High Performance Resins

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Prog. Polym. Sci. 32 (2007) 1344–1391

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Polybenzoxazines—New high performance thermosetting resins:Synthesis and properties

N.N. Ghosh1, B. Kiskan, Y. Yagci�

Department of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469, Turkey

Received 18 January 2007; received in revised form 10 July 2007; accepted 10 July 2007

Available online 2 August 2007

Abstract

Polybenzoxazine is a newly developed addition polymerized phenolic system, having a wide range of interesting features

and the capability to overcome several shortcomings of conventional novolac and resole type phenolic resins. These

materials exhibit (i) near-zero volumetric change upon curing, (ii) low water absorption, (iii) for some polybenzoxazines Tg

much higher than cure temperature, (iv) high char yield, (v) no strong acid catalysts required for curing, and (vi) release of

no toxic by-product during curing. The molecular structure of polybenzoxazines offers enormous design flexibility, which

allows tailoring the properties of the cured materials for a wide range of applications. In this review article, different

synthetic strategies for the preparation of benzoxazine monomers and blends, their polymerization reaction mechanisms,

and the structure–property relationships of the cured materials have been discussed.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: High performance polymers; Polybenzoxazine; Thermoset; Composites; Blends; Macromonomer

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345

2. Chemical methodologies for synthesis of benzoxazine monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1348

2.1. Mono-functional benzoxazine monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1348

2.2. Di-functional and multifunctional benzoxazine monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1349

2.3. Step-wise controlled synthesis of dimers and oligomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351

2.4. Allyl-containing monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1352

2.5. Acetylene containing monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353

2.6. Propargyl ether containing monomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353

2.7. Nitrile containing monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353

2.8. Maleimide and norbornane containing monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1354

2.9. Adamantane containing monomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355

e front matter r 2007 Elsevier Ltd. All rights reserved.

ogpolymsci.2007.07.002

ing author.

ess: [email protected] (Y. Yagci).

ddress: Chemistry Department, Birla Institute of Technology and Science-Pilani (Goa Campus), Zuarinagar, Goa 403726,

www.elsevier.com/locate/ppolysci

dx.doi.org/10.1016/j.progpolymsci.2007.07.002

mailto:[email protected]

ARTICLE IN PRESSN.N. Ghosh et al. / Prog. Polym. Sci. 32 (2007) 1344–1391 1345

2.10. Coumarin containing monomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356

2.11. Epoxy containing monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356

2.12. Naphthoxazine monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356

3. Combination of polybenzoxazines with other polymeric and inorganic materials . . . . . . . . . . . . . . . . . . . . . 1357

3.1. Preparations of blends and composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357

3.1.1. Rubber modified polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357

3.1.2. Polycarbonate blends with polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1359

3.1.3. Poly(e-caprolactone) blends with polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1359

3.1.4. Polyurethane (PU) blends with polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1359

3.1.5. Epoxy blends with polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1360

3.1.6. Phosphorous containing blends with polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1360

3.1.7. Clay-polybenzoxazine composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1361

3.2. Preparation of polymers with benzoxazine moieties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362

3.2.1. Benzoxazine functionalized polystyrene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1362

3.2.2. Benzoxazine functional poly(e-caprolactone) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13633.2.3. Benzoxazine functional poly(methyl methacrylate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364

3.2.4. Alternating maleimide copolymers with pendant benzoxazine groups . . . . . . . . . . . . . . . . . . 1364

3.2.5. Naphthoxazine functional poly(propylene oxide) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365

3.2.6. Benzoxazine functional polyhedral oligomeric silsesquioxane (POSS). . . . . . . . . . . . . . . . . . . 1365

3.3. Polymeric benzoxazine precursors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366

3.3.1. Main-chain precursors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1366

3.3.2. Side-chain precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1367

4. Reaction mechanism of ring opening polymerization of benzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

4.1. Cationic polymerization of benzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

4.1.1. Acid catalyzed polymerization of benzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368

4.1.2. Photoinitiated polymerization of benzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1371

4.2. Thermal polymerization of benzoxazines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1371

5. Properties of polybenzoxazines and their blends and composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373

5.1. Properties of polybenzoxazines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373

5.1.1. Properties of polybenzoxazines with additional functionalities. . . . . . . . . . . . . . . . . . . . . . . . 1377

5.1.2. Properties of rubber-modified polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1379

5.1.3. Polycarbonate (PC)-modified polybenzoxazine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1382

5.1.4. Properties of polycaprolactone (PCL)-modified polybenzoxazine. . . . . . . . . . . . . . . . . . . . . . 1383

5.1.5. Properties of polyurethane-polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1383

5.1.6. Properties of epoxy-polybenzoxazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384

5.1.7. Polybenzoxazines with flame retarding properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

5.1.8. Clay-polymer composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

5.1.9. Boron nitride-polybenzoxazine composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386

1. Introduction

Phenolic resins are widely used in variousapplications, from commodity and constructionmaterials to the needs of the high technologyaerospace industry. Though several desirable prop-erties, such as good mechanical strength, dimen-sional stability, resistance against various solvents,flame retardance, are characteristics of phenolicresins, a number of short-comings are also asso-ciated with these materials. For example, they arebrittle, have poor shelf life, acid or base catalysts are

often used for the preparation of resin, whichcorrode the processing equipments, and they releaseby-products (such as water, ammonia compoundsduring curing) which sometimes affect the proper-ties of cured resins by forming micro voids. Toovercome these problems recently a new type ofaddition-cure phenolic system, polybenzoxazines,has recently been developed. They have gainedimmense interest because they have the capability toexhibit the thermal and flame retardance propertiesof phenolics along with mechanical perfor-mance and molecular design flexibility. Although

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Nomenclature

440-a 6,60-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl)

Amine-POSS primary amine terminated polyhe-dral oligomeric silsesquioxane

ANDAD 4-amino-N,N-dimethyl aniline dihy-drochloride

ATBN amine-terminated butadiene acrylonitrilerubber

ATRP atom transfer radical polymerizationB-35x 6,60-(propane-2,2-diyl)bis(3-(3,5-di-

methylphenyl)-3,4-dihydro-2H-ben-zo[e][1,3]oxazine)

B-a 6,60-(propane-2,2-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

B-a-apa 6,60-(propane-2,2-diyl)bis(3-(3-ethynyl-phenyl)-3,4-dihydro-2H-benzo[e][1,3]ox-azine)

B-a-apa 6,60-(propane-2,2-diyl)bis(3-(3-ethynyl-phenyl)-3,4-dihydro-2H-benzo[e][1,3]ox-azine)

B-af-apa 6,60-(perfluoropropane-2,2-diyl)bis(3-(3-ethynylphenyl)-3,4-dihydro-2H-ben-zo[e][1,3]oxazine)

BAF-apa 6,60-(perfluoropropane-2,2-diyl)bis(3-(3-ethynylphenyl)-3,4-dihydro-2H-ben-zo[e][1,3]oxazine)

B-ala bis(3-allyl-3,4-dihydro-2H-1,3-benzoxa-zinyl)isopropane

B-a-ot 6,60-(propane-2,2-diyl)bis(3-o-tolyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

B-appe 6,60-(propane-2,2-diyl)bis(3-(4-(prop-2-ynyloxy)phenyl)-3,4-dihydro-2H-ben-zo[e][1,3]oxazine)

(2-Benzoxazine) adamantine and methyl aminebased benzoxazine

(3-Benzoxazine) adamantine and aniline basedbenzoxazine

BF-apa bis(3-(3-ethynylphenyl)-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methane

B-m 6,60-(propane-2,2-diyl)bis(3-methyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

B-mt 6,60-(propane-2,2-diyl)bis(3-m-tolyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

BO-apa 6,60-oxybis(3-(3-ethynylphenyl)-3,4-dihy-dro-2H-benzo[e][1,3]oxazine)

BP benzophenoneBP-apa 3,30-bis(3-ethynylphenyl)-3,30,4,40-tetra-

hydro-2H,20H-6,60-bibenzo[e][1,3]oxa-zine

B-pt 6,60-(propane-2,2-diyl)bis(3-p-tolyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

BS-apa 6,60-sulfonylbis(3-(3-ethynylphenyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine)

BZ-apa bis(3-(3-ethynylphenyl)-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methanone

BZPOSS Mono-functional benzoxazine mono-mer containing polyhedral oligomericsilsesquioxane

CAPRO 6-aminocaproic acidCQ camphorquinoneCTBN carboxyl-terminated butadiene acryloni-

trile rubber.CTX chlorothioxanthone.DGEBA diglycidyl ether of bisphenol ADMAc dimethylacetamideDMPA 2,20-dimethoxy-2-phenylacetophenone24DMP-m 3,6,8-trimethyl-3,4-dihydro-2H-ben-

zo[e][1,3]oxazineDODEC dodecylamineDopo dibenzo[c,e][1,2]oxaphosphinine 6-oxideDOPO-Gly glycidyl phosphinateDopotriol dibenzo[c,e][1,2]oxaphosphinine

6-oxide based benzoxazineDSC differential scanning calorimetryEPO732commercial name of a flexible epoxy

resinF-a 6,6-bis(3-phenyl-3,4-dihydro-2H-1,3-

benzoxazineyl) methaneFT-IR Fourier transform infra redHPM N-(4-hydroxyphenyl) malemideHPM-Ba 1-(3-phenyl-3,4-dihydro-2H-ben-

zo[e][1,3]oxazin-6-yl)-1H-pyrrole-2,5-dione

H-POSS hydro-silane functionalized polyhedraloligomeric silsesquioxane

HQ-apabisbenzoxazine derived from hydroqui-none and 3-ethynylaniline

HTBD hydroxyl terminated polybutadieneIHPOGly isobutyl bis(glycidylpropylether) pho-

phine oxideIPDI isophorone diisocyanateIPN Inter penetrating networkITX isopropyl thixanthone15Na naphthalene-1,5-diol based bisbenzoxa-

zine26Na naphthalene-2,6-diol based bisbenzoxa-

zine27Na naphthalene-2,7-diol based bisbenzoxa-

zine

N.N. Ghosh et al. / Prog. Polym. Sci. 32 (2007) 1344–13911346

ARTICLE IN PRESS

2Naa 2,20-p-phenylene-bis-(2,3-dihydro-1H-napth[1,2-e]-m-oxazine)

MIB benzoxazine with maleimide groupMMA methyl methacrylateMMT montmorilloniteNMR nuclear magnetic resonanceNO-apabisbenzoxazine derived from naphtha-

lene-1,5-diol and 3-ethynylanilineNOB p-hydroxyphenylnadimide derived ben-

zoxazineOCNE o-Cresol novolac-type epoxy resinOMMTorganically modified montmorillonite22P-a 8,80-bis(3,4-dihydro-3-phenyl-2H-1,3-

benzoxazine)22P-a PBZ poly(8,80-bis(3,4-dihydro-3-phenyl-

2H-1,3-benzoxazine))P-a N-phenyl-3,4-dihydro-2H-1,3-benzoxa-

zineP-ala 3-allyl-3,4-dihydro-2H-1,3-benzoxazineP-alp 3-phenyl-3,4-dihydro-8-allyl-2H-1,3-ben-

zoxazineP-appe 3-(4-(prop-2-ynyloxy)phenyl)-3,4-dihy-

dro-2H-benzo[e][1,3]oxazinePB-a poly(6,60-(propane-2,2-diyl)bis(3-phenyl-

3,4-dihydro-2H-benzo[e][1,3]oxazine))PB-ala poly(bis(3-allyl-3,4-dihydro-2H-1,3-ben-

zoxazinyl)isopropane)PBO (2,20-(1,3-phenylene)-bis(4,5-dihydro-ox-

azoles))440-a PBZ poly(6,60-bis(2,3-dihydro-3-phenyl-

4H-1,3-benzoxazinyl))PC polycarbonatep-Ca 3-benzyl-3,4-dihydro-6-methyl-2H-1,3-

benzoxazinePCL poly(e-caprolactone)pC-m 3,6-dimethyl-3,4-dihydro-2H-ben-

zo[e][1,3]oxazinePGE phenyl glycidyl etherPh-apa 3-(3-ethynylphenyl)-3,4-dihydro-2H-ben-

zo[e][1,3]oxazine

Ph-apc (4-(3-(4-ethynylphenyl)-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yloxy)phenyl)(4-(phenylethynyl)phenyl)methanone

PHEN p-phenetidinePoly VPpoly(p-vinylphenol)POSS polyhedral oligomeric silsesquioxanePP-a poly(N-phenyl-3,4-dihydro-2H-1,3-ben-

zoxazine)PP-alp poly(3-phenyl-3,4-dihydro-8-allyl-2H-

1,3-benzoxazine)PU polyurethaneSEC size exclusion chromatographySt styrene1,3-Ta 6,60-(4-isopropyl-1-methylcyclohexane-

1,3-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine)

2,8-Ta 6-(1-methyl-4-(2-(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)propan-2-yl)cyclohexyl)-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine

Th thiopheneTHF tetrahydrofuranTMAN 2,4,6-trimethylanilineTMDSC temperature modulated differential

scanning calorimetry235TMP-m 3,5,7,8-tetramethyl-3,4-dihydro-2H-

benzo[e][1,3]oxazine345TMP-m 3,5,6,7-tetramethyl-3,4-dihydro-2H-

benzo[e][1,3]oxazineTP-apa bis(3-(3-ethynylphenyl)-3,4-dihydro-2H-

benzo[e][1,3]oxazin-6-yl)sulfaneTPHT 1,3,5-triphenylhexahydro-1,3,5-triazine.TX thioxanthoneYa mixture of 6,60-(5-isopropylcyclohexane-

1,3-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine), 6,60-(2-isopropyl-cyclohexane-1,4-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine),6,60-(2-isopropylcyclohexane-1,3-diyl)-bis(3-phenyl-3,4-dihydro-2H-ben-zo[e][1,3]oxazine) etc.

N.N. Ghosh et al. / Prog. Polym. Sci. 32 (2007) 1344–1391 1347

benzoxazines were first synthesized by Cope andHoly in 1940s [1], the potential of polybenzoxazineshas been recognized only recently [2]. The molecularstructure of polybenzoxazines offers enormous de-sign flexibility, which allows the properties of thecured materials to be tailored for a wide range ofapplications. These newly developed resins possessunique features, namely (i) near-zero volumetricchange upon curing, (ii) low water absorption,

(iii) for some polybenzoxazine based materials Tg

much higher than cure temperature, (iv) high charyield, (v) no strong acid catalysts required forcuring, and (vi) release of no by-products (even non-toxic) during curing [3]. Though several researchershave reported different synthetic methodologies ofmany benzoxazine containing monomers, blends,composites, and their cure reactions and properties,no extensive and critical review is available solely

ARTICLE IN PRESSN.N. Ghosh et al. / Prog. Polym. Sci. 32 (2007) 1344–13911348

devoted to these materials. A special section hasbeen dedicated to describe the recent trend toincorporate benzoxazine groups into macromolecu-lar chains.

2. Chemical methodologies for synthesis of

benzoxazine monomers

Benzoxazine monomers are typically synthesizedusing phenol, formaldehyde and amine (aliphatic oraromatic) as starting materials either by employingsolution or solventless methods. Various types ofbenzoxazine monomer can be synthesized usingvarious phenols and amines with different substitu-tion groups attached. These substituting groups canprovide additional polymerizable sites and alsoaffect the curing process. Consequently, polymericmaterials with desired properties may be obtainedby tailoring the benzoxazine monomer. In thissection synthesis of different benzoxazine mono-mers is discussed.

2.1. Mono-functional benzoxazine monomers

Holly and Cope [1] first reported the condensa-tion reaction of primary amines with formaldehydeand substituted phenols for the synthesis of well-defined benzoxazine monomers. According to thereported procedure, this reaction was performed ina solvent in two-steps. Later, Burke found that thebenzoxazine ring reacts preferentially with the freeortho positions of a phenolic compound and forms aMannich bridge [4]. The synthetic procedure of theMannich condensation for benzoxazine synthesis ina solvent proceeds by first addition of amine toformaldehyde at lower temperatures to form anN,N-dihydroxymethylamine derivative, which thenreacts with the labile hydrogen of the hydroxylgroup and ortho position of the phenol at theelevated temperature to form the oxazine ring [5](Scheme 1).

As an example, to prepare 3,4-dihydro-3-cyclo-hexyl-6-t-butyl-1,3,2H-benzoxazine, Burke [4] em-ployed two procedures:

(i)

S

Cyclohexylamine was mixed formaldehyde indioxane. After addition of p-butyl phenol the

cheme 1. Synthesis of 3,4-dihydro-2H-1,3-benzoxazines. S

mixture was refluxed for 2 h. Upon cooling toroom temperature, a crystalline product wasobtained, which was then recrystallized from95% ethanol with 78% yield.

(ii)
Paraformaldehyde was dissolved in warmmethanolic KOH solution. The solution wascooled during the portion-wise addition ofcyclohexylamine. After the addition of 4-t-butylphenol, the resulting solution was cooledto room temperature and the product wasrecrystallized from 95% ethanol with 92% yield.Synthesis of a p-cresol based benzoxazine byusing aniline, formaldehyde and p-cresol asstarting materials in dioxane has been reported[6–8].
It has been observed that for some benzoxazines,the ring opening occurs in the presence of com-pounds with active hydrogen (HY), such asnaphthol, indoles, carbazole, imides, and aliphaticnitro compounds, even phenol (which is also one ofthe starting compound for synthesis) [9]; smalloligomers form as by-products. Formation of theMannich bridge structure due to the ring opening ofbenzoxazine in acidic medium (HY) [2] is shown inScheme 2.

The benzoxazines derived from a strongly basicamine and a less acidic phenol were found to bemore stable in the hot alcohols [10]. Substituent onthe benzoxazine ring affects the stability of the ring.The presence of more than one reactive ortho

position in the initial product may lead to anotheraminoalkylation reaction [11]. A significantly higheryield was found when the benzoxazine was derivedfrom phenol having an ortho substituent.

The slow reaction rate, large amount of solventrequired for the synthesis and, in some cases, thepoor solubility of the precursors are the majordisadvantages associated with this procedure.The use of an organic solvent also increases thecost of the products and causes environmentalproblems. Furthermore, the solvent residue in theprecursors also leads to problems during processingof the benzoxazine resins. To overcome these

cheme 2. Ring opening of benzoxazine in acidic medium.


shortcomings, Ishida et al. developed a solventless

synthesis in the melt state [12].The reaction mechanism and kinetics of this

solventless synthesis were proposed by Liu [13]. In atypical synthesis, the reactants, i.e., aldehyde, amineand phenolic precursors are physically mixedtogether, heated to their melting temperature, andthereafter maintained at a temperature sufficient tocomplete the interaction of the reactants to producethe desired benzoxazine. In this connection, itshould be pointed out that formaldehyde is nottypically used as it evaporates easily and losesstoichiometry quickly. Instead, paraformaldehyde isused. The choice for phenols and amines providesthe flexibility in designing monomer structure fortailoring the properties of the resulting polybenzox-azine polymer. The main advantages of the solvent-less synthetic method are improvement of reactiontimes compared with the traditional synthetic routeand formation of fewer unwanted intermediates andby-products.

Although most of the benzoxazines were synthe-sized by using phenol, formaldehyde and primaryamines as starting compounds several other syn-thetic strategies were also reported. To synthesize3,4-dihydro-2H-1,3-benzoxazine, Aversa et al. [14]first synthesized N-(2-hydroxy-3,5-dimethylbenzyl)-aminopropanoic acid via the Mannich reactionbetween 2,4-dimethylphenol, aqueous formalde-hyde, and 3-aminopropanoic acid in ethanol. Thisamino acid was allowed to react in 96% sulfuricacid at room temperature. After neutralization,3-(2-hydroxy-3,5-dimethyl)benzyl-3,4-dihydro-6,8-dimethyl-2H-1,3-benzoxazine was obtained.The reaction steps are shown in Scheme 3.

In this method, the alkylating agent arisesfrom acid-induced deamination of the phenolicMannich base. Thus, the variety of substituent on

Scheme 3. Formation of 1,3-oxazine ring f

the N-3 position of the benzoxazine ring is limited.Benzoxazine can also be obtained by heating themixture of 2,4-xylenol and hexamethylenetetramine(3:4:1mole) at 135 1C for 2 h in air [1]. The reactionof 1mole of 2-hydroxybenzylamine with 2molesof formaldehyde produces bis-(3,4-dihydro-2H-1,3-benzoxazine-3-yl)-methylene [4]. This benzoxazinecan further react with phenol to form 3,4-dihydro-3-(2-hydroxy)benzyl-2H-1,3-benzoxazine [2] (Scheme 4).

Some 3,4-dihydro-2H-1,3-benzoxazines with sub-stituents on C-2 or C-4 such as, 2,2-dibenz-1,3-oxazine, were also synthesized, by the reactionsof salicylamines(o-hydroxybenzylamine) with glyox-al or -diketones in methanol at a temperature lowerthan 20 1C [15]. Another method to synthesizebenzoxazines is directed ortho-metalation metho-dology. This offers a predictable and widelyapplicable synthetic strategy for the regiospecificconstruction of heterocyclic compounds [16]. 3,4-Dihydro-2H-1,3-benzoxazines were synthesizedby directed ortho-lithiation of phenols and by side-chain lithiation of substituted phenols, respectively,in one-pot by reacting with N,N-bis[(benzotriazol-1-yl)methyl]amines as 1,3-biselectrophile synthons(Scheme 5) [17].

2.2. Di-functional and multifunctional benzoxazine

monomers

Curing of mono-functional benzoxazines withphenol resulted in the formation of only oligomericstructures with an average molecular weight around1000Da. Thus, no materials could be made fromthis approach since the thermal dissociation of themonomer competed with chain propagation reac-tion so that high-molecular weight linear structureswere unobtainable [18]. Actually, there is noconvincing evidence reported for the thermal

rom 3-aminopropanic acid derivative.

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Scheme 4. Formation of 1,3-oxazine ring from 2-hydroxybenzylamine.

Scheme 5. Synthesis of 3,4-dihydro-2H-1,3-benzoxazines by ortho-lithiation of phenols.


dissociation theory, though it was mentioned in theliterature. Moreover, Hemvichian K. et al. havereported that the reduction of reactivity is due tohydrogen bonding formation. Such a phenomenonwas observed in the temperature range below thatfor which reverse Mannich reaction occurs inbenzoxazine chemistry [19]. To overcome thislimitation, Ishida and coworkers [8,20] have devel-oped a new class of difunctional or multifunctionalbenzoxazine monomers, and their curing intophenolic materials with the ring opening reactionsbeing initiated by dimers and higher oligomers inthe resin composition. The precursor was synthe-sized using bisphenol-A, formaldehyde and methylamine in different solvents and referred to as B-m

(see Table 2), as a reference to two of its originalingredients: bisphenol-A and methylamine. Themain constituent of the resulting products was amonomer with difunctional benzoxazine ring struc-tures at both ends of the bisphenol A. The rest ofthe composition consisted of a mixture of dimersand oligomers, with both benzoxazine rings and freephenol structures, as detected by NMR, FTIR andSEC. It was observed that the composition of theproducts is, to a large extent, dependent on the

polarity of the solvent. This synthetic methodconsists of a few simple steps and can easily providedifferent phenolic structures with wide designflexibility.

Similar type of difunctional benzoxazine wasprepared using aniline instead of methyl amine[21,22] and the pure monomer was referred as B-a

and oligomers were as oligo-B-a. The structures ofoligo-B-a and B-a were analyzed by 1H-NMRmeasurements. The overall synthetic procedure isshown in Scheme 6 [21]. To achieve successfulprocessing, cure kinetics of this material wasinvestigated by using DSC, which indicated thatthe curing of benzoxazine precursors is an auto-catalyzed reaction until vitrification is occurred, anddiffusion begins to control the curing process [22].

The synthesis of 6,60-(propane-2,2-diyl)bis(3-phe-nyl-3,4-dihydro-2H-benzo[e][1,3]oxazine) (B-a) inhigh yield by the solventless reaction processusing 1,3,5 triphenyl(alkyl) hexahydro-1,3,5 tria-zine, paraformaldehyde and bisphenol A has beenreported [23].

Solventless method was successfully employed forsynthesis of a series of difunctional monomers listedin Chart 1 [12,21,23–26].

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Scheme 6. Synthesis of bisphenol-A and aniline based benzoxazine (B-a) monomer.

Chart 1. Difunctional benzoxazine monomers.


2.3. Step-wise controlled synthesis of dimers and

oligomers

To properly understand the structures of benzox-azines and the polymers formed due to the ring

opening polymerization, several model oligomers(dimers, trimers, tetramers etc.) were synthesizedusing a controlled step-wise route [27–30] via thesynthetic strategy shown in Scheme 7 [28]. From in-depth characterizations of these model benzoxazine

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Scheme 7. Synthetic of model benzoxazine oligomers.


oligomers by 1H-NMR, 13C-NMR and FT-IRspectroscopy a pseudo cyclic structure based onstable –OH–N intramolecular hydrogen bondingand OH–O intramolecular hydrogen bonding hasbeen proposed and the possibility of helicalstructure formation in the longer chain benzoxazineoligomers has been predicted [28].

Laobuthee et al. [31] demonstrated how thestereo-structure of the reactant molecule plays animportant role to control the reaction and synthe-sized an asymmetric product, which was notexpected when considering the chemical formulaof the reactants. The major disadvantages of thetypical polybenzoxazines are their brittleness andthe high cure temperature needed for the ringopening polymerization. Two major approachesare generally considered: (1) by preparing speciallydesigned novel monomers, or (2) by blending with ahigh-performance polymer or filler and fiber.Despite their usual thermal stability, the sidefunctional groups R of the Mannich bridge,–CH2–NR–CH2–, were found to be the weakestpoints in the cross-linked network structures.Thermal decomposition study of the polybenzox-azines revealed that they decompose by loss ofamine fragments [32]. Therefore ‘‘end-capping’’ tothese functionalities by another polymerizablegroup was a promising strategy to stabilize theMannich bridge, with the expectation of furtherimprovement of the thermal stability of the poly-benzoxazines.

In the first approach, the introduction of ethynylor phenyl ethynyl [33,34], nitrile [35], propargyl [36]etc. groups can offer additional cross-linking siteduring polymerization, and were found to be

acceptable choice for that purpose. According tothe second approach, the mechanical and thermalproperties of polybenzoxazines can be improved bythe preparation of copolymers, polymer alloys,composites, and polymer-clay nanocomposites (videinfra) [37–44].

2.4. Allyl-containing monomers

The main advantage of the allyl group [45,46] isnot only that it provides additional crosslinkablesites, but that it can easily be cured at a temperaturelower than that needed for acetylene groups. Allyl-containing monomers have attracted much atten-tion because they are used as reactive diluents ofbismaleimides to improve the toughness of thecured resin [47,48]. Ishida also reported [12] thepreparation of an allyl-containing benzoxazinemonomer, 3-phenyl-3,4-dihydro-8-allyl- 2H-1,3-benzoxazine, from allylphenol, aniline, and paraf-ormaldehyde. A similar benzoxazine monomerbased on allylphenol was reported for silylation ofthe allyl group to enhance the interface between thematrix and glass or carbon fiber in fiber-reinforcedpolybenzoxazine [49] Also, Pei et al. reportedsimilar bifunctional allylphenol-derived polyben-zoxazine [50]. Because of the absence of activatedortho position to the phenolic hydroxyl group, theseallylphenol-based benzoxazine monomers, however,are considered to be difficult to polymerize throughring-opening and are not good candidates forpreparing high performance polybenzoxazines. Thesynthetic approaches adopted by Agag and Takeichi[51] for the preparation of two novel benzoxazinemonomers modified with allyl groups: (i) 3-allyl-3,

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Scheme 8. Synthesis of allyl containing benzoxazine monomers.


4-dihydro 1,3-benzoxazine and (ii) bis(3-allyl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane are shownin Scheme 8.

It was reported that benzoxazines containing allylgroup can polymerize at temperatures below 150 1C.However, this polymerization occurring at lowtemperature was not from the benzoxazine ring-opening reaction, but from the allyl group and ahigh temperature above 250 1C was needed tocomplete the polymerization of benzoxazine rings.Synthesis of a series of allyl group containingmono-functional benzoxazine monomers, wherethe allyl group is attached with nitrogen and derivedfrom cresol and allyl amine by a solventless methodhas been reported and the effect of these allylgroups on polymerization reaction and the perfor-mance enhancement of the cured polymers athigh temperature has been reported by Takeichiet al. [52].

2.5. Acetylene containing monomers

The synthesis of easily processable benzoxazinemonomers with acetylene functionality has beenreported by Ishida et al. [34,53]. It has beenobserved that the high thermal stability of thepolybenzoxazines derived from this class of mono-mers is a combined result of polymerization ofacetylene terminal functional group and oxazinering-opening polymerization. Most of the mono-functional monomers were synthesized by thegeneral solvent method whereas the multifunctionalmonomers were obtained by solventless methods.

The synthesis of various difunctional monomers isdepicted in Scheme 9.

2.6. Propargyl ether containing monomers

Propargyl ether group, as a thermally reactiveend-capping agent, has attracted much attentionbecause these monomers can be synthesized in highyield with low cost, in contrast to ethynyl-contain-ing monomers which the preparation procedure is inlow yield and high price [54]. Agag and Takeichihave prepared novel benzoxazine monomers con-taining a propargyl ether group as the cross-linkablefunctional group according to Scheme 10 andobtained novel polybenzoxazines with attractivethermal properties [36]. The ring-opening polymer-ization of oxazine ring and cross-linking of propar-gyl ether group occurred at almost the sametemperature range, at 230 1C for mono-functionaland 249 1C for bifunctional monomer. Polybenzox-azines derived from these monomers exhibitedsignificantly improved thermal properties than thetypical polybenzoxazines.

2.7. Nitrile containing monomers

Development of high performance phthalonitrilefunctional polybenzoxazines was another attempttaken by Ishida et al. to achieve highly thermalstable resin [35]. It was expected that side func-tionality, phthalonitrile, would contribute to thecross-linked network formation by its own poly-merization. This attempt was taken because nitrile

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Scheme 9. Synthesis of ethynyl containing benzoxazine monomers.

Scheme 10. Synthesis of propargyl ether containing benzoxazine monomers.


group reacts during pyrolysis of polyacrylonitrile. Ithas been reported that this thermal polymerizationcan be initiated by nucleophilic species that attacknitrile groups and form active species –C+

¼ N�.The active species continue the reaction with theneighboring nitrile group and a ladder like polymeris formed with tetrahydronaphthiridine ring struc-ture [55]. Benzoxazines with one or more nitrilefunctionalities of the following structures weresynthesized. Chart 2 represents several phenyl nitrilecontaining benzoxazine monomers [35]. Notably,the polymers obtained from monomers with two

–CN groups exhibited better thermal properties [56](See Section 5).

2.8. Maleimide and norbornane containing

monomers

A benzoxazine compound with a maleimidependant (HPM-Ba) was prepared to achieve attrac-tive processing and thermal properties. It was pre-pared from N-(4-hydroxyphenyl) malemide (HPM),formaldehyde and aniline in dioxane mediumat 30% yield. Another reported method uses 1,3,

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Chart 2. Phenylnitrile containing benzoxazine monomers.

Chart 3. Maleimidyl and norbornyl functional benzoxazines.


5-triphenylhexahydro-1,3,5-triazine (TPHT) . Thereaction was performed through a solventlesssynthesis route where TPHT, aniline and parafor-maldehyde was mixed together and heated at 150 1Cfor 1.5 h. The yield of the final product, HPM-Ba,after washing and precipitation was 70% [57,58].Similarly, mono-functional benzoxazine with nor-borane functionatility, NOB, was synthesized [59]using p-hydroxyphenylnadimide, p-formaldehydeand aniline in DMF at 90 1C. Also, a nitrile groupcontaining maleimide benzoxazine was synthesizedto further improve thermal properties of polyben-

zoxazine resin [60].The structures of benzoxazinemonomers having malemide and norbornane func-tionality are shown in Chart 3.

2.9. Adamantane containing monomers

The synthesis of adamantyl modified benzoxazinemonomers shown in Chart 4 has been reportedusing 4-(1-adamantyl)-phenol, formaldehyde andaniline (or methylamine) in dioxane [61,62].

It was expected that the rigid structure ofthe adamantane would hinder the chain mobility


(boat anchor effect) and substantially enhancethe thermal properties of the resulting polymer,including the glass-transition temperature anddecomposition temperature, especially for poly(6-adamantyl-3-methyl-3,4-dihydro- 2H-1,3-ben-zoxazine) (poly(3-benzoxazine). In the poly(6-adamantyl-3-phenyl-3,4-dihydro-2H-1,3-benzox-azine) (poly(2-benzoxazine) system, however, theopposite result for the glass-transition temperaturewas observed and explained by lowering of cross-linking density. As the phenyl group was bulkierthan the methyl group, the movement of themolecular chain was hindered between bridgingpoints during the curing process; this resulted in alower cross-linking density.

2.10. Coumarin containing monomers

A monomer, 4-methyl-9-p-tolyl-9,10-dihydrochro-meno[8,7-e][1,3]oxazin-2(8H)-one, possessing bothbenzoxazine and coumarin rings in its structure wassynthesized by the reaction of 4-methyl-7-hydroxy-coumarin, paraformaldehyde, and p-toluidine in 1,4-dioxane. Upon photolysis around 300nm, where thebenzoxazine ring has no absorption, this monomer

Chart 4. Adamantyl fuctional benzoxazine.

Scheme 11. Synthesis of coumarin con

underwent dimerization via the [2ps+2ps] cycloaddi-tion reaction (Scheme 11). The process was followedby UV–VIS spectroscopy. UV absorption versus thewavelength showed a decrease in the absorbanceat 320nm resulting from the dimerization of thecoumarin group. As the coumarin dimerizes, the levelof unsaturation decreases because of the formation ofthe cyclobutane ring

The 1H-NMR investigation of the irradiatedsamples revealed that the microstructure of thedimers was rather complex. Various isomers withdifferent syn and anti and head-to-head and head-to-tail configurations as well as cis–trans structuresarising from the methyl groups were identified [63].

2.11. Epoxy containing monomers

Andreu et al. reported the synthesis and poly-merization of glycidylic derivatives of benzoxazinesobtained from aniline and 4-hydroxybenzoicacid and from phenol and 4-aminobenzoic acid(Scheme 12) [64]. By introducing epoxy groups intothe molecular structure of benzoxazine, anotherattractive way of improving the thermal stabilityand glass-transition temperatures of the resultingpolybenzoxazines was achieved.

2.12. Naphthoxazine monomers

On replacement of the benzene ring by thenaphthalene, the corresponding oxazine becomes anaphthoxazine. Naphthoxazines were synthesizedemploying the similar strategy, i.e., reaction ofnapthol, formaldehyde and primary amines. Butalong with it alkylaminomethyl-2-napthol alsoformed as by-product as shown in the Scheme 13[9].

Solvent, temperature and basicity of amine playimportant roles upon the yield of the correspondingnaphthoxazine monomer formation. Difunctional

taining benzoxazine monomers.

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Scheme 12. Synthesis of epoxy containing benzoxazine monomers.

Scheme 13. Synthesis of naphthoxazines.


amines like p-phenylenediamine when reactedwith formaldehyde and napthol (1:4:2molar ratio)it produced 2,20-p-phenylene-bis-(2,3-dihydro-1H-napth[1,2-e]-m-oxazine) (2Na-a). Several other di-functional naphthoxazines were also synthesizedfrom dihydroxynapthalene, formaldehyde and pri-mary amines (Chart 5) [9,65–69]

Apart from naphthoxazine some fluorinatedbenzoxazine [70–72] and furan containing benzox-azine [73] have also been reported in the literature.

3. Combination of polybenzoxazines with other

polymeric and inorganic materials

As stated previously, several approaches to over-come some of the shortcomings of polybenzoxa-zines, such as mechanical properties, high curingtemperature and low process ability, have beenproposed. These include modification of the mono-

mer, preparation of polymer blends and composites,hybridization with inorganic materials and chemicalincorporation of benzoxazine structure into poly-mers. The first approach which concerns themodification of monomer in the synthesis step hasbeen discussed in detail in the previous section. Themethods described open the possibility of prepara-tion of a wide range of monomers with additionalfunctionalities, if not to completely meet targetedproperties, at least to improve them. In thefollowing sections, we will discuss the combinationof polybenzoxazines with the other polymeric andinorganic materials.

3.1. Preparations of blends and composites

3.1.1. Rubber modified polybenzoxazine

One of the successful approaches to overcome theinherent brittleness of the thermosets is modification

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Chart 5. Difunctional naphthoxazines.


by rubber [74,75]. The toughening mechanism mayinvolve cavitation of the rubber particles, followedby plastic deformation of the matrix [76–78].Though cavitation is not the only source theconsiderable toughening, its importance on theplastic deformation of the matrix has been widelyrecognized [79,80]. Two mechanisms for the plasticdeformation induced by rubber particles have beenproposed: (i) shear yielding of matrix between theneighboring rubber particles, and (ii) plastic voidgrowth of the matrix surrounding the particle. It hasalso been shown that the rubber particles in thematrix phase relieve the constraint in front of cracktips by rubber cavitations, which triggers theformation of shear bands [79,81]. Various morpho-logical parameters, such as particle size, particle sizedistribution, and matrix-to-particle adhesion, playimportant roles in toughening [80,82–85]. Liquidrubber, commonly used for epoxy modifications, isthought to be appropriate due to its low viscosity,and its polarity can easily be monitored by changingthe ratio of polybutadiene and acrylonitrile. Thepolarity control of additives is important becausepolarity effects the distribution of rubber in thematrix. It has been reported that the phaseseparation of rubber and matrix is necessary andthe size of distributed rubber particle has to be102–103 nm to obtain substantial improvement intoughness [86].

Polybenzoxazine was modified with amine-termi-nated butadiene acrylonitrile rubber (ATBN) andwith carboxyl-terminated butadiene acrylonitrile(CTBN) rubber in order to improve its mechanicalproperties [81]. Rubber modification of polybenzox-azine was carried out by adding liquid rubber to amolten benzoxazine monomer (bisphenol-A baseddifunctional benzoxazine) at 120 1C with mechan-ical stirring. The molten mixture was then cast in asilicon rubber open mold and cured at a well-defined curing cycle. In that investigation theformulation of ATBN and CTBN series were variedfrom 0 to 3wt%.

In another investigation, Ishida et al. usedhydroxyl terminated polybutadiene (HTBD) rub-ber, having various epoxy content, as the toughen-ing modifier [85]. The epoxidized polybutadienerubber can undergo copolymerization with thehydroxyl groups produced upon ring opening ofbenzoxazine, and thus can be chemically graftedinto the matrix network [87], a toughened compositewith a higher compatibility was obtained. A meltmixing method was used to obtain rubber-modifiedpolybenzoxazines.

Agag and Takeichi [88] reported the preparationof hydroxyphenylmaleimide (HPM) and ATBN-modified polybenzoxazine by mixing benzoxazinemonomer (B-a), HPM and ATBN in melting state,followed by film casting and curing.


3.1.2. Polycarbonate blends with polybenzoxazine

Due to the relatively high toughness and thecapability of intermolecular hydrogen bond forma-tion with polybenzoxazine main chain, polycarbo-nate (PC) was chosen as a blending material toimprove the toughness of polybenzoxazines [89].According to Ishida and Lee, the driving force thatresults in the miscibility of the PC/benzoxazineblend in the entire composition range is theinteraction between the hydroxyl groups of poly-benzoxazine and the carbonyl groups of the PC.A solution blending method was employed for thepreparation of all the blend samples. Solutions ofthe purified benzoxazine monomer based onp-cresol and aniline, 3-benzyl-3,4-dihydro-6-methyl-2H-1,3-benzoxazine (abbreviated as p-Ca), and PCwere blended at room temperature to form ahomogeneous mixture with the aid of chloroformto obtain a transparent yellow solution. The solventin the blended mixture was first evaporated in anambient environment until most of the solvent wasdriven off, followed by removal of the residualsolvent and moisture in a vacuum oven at roomtemperature for at least 48 h. The sample obtainedabove was isothermally polymerized in an air-circulated oven at 180 1C for various periods oftime. It should be noted that phase separationoccurs with increase of the PC content [90].

3.1.3. Poly(e-caprolactone) blends with

polybenzoxazine

Though poly (e-caprolactone) (PCL) possessesvery low Tg (�55 1C), its thermal stability is muchhigher compared to the other low Tg modifiers. Thisunique property makes PCL a potential candidatefor blending with polybenzoxazine to achieve easypossibility and improved thermal properties. Apartfrom that, as intermolecular hydrogen bondingbetween hydroxyl groups of polybenzoxazine mainchain and the carbonyl groups of PCL may form, itcan enhance the miscibility of PCL with polyben-zoxazine [91,92].

The preparation and characterizations of PCL-polybenzoxazine (PB-a) blends by melt blendingprocess was reported by Ishida and Lee [91,92].Different concentrations of PCL were added to B-aat 120 1C. After thorough mixing, a clear homo-geneous mixture was obtained. This mixture wasthen step-cured in a compression molder afterdegassing.

Zheng et al. [93] prepared the blends of B-awith PCL by casting from chloroform solution at

room temperature followed by removal of solventsby drying in a vacuum oven at 60 1C for 2 d. A meltblending method was applied to prepare PB-a/PCLblends from B-a and PCL.

A solution blending method was used by Huangand Yang [94] to obtain B-m/PCL blends havingdifferent compositions where THF was used assolvent.

3.1.4. Polyurethane (PU) blends with

polybenzoxazine

Good abrasion resistance, outstanding oilresistance, excellent low-temperature flexibility,and extraordinary processibility make polyure-thane (PU) elastomers (with soft segments derivedfrom polyols and hard segments from isocya-nates and chain extenders) one of the mostattractive class of elastomers. They also exhibit awide range of hardness and elastic moduli that justfills the gap between plastics and rubbers. Inanother words, they have the potential to tailorthe materials with characteristics of either highmodulus or good elasticity [95,96]. However,low resistance to moisture and hydrolysis, lowresistance to polar solvents, and poor thermalstability are limitations associated with theseelastomers. Generally, the acceptable thermal dur-ability for PUs ranges from 80 to 90 1C, and thethermal degradation of PUs occurs at ca. 200 1C[97]. The phenolic hydroxyl groups present inthe polybenzoxazine have a strong capability forreacting with PUs or their prepolymers withterminal reactive �NCO groups, which draws themotivation to prepare PU/polybenzoxazine blends[98,99].

Poly(urethane-benzoxazine) films were preparedby Takeichi et al. by a solution blending method inwhich the PU prepolymer was mixed with variousamount of a benzoxazine monomer, B-a, in THFand followed by casting on glass plates and curingby thermal treatment [100].

Inter Penetrating Networks (IPN) of PU/PB-awas prepared by mixing B-a with PU in warm N,

N- dimethylacetamide (DMA). The mixture wasprocured at 120 1C for 1 h and was coated into apreheated Teflon mold at 180 1C. The mold wasthen kept in a vacuum oven at 120 1C for 2 h andthen cured at 200 1C for 2 h [101].

A melt blending technique was used by Rimdusitet al. for alloying polybenzoxazine with PU andepoxy [102].


3.1.5. Epoxy blends with polybenzoxazine

Benzoxazines were first copolymerized with anepoxy resin in order to modify their performance byIshida et al. [103]. The addition of epoxy to thepolybenzoxazine network greatly increases thecrosslink density of the thermosetting matrix andstrongly influences its mechanical properties. Copo-lymerization led to significant increase in the glass-transition temperature, flexural stress, and flexuralstrain at break above those of the polybenzoxazinehomopolymer, with only a minimal loss of stiffness.

Copolymers from polybenzoxazines and epoxyresins were also designed keeping in mind that thering opening reactions of benzoxazines producesphenolic hydroxyl groups, which can react withepoxy resins and provide additional cross-linkingpoints into the matrix offer a network structure[104]. Kimura et al.. prepared the samples contain-ing 50mol% B-a and 50% diglycidyl ether ofbisphenol A (DGEBA) and cured in a mold in theoven using the curing condition of 150 1C/1 h+170 1C/1 h+190 1C/2 h+200 1C/2 h+220 1C/2 h [6]. As it is reported that terpenediphenol-formaldehyde resin possesses superior heat resis-tance, water resistance, and mechanical properties,terpenediphenol based benzoxazine monomers weresynthesized (Scheme 14) and cured blend samplescontaining 50mol% DGEBA and 50mol% benzox-azine monomers were prepared employing theabove mentioned cure conditions [105]. The mold-ing compounds were prepared by hot roll-kneadingof a mixture of 50 phr (per hot roll-kneading) Ya,

Scheme 14. Synthesis of terpenediphen

50 phr OCNE (o-Cresol novolac-type epoxy resin)wax and 100 phr fused silica. Test pieces of themolding compounds were prepared by compressionmolding at 190 1C for 20min after preheating torequired moldability for compression molding. Alltest pieces were postcured at the same cureconditions to complete the cure reactions, and wereused for the various measurements. Rao et al.prepared copolymers of chain extended epoxy(40mol%) with benzoxazine (bisphenol A andaniline based) (60mol%) using a solution mixingmethod in acetone, and investigated the effects ofmolecular weight of the added epoxy resins [106].

3.1.6. Phosphorous containing blends with

polybenzoxazine

Organo-phosphate compounds have attractedattention for their use as flame retardant polymers.Espinosa et al. suggested two different routes for thepreparation of flame retardant polymers. [107,108]:(1) modified novalac resins with benzoxazines werecopolymerized with a glycidyl phosphinate, (2)modified novalac resins with benzoxazines werecured with isobutyl bis(glycidylpropylether) pho-phine oxide (IHPOGly) as cross-linking agent.Mixtures of novolac resin, diglycidylethers andPPh3 were made by dissolving the components inacetone and then evaporating the solvent at roomtemperature under a vacuum. The resin was placedin a 60� 40� 0.5mm mold and compressionmolded at 180 1C for 2 h under 0.1Mpa pressure.Post-curing was carried out at 220 1C for 5 h.

ol based benzoxazine monomers.

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Chart 6. Flame retardant materials.


Lin et al. [109] applied three approaches to obtainflame-retardant benzoxazines. In the first approach,a novel phosphorous containing dopotriolbenzox-azine was copolymerized with a commercial ben-zoxazine [6,6-bis(3-phenyl-3,4-dihydro-2H-1,3-ben-zoxazineyl) methane (F-a)] or diglycidyl ether ofbisphenol A (DGEBA). In the second case, theelement phosphorus was incorporated into benzox-azine via curing reaction of dopotriol and F-a. Inthe third approach, dopo reacted with benzoxazineto incorporate the element phosphorus. (SeeChart 6)

3.1.7. Clay-polybenzoxazine composites

Smectite clays are good candidates for thepreparation of organic–inorganic nanocompositesbecause they can be broken down into nanoscalebuilding blocks and act as reinforcing phase inorganic–inorganic hybrid nanocomposites [110–112]. The design and creation of new materialsfrom polymer and layered silicates composites hasbecome an extremely interesting field of research,because they typically exhibit properties far superiorto those of separate components and are capable ofachieving the recent technological requirements.Thus, the general perception that clays act as lowcost fillers in polymers has been changed because oftheir ability to enhance the properties of the finalmaterials.

A nanocomposite composition comprising clayand benzoxazine monomer, oligomer and/or poly-mer was first developed by Ishida et al. The presenceof benzoxazine in the clay resulted in an at least

about 5% increase in the spacing between plateletsof the clay [113]. In another study, Agag andTakeichi [39] prepared the polybenzoxazine–clayhybrid nanocomposites from a polybenzoxazineprecursor (B-a) and organically modified montmor-illonite (OMMT), as a layered silicates. OMMTswere prepared by surface treatment of montmor-illonite (MMT) by octyl, dodecyl or stearyl ammo-nium chloride. A melt of B-a and OMMT wasmixed using a mechanical stirrer at 100 1C, with theaddition of a small amount of methylene chloridewas added to achieve better dispersion. The mixturewas then heated at 120 1C for 2 h to removesolvents, followed by film casting on glass plates.Then film was cured by step-wise increase of heatingup to 230 1C.

Poly(urethane-benzoxazine)-clay hybrid nano-composites (PU/P-a–OMMTs) were prepared insitu in a copolymerization of a polyurethane (PU)prepolymer and a mono-functional benzoxazinemonomer, 3-phenyl-3,4-dihydro- 2H-1,3-benzoxa-zine (P-a), in the presence of an organophilicmontmorillonite (OMMT), by solvent method usingDMAc [114]. OMMT was prepared by the cation-exchange reaction between Na+ cation and dodecylammonium chloride. An OMMT suspension inDMAc was added to a solution of P-a in DMAcat 60 1C, followed by the addition of a PUprepolymer with continuous stirring. The homo-geneous solution was cast on a glass plate, followedby thermal treatment for curing.

Phiriyawirut et al. [115] prepared another type oforganically modified montmorillonite (OMOM) byion-exchange reaction between Na+-montmorillo-nite and various protonated amines. The aminesused as the modifying agent were dodecy-lamine (DODEC), 6-aminocaproic acid (CAPRO),4-amino-N,N-dimethyl aniline dihydrochloride(ANDAD), p-phenetidine (PHEN) and 2,4,6-tri-methylaniline (TMAN). Mixtures of 3wt% OMOMwith benzoxazine monomers were prepared usingsolvent, binary solvent or nonsolvent systems. Allsamples were cast on aluminium foil surface, andsolvents were allowed to evaporate and then curedat 230 1C for 90min.

For preparation of nanocomposites, Chen et al.mixed OMMT with B-a and PBO (2,20-(1,3-pheny-lene)-bis(4,5-dihydro-oxazoles)) in their melt state(Scheme 15) [116].

Ishida and coworkers have used carbon fiber,glass fiber and natural fiber to develop high perfor-mance fiber-reinforced polybenzoxazine composites

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Scheme 15. Thermosetting resin from 2,20-(1,3-phenylene)-bis(4,5-dihydro-oxazoles) (PBO) and bisbenzoxazine (B-a).

Scheme 16. Synthesis of poly(p-vinylphenol) based benzoxazine.


and reported their properties [49,117–119]. Theyalso investigated the use of CaCO3 as filler [120].The preparation of titania-polybenzoxazine asorganic inorganic hybrid material by using sol–gelprocess was reported by Agag et al. [121].

3.2. Preparation of polymers with benzoxazine

moieties

A macromonomer technique was applied forchemical linking of polybenzoxazines with otherconventional polymers. The benzoxazine groupswere introduced by initiation of a selected poly-merization or synthesizing benzoxazines from ami-no or phenol functional prepolymers. In the formercase, the propagating species should be unreactivetowards the benzoxazine ring and N and O heteroatoms.

3.2.1. Benzoxazine functionalized polystyrene

Poly(p-vinylphenol) (Poly(VP)) based benzoxa-zine was prepared from Poly(VP), formaline, andaniline (Scheme 16). The curing behavior of thebenzoxazine with the epoxy resin and the propertiesof the cured resin were investigated.

The curing reaction did not proceed at lowtemperatures, but it proceeded rapidly at highertemperatures, without a curing accelerator. Thereaction induction time or cure time of the moltenmixture from Poly(VP) based benzoxazine andepoxy resin was found to decrease, compared withthose from conventional bisphenol A based benzox-azine and epoxy resin. The curing reaction rate ofPoly(VP) based benzoxazine and epoxy resinincreased more than that of conventional bisphenolA based benzoxazine and epoxy resin. The proper-ties of the cured resin from neat resins and from

reinforced resins with fused silica were evaluated.The cured resins from Poly(VP) based benzoxazineand epoxy resin showed good heat resistance,mechanical properties, electrical insulation, andwater resistance compared to the cured resinfrom VP and epoxy resin using imidazole as thecatalyst [41].

A unique synthetic route was reported by Kiskanet al. [122] for the synthesis of a macromonomer forwhich the benzoxazine ring was anchored to thepolystyrene polymer. Dibromophenyl terminatedpolystyrene was synthesized using Atom TransferRadical Polymerization (ATRP), which was thenfollowed by Suzuki coupling reaction to prepareamino functional polymers. These amino func-tional polymers when reacted with phenol andparaformaldehyde at 110 1C for 2 h producedbenzoxazine functionalized polystyrene macromo-nomer. The synthetic strategy is illustrated inScheme 17.

More recently, Ergin et al. [123] used coppercatalyst 1,3-cycloaddition reaction (called a ‘‘ClickReaction’’) to synthesize side-chain benzoxazinefunctional polymers. This route has the uniquefeature of being quantitative and at the sametime preserving the benzoxazine ring structure.

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Scheme 17. Synthesis of benzoxazine functional polysterene macromonomer.

Scheme 18. Synthesize of side-chain benzoxazine functional polystyrene by copper catalyzed 1,3-cycloaddition reaction.


The benzoxazine groups have been shown to readilyundergo thermal ring opening reaction to formcross-linked polymer networks (Scheme 18).

3.2.2. Benzoxazine functional poly(e-caprolactone)

A novel benzoxazine ring-containing PCL wassynthesized by Sue et al. For this purpose, first

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Scheme 19. Synthesis of benzoxazine functional poly(e-caprolactone).

Scheme 20. Synthesis of napthoxazine functional poly(e-caprolactone).


hydroxyl functional benzoxazine was prepared.Subsequently, this benzoxazine was used as the co-initiator for the stannous-2-ethylhexanoate (Sn(Oct)2) catalyzed living ring-opening polymerizationof CL. The synthesis of the initiator and ben-zoxazine ring-containing (PCL) are shown inScheme 19.

These authors also prepared porous polybenzox-azine materials by using this macromonomertogether with bisbenzoxazine [124]. Films were castand thermally cured, which resulted in the nanoscalemicrophase separation of these two dissimilar blocks.Then, the labile PCL constituent was removedselectively through hydrolysis using NaHCO3, whichcreated a nanoporous morphology.

A similar synthetic strategy to prepare napthox-azine functional PCL was followed by Kiskan et al.(Scheme 20) [125].

3.2.3. Benzoxazine functional poly

(methyl methacrylate)

It is well known that photosensitized aromaticcarbonyl compounds in conjunction with hydrogendonors can readily initiate free radical polymeriza-tion of appropriate olefinic monomers. Amongvarious hydrogen donors tertiary amines werefound to be the most suitable co-initiators [126].Depending on the substituents, dialkyl anilinederivatives are also used in these systems. Besidesthe oxazine ring, benzoxazines possess substituteddimethyl aniline groups in the structure. It seemed,therefore, appropriate to test whether they wouldalso act as hydrogen donor in photoinitiated free

radical polymerization using aromatic carbonylsensitizers. Accordingly, Tasdelen et al. demon-strated free radical polymerization of methylmethacrylate (MMA) [127]. Polymerization wasinitiated upon irradiation at l 4 350 nm in CH2Cl2solution containing benzoxazine (P-a) and one ofthe following photosensitizers: benzophenone (BP),thioxanthone (TX), isopropyl thixanthone (ITX),chlorothioxanthone (CTX) and camphorquinone(CQ) (Scheme 21). The postulated mechanism isbased on the intermolecular reaction of excitedphoto-sensitizer with the tertiary amino moiety ofground state benzoxazine and subsequent hydrogenabstraction reaction. The resulting aminoalkylradicals initiate the polymerization. The possibilityof deep curing using described photo-initiatingsystem followed by the thermal ring opening ofthe incorporated benzoxazine groups was alsodemonstrated.

3.2.4. Alternating maleimide copolymers with

pendant benzoxazine groups

It was recently reported that alternating copoly-mers of maleimide-benzoxazine with styrene (St)can readily be prepared by photo-induced radicalpolymerization at room temperature using 2,20-dimethoxy-2-phenylacetophenone (DMPA) asphoto-initiator (Scheme 22). The photochemicalmethod was deliberately chosen to preserve thebenzoxazine ring structure. Copolymer com-positions and monomer reactivity ratios suggestedthe alternating copolymerization. These poly-mers underwent cross-linking through the thermal

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Scheme 21. Synthesis of benzoxazine functional poly(methyl methacrylate) by photoinitiated free radical polymerization.

Scheme 22. Synthesis of alternating copolymers of maleimide-benzoxazine with styrene by photoinitiated free radical polymerization.


ring opening reaction of pendant benzoxazinegroups [128].

3.2.5. Naphthoxazine functional poly(propylene

oxide)

Thermally curable naphthoxazine-functionalizedpolymers were synthesized by the reaction of linear(Diamines) and branched (Triamines) poly(propy-lene oxide)s (Jeffamine series) having variousmolecular weights, with p-formaldehyde, and2-naphthol (see Scheme 23). Properties and mor-phologies of the products before and after curingwere investigated [129].

3.2.6. Benzoxazine functional polyhedral oligomeric

silsesquioxane (POSS)

Recently, a novel class of organic–inorganichybrid materials has been developed containingpolyhedral oligomeric silsesquioxane (POSS)[130–134] which contains an inorganic Si8O12 coresurrounded by eight hydrocarbon substituents, orseven of them plus a functional group. The uniqueand well-defined structure of POSS moiety providesthe possibility of preparing hybrid materials withinteresting structures. Several reports during lastfew years have reported the synthesis and char-acterization of mono-substituted POSS derivatives.

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Scheme 23. Synthesis of naphthoxazine functional poly(propylene oxide).

Scheme 24. Synthesis of benzoxazine functional polyhedral oligomeric silsesquioxane by hydrosilylation.


Synthesis of benzoxazine monomer containing aPOSS moiety (BZPOSS) by two different routes hasbeen reported by Chang et al. as described below:

(i)
Benzoxazine-POSS (BZ-POSS-1) was synthe-sized from the reaction of hydro-silane functio-nalized POSS (H-POSS) and allyl functionalbenzoxazine (3: 4molar ratio) in toluene in thepresence of a Pt catalyst at 80 1C under nitrogenatmosphere (Scheme 24).
(ii)
Another structurally similar macromonomerwas synthesized from the reaction of primaryamine terminated POSS (Amine-POSS), phenoland paraformaldehyde in THF medium at 90 1C(Scheme 25) [130–134].
3.3. Polymeric benzoxazine precursors

3.3.1. Main-chain precursors

High-molecular weight polybenzoxazine precur-sors can be synthesized from aromatic or aliphaticdiamine and bisphenol-A with paraformaldehyde(see Scheme 26).

The possibility of the preparation of polymerscontaining oxazine ring in the main chain was firstdiscussed by Liu et al. [2]. Later, more detailed workon the effect of water, solvents, catalyst, ratio ofreactants and temperature was reported by the sameresearch group [13]. The major problems associatedwith the preparation of such main-chain ben-zoxazine precursor polymers were low molecularweight and cross-linking arising from the Mannich

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Scheme 25. Synthesis of benzoxazine functional polyhedral oligomeric silsesquioxane by oxazine ring formation.

Scheme 26. Synthesis of polybenzoxazine precursors.

Scheme 27. Synthesis of semi-conductive side-chain polybenzox-

azine precursor.


reactions of multiple functional groups. The choiceof the right conditions for a Mannich reaction iscritical for achieving high yields with a minimum ofside reactions. In this type of Mannich polymeriza-tion, partially ring-opened structures were alsoobserved, but the ratio of the ring-closed structurein the precursor was high enough to be used aspolybenzoxazine precursors. The precursor solutionwas cast on a glass plate, giving transparent andself-standing precursor films, which were thermallycured up to 240 1C to give brown transparentpolybenzoxazine films. The toughness of the cross-linked polybenzoxazine films from the high-mole-cular weight precursors was greatly enhancedcompared with the cured film from the typical lowmolecular weight monomer. Tensile measurementof the polybenzoxazine films revealed that poly-benzoxazine from aromatic diamine exhibited thehighest strength and modulus, while polybenzox-azine from longer aliphatic diamine had higherelongation at break. The viscoelastic analysesshowed that the glass-transition temperature of thepolybenzoxazines derived from the high-molecularweight precursors were as high as 238–260 1C.Additionally, these novel polybenzoxazine thermo-sets showed excellent thermal stability [135,136].

3.3.2. Side-chain precursor

The only reported side-chain polymeric benzox-azine precursor is based on polyphenylene structure.

Soluble and thermally curable conducting high-molecular weight polybenzoxazine precursors wereprepared by oxidative polymerization 3-phenyl-3,4-dihydro-2H-benzo[e][1,3] oxazine (P-a) alone and inthe presence of thiophene (Th) with ceric ammo-nium nitrate in acetonitrile (Scheme 27). Theresulting polymers exhibit conductivities around10�2 S cm�1 and undergo thermal curing at varioustemperatures. The partially ring-opened structurewhich was formed during the oxidative polymeriza-tion affects the thermal curing behavior of thepolymers. The cured products exhibited highthermal stability, but lower conductivity than thoseof the precursors [137].


4. Reaction mechanism of ring opening

polymerization of benzoxazine

To understand the polymerization reaction me-chanism of benzoxazines, an understanding of thechemical structure of its oxazine ring is veryimportant. A single crystal X-ray crystallographicstudy revealed that the preferential conformation ofa mono-oxazine ring containing benzoxazine is adistorted semi-chair structure, with the nitrogen andthe carbon between the oxygen and nitrogen on theoxazine ring sitting, respectively, above and belowthe benzene ring plane. The resulting ring strainfrom this molecular conformation helps this type ofsix-membered ring undergo ring-opening reactionunder specific conditions. In addition, due to theirhigh basicity (by Lewis definition) both the oxygenand the nitrogen of the oxazine ring can act aspotential cationic polymerization initiation sites andmakes the ring very likely to open via a cationicmechanism [138,139]. The electron charge calcula-tion after energy minimization predicts that oxygenmight be the preferred polymerization site overnitrogen due to its high negative charge distribution(O, �0.311; N, �0.270).

The ring opening reaction of the benzoxazine wasfirst reported by Burke et al. [5]. In the reaction of1,3-dihydrobenzoxazine with a phenol, having bothortho and para positions free, it was found thataminoalkylation occurred preferentially at the freeortho position to form a Mannich base bridgestructure, along with small amount reaction at para

position. To explain this ortho preference theformation of an intermolecular hydrogen-bondedintermediate species was proposed. Riese et al.also observed the high reactivity of the ortho

position when following the kinetics of mono-functional benzoxazines with 2,4-di-tert-butylphe-nol catalyst [18]. The typical method of polymeriza-tion of benzoxazine monomers is thermal curingwithout using any catalyst [7,22,25,140,141]. Itshould be emphasized that the polymerizationmechanism of benzoxazine resins is still not wellestablished.

4.1. Cationic polymerization of benzoxazine

4.1.1. Acid catalyzed polymerization of benzoxazine

Some investigations on catalyst assisted benzox-azine curing showed that the presence of catalystsinfluence to reduce the induction time and acceleratethe reaction rate [142]. However, no significant

polymerization was observed below 100 1C. Ishidaand Rodriguez [22] have surveyed various acidsranging from strong acids to weak carboxylic acidsto phenols as catalyst for this type of polymerizationreaction. It has been observed that polybenzoxa-zines cured with strong carboxylic acids wereinferior to those cured with weak carboxylic acids[143]. Several initiators, such as PCl5, PCl3, POCl3,TiCl4, AlCl3 and MeOTf, were also reported aseffective catalyst for polymerization which providespolybenzoxazines with high Tg and high char yield[138].

From the investigations on use of variouscationic, anionic and radical initiators it has beenproposed that the ring opening polymerization ofthe benzoxazine proceeds through a cationic me-chanism [138,139,144,145]. McDonagh and Smith[11] reported that 3,4-dihydro-2H-1,3-benzoxazineexhibits ring/chain tautomerism when protonated,by migration of the proton from the nitrogen to theoxygen atom, and thereby produce iminium ions inthe chain form. Ring opening mechanism byprotonation of the oxygen atom to form an iminiumion, followed by electrophilic aromatic substitution,as shown in Scheme 28 was proposed by Dunkersand Ishida [143].

But this mechanism does not take into accountthe effect of the pKa of the acid, which controls thestructure of the reactive intermediate. The effects ofstrong and weak carboxylic acids and phenols ascatalysts on curing 3,4-dihydro-3,6-dimethyl-2H-1,3-benzoxazines to polybenzoxazines has beendescribed [143]. The curing reaction was monitoredin situ by using Fourier transform infrared (FTIR)spectroscopy. The IR bands, used to evaluate thecuring reaction, were (i) 1050 cm�1, representativeof the oxazine ring, (ii) 813 cm�1, associated with1,2,4 substitution of the monomeric benzene, and(iii) 1030 cm�1 , attributed to the methyl rocking onthe para position of the benzene ring and used as aninternal standard.

In the presence of strong organic acid, such astrifluroacetic acid, benzoxazine monomer convertsto polybenzoxazine immediately at low tempera-tures after ring opening. The formation of theiminium ion as intermediate was proposed, becausetrifluoroacetic acid can provide a counter ion,capable of existing in the ionic form rather thanthe covalent form and can give stability of theintermediate. As the curing temperature increases,side reactions also took place, which also leads tocuring.

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Scheme 28. Acid catalyst ring opening polymerization of 3,4-dihydro-2H-1,3-benzoxazines.


But when sebacic acid, a weak acid, was used ascatalyst, the polymerization reaction was slow in theearly stage of the reaction. The ring openingpolymerization of benzoxazines when catalyzed bya weak carboxylic acid was proposed to be an auto-accelerated reaction, where aminomethyl esterspecies were initially formed as an intermediate.At the beginning of the reaction a covalentlybonded aminomethyl species existed in equilibriumwith the iminium ion form of the intermediate. Thisexplains the large difference seen in the early stagesof the reaction. Since the reaction of this inter-mediate with another benzene ring to form theaminomethylene bridge occurred very slowly, cata-lyst was consumed, but could not be regenerated.As the dielectric constant of the medium increasedthrough the appearance of hydroxyl groups due tothe ring opening, the equilibrium shifted toward thereactive carbocation form. Thus, the consumptionof trisubstituted benzene was accelerated by thisshift in the equilibrium. Then, electrophilic aromaticsubstitution occurred and regenerated the acidcatalyst. This explains how the pKa value of theorganic acid effects the polymerization of benzox-azine. In the early stages of the reaction, the acids,having pKa in the range of 0.70–4.43, provide astable counterion for the intermediate iminiumcation where as adipic acid and the acids withhigher pKa values do not provide support for theiminium ion and this factor influence the reaction.

When pure benzoxazine was cured withoutcatalyst at 160 and 170 1C, the curing may be

catalyzed by phenols, which can be formed by thering opening from trace impurities. The ring open-ing and the Mannich bridge formation wereconsecutive reactions, whereby the consumption ofone benzoxazine ring and one trisubstituted benzenering should be occurred simultaneously. This isreflected in the FTIR study of the early part of thereaction. In the later stages the ring openingreaction occurred by termination.

Based on the results obtained from PCl5 initiatedpolymerization of different mono-oxazine ringcontaining substituted 3,4-dihydro-2H-1,3 benzox-azines, Wang and Ishida proposed three differentmechanisms [139] and explained the dependency offormation of different polymeric structures on thenumber and the position of substitutions in thebenzene ring of the monomer. The structures of fourtypes of investigated monomers, pC-m, 24DMP,235TMP and 345TMP, are shown in Chart 7.

1H-NMR, 13C-NMR and FT-IR study of thepolymers obtained from the PCl5 initiated poly-merization of the above mentioned monomersrevealed that (i) the polymers having Mannich basephenoxy-type structure (Type I) forms by polymer-ization of from 24DMP-m and 235TMP-m mono-mers, (ii) the Mannich base phenolic-type structure(Type II) polymer produce from pC-m monomer,and (iii) the mixed polymers result from 345TMP-mmonomer, with the phenoxy type (Type I) as themajor component. These results demonstrate howthe change of the position of substitute of thebenzene ring affects the nature of the resulting


polymers. The proposed reaction mechanisms areshown in Schemes 29 and 30 [139].

Scheme 29 illustrates the mechanism of formationof Type I polymer, having the Mannich basephenoxy-type structure, from 24DMP-m and235TMP-m monomers. It was proposed [139] thatthe oxygen on the oxazine ring acts as the initiationsite and due to the attack of a cationic initiator(H+) cyclic tertiary oxonium ion intermediate form.The polymerization then proceeds via the insertionof monomers through the reaction between theintermediate and the oxygen of another oxazine ringand results the formation of Mannich base phe-noxy-type (Type I) polybenzoxazine structure. Analternative polymerization route for Type I struc-ture formation was also suggested as shown in

Chart 7. Methyl substit

Scheme 29. Cationic ring opening mechanisms of

Scheme 29, similar to the mechanism A, but in thiscase N acts as the initiation and as well aspropagation sites. The formation of Mannich basephenolic-type structure (Type II) polymer from pC-m monomer was explained by assuming that uponinitiation by a cationic initiator, the propagationproceeds by the incorporation of monomersthrough the reaction of the unobstructed ortho

position of benzene and eventually produces aMannich base phenolic-type (Type II) polymer.This proposed mechanism is illustrated as mechan-ism B in Scheme 30.

Moreover, in this case, the monomers propa-gate via reasonably stable carbocations, i.e., theintermediate oxonium cation is stabilized by in-tramolecular hydrogen bonding, which could lead

uted benzoxazines.

3,4-dihydro-2H-1,3-benzoxazines (Type I).

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Scheme 30. Cationic ring opening mechanisms of 3,4-dihydro-2H-1,3-benzoxazines (Type II).


to high-molecular weight polymer formation. It hasbeen observed that the polymer having highestmolecular weight, was formed from pC-m amongstthese four type monomers.

In case of 345TMP major polymerization pro-ceeds via mechanism A (formation of Type Ipolymer), the unobstructed ortho position on thebenzene ring also partially participate in thepolymerization through mechanism B, resulting asmall portion of the phenolic type (Type II) polymerstructure formation. Quantitative analysis by NMRof two different polymer structures revealed a 9:1ratio for the Mannich base phenoxy-type (Type I)and the Mannich base phenolic-type (Type II)polybenzoxazines.

It was also mentioned that polybenzoxazinestructure via thermal curing could also be thoughtof as the Type II polymer structure, which can begenerated through mechanisms similar to mechan-ism B [139].

Phenols (trace amount of which may present asimpurity) with free ortho positions can act asinitiators in the oligomerization of benzoxazinecompounds. It can be speculated that at elevatedtemperatures, the self-dissociation of the benzox-azine ring can produce free phenol structures andalso initiate the ring opening reaction.

4.1.2. Photoinitiated polymerization of benzoxazine

The photoinitiated ring-opening cationic poly-merization of a mono-functional benzoxazine, 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (P-a), withonium salts such as diphenyliodonium hexafluor-ophosphate and triphenylsulfonium hexafluoropho-sphate as initiators was investigated by Kasapogluet al. [146]. In this work, both direct and indirectactivation using radical sources and photosensitizerswere reported. 1H-NMR and FT-IR study revealedthe complex structure of the resulting polymerswhich was related to the simultaneous ring-openingprocess of the protonated monomer either at theoxygen or nitrogen atoms. The phenolic mechanism

also contributed, but its influence decreased withdecreasing monomer concentration. Free radicalpromoted cationic polymerization of benzoxazineswas also examined. In this case, the polymerizationcan be performed at much higher wavelengths andcarbon-centered radicals formed from the photo-lysis of 2,2-dimethoxy-2-phenylacetophenone(DMPA), were oxidized to produce carbocations.These carbocations are capable to initiate benzox-azine polymerizations. Scheme 31 describes thatafter addition of a proton (or carbocation) to theeither heteroatom (oxygen or nitrogen) yieldsoxonium or ammonium cations, respectively. Forthe next step, several probable routes were proposedby which polymerization can proceed and producedifferent polymeric structures.

4.2. Thermal polymerization of benzoxazines

A cross-linked network structured polybenzoxa-zines, with higher Tg and degradation temperature,can be obtained when difunctional or multifunc-tional benzoxazines undergo polymerization. Thepolymeric structures form due to curing of mono-functional and difunctional benzoxazines are shownin Scheme 32 [51]. Obviously, difunctional benzox-azines derived from diamines are expected toundergo similar cross-linking [44,85].

In the DSC thermogram of a mono-functionalbenzoxazine, P-a, a sharp exotherm was observedwith onset and maximum temperatures of theexotherm at 202 and 230 1C, respectively, corre-sponding to the ring-opening polymerization, andan exotherm for P-a of 62 cal/g. In case ofdifunctional benzoxazine, B-a, DSC showed anexotherm onset at ca. 223 1C and maximum at a249 1C corresponding to the ring-opening polymer-ization of benzoxazine. The amount of exotherm forB-a was 79 cal/g [51].

It has been observed that during synthesis of adifunctional benzoxazine (from bisphenol A, for-maldehyde and methyl amine) not only bisphenol-A

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Scheme 31. Photoinitiated polymerization of N-phenyl-3,4-dihydro-2H-1,3-benzoxazine.

Scheme 32. Curing of monofunctional and difunctional benzox-

azines.


based benzoxazine (B-m) monomer forms as majorproduct, but also dimers and small oligomers formby the subsequent reactions between the rings andortho position of bisphenol A hydroxyl groups.These free phenolic hydroxy structure containingdimers and oligomers trigger the monomer to beself-initiated towards polymerization and cross-linking reactions [8].

Attempts have been taken to understand the curemechanism and kinetics of the thermal curing ofmono and difunctional benzoxazines utilizing DSC,FTIR, DMA, 13C and 15N solid sate NMRspectroscopic measurements [136,147–153].

It has been proposed that the ring-openinginitiation of benzoxazine results the formation of acarbocation and an iminium ion, which exist inequilibrium [147] (Scheme 33). Polymerization


proceeds via the electrophilic substitution by thecarbocation to the benzene ring. This transferoccurs preferentially at the free ortho and para

position of the phenol group. The stability of theiminium ion greatly affects the propagation ratebecause carbocation is responsible for propagation.Further, the reactivity of the equilibrium pairdepends on the basicity of the amine group. Themore basic the amine, with more the free electrondensity of the nitrogen, has the capability tostabilize more the positive charge of the iminiumion. If the iminium ion is more stable, theequilibrium shifts toward it, causing lowering inpropagation rate. If the iminium ion is unstable, theequilibrium will be shifted toward the carbocation,resulting in a higher propagation rate.

It should be noted that since the propagationreaction involves chain transfer to a benzene ringtemperature should have a great impact on the rateof propagation. Kinetic study indicated that in theearly stages of polymerization, the reaction might berelatively independent of the cure temperature.As the reaction proceeds, the temperature effecton propagation becomes more evident in thereaction kinetics.

Curing reactions at two different temperatures,below and above Tg temperature, demonstrate thatthe kinetics are significantly different for the twocure temperatures. Vitrification occurs sooner athigher cure temperature than the lower curetemperature, especially below the Tg. As vitrifica-tion causes a large increase in the viscosity of thesystem, at the reaction becomes largely diffusion-controlled, and greatly affect the curing kinetics[147]. Scheme 34 illustrates the thermal polymeriza-tion of B-a through cationic mechanism.

Solid State 15N-NMR study identified the forma-tion of a structure generated possibly due to the

Scheme 33. Initiation of ring-opening

electrophilic substitution reaction between ortho

position of the aniline and carbocation. Similar tophenol, the electron-donating nature of nitrogen ofthe aniline makes its ortho and para position aspossible sites for electrophilic substitution with thecarbocation. The formation of this structure isshown in Scheme 35 [148].

5. Properties of polybenzoxazines and their blends

and composites

5.1. Properties of polybenzoxazines

A typical polybenzoxazine, prepared from mono-functional 3-phenyl-3,4-dihydro-2H-1,3-benzoxa-zine (P-a), exhibits Tg at 146 and 161 1C, obtainedfrom maximum of loss modulus and the maximumof tan d, respectively, of DMA results. The storagemodulus decreases sharply at about 110 1C. TheTGA profile showed 5% and 10% weight loss fortemperatures of 342 and 369 1C, respectively, and achar yield of 44% [51].

A comparative investigation on several physicalproperties of polybenzoxazines (PB-a and PB-m),prepared by thermal curing of difunctional B-a andB-m monomers, has been reported by Ishida andAllen [87]. They exhibit high Tg and significantlyhigher tensile moduli than both phenolics andepoxies and at the same time maintain adequatetensile strength and impact resistance.

From the DMA study of these cured polybenzox-azine materials; it has been observed that theypossess the characteristic features of cross-linkedthermosetting materials. The PB-a has a higherstorage modulus in the glassy region than the PB-m,as observed from their respective room-temperaturevalues of 2.2 and 1.8GPa. The glass-transitiontemperature of the PB-m (180 1C), however, is

polymerization of benzoxazines.

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Scheme 35. Electrophilic substitution reaction of aniline moiety.

Scheme 34. Thermal polymerization of B-a through cationic mechanism.


significantly higher than that of the cured PB-amaterial (150 1C), as determined from the maximaof the loss spectrum. As the presence of high freevolumes responsible for lowering of Tg, it waspostulated that the PB-a might contain a greaterfree volume than the PB-m. The crosslink density ofcured PB-a was estimated of about 1.1� 10�3

mol/cm3 where as that of PB-m was not able to bedetermined, because the torque in the plateau regiondropped below the minimal sensitivity of thetransducer. But as the storage modulus was atthe level of the PB-a plateau and still decreasedat its last measurable point, it was assumed thatthe PB-m has an even lower crosslink density thanthe PB-a.

For these polybenzoxazines the concentration ofnetwork chains is significantly lower than istypically seen in cross-linked epoxides. Though thepolybenzoxazines posses low cross-linking density,they exhibit higher Tgs. The intra and intermole-cular hydrogen bonding in the network of the

polybenzoxazines and the cured materials areresponsible for low crosslink density [154,155].However, these hydrogen bindings are sufficientlystrong to confine segmental mobility and contributerigidity in the glassy state, which would normally beexpected only from a much tighter networkstructure. In this connection it should be pointedout that the higher value of storage modulus of PB-a than that of PB-m should not be explained fromthe crosslink density point of view. According tomany authors for epoxy resins, the crosslink densityhas little or no influence on stiffness or rigidity inthe glassy state [156–158]. Free volume, chaininteraction, and intermolecular packing influencethe small strain properties of a material in its glassystate, including the modulus. Hydrogen bondingshould decrease the flexibility of a cross-linkednetwork as it hinders rotational isomeric configura-tional changes and other segmental motion of chain.Thus, the higher glassy modulus of PB-a indicatethat the hydrogen bonding is more prevalent in the

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Table 1

Volumetric expansion coefficient values of polymers obtained

from difunctional benzoxazine and epoxy resins

Polymer Volumetric expansion

coefficient (cm3/cm31C)

PB-a 1.7� 10�4

PB-m 2.1� 10�4

Epoxy resin 1.7–2.8� 10�4


PB-a than in the PB-m (see Scheme 29 for thestructure).

The reported values of notched Izod impactstrengths for PB-a and PB-m are 18 and 31 J/m,which are higher than for these phenolic materials(�17 J/m) and similar to epoxy resins (�32 J/m).Because of the difference in the crosslink densitiesthe lower value of impact strengths for PB-a thanPB-m was expected. A more highly cross-linkedmaterial behaves in a more brittle manner becausehigh cross-linking lowers segmental mobility.

Generally, intermolecular packing, free volume,molecular architecture, and molecular weight be-tween cross-links influence the large-strain glassystate properties, namely tensile strength and elonga-tion at break. Higher free volume tends to enhancethe mobility of network segments under load toincrease ultimate elongation. PB-a exhibits brittlefracture at a higher strain than PB-m. PB-apossesses superior tensile strength and elasticmodulus than those of PB-m, which indicate thatthe regularity and perfection of the network formedfor the PB-a are superior to those of the PB-mnetwork. These two materials exhibit near-zeroshrinkage due to curing at about 200 1C where astypical epoxy resins show higher cure shrinkage.One possible explanation might be the relieving ofring strain during the ring opening polymeriza-tion of benzoxazines. However, ring strain alonecannot explain the near-zero volumetric shri-nkage. Chain conformation influenced by strongintramolecular hydrogen bonding is also an im-portant factor for the volumetric expansion. It hasbeen observed that the volumetric expansioncoefficients for PB-a and PB-m are competitive withthat of epoxides; values are listed in Table 1[159,160].

It has been observed that after 600 days in waterat room temperature PB-a absorbs water up to1.9%, whereas PB-m gains up to 1.3% by weight;the former material absorbs water at a slower rate.Despite the presence of hydrophilic phenolic andtertiary amine groups, polybenzoxazines do notabsorb water as much as do phenolic or epoxyresins [24]. The mode of sorption of water of thesematerials was determined by plotting the log of theamount of the amount of water vs. logarithmic time,which indicated the occurrence of a very nearFickian behavior. The rate of diffusion of PB-m(diffusion coefficient ¼ 4.9� 102 cm2/s) is higherthan that of PB-a (diffusion coefficient ¼ 3.6�102 cm2/s).

Bellenger et al. [161] reported that for epoxy-amine systems the rate of water transport in thenetworks is governed by polymer–water interactionsand is inversely related to the extent of intermole-cular hydrogen bonding. The presence of inter- andintramolecular hydrogen bonding within the poly-benzoxazine systems, which shield the hydroxylgroups (present abundantly in the network) frominteraction with water molecules is the probablemain cause for the low water diffusivities andsaturation contents of the polybenzoxazines. Thelower diffusion rate of PB-a, despite of its higheroverall absorption, than that of PB-m is consistentwith their finding that diffusivity decreases withhydrophilicity.

The dielectric constant value of PB-a is 3.6 andhas only a slight dependence on frequency attemperatures below approximately 120 1C. It de-creases less than 3% as the testing frequencyincreases from 428Hz to 1MHz. Thus, the poly-benzoxazine not only has a lower electrical capaci-tance than other thermosetting materials (forconventional phenolic resins dielectric constant is4.8–5 and for epoxides 3.7–4), but also is lesssensitive to the changes in frequency. The change ofloss factor with temperature shows the B-a materialwithstands electrical power loss at least as well asepoxies, which have loss factors that are typicallybetween 0.01 and 0.08 [87].

The relaxation process of PB-a begins at about Tg

(150 1C), and the dielectric properties of the resinbegin to deteriorate. Even so, the polybenzoxazinematerial appears to possess excellent electricalperformance up to service temperatures (150 1Cfor B-a) beyond those of most other polymer resins.

In another paper it has been reported that DMAanalysis of polybenzoxazines from B-m (synthesizedfrom bisphenol A and methyl amine) shows Tg at215 1C when the sample was cured at 210 1C [162].

A systematic study of thermal and mechanicalproperties for a series of polybenzoxazines, basedupon alkyl substituted aryl amines, has been


reported by Ishida and Sanders [26]. Difunctionalbisphenol A based benzoxazines monomers weresynthesized from different methyl substitutedamines. The curing of these monomers was per-formed by following the step profile: 140 1C for30min, 160 1C for 30min, 170 1C for 45min, 180 1Cfor 45min, 190 1C for 75min, and 200 1C for 90min.The substitution on different positions show pro-nounced effect on polymerization. Due to theactivation of ortho and para positions on thependent ring by placing electron-donating alkylsubstituent groups on one or both meta positions,the oxazine ring-opening polymerization occurs atlower temperatures. In addition, significant num-bers of arylamine Mannich bridges and methylenebridges were formed during the cure of thesemonomers [163]. DSC results showed that B-a-otexhibits the lowest glass-transition temperature of114 1C and comparatively lower extent of reactionin this material is possibly due to the lower basicityand greater steric hindrance of the arylamine. B-aand B-pt showed Tg at 168 and 158 1C, respectively,where as B-mt and B-35x exhibit at 203 and 205 1Cafter the initial cure. BA-a and BA-pt after curingan additional 30min at 240 1C, exhibit the final Tg

at 209 and 238 1C, respectively. Further curing athigher temperatures did not increase the Tg of thesematerials appreciably [26].

Among these series of materials B-mt possess thehighest moduli, with a storage modulus 1.78GPa at28 1C and a plateau modulus of 11.9MPa at 265 1C.These are much higher than for other polybenzox-azines, which show definable rubbery plateaus. Inaddition, it was quite stable in the rubbery region,with no void forming observed even after 2 h attemperatures above 260 1C, whereas many poly-benzoxazines undergo degradation and weight losssoon after reaching temperatures above Tg. B-mt isone of the few polybenzoxazines which shows such alarge window of thermal stability in the rubberyregion [26].

Three major events were observed via a thermo-gravimetric analysis of these materials. Ishida andcoworkers analyzed the evolved gases to determinethe nature of these weight loss events and alsoproposed degradation mechanism [19,32,164]. Thefirst event near 310 1C was due to the breakage ofMannich bridge in the phenolic Mannich bridgenetwork which produced free aniline via a deamina-tion reaction, along with some N-methyl anilines bydeaminomethylation. During the second event atabout 400 1C, the breakup of the isopropylidene

linkage of the bisphenol A occurred. The primaryweight loss products were aniline and variousphenolic species. Finally, the last weight loss,centered near 460 1C, was attributed to the degrada-tion of char, with release of traces of phenolic andsignificant amount of substituted benzene com-pounds. The meta substituted materials, B-mt andB-35x, achieved the highest thermal stability andshowed a different weight loss behavior where thefirst weight-loss event was absent. For BA-35x anew peak appeared at 350 1C, which was due to therelease of amine. It was proposed that these twomaterials possess a polymeric structure which is nota pure phenolic Mannich bridge network, butcontains additional arylamine Mannich bridge net-work and various methylene bridges similar to thosein a phenolic network [165]. The thermal stability ofdifferent polybenzoxazines has also been reported inseveral papers [32,33,35,53,56,166,167].

Two difunctional polybenzoxazines, 22P-a PBZand 440-a PBZ, were prepared by curing thebenzoxazine monomers, 8,80-bis(3,4-dihydro-3-phe-nyl-2H-1,3-benzoxazine), abbreviated as 22P-a, and6,60-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl)ketone, abbreviated as 44O-a, respectively [25]. Tgsof these polybenzoxazine materials increase linearlywithout showing the ultimate value with theincrease of postcure temperature. Although 440-aPBZ cured at 316 1C for 1 h exhibited Tg of 410 1C,it showed decomposition starting at 300 1C accom-panied by weight loss. The high Tg may be due tosecondary reactions involving bisphenolic methy-lene bridge formation and some other unknownstructures [148]. Therefore, the recommended post-cure temperature was 290 1C rather than 316 1C. Asthe Tgs of the 440-a PBZ are always higher than thecure temperatures applied (Tg was 365 1C atpostcure temperature 300 1C), it provides a greatadvantage in processibility. There are only a fewthermosetting polymers that exhibit such behavior[168]. The Tg higher than Tcure behavior might bedue to cross-linking reaction, which is not comple-tely quenched in the glassy sate and surpasses thecuring temperature.

DMA results of the samples cured at 180 1Cindicate that further curing at higher temperaturewas necessary by showing the increase of G0 and G00

after the a transition (Tg). But this behaviordisappeared when the samples were cured at highertemperature (ca 240 1C). The storage moduli atroom temperature of these polymers are approxi-mately 2.0GPa.


The TGA of the 22P-a PBZ (cured at 240 1C) and440-a PBZ (cured at 290 1C) indicate that they startdecomposing at 200 1C in both air and nitrogen,followed by oxidation in air at 550–600 1C. The highchar yield of 440-a PBZ makes it a good candidatefor the precursor of carbon–carbon composites [25].

It has been reported that incorporation of severaltransition metal salts (2mol%) gives improvementof the char yield of the polybenzoxazines by10–20%. The metal salts initiate the ring opening,but do not catalyze the polymerization, andpromote the carbonyl group formation duringpolymerization [169].

Degradation of polybenzoxazines (derived fromdifferent phenols and amines) by UV radiation hasbeen reported and degradation mechanism has beenproposed by Macko and Ishida [170–172]

5.1.1. Properties of polybenzoxazines with additional

functionalities

As stated in the synthesis part (Section 2), theincorporation of several other functionalities caninfluence the curing behavior of benzoxazine.Obviously, this would result different microstruc-ture and consequently the thermal and mechanicalproperties of the cured products. In the followingsection, the effect of different functional groups onthe properties of both precursor benzoxazinemonomers and the corresponding polymers will bedescribed.

(i)
Benzoxazine with acetylene group.The nonisothermal DSC thermograms of Ph-apa resins show that oxazine ring openingpolymerization exotherm overlaps with acety-lene polymerization exotherm at the tempera-ture range of 220–235 1C. However, Ph-apcexhibited two well-resolved exotherms for bothprocesses (i) the sharp exotherm at 230 1C forthe benzoxazine polymerization, and (ii) thebroad exotherm at 350 1C because of theacetylene polymerization. These assignmentswere supported by the FT-IR studies of thepolymerization of this compound [33,53]. It wasalso reported that polymerization of disubsti-tuted arylacetylenic monomers occurs at thehigher temperature of 350 1C, as identified byDSC [173].Char yield of polybenzoxazines from purifiedacetylene functionalize benzoxazine monomerswere 5–10% lower than the char yield of resinsfrom as-synthesized monomers, as determined
from TGA. Very high char yield of 80wt% wasachieved for this type of polybenzoxazines. Thehigh char of these polymers are due tointroduction of another polymerizable func-tional group, acetylene, by which a more cross-linked network structure forms due to poly-merization. The char yield of the analogouscompound (B-a) containing aniline instead of 3-aminophenylacetylene is 32wt%. Side phenylgroups present in the structure of polybenzox-azines from unfunctionalized monomers (B-a)can easily be volatilized during thermal degra-dation. Linking these weak groups by introdu-cing polymerizable acetylene group contributedto improve the thermal stability of thesematerials.These polybenzoxazines exhibit very highglass-transition temperatures (Tg) rangingfrom 320 to 370 1C and high values of shearmodulus (G0), up to 2.3GPa, as determined bythe DMA [34].

(ii)
Benzoxazine with propargyl ether functionalgroupThe DSC exotherm for monomers with thepropargyl group, P-appe, starts at 191 1C with amaximum 235 1C, indicating that the ringopening polymerization and cross-linking ofpropargyl group took place within the sametemperature range. The appearance of anotherexotherm, starting at 325 1C with maximum at341 1C, is due to the degradation of cross-linkedstructure. For bifunctional benzoxazine withpropargyl group, B-appe, the similar behaviorwas observed. From DMA results revealed thatthe Tg of these polymers were increased byabout 100–140 1C and the storage moduli weremaintained constant up to �100 1C highertemperature than the typical unfunctionalizedpolybenzoxazines. Excellent thermal stability ofthese polymers, as reflected from TGA results,was due to the prevention of volatilization ofaniline derivatives as a degradation product byanchoring the aniline component in the net-work structure through cross-linking by thepropargyl ether groups. The char yield of thesepolymers was also increased by ca. 22–29%[36].
(iii)
Benzoxazine with allyl groupDSC investigations [51] reveled that for 3-allyl-3,4-dihydro-2H-1,3-benzoxazine (P-ala) thethermal curing of the allyl group occurredfirst, showing an exotherm with an onset


temperature at 145 1C with exotherm peak at207 1C, followed by the ring-opening of theoxazine ring, which appeared as secondexotherm, onset at 225 1C with a maximum at260 1C. The total exotherm of P-ala was 84 cal/g. DSC thermograms after each cure for P-alashowed that the first exotherm for the cross-linking of allyl groups disappeared after curingat 200 1C and the second exotherm decreasedwith the increasing cure temperature anddisappeared after 240 1C.On the other hand, P-alp (3-phenyl-3,4-dihy-dro-8-allyl-2H-1,3-benzoxazine) showed onlyone exotherm, the onset of which was at241 1C and maximum at 263 1C, without show-ing any exotherm at lower temperature rangefor to the thermal cure of the allyl group. Theamount of exotherm was 20 cal/g, much smallerthan P-ala. The difficulty of the radical poly-merization of the allyl phenyl group is due tothe stability of the radical [174]. In the case ofP-alp, the ortho position, the primary reactionsite to form phenolic Mannich bridge structurevia the ring-opening polymerization, is blockedby allyl group. Therefore, this exotherm at hightemperature might be due to the cleavage of theoxazine ring that leads to degradation [163].The curing of a bifunctional allyl-containingbenzoxazine, B-ala (bis(3-allyl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane), was investigatedby Agag and Takeichi and compared with thetypical bifunctional benzoxazine, B-a [51].When the DSC plots of B-a showed anexotherm with onset at ca. 223 1C with max-imum at 249 1C corresponding to the ring-opening polymerization of benzoxazine, B-alaexhibited an unsymmetrical broad exothermwith the onset at 145 1C and maximum at265 1C corresponding to both the cross-linkingof allyl group and the ring-opening polymeriza-tion of benzoxazine. The heat of polymeriza-tion for B-a was 79 cal/g and that for B-ala was127 cal/g. Thermal polymerization of N-allylgroup is known to occur at lower temperature[45,46]. In the case of P-ala, it was consideredthat the thermal polymerization of allyl groupoccurred first, followed by the ring-openingpolymerization of benzoxazine at slightly high-er temperature than P-a. The shift of the ring-opening polymerization to higher temperaturerange was due to the restricted mobility of P-alabecause of the polymerization of allyl group.

A significant increase in Tg was observed due tothe introduction of allyl groups in the mono-mers. For example, when the typical polyben-zoxazine, PP-a (from mono-functionalbenzoxazine without acetylene group), exhib-ited the Tg at 146 1C, that for PP-ala was shiftedto as high as 285 1C. Since the introduced allylgroups provide additional cross-linking sitesinto polybenzoxazine, the rigidity of the poly-mer backbone was increased with cross-linkingdensity, and hence the damping was signifi-cantly decreased. However, for PP-alp, the Tg

was as low as 107 1C. The poor thermo-mechanical properties for PP-alp were due toits low cross-linking density, which arises fromthe difficulty in the polymerization of themonomer as described above. Bifunctionalpolybenzoxazines, PB-ala (with acetylene sidegroup) and PB-a (without acetylene sidegroup), showed similar behavior exhibitingTgs at 298 and 154 1C, respectively, indicatingthe beneficial effect of additional cross-linkingoffered by the introduction of allyl group asanother cross-linkable site.TGA showed that for PP-ala and PB-ala, thethermal stability was improved compared to thecured samples of the corresponding benzoxa-zines without allyl functionality (PP-a and PB-a). This was inferred from their increase in 5%and 10% weight loss temperatures. Notably,these temperatures were decreased for thebenzoxazines possessing allyl group on thephenyl ring (PP-alp). The observed increasefor PP-ala and PB-ala was due to the preven-tion of amines from volatizing at the initialstages of the degradation because of theadditional cross-linked structure. The charyields of PP-a PP-ala and PP-alp were almostthe same (�44%) [32,51].

(iv)
Benzoxazine with nitrile functional groupPhenylnitrile- and phthalonitrile- functionalbenzoxazines and their copolymers possess highthermal stability because terminal phthaloni-trile group introduce extra cross-linking in thenetwork structure. It was reported that theortho nitrile group in the ortho-phenyl nitrilefunctional benzoxazine is more reactive duringpolymerization than meta- and para-nitrileanalog [56]. TGA-FTIR analysis revealed thatsome portion of the nitrile groups present in themonomer undergoes cross-linking reaction dur-ing curing and the rest react during char


formation and results in high char yields. Thesehighly cross-linked materials also possess high-er Tg in the range of 275–300 1C and Tg which ishigher than Tcure. The neat phthalonitrilebenzoxazine resins have high melting point(160 1C) and higher melt viscosity than un-functionalized benzoxazines, whereas phenylni-trile mono-functional benzoxazines are viscousliquids at room temperature with viscosity6� 105 Pa s and 1 Pa s at 80 1C [35,56].

(v)
Benzoxazine with maleimide & norbornanefunctional groupBenzoxazine monomers with imide functional-ities, maleimide (MIB) and norborane (NOB)showed improved thermal properties. The DSCand FTIR studies of maleimide containingmonomer, HPM-Ba (see Section 2.8) , revealedthat polymerizations occurs in two stages in thetemperature range from 120 to 250 1C (i)polymerization of CQC bonds of maleimidegroup at about 150 1C by free radical mechan-ism, and (ii) the ring opening polymerization ofoxazine at about 230 1C [57]. DSC thermo-grams of MIB and NOB (see Section 2.8)showed benzoxazine polymerization occurredat 213 and 261 1C, respectively. In case of NOBthe cross-linking reaction of nadimide groupproceeds via reverse Diels-Alder reaction athigher temperature ca 271 1C. The char yieldsand Tg of the benzoxazine based polymers hasalso been increased due to incorporation ofthese additional functionalities, since they im-prove the network structure by providing extracross-linking [58,59].
(vi)
Benzoxazine with adamantine functional group
Polymers obtained by thermal curing of benzox-azines with adamantine functional group exhibiteddifferent Tg values depending on the substituents onthe benzoxazine ring. The lower Tg noted withpoly(2-benzoxazine) was attributed to the presenceof bulkier phenyl group in the structure whichcauses hindrance in the molecular chain movementin the network structure. Due to the same reason,poly(3-benzoxazine) possesses higher cross-linkingdensity and also higher decomposition temperature.However, due to the incorporation of adamantanegroup into the polybenzoxazine backbone, thecrosslink density of these polymers becomes lowerthan that of unmodified polybenzoxazines, which isreflected in the comparatively lower char yield ofadamantane functionalized polymers. Interestingly,

they show high decomposition temperature [61,62].The thermal properties of polybenzoxazines pre-pared from different benzoxazine monomers arelisted in Table 2.

5.1.2. Properties of rubber-modified polybenzoxazine

It has been reported by Jang and Seo [81] that thestress intensity factor, KIc, was increased whenpolybenzoxazine was modified with amine termi-nated butadiene acrylronitrile rubber (ATBN) orwith carboxyl-terminated butadiene acrylronitrilerubber (CTBN). For toughening polybenzoxazinewith liquid rubber, the particle size and the contentof rubber dissolved in matrix phase are the mainfactors of the toughness improvement. Improve-ment of toughness is shown better by ATBN thanCTBN and the trend of change of KIc values withthe rubber content was different in both the cases.The KIc of polybenzoxazine increased from 0.6 to1.8MP.m1/2 with the increase of rubber content.Due to the highly cross-linked nature of thestructure, the crack propagation rate is very fastfor neat polybenzoxazine. However, a rough frac-ture surface, which may cause multiple crackinitiation, was observed in both CTBN andATBN-modified systems. In addition, several dif-ferent features were observed in the morphologies ofCTBN- and ATBN-modified cases. It has beenobserved that the flexural strength of the ATBN-modified polybenzoxazine was increased with in-creasing rubber content, but decreased slightly forCTBN-modified polybenzoxazine.

The flexural strength of polybenzoxazine in-creased slightly or was maintained, and its flexuralmodulus decreased up to 2.4GPa as the rubbercontent increased.

From DSC study of the cure reaction of CTBNand ATBN-modified systems; it was observed thatcure peak temperature decreased with the increaseof rubber content. By acting like an acid catalyst.CTBN helps the ring opening and this effect resultsthe decrease of cure temperature. But ATBN, anamine terminated rubber, acts as a stabilizer of thering-opened compound and helps to reduce the curetemperature. Tg was also found decrease with theincrease of CTBN and ATBN concentration [81].

Agag and Takeichi [88] reported that modifica-tion of polybenzoxazine by incorporation of ATBNlowers onset temperature and the maximumexotherm of the ring opening of benzoxazine to180 and 216 1C, respectively, whereas on incorpora-tion of hydroxyphenylmalemide (HPMI) those

ARTICLE IN PRESS

Table 2

Thermal properties of polybenzoxazines

Monomers Tg (1C) T5% (1C) T10% (1C) Char yield (%) Reference

O

N

(P-a)

146 342 369 44 [51,87]

O

N

O

N CH3

H3C

(B-a)

150a 310 327 32 [51,87]

170b

O

N

O

NH3C

CH3

H3C

CH3

(B-m)

180 – – – [51]

H3C

CH3

O

N

O

N

CH3

CH3(B-ot)

114 228 – 32 [26]

H3C

CH3

O

N

O

N

CH3

CH3 (B-mt)

209 350 – 31 [26]

CH3

H3C

O

N

O

N

H3C

CH3

(B-pt)

158 305 – 32 [26]

CH3

H3C

O

N

O

N

H3C

CH3

CH3

CH3

(B-35x)

238 350 – 28 [26]

ON

O N

(22P-a)

200 250 260 45 [25]

O

O

NN

O(44O-a)

340 290 370 65 [25]

Acetylene functionalized monomers

O

N

CH

(Ph-apa)

329 491 592 81 [53]


ARTICLE IN PRESS

Table 2 (continued )


O

N

O

H3C

CH3N

CH

CH(B-apa)

350 458 524 74 [53]

O

N

O

F3C

CF3N

CH

CH (B-af-apa)

368 494 539 71 [53]

Allyl functionalized monomers

O

N

CH2

(P-ala)

285 348 374 44 [51]

ON

CH2

(P-alp)

107 288 356 45 [51]

O

N

O

NH2C

CH3

H3C

CH2

(B-ala)

298 343 367 28 [51]

Phenyl propargyl functionalized monomers

O

N

O

CH

(P-appe)

249 362 400 66 [36]

O

N

O

N CH3

H3CO

O

HC

CH

(B-appe)

295 352 388 66 [36]

Nitrile functionalized monomers

O

N

CN(I)

175 332 371 60 [35]

O

N CN

CN

(x)

278 450 560 76 [56]


ARTICLE IN PRESS

Table 2 (continued )


O

N

O

N CH3

H3C

CN

NC

CN

CN

(IV)

300 423 468 68 [56]

Malemide functionalized monomers

N

O

O

O

N

(MIB)

252 375 392 56 [59]

NO O

O N

n

(HPM-BaI)

204 330 366 49 [57,58]

N

O

O

O

N

(NOB)

Above 250 365 383 58 [59]

Adamantane functionalized monomers

N

O (2-benzoxazine)

109 335 365 24.2 [61]

N

O

H3C

(3-benzoxazine)

189 399 439 30.8 [61]

aSynthesized by the solventless method.bSynthesized in 1,4-dioxane.


values were 160 and 200 1C, respectively. Viscoelas-tic measurements showed that the incorporation ofHPMI increased Tg and the storage moduluscompared to that of the unmodified polybenzox-azine and ATBN-modified polybenzoxazines. TGAthermograms indicated that these modifications didnot increase the thermal stability remarkably.However, thermal stability was slightly decreasedwith incorporation of ATBN whereas the incor-poration of HPMI into PB-a or into ATBN-modified PB-a thermal stability slightly increased.It has also been observed that the incorporation ofHPMI into ATBN-modified polybenzoxazine im-proved the thermal and mechanical properties of thematerials.

An atomic force microscopy (AFM) study wasemployed to investigate the hydroxyl terminatedpolybutadiene rubber (HTBD) modified polyben-zoxazine [85]. Both the dissolved rubber and phase-separated rubber were found to facilitate the energydissipation upon mechanical deformation, yet thelater was appeared to be much more effective, asonly 40% of extra damping was observed from theformer compared with 80% from the latter.

5.1.3. Polycarbonate (PC)-modified

polybenzoxazine

Polycarbonate (PC) was found to be completelymiscible with the cured polybenzoxazine resin in aDSC analysis, which showed the presence of a single


glass-transition temperature and the disappearanceof the PC melting behavior. Ishida and Leeconcluded that the main reason for this miscibilityof PC in the PC-polybenzoxazine blend is thehydrogen-bonding interaction, which occurs be-tween the hydroxyl groups of polybenzoxazine andthe carbonyl groups of the PC [89]. It was observedthat hydrogen-bonding of carbonyl groups did notoccur until 1 h of curing at 180 1C, because of theexistence of rather stable intramolecular hydrogenbonding within the flexible polybenzoxazine mainchain at an early stage of curing. The content ofhydrogen-bonded carbonyls gradually increasedafter prolonged heating because the hydroxylgroups became more accessible to the mobile PCchains after gelation. Moreover, both the fraction ofhydrogen-bonded carbonyls of PC and the strengthof the hydrogen-bonded hydroxyl groups of poly-benzoxazine were greater in the blends with a lowerPC concentration. DSC experiments revealed thatdue to the addition of PC modifier, the ring-openingand polymerization reactions became slow at anearly curing stage and a smaller polymerizationconversion was observed in the blend with a higherpercentage of PC. For this reason the exothermicpeak of the polymerization shifted toward a highertemperature and the glass-transition temperaturesof PC blends appeared to be lower than thepredicted values from the Fox equation.

5.1.4. Properties of polycaprolactone (PCL)-

modified polybenzoxazine

FTIR investigation of PCL-polybenzoxazineblends, with a wide range of compositions, byIshida and Lee [91] indicated the existence ofhydrogen bonding between hydroxyl groups ofpolybenzoxazines and carbonyl groups of PCL.DSC results of various PCL-polybenzoxazinesblends revealed that the addition of PCL delaysthe polymerization reaction, which was reflected bythe appearance of onset and peak temperatures ofbenzoxazine exotherms at higher temperatures asmore PCL added into the benzoxazine monomers.The Tgs of the blends with PCL concentrationsgreater than 55%, were located in the range of thePCL Tg, whereas the blends with a PCL content lessthan 33% exhibited final Tgs in the benzoxazinerange. The Tgs of the blends increased continuouslywith increasing concentration of PCL until 33wt%.This is due to the fact that in presence of PCL,higher polymerization conversion occurred, asshown by FTIR results. The addition of PCL

improved the flexural properties of the blends andas well as thermal properties [92]. Phase separation,thermal properties and morphological features ofPCL- polybenzoxazine blends were reported byGuo et al. [93] and Yang et al. [94].

5.1.5. Properties of polyurethane-polybenzoxazine

Poly(urethane-benzoxazine) films were preparedby blending the PU prepolymer with a benzoxazinemonomer, B-a, (derived from bisphenol A) [100].The PU prepolymer was blended with variousamount of B-a in THF and followed by thermaltreatment. It was believed that the cross-linkingbetween -NCO of the PU prepolymer and phenolicOH, from ring-opening polymerization of B-a, andthe allophanate formation via the intermolecularreaction of the PU prepolymer construct the mainstructures of the PU/B-a composite. The transpar-ent nature of the cured PU/B-a films suggested thegood compatibility between PU and B-a compo-nents. The appearance of a single Tg in viscoelasticmeasurements indicated that no phase separation inpoly(urethane-benzoxazine) occurred during the insitu polymerization. Tg increased with increasingB-a content. Elasticity characteristics with a goodelongation with excellent reinstating behavior wasexhibited by the films containing less than 15% ofB-a, while those containing more than 20% of B-aexhibited plastic characteristics.

The films possessed excellent resistance to organicsolvents such as THF, DMF, and NMP. Comparedwith PU these films showed an improvement inthermal stability. The decomposition temperature ofPU/B-a films increased with the higher B-a content.

But, FTIR study of the PU/polybenzoxazinebased IPN by Tang et al. [101] indicated that noapparent graft reaction occurred between the twocomponents during IPN formation. SEM and TEMstudies showed that although PU/polybenzoxazineIPN film was transparent, phase separation occursto a certain level regardless of the composition. Itwas concluded that the structure of PU significantlyinfluenced the B-a monomer distribution in PUnetwork and subsequently affected the ring openingpolymerization. The B-a monomers were welldistributed in a noncompact PU network and withthe increase of the degree of cross-linking thisdistribution of monomers was probably disturbed.During the thermal polymerization, rearrangementof B-a oligomers was hindered, resulting from thehydrogen bonding between the renascent hydroxylgroups of PB-a and the PU segments. That


interaction becomes more difficult with increasingcross-linking degree of the PU network.

5.1.6. Properties of epoxy-polybenzoxazine

For the improvement of the mechanical andwater resistance properties of the cured resins frombenzoxazine compounds and epoxy resins, terpen-diphenol-based benzoxazines were synthesized andtheir curing with epoxy resins were investigated[105]. It has been observed that the curing reactiondid not proceed below 150 1C, but it proceededquantitatively without curing accelerators above180 1C. The cured resins derived from terpendiphe-nol-based benzoxazines and epoxy resins exhibitedhigher Tg, because of the hindrance of molecularchain mobility by the rigid and bulky cyclohexanering from terpen backbone. The cured resinsshowed superior heat resistance, electrical insula-tion, and specially water resistance propertiescompared with the epoxy resins cured by a bi-sphenol A Novalac resin or B-a.

According to Rimdusit et al. [175] the appearanceof two exotherms in the DSC plots of binarymixture of benzoxazine and epoxy resins was due tothe existence of at least two reactions: (i) curingreaction among benzoxazine monomers for the firstexotherm, at the temperature range of about240–250 1C, and (ii) the second exotherm wasattributed to the reaction between benzoxazineand epoxy resins, which occurred at temperaturesof about 290–300 1C [108,176].

Agag et al. [104] described the curing behavior ofan epoxy resin and benzoxazine resin. The epoxyrings opened when they reacted with the hydroxylgroups that resulted from the ring opening ofbenzoxazines, and construct a network structure.For blends with equal functionality of oxirane tooxazine, the ring opening of benzoxazine and thepartial curing of epoxy with hydroxyl functionalitieswas indicated by a single exotherm at temperaturesof about 240 1C in DSC thermograms. For theblends with higher molar ratio of epoxy, thehomopolymerization of the residual epoxy resinswith secondary hydroxyl groups, resulting fromthe ring opening of epoxide, [177] was observed bythe second exotherm appears at 300 1C in the DSCplot.

For better understanding of the curing behaviorof the epoxy resins by bisphenol A based benzox-azine Fukuda et al. [6] investigated the curingreaction of model reactions of phenyl glycidylether (PGE) and a mono-functional benzoxazine,

p-Ca (synthesized from p-cresol, formaldehyde andaniline). Curing reaction at different temperatureswere monitored by 13C-NMR spectroscopy, whichconfirmed that the phenolic hydroxyl groupsproduced by the ring opening of p-Ca reactedrapidly the epoxy groups of PGE at highertemperature, especially above 190 1C, without acatalyst. It was postulated that the tertiary aminegroup produced by ring opening of benzoxazineaccelerated the reaction. A set of curing reactionswith DGEBA and bisphenol A based benzoxazine(B-a) was carried out to compare the curingbehavior of DGBA with bisphenol A type Novalachardener. It was observed that epoxy resin cured byB-a possess higher Tg (175 1C) along with superiorheat resistance, water resistance and electricalinsulation to those of the epoxy cured by BisA-N.

The effects of epoxy concentrations on theproperties of benzoxazine-epoxy copolymers havebeen extensively studied by Ishida and Allen [103].The effect of molecular weight epoxy resins inepoxy-benzoxazine was reported by Pasala et al.[106]. Epoxy resins having different molecularweights were synthesized by the chain extension ofglycidyl ether of bisphenol A with bisphenol A andtetrabromobisphenol A. Copolymers having highercrosslink density and Tg resulted due to theincorporation of epoxy in the polybenzoxazinenetwork. The reduction of Tg with increasingmolecular weight due to reduced crosslink density,whereas a marginal increase in storage moduluswith chain extension was observed from DMTAstudies. TGA results indicated that the samples werestable up to 300 1C. Copolymerization with epoxy infact causes reduction of char yields compared withpure polybenzoxazine, but chain extension causedslightly increase in the char yield. Increasingmolecular weight between epoxy groups by chainextension of bisphenol-A and tetrabromobisphenolA has afforded copolymers with reduced crosslinkdensity, improved storage modulus, reduced glass-transition temperature and a slight increase in thechar yield.

Damrongsakkul et al. reported a comparativestudy of the properties of polybenzoxazine alloyingwith urethane prepolymer and epoxy resins [102].According to their report the toughness of poly-benzoxazine was effectively improved by alloyingwith isophorone diisocyanate (IPDI)-based ur-ethane prepolymers (PU) or with flexible epoxy(EPO732). The flexural testing and dynamic me-chanical analysis revealed that due to the addition


of more flexible molecular segments in the polymerhybrids, the toughness of the alloys of the rigidpolybenzoxazine and the PU or the EPO732systematically increased with the amount of eithertoughener. The curing temperature of the benzox-azine resin (B-a) at about 225 1C shifted to highervalue when the fraction of B-a in alloy decreased.Interestingly, Tg of the B-a/PU alloys was signifi-cantly higher (Tg beyond 200 1C) than those of theparent resins, i.e., 170 1C for BA-a and �70 1C forPU, whereas decreases of the Tg was observed as thecontent of epoxy fraction increased. Furthermore,the degradation temperature of the B-a/PU alloysimproved with the presence of the PU, though theopposite trend was observed in the B-a/EPO732systems. The char yield of both alloy systems wassteadily enhanced with the increased benzoxazinecontent because the char yield of the polybenzox-azine was inherently higher than that of the twotougheners.

5.1.7. Polybenzoxazines with flame retarding

properties

Espinosa et al. synthesized modified novolacresins with benzoxazine rings and copolymerized itwith glycidyl phosphinate (DOPO-Gly) [108]. DTAresults showed that the storage modulus, cross-linking densities and Tgs of the blends decreasedwith increasing DOPO-Gly content. The reason ofthis trend may be the presence of bulky DOPOgroup, which decrease the cross-link density andappear to be less able to restrict segmental motions.These phosphorylated resins showed high charyield, which increases with increasing phosphorouscontent. This also indicates that their flame retar-dancy would be high. The thermal stabilities ofDOPO-benzoxazine-novolac resins are relativelypoor compared to the phosphorous free benzox-azine-novolac resins, because phosphorous DOPOgroup degrades at relatively low temperatures. Theburn tests (UL-94) of these materials indicate thatnovolac modified benzoxazines are V-1 materialswhere as high phosphorous content polymersbelong to V-0 category.

When novolac resins with benzoxazine ringscured with isobutyl bis(glycidylpropylether) phos-phine oxide) (IHPOGly), they produce flameretardant polymers of V-0 grade [107]. Thermo-gravimetric analysis of these materials showed thatthe temperature of 5% weight loss decreaseswith increase of phosphorous content and charyields were around 20%. The phosphorous contain-

ing materials showed higher Tgs, because of thepresence of strong polar PQO group.

5.1.8. Clay-polymer composite

Takeichi et al. prepared polybenzoxazine-clay (B-a-OMMT) [39] and poly(urethane-benzoxazine)-Clay (PU/P-a-OMMT) [114] nanocomposites withvarious compositions. It has been observed that dueto the catalytic effect of OMMT, the ring openingtemperature of benzoxazines was reduced for thesecomposites compared to the pristine polymer. Tgsand char yield of these hybrid materials were alsohigher and increased with increasing OMMTcontent. The initial decomposition temperatures(5% and 10% weight loss temperatures) wereenhanced by hybridizing with OMMT. In the caseof Pu/P-a- OMMT composites, the tensile strengthand modulus increased, while the elongation de-creased with the increase of OMMT loading. Due tothe addition of OMMT, the solvent resistance wasalso improved. This may be because of the layeredsilicate structures in OMMT which acts as aprotecting wall and prevents solvent penetrationinto the nanocomposites.

TGA of the polybenzoxazine-OMOM compositesprepared by Phiriyawirut et al. indicated that thechar yield of the composites is greater than that ofpolybenzoxazines (except for MOM-dodecylamine-polybenzoxazine, which may undergo some decom-position during curing). The heat resistance of thesecomposites has been improved [115].

Yu et al. reported that in the case of PBO-Bz-OMMT composites, the inclusion of OMMTdecreases the curing temperature and increases Tg

and the storage modulus of these nanocompositeswas maintained up to higher temperatures [116].

5.1.9. Boron nitride-polybenzoxazine composites

To develop highly conductive molding com-pounds for electronic packing applications Ishidaet al. prepared boron nitride filled polybenzoxazines[178,179]. These materials exhibited a very highconductivity along with high and stable mechanicalstrength up to 200 1C with a high Tg of ca. 220 1Cand a very low water absorption property. Thespecific heat capacity of boron nitride filled poly-benzoxazines has been investigated using tempera-ture modulated differential scanning calorimetry(TMDSC) and it was observed that filler loading isthe critical factor that can change the heat capacityof the composite. A linear relationship between thecomposite heat capacity and filler loading was


found out [180]. During investigation of the inter-phase of boron nitride-polybenzoxazine, it has beenobserved that the boron nitride surface inhibitscuring of benzoxazine coatings in the interfacialregion. DMA results indicated a slightly higheractivation enthalpy of the glass-transition process,as well as slightly higher Tg for the cured compositespecimens [181].

6. Conclusions

The previous short review articles [2,182,183]focused on the earlier developments and specificaspects of the benzoxazine chemistry. In this review,we have discussed the synthetic strategies to preparebenzoxazine monomers, polymerization reactionmechanisms and structure property relationship ofthe cured polybenzoxazines and related blends andcomposites. Phenols, amines and formaldehyde arecommonly used starting compounds for the synth-esis of benzoxazine monomers. Using substitutedphenols and amines, it is possible to incorporateadditional polymerizable sites into the monomer.This provides flexibility to design a wide range ofmonomers with tailored structure, which are cap-able to produce polymeric materials upon curingwith desired properties. Polymerization of benzox-azine monomers in the presence of catalystsproceeds through a cationic ring opening mechan-ism. During thermal curing, it also follows thecationic mechanism, but traces of impurities can actas an initiator. A deeper understanding of thereaction mechanism of benzoxazine ring openingreaction still requires further investigations. Poly-benzoxazine materials generally exhibit such prop-erties which are combinations of thermal and flameretardance properties of phenolic resins along withgood mechanical performance. They overcomeseveral shortcomings associated with conventionalnovolac and resol-type phenolic resins. The impor-tant features of polybenzoxazines include theirnearly zero shrinkage upon curing and very lowwater absorption with good thermal stability.

Incorporation of benzoxazine moieties in polymerchain to obtain better processibility and bettermechanical properties are future research trends inthis area.

Acknowledgements

N N Ghosh gratefully acknowledges the visitingscientist scholarship support provided by TUBI-

TAK (Turkish Scientific and Technological Re-search Council).

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High Performance Resins

Documents

polyhedral

mannich base

weak carboxylic

atomic force

mannich base

free ortho

modied novalac

reduced crosslink