Recent developments in the chemistry of cyanate esters

Polymer International 47 (1998) 465È473

Recent Developments in the Chemistry ofCyanate Esters¹

Ian Hamerton* & John N. Hay

Department of Chemistry, School of Physical Sciences, University of Surrey, Guildford, Surrey GU2 5XH, UK

(Received 4 June 1998 ; accepted 22 July 1998)

Abstract : During the last decade, aromatic cyanate esters (CE) have becomeestablished as a new and unique class of high-performance thermosetting resinsfor use as prepreg matrices in both the electronic and aerospace industries. Thebasic properties of CE resins, such as low moisture absorption, excellent electri-cal properties and good Ñammability characteristics, make them attractive com-posite matrices and di†erentiate them from standard epoxy resins (epoxies) andbismaleimides. Although they are relative newcomers to the composites industry,CE resins are enjoying unprecedented success for certain applications. Severalmajor space and radome manufacturers have qualiÐed CE resins despite theextensive database on epoxies and the inherently conservative nature ofindustry. Increasing demands on the materials used in these areas have stimu-lated the use of CE resins over other more conventional polymers. The aim ofthis review is to bring to the readerÏs attention the more recent developments inthe science of cyanate esters. 1998 Society of Chemical Industry(

Polym. Int. 47, 465È473 (1998)

Key words : cyanate esters ; synthesis ; cyclotrimerization ; thermoset blends ;simulation

SYNTHESIS AND MANUFACTURE

Historically, attempts to prepare organic cyanate esters(CEs) date back to the work of Cloez in 18571 when thereaction of alkoxide species with cyanogen chloride wascarried out. This procedure and later attempts with ary-loxides were not successful because the use of excessalkoxide led to the production of mixtures containingboth the desired cyanates and also imidocarbonates asby-products (via a competing reaction between the alk-oxide and the organic cyanate ; Fig. 1, reaction (i) showsthe corresponding aryl species). In 1960, a modiÐedversion of this approach was successful when ortho-substituted phenols were employed, and the Ðrst ortho-alkylated aryl cyanate was isolated.2 In the revisedmethod, the increased steric hindrance a†orded by the

* To whom all correspondence should be addressed.¤ The second part of this review is I. Hamerton & J. N. Hay,High Performance Polymers, 10 (1998) 163È174.

substituent prevented the reaction of the excess arylox-ide from consuming the product under the reaction con-ditions employed. In fact, whilst the preparation ofesters of cyanic acid has been reviewed in greatdepth,3h9 so far the only synthetic method having anycommercial importance for the preparation of high-temperature thermosetting resins (Table 1) has been thereaction between a cyanogen halide and a phenol in thepresence of a base (Fig. 1a). This preparation methodwas originally patented by Farbenfabriken Bayer AG in1963 ;10,11 unlike a number of the other preparativeroutes, it was successful with mono- and polyphenolsand with a number of partially halogenated aliphatichydroxyl compounds. Furthermore, it could be carriedout on an industrial scale. This method may be appliedto a large number of aryl and haloalkyl cyanates, but isnot applicable for alkyl cyanates. However, the methodsof Jensen and Holm12 or Martin,13 involving the ther-molysis of a thiotriazole intermediate (Fig. 1b), may beemployed to produce alkyl cyanates (although the

4651998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

466 I. Hamerton, J. N. Hay

Fig. 1. Commercial preparation of cyanate ester monomers.

resulting products are still apt to undergo exothermicisomerization to the corresponding isocyanates).

NEW SYNTHETIC DEVELOPMENTS:

HETEROATOMIC CYANATE ESTERS

Within the past 5 years, one development in the synthe-sis and characterization of CE monomers and polymerswhich merits discussion is the incorporation of hetero-atoms. One example of the formation of covalent bonds

between a cyanate group and an element other thancarbon has been accomplished recently with silicon.Eaborn and co-workers14h16 have successfullyemployed the reaction of numerous iodides containinga bulky trimethylsilyl group, with(Me3Si)3C,AgOCN solvated with dichloromethane. Unfortunately,whilst it has been possible to isolate the cyanate pro-ducts, successful trimerization has yet to be reportedbecause the resulting cyanates (Me3Si)3CSiRR@(OCN)(where R and R@ may be and Me(NCO)),Me2 , (OMe)2in common with alkyl cyanates, undergo rapid isomer-ization to the corresponding isocyanates (Scheme 1).

POLYMER INTERNATIONAL VOL. 47, NO. 4, 1998

Recent developments in cyanate ester chemistry 467

TABLE 1. Chemical structure of commercial or development cyanate esters available in monomer and/or

prepolymer form

Polycyanate monomer Tradename/ 1994 Melting Homopolymer propertya

structure/pecursor supplier cost point/(£/kg) viscosity T

gH

2O D

kat G

ICat 25¡C (¡C) (%) 1 MHz (J/m2)

AroCy B/ 29 Crystal 289 2·5 2·91 140

Ciba-Geigy 79¡CBT-2000/Mitsubishi

bisphenol A GC

AroCy M/ 44 Crystal 252 1·4 2·75 175

Ciba-Geigy 106¡C

tetramethylbisphenol F

AroCy F/ 99 Crystal 270 1·8 2·66 140

Ciba-Geigy 87¡C

hexafluorobisphenol A

AroCy L/ 66 Liquid 258 2·4 2·98 190

Ciba-Geigy 90–120 cP

Semi-solid

29¡Cb

bisphenol E

XU-366/ 68 Liquid 192 0·7 2·64 210

Ciba-Geigy 8000 cP

Semi-solid

bisphenol M 68¡Cb

Primaset 68 Semi-solid 270– ¿350 3·8 3·08 60

PT/Allied 250 000

Signal cpc

REX-371/Ciba-Geigy

novolac resin

XU-71787 – Semi-solid 244 1·4 2·80 125

Dow 1000 cPd

Chemical

dicyclopentadienyl bisphenol

a Cured state properties : glass transition temperature (DMA); (%), water absorption at saturation (100¡C);Tg, H

2O D

k,

dielectric constant ; fracture energy (double torsion method).GIC

,

b Melting point of supercooled liquid.

c PT30.

d XU71787.02 (at 82¡C).

Future research may concentrate on the use of di†erentinorganic cyanates to yield polymers with improvedthermomechanical properties.

CYCLOTRIMERIZATION REACTION

MECHANISM

CEs are remarkable in that they polymerize via a cyclo-

trimerization reaction to form a cyanurate network witha high degree of efficiency (Fig. 2) ; the use of metallic oramine catalysts can reportedly achieve conversionsgreater than 98%. In practice, CE resins are usuallycured by a transition metal carboxylate, or chelate cata-lyst, in the presence of an active hydrogen co-catalystsuch as nonylphenol. Certain transition metal ions suchas zinc show particularly high catalytic activity due to



Scheme 1

their low coordination number and high ligand mobilityduring the cure process.

Despite the importance of cyanate cyclotrimerization,the mechanism of the reaction is uncertain and a matterof some speculation ;17 theories range from cyclo-trimerization of a species where three cyanates are co-ordinated to a metal ion18 in sufficiently close proxim-ity to allow ring closure (to form the cyanurate) tooccur by either a step-growth, anion-initiated ionicmechanism,19 to those proposing stepwise reaction of acyanate with a reactive species such as a carbonic esterimide,20 a pseudo-imidoyl halide,21,22 a pseudo-amidine,23 or a range of cyanateÈLewis acid complex-

es.17,21 It has been reported that soluble transitionmetal cations are of the order of 103 times more e†ec-tive at promoting rapid gelation than are active hydro-gen accelerators,18 but that after the gel-point, thistrend is reversed and active hydrogen compounds arebetter than transition metals at achieving high degreesof conversion. It is believed that this is due to theslowed rate of formation for the bulky, three-memberedgrouping necessary to accelerate cyclotrimerization in agelled network. This hypothesis is supported by theobservation of a low conversion of high (99.7%) puritydicyanate in the absence of alkyl phenol co-catalyst.18Hence, it was believed that a di†erent mechanism prob-

Fig. 2. Schematic representation of the thermal polymerization of dicyanate monomers (in the presence of catalysts).



ably dominated during the later stages of reaction. Inci-dentally, independent researchers24 reported values forthe conversion of bulk dicyanates at gel-point ofbetween 50 and 64%, which in turn inÑuence the Ðnalproperties of the crosslinked resins. Two explanationso†ered for deviations from the ideal gel-point value of50% predicted from theory involve (i) the presence ofintramolecular reactions before gelation and (ii) heter-ogeneous reaction in the form of localized inactivecentres.

Much research into cyanate cyclotrimerization, par-ticularly with respect to the polymerization of dicya-nates, has centred on the search for dimericintermediates25 and Fang and Shimp26 have postulatedthat diazacyclobutadiene (DACB), or a derivativethereof, may be the reactive intermediate.

However, despite, extensive work in this area, it is par-ticularly notable that no actual “dimericÏ intermediatespecies have been isolated, making it impossible tomake a deÐnitive statement concerning the mechanism.Both Jones and co-workers25a and Fyfe et al.25b havereported results of nuclear magnetic resonance (NMR)spectroscopic studies of the cyclotrimerization mecha-nism involving model cyanate esters (in the latter studybearing stable isotopes, 13C and 15N atoms). Whilststudies, under both solution and solid state conditions,revealed that the reaction yields almost quantitativeconversion, no NMR data could be found to supportthe presence of intermediate species under the experi-mental conditions. Fang and Houlihan27 have reportedmass spectral data for the presence in the cyclo-trimerization mixture of a hydrated cyanate dimer(DMH), which has been proposed to act as a catalyticagent in the polycyclotrimerization reaction of dicya-nates (i.e. in the absence of a formal catalyst such as anactive hydrogen or transition metal compound). It hasbeen suggested that the autocatalytic nature of thispolycyclotrimerization reaction can be explained by thepresence and accumulation of this DMH species,through reversible cyanate dimer formation and itsattack by adventitious water from the surroundings. Ithas been suggested that this hypothesis readily explainsthe lack of an isolable dimer in both cases.

Recently, Brownhill and co-workers28 have postu-lated an alternative stepwise cyclotrimerization mecha-nism involving a rate-limiting nucleophilic attack of thecyanate nitrogen on the cyanato carbon of a cyanateÈtitanium tetrachloride complex pseudonitrillium inter-mediate (Fig. 3). Spectroscopic evidence revealed thatsubsequent steps are fast, with no evidence for dimericor acyclic trimeric intermediates ; the highly reactive

Fig. 3. Proposed mechanism for the cyclo-TiCl4-catalysedtrimerization of cyanate monomers (after ref. 28).

nature of the pseudonitrillium species is thought toexplain the lack of success in isolating dimeric specieswhich previous workers have experienced.

INFLUENCE OF MOLECULAR ASSOCIATION

ON CYCLOTRIMERIZATION

The question of molecular association in cyanate mono-mers remains unresolved. Crystal structure data forbisphenol A dicyanate monomer have been reported tobe in agreement in two independent studies,25b,29 butwhilst the independently determined X-ray crystal struc-tures are virtually identical, di†erent interpretationshave been proposed concerning the formation of adipole-induced dimer complex. Both studies report thatthe closest intermolecular nitrogen to carbon approachdistance between adjacent cyanate groups is 3É48 Ó.There are always difficulties in extrapolating data fromthe solid to liquid states, but dilatometric data30 indi-cate a calculated fractional volume increase of 12É6% onmelting, and the packing coefficient determined for aseries of thermosets is claimed9 to indicate that thedensity of repeat units does not vary when this molecu-lar ordering is observed. Fang25d,31 has also presentedspin-relaxation time NMR data for bisphenol A dicya-nate monomers and oligomers which show that themonomer appears to show a strong temperature depen-dence, and apparently possesses some form of “poly-mericÏ characteristics under reaction conditions. These



data have been o†ered as evidence that the repeat unitmolecular symmetry promotes an orientational order-ing in the thermoset matrix and contributes to the highdegree of cyclotrimerization efficiency, but further workis necessary to conÐrm this contentious issue.

The use of molecular orbital calculations to resolvethe question of cyanurate ring association is an areawhich has received relatively little attention to date andthis may serve to further elucidate these observations.The earlier discussion of cyclotrimerization mechanismssuggests that the presence of a transition metal catalystmay serve to greatly accelerate the ring-forming reac-tion by promoting close association of the CE mono-mers. This in itself suggests that the association betweenindividual cyanate groups is not a strongly favouredinteraction.

BLENDS WITH OTHER THERMOSETS

Blends of di†erent cyanates are common32 and arereviewed elsewhere.33 Monofunctional cyanates canalso be used to modify the properties of a cured CE.34Dinonylphenol cyanate was used as a reactive diluent ina Ñuorinated bisphenol A dicyanate in an attempt toproduce a resin processable under the same conditionsas an FR-4 epoxy. Incorporation of a monocyanateproduces a dramatic reduction in the of the curedT gCE.

Blends with other thermosets include epoxy and bis-maleimide (BMI) resins. EpoxyÈcyanate blends are themost common and are found in most commercial CEprepreg systems. The nature of any epoxyÈcyanate co-reaction has been the subject of much argument, conjec-ture and in-depth study in recent years. Doubtless thearguments will continue for some time to come. Wepropose merely to summarize the current position. Themechanism of the co-reaction of epoxy with cyanatewas studied by Shimp and Wentworth,35 who con-Ðrmed the results of an earlier study by Bauer and co-workers.36 The proposed mechanism is complex butinvolves, in part, formation of an oxazolidinone ringfrom the reaction of a cyanate group with epoxyrings. Using FTIR spectroscopy, it was demonstratedthat a greater proportion of oxazolidinone rings isformed from the cyanate monomer than from thecyclized prepolymers.35 The preferred catalyst for maxi-mizing ring formation is a titanium chelate. Somemechanical properties are improved in these networkscompared with those prepared using more conventionalcatalysts. Incorporation of an epoxy cure catalyst suchas 2-ethyl-4-methylimidazole (2E4MI) in an epoxyÈcyanate blend results in formation of less oxazolidinoneand isocyanurate than in uncatalysed blends.37

Fyfe et al.38 have studied the cross-reaction in somedepth using a range of techniques, including 1H, 13C

and 15N NMR spectroscopy and mass spectrometry,together with monofunctional epoxy and cyanate modelcompounds. This study conÐrms the formation of oxa-zolidinone from reaction of one cyanate with two epoxyrings (Fig. 4).

The proposed mechanism involving the unstablebicyclic intermediate is unlikely, but the authors refer tothis mechanism only as “possibleÏ, with no evidence pre-sented for this reaction path. Not surprisingly, becauseit requires the reaction of two epoxy molecules with onecyanate, the oxazolidinone is formed in only 12% yieldwhen the epoxy and cyanate are heated at 180¡C in 1 : 1molar ratio. Minor cross-reaction products arose fromreaction of the cyanate with epoxy hydroxyl groups andreaction between carbamate intermediates and epoxygroups. Previous work noted the formation of up to32% oxazolidinone from an equimolar monomermixture, depending on the catalyst.35 Recently, Grenier-Loustalot et al.39 studied the molten state reaction ofcyanate with epoxy and were able to detect not only theexpected oxazolidinone, but also a species to which thestructure of a four membered ring cyanate dimer(C2N2)was assigned on the basis of 1H and 13C NMR spec-troscopy. ConÐrmation of this structure by other tech-niques such as mass spectrometry would have beenuseful. As these workers suggest, the precise course ofthe epoxyÈcyanate reaction is probably dependent on

Fig. 4. Proposed mechanism for the cyanate/epoxy co-reaction in which and are typically aryl residues (afterR1 R2

ref. 35).



Fig. 5. Proposed reaction mechanism for allyl-substitutedcyanates and bismaleimides where Rr, Rr@\ H or an aryl

residue (after ref. 42).

the relative proportions of each monomer, thus addingto the complexity. Disregarding these problems, a usefuldevelopment might be the identiÐcation of catalystswhich promote a speciÐc reaction path leading to for-mation of a speciÐc heterocyclic product in high yield.This might result in resins showing truly synergistice†ects compared with current epoxyÈcyanate systems.

Fyfe et al.40 have applied the concept of linked inter-penetrating networks (LIPNs) (see below) to epoxyÈcyanate blends. An AB monomer was formed bymonoepoxidation of bisphenol A, followed by cyanationof the remaining phenol group, to give almost purebisphenol-AÈmonocyanate monoglycidyl ether. Thecyanate group can be cured independently withouta†ecting the epoxy group, but not vice versa. Themonomer was used as a crosslinking agent in cyanateÈepoxy blends to give a product which was muchtougher and stronger than the product of the cyanateÈepoxy blend alone.

Hamerton and co-workers41h44 earlier synthesizedallyl cyanate monomers as co-monomers for use incyanateÈBMI blends. These blends are important con-stituents of commercial resin systems (e.g. MitsubishiÏsSkylex/BT resins), but early postulates of co-reactionbetween the cyanate and the BMI functional groupshave since been discredited, for example by use of 15NNMR spectroscopy to study mechanisms in thesesystems.44 The LIPNs produced by copolymerization ofallyl cyanates with BMIs exhibit very high valuesT g

of up to about 350¡C, while achieving fracture tough-ness values in excess of either homopolymer.45 Thisapproach has been extended to allyl cyanates based onarylene ether sulphone oligomers.43 An allyl cyanatemonomer was reported subsequently and independentlyby Fyfe et al.,40 and shown to copolymerize with methylmethacrylate. These allyl cyanates can be cyclo-trimerized to form allyl triazines without a†ecting theallyl group.41 The co-reaction between the allyl groupand the BMI is believed to proceed via an ene/DielsÈAlder mechanism, as shown in Fig. 5,46 which alsotakes into account some recent observations of BMI/allylphenol co-reactions by Reyx et al.47 Related studieshave shown that propenyl cyanates can act in the sameway to improve the properties of cyanateÈBMIblends.45

MOLECULAR SIMULATION

To date, comparatively little work has been publishedconcerning the simulation of mechanical properties ofthermosetting polymers. This is largely due to the com-plexity of the network formed during the poly-merization process and hence the difficulty in producinga representative model for simulation. Fang andShimp26 discussed the implications of cyanate conver-sion data on network growth from a brief review of theliterature and molecular mechanics simulations per-formed on monomers and oligomers of bisphenol Adicyanate (an experimentally well determined and wellknown commercial polymer system). Unfortunately,they were unable to reproduce the bisphenol A dicya-nate monomer accurately because the OwCN bondlength was drastically shortened from 1É273 to 1É095 Ó(the accepted bond length from single crystal X-raydata25b,29) and calculations were performed in vacuo.This would in turn lead both to over-estimations of thesteric energies and charge interactions of the resultingoligomers. The calculations of Fang and Shimp,26carried out on bisphenol A dicyanate trimers, yieldedenergies of 26É6 kcal mol~1 for the open-chain trimerand 18É96 kcal mol~1 for a cage-like conformer, suggest-ing that both states are relatively strain-free. However,no inference should be made concerning the preferredmechanism because only two conformers (of a muchlarger ensemble) were considered. The apparent prefer-ence for the cage-like structure was attributed to thecontribution from excessive non-bonding attraction, butno other evidence was put forward in the publication tosupport this. Calculations made on higher oligomers(tetramers and pentamers) produced some very close-packed molecules (in which the closest contact betweencyanate groups was reported as in which no3É76 Ó)steric repulsion between the end-groups was observedfor calculations on oligomers containing up to 20



Fig. 6. (a) Idealized polycyanurate structural repeat unitwhere 1a is linked with 1b, 2a with 2b and 3a with 3b toconform to periodic boundary conditions (N.B. — denotes abisphenyl moiety of the form ph.x.ph, e.g. (b)x \ [CH3]2C).Three-dimensional representation of the same as ball andspoke model (N.B. hydrogen atoms have been omitted for

clarity). (c) Diagrammatic representation of (b).

monomer units. In fact, the presence of any steric inter-actions was reported to favour chain propagationrather than chain inhibition (the extremely low segmentactivation energy (1 kJmol~1) of the polycyanurate ofbisphenol A48 was cited as evidence).

Recently, Howlin and co-workers49,50 have suc-cessfully performed a series of experiments on the poly-cyanurates of bisphenol dicyanate and a six-ringedpolyaromatic dicyanate. These experiments yield bothphysical and mechanical measurements to support theÐndings of computer simulations. For bisphenol A poly-cyanurate, the YoungÏs modulus from simulation was4É04 GPa (experimental 3É39 GPa) and PoissonÏs ratiofor the same system was 0É39 (experimental 0É35). Theonly constant to have an error margin greater than 50%was the Lame� constant j (which has no physicalmeaning). The of polycyanurate of the six-ringedT gpoly(arylene ether sulphone) was calculated to be 490 Kfrom a plot of total energy versus temperature ; theexperimental value for this system was 413 K as deter-

mined by dynamic mechanical thermal analysis. Thesimulated elastic constants were generally higher thanthe experimental values and always at the upper limit ofthese values. One problem with the model was that itfailed to address the formation of triazine bicyclophanecage structures arising from intramolecular cyclizationwhich has been reported by Fang and Houlihan27 usingmass spectral data ; these structures may account for theobserved low crosslink density of these polymersystems. Because the simple model of Howlin et al.49,50consisting of 200 atoms, assumes a “perfectÏ polymer(lacking voids, defects or structures like those men-tioned above), the results (which of course relate to amuch more complex network) are still well reproduced.

A step forward has been made with the developmentof a more realistic force Ðeld for the cyanurate ring.This has been validated against fourteen X-ray crystalstructures which contain this moiety.51 Associated workhas also been carried out52 to produce a model whichrepresents the minimum required structure to replicatethe three-dimensional polycyanurate network (Fig. 6).In the future, much larger and more realistic modelswill be possible with improvements in software, andwork continues in this rapidly expanding research area.

ACKNOWLEDGEMENTS

The authors wish to thank Dr Brendan J. Howlin andMr Richard D. Allington (University of Surrey) foruseful discussions concerning molecular simulationstudies, and Professor Bill Wright for critical analysis ofthe manuscript.

BACKGROUND READING

Graver, R. B., in High Performance Polymers : T heir Origin and Devel-opment, eds R. B. Seymour & G. S. Kirshenbaum, Elsevier, NewYork, 1986, pp. 309È316.

Lin, S.-C. & Pearce, E. M., High Performance T hermosets, Chemistry,Properties, Applications, Hanser, Munich, 1993, pp. 65È108.

Pilato, L. A. & Michno, M. J., Advanced Composite Materials,Springer-Verlag, Berlin, 1994, pp. 39È42.

Hamerton, I. (ed.) Chemistry and T echnology of Cyanate Ester Resins,Blackie Academic and Professional, Glasgow, 1994.

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52 Allington, R., University of Surrey, personal communication, 1997.


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Recent developments in the chemistry of cyanate esters