the
alternating
copolymerization of CO2 and CHDO with
tetraphenylporphyrinatocobalt(III)
chloride/dimethylaminopyridine as the catalyst system. The
subsequent bromine
addition to the double bonds afforded brominated PCHC, regarded
as a promising
material for optics, owing to its high transparency and expected
high refractive index
(Scheme 1.8, Route a).45
Darensbourg et al. also investigated the CHDO/CO2
copolymerization and the post-modification of PCHDC.46
They employed a
(salen)CoDNP/PPNDNP (DNP = 2,4-dinitrophenolate) catalyst system
to produce high
molecular weight PCHDC with 100% selectivity. PCHDC was then
modified by
quantitative thiol-ene addition of thiolglycolic acid to afford
an amphiphilic copolymer,
which was converted further into a water-soluble polymer upon
deprotonation with
ammonium hydroxide (Scheme 1.8, Route b). A similar modification
has also been
applied to poly(1,3-cyclohexadiene carbonate). Winkler et al.
likewise reported the
preparation of PCHDC using di-zinc/magnesium or chromium
(III)/cobalt(III) salen
complexes.47
The ring-opening copolymerizations of CHDO/phthalic anhydride
were
also studied, affording renewable, unsaturated polyesters with
Tgs up to 128 oC.
Limonene 1,2-monoepoxide (LO), derived mainly from the
(R)-limonene isomer
present in orange oils, is used as a flavor agent in the
fragrance industry and as a green
solvent. Since the first report in 2002 on the alternating
copolymerization of LO and
CO2 catalyzed by a -diiminate (BDI) zinc acetate complex,48
efforts have been made
to study the specific properties and potential applications of
limonene-based
polycarbonates. Hauenstein et al. improved the monomer quality
by efficient removal
of protic impurities to achieve a high MW poly(limonene
carbonate) (PLC, Mn >100
kDa) and almost quantitative conversion of LO. The polymer
showed also a high Tg
(130 oC), excellent thermal resistance, hardness and
transparency. The further multiple
modifications significantly tuned the properties of the PLC in
many directions.49
The
thiol-ene reaction with butyl-3-mercaptopropionate transformed
PLC, a potential
engineering thermoplastic, into a rubber. Moreover, the
modification with 2-
(diethylamino)ethanethiol and the subsequent quaternization of
the amine with an aryl
halide afforded materials with antibacterial activity. The
hydrophilicity and water
solubility of the PLC were enhanced by reacting the pendant
double bonds with
mercaptoethanol and mercaptoacetic acid, respectively (Scheme
1.9, Route a). The
acid-catalyzed electrophilic addition of monohydroxylated
poly(ethylene glycol) (PEG)
to PLC led to a PLC exhibiting a remarkably enhanced
hydrophilicity (Scheme 1.9,
Route b). A completely saturated counterpart of PLC exhibited
improved heat
processability.
-
13
Scheme 1.9. Synthesis and post-modification of PLCs.48-50
Terpolymerizations of LO, CHO and CO2 were carried out using an
Al(III)
aminotriphenolate
complex/bis(triphenylphosphoranylidene)ammonium chloride
(PPNCl) catalyst system.50
The thermal properties of the terpolymers were improved by
thiol-ene crosslinking reactions with 1,2-ethanedithiol,
affording interconnected
networks with decomposition temperatures, Tds, in the range of
250-280 oC and Tgs of
up to 150 oC.
The alkynyl group is also a very interesting functionality
utilized in many organic
reactions, in particular in the 1,3-dipolar cycloaddition or
click reaction. Frey et al.
synthesized propargyl-functional polycarbonates with different
contents of functional
units via the terpolymerization of glycidyl propargyl ether,
glycidyl methyl ether and
CO2.51
The reaction between the pendant propargyl groups and benzyl
azide was highly
efficient, undergoing the copper-catalyzed Huisgen-1,3-dipolar
addition (Scheme 1.10).
A wide range of functional APCs can be produced via this
strategy.
Scheme 1.10. Synthesis and post-modification of
propargyl-functional APCs.51
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14
Furfuryl-functionalized epoxides are potentially available from
renewable resources. The furfuryl groups are used for crosslinking
and network formation via thermally
reversible Diels-Alder (DA) reactions. Thus, Hu and coworker
prepared the first
furfuryl glycidyl ether (FGE)/CO2 copolymer (PFGEC) using a rare
earth ternary
catalyst (Scheme 1.11, Route a).52
The new polycarbonate became yellowish at ambient
atmosphere due to post-polymerization via crosslinking of the
highly reactive furan
rings. The effective DA reaction of PFGEC with N-phenylmaleimide
was able to
stabilize the polymer by reducing the number of furan rings and
introducing bulky
groups. Similarly, Hilf et al. synthesized well-defined
FGE/glycidyl methyl ether
(GME)/CO2 terpolymers (PDI~1.2-1.4) of various compositions
under solvent-free
conditions.51
The post-modification with a monofunctional
maleimide-containing
catechol resulted in polycarbonates that may be useful for a
wide range of applications
such as adhesives, coatings, sensors and smart hydrogels. The
sol-gel transformation
via the DA reaction with
1,1-(methylenedi-4,1-phenylene)bismaleimide was fully
thermally reversible (Scheme 1.11, Route b), making this
polycarbonate a possible
candidate for application as self-healing material.
Scheme 1.11. Synthesis and post-modification of
furfuryl-functional APCs.51, 52
1.3.2.2 APCs with pendant ester/carbonate groups
Ester/carbonate-functionalized epoxides are seldomly used,
particularly in the
copolymerization with CO2, for several reasons. First, many
catalysts are often
-
15
promoting transesterifications and transcarbonations. As a
consequence, branched and cyclic polymers are formed, which retards
the polymer chain growth and causes a
broad molecular weight distribution. Duchateau and coworkers
proved such behavior
by MALDI-ToF-MS studies on ester-functionalized polycarbonates
obtained by
copolymerization of 3,4-cyclohexene-oxide-1-carboxylic acid
methyl ester or 3,4-
cyclohexene-oxide-1-carboxylic acid phenyl ester and CO2.53
Moreover, due to the
similarity between the main chain carbonate linkages and the
pendant ester/carbonate
groups, the post-modification via transesterification or
hydrolysis can lead to the
breakdown of the polymer. Takanashi reported the first example
of carbonate-
functionalized epoxide/CO2 copolymerization using a ZnEt2/water
system as catalyst.54
The structures of the monomers are shown in Scheme 1.12 (Route
a). The hydrolysis of
the resulting polycarbonates was investigated under basic or
acidic conditions, during
which the compound attached to the pendant group was released.
Those materials are
thus potential drug carriers for controlled drugs release.
A fatty acid has been modified to make an epoxide-carrying ester
functionality. The
fatty acid-based epoxide, epoxy methyl-10-undecenoate, was
copolymerized with CO2
using a zinccobalt(III) double metal cyanide complex [ZnCo(III)
DMCC],
generating a dihydroxyl-terminated-polycarbonate with Tgs of -38
to -44 oC.
55 The fully
biobased polycarbonate was then used to initiate ring-opening
polymerization of L-
lactide, affording a biodegradable triblock copolymer (Scheme
1.12, Route b). The
pendant ester functionalities remained untouched during the
process.
Scheme 1.12. Synthesis and post-modification of APCs containing
pendant
ester/carbonate groups.53-55
-
16
1.3.2.3 APCs with pendant hydroxyl and carboxylic acid groups
The preparation of APCs bearing pendant hydroxyl and carboxylic
acid groups with
protic functionalities, introduced directly via the
copolymerization of CO2 and epoxides,
is rather challenging, since the protic species may deactivate
the catalyst or act as chain
transfer agents, causing branching, crosslinking or limiting the
growth of the polymer
chains. Such epoxides must then be modified by appropriate
hydroxyl- or carboxyl
protective groups, before copolymerization with CO2, and
subsequent deprotection is
required. The latter step should proceed quantitatively without
degradation of the
polycarbonate backbone.
Protected monohydroxy-functional glycidyl ether
Scheme 1.13. Synthesis and degradation of of poly(1,2-glycerol
carbonate).56-60
Glycidol/CO2 copolymerization attempts by Inoue using the
ZnEt2/H2O system as
catalyst led exclusively to the cyclic glycerol carbonate.56
The hydroxyl group was then
protected by a trimethylsilyl group. The copolymerization of the
new epoxide with CO2
afforded alternating copolymers with high molecular weight. The
hydrolysis of the
pendant trimethylsilyl ethers under acidic conditions gave
poly(1,2-glycerol carbonate)
(1,2-PGC), which readily degraded into cyclic glycerol carbonate
while exposed to
water (Scheme 1.13, Route a). Geschwind and Frey later described
the synthesis of 1,2-
PGC via two different approaches based on the use of ethoxy
ethyl glycidyl ether
(EEGE) and benzyl glycidyl ether (BGE).57
The protecting groups were cleaved either
by acidic treatment (PEEGEC) or by hydrogenation (PBGEC) (Scheme
1.13, Route b).
The former method caused the degradation of the polymer backbone
while the latter
did not. Isotactic linear 1,2-PBGECs was also prepared by Zhang
and Grinstaff using
the same method based on butyl glycidyl ether.58
Poly(butyl ether 1,2-glycerol
carbonate) was investigated as a potential (thermally) stable
solid polymer electrolyte.
-
17
It exhibited temperature-dependent conductivity with values
comparable to those of optimized PEO-based electrolytes.
59 Wu and coworkers proposed a simple way for the
preparation of PPC-co-1,2-PGC terpolymers, using
2-[[(2-nitrophenyl)methoxy]-
methyl]oxirane (Scheme 1.13, Route c).60
The o-nitrobenzyl (ONB) protecting groups
were cleaved efficiently under UV light within minutes,
affording hydroxyl-
functionalized PPCs with strong hydrophilicity and high Tgs.
Protected bishydroxy-functional epoxide
Geschwind and Frey also synthesized the glycerol-derived monomer
1,2-
isopropylidene glyceryl glycidyl ether (IGG, Scheme 1.14, Route
a). IGG was
terpolymerized with GME and CO2, giving random terpolymers with
different contents
of IGG units.61
The removal of the protecting acetal groups under acidic
hydrolysis
resulted in a new type of hydroxyl-functional polycarbonates
with diol side groups.
These polymers are rather hydrophilic materials with good
hydrolytic stability.
Moreover, the pendant two vicinal hydroxyl groups allows their
use as potential
substrates for the attachment of molecules with aldehyde or
ketone functionalities via
the formation of cyclic acetal- or ketal-groups. The facile
release of binding molecules
makes this class of compounds attractive for applications such
as controlled drug
release.
Scheme 1.14. Synthesis of APCs with bishydroxyl
functionalities.61-62
Liu and coworkers reported the synthesis of highly isotactic PCs
via copolymerization
of meso-3,5-dioxaepoxides and CO2 using a chiral dinuclear
Co(III) catalyst (Scheme
1.14, Route b).62
These isotactic PCs were semicrystalline with melting points
(Tm)
between 179 and 257 oC. The polymer derived from
4,4-dimethyl-3,5,8-
trioxabicyclo[5.1.0]octane (CXO) was hydrolyzed into a
stereoregular poly(1,2-
bis(hydroxymethyl)ethylene carbonate) with two primary hydroxyl
groups per
monomer unit upon acid treatment. This hydroxyl-functional
polymer was further used
as macro-initiator for grafting polymerization of lactide,
affording fully degradable
brush polymers, which may be used in biomedical and
pharmaceutical applications. In
-
18
addition, the optically active terpolymers of CHO, CXO and CO2
could be potential optical materials for applications like fiber
optics.
Protected carboxyl-functional glycidyl ether
Tsai et al. described the synthesis of an interesting
polycarbonate with pendant
carboxylic groups.63
In this report, tert-butyl 3,4-epoxybutanoate, derived from
3,4-
dihydroxybutyric acid, a normal human metabolite, was
copolymerized with CO2 using
bifunctional cobalt (III) salen catalysts, followed by the
removal of the tert-butyl
protecting group (Scheme 1.15). The resulting
poly(3,4-dihydroxybutyric acid
carbonate) was further modified for evaluation as carrier for
platinum-based anticancer
drugs. These results suggest that poly(3,4-dihydroxybutyric acid
carbonate) and the
related derivatives have the potential to serve as platinum drug
delivery carriers for
future anticancer pharmaceuticals.
Scheme 1.15. Synthesis and post-modification of APCs carrying
carboxylic acids.63
These functional polymers possess the following advantages in
the biomedical field: (i)
free functional groups for the binding of chemotherapeutic
agents, antibacterial
compounds, anti-inflammatory agents, fluorescent tags, or
material property modifiers;
(ii) a defined biodegradation route to afford nontoxic and
nonacidic byproducts, e.g.,
glycerol and carbon dioxide; (iii) physical properties ranging
from semicrystalline or
amorphous materials based on the polymer or copolymer
composition; (iv) potential
processability for manufacturing methods such as
electrospinning.
1.3.2.4 APCs with pendant hydrophilic/hydrophobic groups
CO2-based polycarbonates with rapid and reversible thermal
response at body
temperature were synthesized by alternating copolymerization of
CO2 and epoxides
-
19
with pendant hydrophilic (methoxyethoxy)ethoxy or methoxyethoxy
chains (see Scheme 1.16).
64 These novel polymers possess interesting properties such
as
biodegradability, well-defined thermo-sensitivity, nontoxicity
and anti-immunogenicity,
making them promising materials for various biomedical
applications.
Scheme 1.16. Synthesis of hydrophilic groups-containing
APCs.64
Kim and Coates reported a method for the synthesis of
multi-segmented polycarbonate
graft copolymers (Scheme 1.17).65
First, norbornenyl-terminated macromonomers with
variable block sequences were generated via living
copolymerization of functionalized
CHOs and CO2, using a -diiminate zinc catalyst with a norbornene
carboxylate
initiator. The segmented graft copolymers were then produced by
the subsequent ring-
opening metathesis polymerization of these macromonomers. This
method provides a
highly controlled way for the synthesis of core-shell molecular
brushes, which could be
useful in fields of phase separation, self-assembly and
nanostructure formation.
Scheme 1.17. Synthesis of norbornenyl-terminated PCHC from
functional CHO
carrying a hydrophilic or hydrophobic group and subsequent
ROMP.65
1.3.2.5 APCs with pendant epoxy/cyclic carbonate
Aliphatic polycarbonates with dual cyclic carbonate and epoxide
functionalities were
synthesized via an ethylene-bridged imine-thiobis(phenolate)
chromium complex
[(ONSO)CrCl]-mediated copolymerization of 4-vinyl-1-cyclohexene
diepoxide with
carbon dioxide in one pot (see Scheme 1.18).66
However, the broad molecular weight
distributions of these polycarbonates, even for relatively low
monomer conversions,
indicate a limited chemo-selectivity. Such dual functional APCs
have Tgs up to 154 C.
The content of pendant cyclic carbonate and epoxy groups could
be conveniently tuned
by varying the molar ratio of cocatalyst to metal complex and
the reaction temperature.
-
20
Furthermore, these polycarbonates can undergo multiple
post-modifications, providing an extensive library of
functionalized polycarbonate derivatives with hydroxyl- or
azidoalcohol-, and hydroxyurethane-functional groups.
Scheme 1.18. Synthesis and post-modification of APCs carrying
pendant cyclic
carbonate and epoxy groups.66
1.3.2.6 Side-chain liquid crystalline (SCLC) APCs
Side-chain liquid crystalline (SCLC) polycarbonates were
prepared by
copolymerization of carbon dioxide with epoxides carrying a
nitrostilbene mesogenic
group and spacers of different lengths (Figure 1.20, Route
a).67
The corresponding five-
membered cyclic carbonates was also formed as side products. No
clear relation
between phase transition temperatures and spacer length was
found, probably due to
the quite broad molecular weight distribution (5.9-80). Similar
observations were
reported for the SCLC polycarbonates with alkoxyphenylbenzoate
side groups (Scheme
1.19, Route b).68, 69
The high Tg of these polymers (110 oC) may be interesting
for
nonlinear optics applications or for optical data storage,
provided that the mesophase is
nematic and can easily be aligned.
Scheme 1.19. Synthesis of APCs containing LC side
groups.67-69
-
21
1.4 Aim of this study Terpenes form a large and diverse class of
organic compounds, mainly existing as the
primary constituents of the essential oils of many types of
plants and flowers. The use
of abundant, naturally occurring compounds for chemical
synthesis is an important
strategy for reducing our dependence on petroleum-derived raw
materials and for
enhancing the sustainability of chemical products. The aim of
this project is to develop
new materials from copolymers based on monomers derived from
natural terpenes,
particularly epoxides, and CO2. Among the available epoxides,
limonene oxide (LO)
and limonene dioxide (LDO), derived from the natural cyclic
monoterpene limonene,
are very promising monomers. Despite the enormous efforts to
develop new catalysts
and monomers for the copolymerization of epoxides and CO2, the
applications of the
traditional APCs like PPC and PCHC are still limited. The lack
of functionalities in
these APCs makes it impossible to improve their properties via
chemical modifications.
In comparison, the limonene-based polycarbonates can be fully
biobased functional
polycarbonates, serving as the platform for multiple
modifications or crosslinking
reactions, thereby making them suitable for various
applications.
To address these issues, the following goals were defined:
1) Synthesis and characterization of polycarbonates from LO and
LDO with moderate
molecular weight and proper functionalities.
2) Post-modifications of these limonene-based polycarbonates and
exploration of the
structure-properties relationships.
3) Evaluation of some typical functional polycarbonates in
(powder) coating
applications.
4) Studies on the degradation behavior during depolymerization
of these polymers or
crosslinked networks with the aim of exploring their
recyclability.
1.5 Outline of this thesis
APCs have received increasing attention due to their potential
recyclability and
biodegradability. An attractive method to prepare APCs lies in
the catalyzed alternating
copolymerization of epoxides with carbon dioxide. Among the
available epoxides, LO
and LDO, derived from the naturally occurring cyclic
monoterpene-limonene, are very
promising monomers. The rigid structure of the monomer units
result in high Tg
polymers and their natural origin provides the advantage of
renewability. Moreover, the
extra functionalities, viz. alkenyl and methyloxiranyl groups,
in the resulting
amorphous, transparent and hard polymers allow the use of
multiple curing and
-
22
modification chemistries and methods. This thesis describes the
synthesis, post-modifications and the evaluation of limonene-based
APCs as novel coating resins.
This chapter (Chapter 1) presents an introduction to the
development of APCs by
epoxide/CO2 copolymerization and summarizes the literature
describing the
functionalization of APCs. Also the aim and the outline of the
thesis are stated.
In Chapter 2, the synthesis and the coating evaluation of
hydroxyl-functionalized
PLCs will be discussed. First, hydroxyl-terminated poly(limonene
carbonate)s (PLCs)
with desired molecular weight were prepared from LO and CO2 by
two different
methods, including 1) the breaking down of high molecular weight
polycarbonates via
transcarbonation using polyols and by 2) applying
multifunctional chain transfer agents
(CTAs) during the LO / CO2 copolymerization. The curing of
dihydroxyl PLCs via
urethane chemistry gave poorly performing coatings with moderate
properties, which
could be explained by the limited reactivity of the secondary
and tertiary OH end
groups with polyisocyanates. In order to increase the number of
isocyanate-reactive
OH groups, post-modifications of PLCs with different
mercapto-alcohols via thiol-ene
click reactions were performed. PLCs with tunable properties
were obtained by
controlling the stoichiometry of these reactions. Some typical
PCs were evaluated as
coating resins by solvent casting and subsequent curing with
polyisocyanates. The
resulting coatings showed good acetone resistance and high
hardness.
In Chapter 3, the direct curing of PLCs with a multifunctional
thiol compound via
thiol-ene chemistry will be discussed. In-depth kinetic studies
determined the optimal
conditions for thiol-ene curing after solvent-casting. Both
thermal and UV curing were
employed. The crosslink density was controlled by using
different amounts of curing
agent, and thereby, varying the thiol/ene ratio. All the cured
coatings showed good
acetone resistance and high hardness. UV curing appeared to be
more efficient than
thermal curing, as indicated by Knig hardness measurements. The
high Tg of PLC
makes it a suitable candidate for application as a powder
coating resin.
In Chapter 4, the synthesis, the post-modification and an
application example of
poly(limonene-8,9-oxide carbonate) (PLOC) will be presented. The
chemo-selective
copolymerizations of limonene dioxide with CO2 using an Et-BDI
zinc amido complex
as catalyst generated PLOC, which exhibits Tg values up to 135
oC. Besides, PLOC
bears one pendant epoxide group in every repeat unit, which
readily underwent
chemical modification by catalyzed epoxide ring-opening
reactions with thiols and
carboxylic acids or by catalyzed CO2 insertion, without
affecting the polycarbonate
main chain. Finally, a fatty acid-modified PLOC was evaluated as
comb-like alkyd
resin.
-
23
In Chapter 5, the high Tg thiol-epoxy network formation based on
PLOC and the corresponding coating evaluation will be described.
PLOCs with different molecular
weights were cured by two multifunctional thiols. The optimal
curing conditions were
determined by a kinetic study of the curing at different
temperatures and with different
catalyst loadings. The stoichiometry of the curing reaction
determined the crosslink
density of the networks as indicated by DMTA results obtained
from the corresponding
free-standing films. The resulting coatings showed good acetone
resistance and
variable hardness and Tgs, depending on the structure of the
curing agents.
Chapter 6 will present the powder coating evaluations based on
several formulations
containing either PLC or PLOC. The UV-cured powder coatings
prepared from PLCs
and PLOCs with Mn values ranging from ca. 3 to 6 kDa showed
promising properties,
like good metal adhesion, acetone resistance and high hardness.
The results of the
coating tests indicated that these two PCs are promising
functional thermoset coating
resins.
In Chapter 7, the recyclability and degradability of PLC and
PLOC will be discussed.
The depolymerization of PLCs and PLOCs was investigated,
employing various
catalysts, including strong organic or inorganic bases and
several metal complexes. The
(metal-assisted) base-initiated depolymerization of PLC led to a
quantitative
conversion of the polymer into LO. However, the full
depolymerization of PLOC
resulted in a mixture of LDO and 1,2-epoxy cyclic limonene
8,9-carbonate, which can
be used for the synthesis of a multifunctional cyclic carbonate
as intermediate for non-
isocyanate based polyurethanes.
In Chapter 8 a technology assessment of the possible industrial
use of the APCs
described in this thesis will be provided, together with some
comments and suggestions
for future research in this field.
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2
Hydroxy-functional poly(limonene carbonate)s
Abstract
,-Dihydroxyl poly(limonene carbonate)s (PLCs) have been prepared
by
copolymerization of limonene oxide with CO2, using a -diiminate
zinc-
bis(trimethylsilyl)amido complex as the catalyst, and subsequent
transcarbonation
reactions with various (metallo)organic catalyst/polyol systems,
viz. stannous octoate,
(salen)AlEt (salen=2,2'-ethylenebis(nitrilomethylidene)diphenol)
and 1,5,7-
triazabicyclo-[4.4.0]dec-5-ene (TBD) as the transcarbonation
catalysts, combined with
1,3-propanediol, 1,10-decanediol, isosorbide,
2-(hydroxymethyl)-2-ethylpropane-1,3-
diol and pentaerythritol as the transcarbonation agents. The
structure and end groups
of the polymeric species of the PLCs were identified by
MALDI-ToF-MS. For the first
time, the ,-dihydroxyl PLCs were used in solvent casting and
curing. Nevertheless,
only moderate properties were achieved due to incomplete curing.
A partial post-
modification of these polycarbonates was fulfilled via thiol-ene
chemistry using two
mercaptoalcohols with different chain lengths, viz.
2-mercaptoethanol and 6-
mercaptohexanol. The thermal properties and hydroxyl values
(OHVs) of the resulting
hydroxyl PLCs were modulated by controlling the type and the
amount of incorporated
thioether species. The curing kinetics of the reactions between
these PLCs and
blocked/non-blocked multifunctional isocyanates was studied by
ATR-FTIR, followed
by solvent casting and curing under optimized conditions. The
good acetone resistance
and high transparency and hardness of the coatings demonstrated
that the fully
biobased PLCs with adequate molecular weights and OHVs are
promising resins for
coating applications.
This work has been published as: 1) C. Li, R.J. Sablong, C.E.
Koning, Eur. Polym. J., 2015, 67,
449-458; 2) C. Li, S. van Berkel, R.J. Sablong and C.E. Koning,
Eur. Polym. J., 2016, 85, 466
477.
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28
2.1 Introduction
The manufacturing of CO2-derived polycarbonates is one potential
methodology for
utilizing CO2 as C1 feedstock for developing novel polymers and
materials.1, 2
Since
the first report of the CO2 copolymerization reaction with
propylene oxide by Inoue
and coworkers, such polycarbonates have attracted great
attention.3 New epoxides and
different catalyst systems have been investigated in order to
achieve various polymer
structures and properties.4, 5
The most commonly used epoxides are propylene oxide
(PO), cyclohexene oxide (CHO) and styrene oxide (SO). Many other
petroleum-
derived epoxides carrying functional groups, like
4-vinyl-cyclohexene oxide (VCHO),6,
7 butadiene monoxide,
8 and the renewable 1,4-cyclohexadiene oxide (CHDO)
9, 10 and
limonene oxide (LO)11
have also been copolymerized with CO2, after which the
remaining functional groups were used for further modification
or crosslinking
chemistry.
One important challenge is to find promising applications for
the different
polycarbonates in line with their specific properties and
functionalities. So far,
extensive studies have been carried out on the modification of
CO2-derived PCs
bearing pendant functionalities. Coates and his coworkers
successfully transformed
linear vinyl-containing polycarbonates into nanoparticles of
controlled dimensions
using a second-generation Grubbs catalyst, which effectively
cross-metathesized the
vinyl groups along the polymer chain. The resulting polymeric
nanoparticles may be
applied in electronics as nanoporous insulators.12
Koning et al. reported a new
generation of polycarbonate resins, synthesized from CHO and
VCHO, which were
further evaluated as powder coating resins, using thiol-ene
chemistry for the
crosslinking reactions involving the pendant vinyl groups. The
resulting coatings
showed promising properties, like processability, high pencil
hardness and good
acetone resistance.6 Recently, Darensbourg and coworkers
described a practical method
to prepare amphiphilic/water-soluble polycarbonates by
post-polymerization
functionalization of PCs made from CO2 and 2-vinyloxirane.
Poly(2-vinyloxirane
carbonate) (PVIC) was first functionalized by thiol-ene coupling
with hydroxyl and
carboxylic groups and accordingly, became amphiphilic. The
amphiphilic
polycarbonates were further modified by ring-opening and/or
deprotonation.8 These
new PCs are expected to provide a powerful platform for
bio-conjugation. All these
reports described potential applications for each kind of
polycarbonate, but the great
dependence on the petroleum sources might limit their use in the
future when the fossil
feedstock will dry up.13
Thus, the development of new PCs based on bio-renewable
resources is not only promising but even necessary.
Poly(limonene carbonate) (PLC) prepared from LO, a derivative of
orange peel oil or
turpentine oils, is completely biobased and shows attractive
thermal and optical
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29
properties.14
Since the first report by Coates on the alternating
copolymerization of
limonene oxide with CO2, to our knowledge, limited work related
to the applications of
this new, fully renewable polycarbonate has been reported.15
As an amorphous and
highly transparent polymer, it is a promising thermoset resin
for coating applications.
The presence of pendant isopropenyl groups in PLCs, like the
previously mentioned
pendant vinyl groups in PVCHCs, allows relatively easy
modifications that will alter
the properties of the polymers, and thus extend their potential
applications.16-21
Several
studies have already demonstrated that those isopropenyl groups
could be modified by
the earlier mentioned thiol-ene click reaction, a powerful and
very versatile tool for
polymer functionalization. 15
For coating applications, the polymer resins should contain at
least two functional
groups (hydroxyl, carboxylic acid or epoxy) for the crosslinking
reactions with the
multifunctional curing agents. When the curing reactions are
restricted to the functional
end groups, moderate molecular weights (MWs) (2-4k) are
necessary to exhibit low
solution or melt viscosities and to obtain adequate crosslink
densities. Two approaches
can be used to synthesize the relatively low MW,
hydroxy-terminated polycarbonates,
either by applying multifunctional chain transfer agents (CTAs)
in epoxide/CO2
copolymerizations or by breaking down pre-made high Mw
polycarbonates via
transcarbonation reactions with polyols. Water and other protic
species have been
proven to act as chain transfer agents (CTA)s for some catalyst
systems, resulting in
relatively low MW polycarbonates. 22-24
Thus, in principle hydroxyl-terminated
polycarbonates could be produced by using dihydroxyl chain
transfer agents during the
polymerization.
In this work, we describe a promising approach to make fully
renewable ,-
dihydroxyl polycarbonate resins from LO and CO2 for coating
applications (Scheme
2.1). A new generation of end-functionalized PLC resins was
synthesized by
alternating polymerization of LO and CO2 and subsequent
transcarbonation reactions
using various (metallo)organic catalyst/(renewable) polyol
systems, as shown in
Scheme 2.1. Representative PLCs were evaluated as potential
coating resins by solvent
casting and subsequent curing. Some properties of the resulting
coatings were then
evaluated. We believe this is the first time that such fully
biobased polycarbonates have
been evaluated for practical applications. Furthermore, the
post-modification of high
MW PLCs with different mercaptoalcohols carrying primary OH
functionalities was
also investigated. PLCs with tunable properties were obtained by
controlling the
stoichiometry of the reactions and the nature of the
mercaptoalcohols. Some typical
mercaptoalcohol-modified PLCs were evaluated as coating resins
by solvent casting
and subsequent curing with polyisocyanates. As we will show in
this chapter, the PLCs
carrying reactive primary OH groups can form fully crosslinked
polycarbonate-
urethane network structures.
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30
Scheme 2.1. Synthesis and post-modification of
dihydroxyl-terminated PLCs.
2.2 Experimental section
2.2.1 Materials and general considerations
All reactions involving air- or water-sensitive compounds were
carried out under dry
nitrogen using MBraun glove boxes or standard Schlenk line
techniques. The
copolymerizations were performed in either a 10 mL high-pressure
reactor (homemade)
or a 200 mL stainless steel reactor (Bchi). Toluene,
tetrahydrofuran (THF),
dichloromethane (CH2Cl2) and diethyl ether were purchased from
Biosolve and
purified using an activated alumina purification system.
cis/trans (45/55)-R-Limonene
oxide (98% purity) and cis/trans-S-limonene oxide (99% purity)
were purchased from
Aldrich, distilled from calcium hydride and stored under
nitrogen. Carbon dioxide
(99.999% purity) from Linde Gas was used without further
purification.
Hexamethylene diisocyanate-based polyisocyanate (trade name:
Desmodur N3600) and
its corresponding caprolactam-blocked polyisocyanate (trade
name: Desmodur BL3272)
were gifts from Bayer AG. Other chemicals were also obtained
from Aldrich and used
as received. Zinc bis[bis(trimethylsilyl)amide] was synthesized
as described in the
literature.25
(Et-BDI)Zn[N(SiMe3)2][Et-BDI =
2-((2,6-diethylphenyl)amido)-4-((2,6-
diethylphenyl)imino)-2-pentene)] and (salen)AlEt (salen =
2,2'-
ethylenebis(nitrilomethylidene)diphenol) have been synthesized
according to published
procedures.26, 27
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31
2.2.2 Characterization
NMR spectra were recorded on a Varian Mercury Vx (400 MHz)
spectrometer at 25 C
in chloroform-d1 and referenced versus residual solvent shifts.
Gel permeation
chromatography (GPC) analyses were carried out using a Waters
Alliance system
equipped with a Waters 2695 separation module, a Waters 2414
refractive index
detector, a Waters 2487 dual absorbance detector, and a PSS SDV
5 m guard column
followed by two PSS SDV linearXL columns in series of 5 m (8
300) at 40 C. THF
with 1% v/v acetic acid was used as eluent at a flow rate of 1.0
mL min-1
. The columns
were calibrated using a series of polystyrene standards (Polymer
Laboratories, Mp =
580 Da up to 7.1 106 Da). Before the analyses, the samples were
filtered through a
0.2 m PTFE filter (13 mm, PP housing, Alltech). MALDI-ToF-MS
analyses were
performed on a Voyager DE-STR from Applied Biosystems equipped
with a 337 nm
nitrogen laser. An accelerating voltage of 25 kV was applied.
Mass spectra of 1000
shots were accumulated. The polymer samples were dissolved in
THF at a
concentration of 1 mg mL-1
. The cationization agent used was potassium
trifluoroacetate (Fluka, >99%) dissolved in THF at a
concentration of 5 mg mL-1
. The
matrix
trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile
(DCTB)
(Fluka) was dissolved in THF at a concentration of 40 mgmL-1
. Solutions of matrix,
salt, and polymer were mixed in a volume ratio of 4:1:4,
respectively. The mixed
solution was hand-spotted on a stainless steel MALDI target and
left to dry. The
spectra were recorded in the reflection mode. All MALDI-ToF-MS
spectra were
recorded from the crude products. An in-house developed software
was used to
characterize the polymers in detail and allowed us to elucidate
the individual chain
structures and the end groups of the polymer chains.
Differential Scanning Calorimetry
(DSC) analyses of polymer samples were performed on a DSC Q100
from TA
Instruments. Between 4 and 6 mg of polymer were placed in an
aluminum pan. The
samples were generally heated from room temperature to 160C at
10C min1
under
nitrogen. The reported DSC data from the second run were used to
determine the glass
transition temperatures of the polymers. End group titrations:
the acid values (AV, mg
KOH/g of polymer) and the hydroxyl values (OHV, mg KOH/g of
polymer) were
measured titrimetrically according to ISO 2114-2000 and ISO
4629-1978, respectively.
The efficiency of the crosslinking reaction and the coating
performance of the cured
coatings were evaluated at room temperature via the acetone rub
and rapid deformation
tests (reverse impact test, ASTM D 2794), see below.
To examine the physical properties of the cured free-standing
films, dynamic
mechanical thermal analysis (DMTA) was performed on a Q800 DMTA
(TA
instruments), equipped with a film fixture for tensile testing.
Film tension DMTA
measurements were performed on rectangular dry film samples
between 20 and 180 oC,
with a heating rate of 3 oCmin
-1. The tests were performed in the controlled strain
-
32
mode with a frequency of 1 Hz, an oscillating amplitude of 0.12
m, and a force track
of 125%.
2.2.3. Copolymerizations of limonene oxide with CO2
The copolymerizations were performed at 25 C in bulk with
different epoxide/catalyst
ratios. The catalyst was dissolved in LO and the solution was
transferred into a 5 mL
glass insert placed in the high pressure reactor. The reactor
was sealed, pressurized
with CO2 and allowed to react for the desired time. The LO
conversion was determined
by 1HNMR. The reaction mixture was dissolved in a small amount
of dichloromethane
and precipitated with a large excess of methanol. The polymer
was washed with
methanol to remove the catalyst and the unreacted epoxide and
dried in vacuo.
2.2.4. Transcarbonation reactions of PLCs with polyols
The PLCs and polyols (with monomer units/polyol = 10/1 mol/mol.)
were dissolved in
toluene under nitrogen at the desired temperature. After a
homogeneous mixture was
formed, the catalyst (1 mol % with respect to monomer units of
the PLCs) was added.
The solution was allowed to react until the target MW was
reached. The development
of molecular weight and its distribution was monitored by GPC.
The reaction mixture
was dissolved in a small amount of CH2Cl2 and precipitated with
methanol. The
resulting solid was redissolved in CH2Cl2 and reprecipitated in
methanol two times.
The final product was dried in a vacuum oven to give a white
powder.
2.2.5 Thiol-ene modification
A typical procedure for the synthesis of primary
OH-functionalized PLC polymer
(PLC-OH) was as follows. The modification reactions were carried
out with molar
ratios of reagents [C=C]0/[mercaptoalcohol]0/[AIBN]0 = 1/x/0.3.
Thiol-ene click
reactions between PLC (2 g, 10.2 mmol of pendant isopropenyl C=C
groups) and
mercaptoalcohol were conducted in a 100 mL Schlenk flask under
nitrogen atmosphere
with 10 mL 1,4-dioxane as solvent and AIBN (0.557 g, 3.4 mmol)
as initiator. The
reaction mixture was allowed to stir overnight at 80 oC. After
filtration, the solvent was
removed by rotary evaporation. The residual solid was dissolved
in THF and
precipitated in methanol/H2O (9/1). The product was
re-precipitated twice and PLC-
OH was obtained after drying in vacuum oven for 2-3 days at 60
oC.
2.2.6. Solvent casting and curing of hydroxyl-functional
polycarbonates
The PLC-OHs were cured using either a conventional
polyisocyanate curing agent, viz.
a trimer of hexamethylene diisocyanate (curing agent I, Desmodur
N3600, NCO
equivalent weight = 183 g/mol) or its derivative blocked with
-caprolactam (curing
-
33
agent II, Desmodur BL3272) (Scheme 2.2). PLC-OHs with high OHVs
were always
cured using the blocked polyisocyanate in order to prevent fast
gelation at room
temperature before curing in the oven. The other curing
experiments were carried out
using Desmodur N3600.
A solution of polycarbonate (0.3 g) and 0.5 wt% (calculated on
polymer mass) of
dibutyltindilaurate (DBTDL) in anhydrous NMP (0.6 mL) was
prepared along with a
separate solution of 1.05 molar equivalent of crosslinker
(calculated according to the
titration results) in NMP (0.3 mL). The two solutions were mixed
and a wet film of 220
m thickness was subsequently applied onto an aluminum panel,
using a doctor blade.
The film was left to dry at room temperature and then cured at
180 C for 30 minutes
under nitrogen.
Scheme 2.2. Approximate structures of the curing agents.
2.2.7. Evaluation of the cured coatings by the acetone double
rub test and
the reverse impact test
The coating performances were evaluated at room temperature
using several tests. The
solvent resistance was evaluated using the acetone rub test.
Hereto, the sample was
rubbed back and forth with a cloth drenched in acetone. If no
damage was visible after
more than 150 rubs, i.e. 75 double rubs (DRs), the acetone
resistance of the coating
was considered as good. The so-called reverse impact test is a
rapid deformation test,
performed by dropping a 1 kg ball on the backside of a coated
pan