Thermal Polymerization of Cyanate Ester-Benzoxazine：Study of a Functional Model Compound

WANG Fan，ZHOU Hui，WAN Li，DONG Bin，LI Shengchao，QI Huimin

（1.Key Laboratory of Specially Functional Polymeric Materials and Related Technology of Ministry of Education，School of Materials Science and Engineering，East China University of Science and Technology，Shanghai 200237，China；2.Shanghai Composites Science &Technology Co.，Ltd.，Shanghai 201112，China）

Abstract:A novel benzoxazine（BOZ）monomer is synthesized by a pot method with solvent-free to blend with cyanate ester（CE）.A soluble intermediate is obtained after being cured for 20 h at 80 ℃.The two model compound and the blends are analyzed with the infrared radiation（IR），nuclear magnetic resonance（NMR）spectroscopy，and differential scanning calorimetry（DSC）.The results show that an intermediate of the iminocarbonate and BOZ structures is formed by the ring-open BOZ reacting with the cyanate groups and ring-unopened BOZ.Moreover，rearrangement and ring-opening occur in the postcure of the intermediate to form the alkyl isocyanurate structure with polybenzoxazine.

Key words:thermosetting resin；benzoxazine（BOZ）；cyanate ester（CE）resin；polymerization mechanism

0 Introduction

The blends of cyanate ester（CE）and benzoxa‑zine（BOZ）are high-performance thermosetting res‑ins，and can be fine-turned through compositional variations and innovation processing schedules.Many blends of CE and BOZ contain curable sites.Cocured resins have attracted wide attention of research‑ers since they can be processed into void-free compo‑nents and possess further enhancement in high ther‑mal stability，high strength，low dielectric constants over a wide range of frequencies and temperature，high glass transition temperature as well as low water absorption properties.These properties render the blends useful as matrices for a wide range of ad‑vanced applications as the substrates of printed circuit boards，heat-resistant materials，and matrices of ad‑vanced composites in aerospace and astronautic.Al‑though some blends of oligomers are common，and have been found in many commercial prepreg systems，the mechanism of the polymerization of the BOZ and CE groups is not yet fully established since the cross-linking and insoluble make the solution spectroscopic analysis difficult.The actual cure chem‑istry is subject to the conjecture and uncertainty.

Though the reaction mechanisms have been poor‑ly understood，studies on such subjects have been widely investigated based upon observations involving model compounds and selected monomers，and some insight has been obtained.It is shown that a miscible and homogenous structure derived by cyclotrimeriza‑tion，ring-opening polymerization of BOZ，or form imidocarbonate could lead to the polycyanurate struc‑ture and triazine as a part of the polybenzoxazine ma‑trix.SANTHOSH-KUMAR et al.，KIMURA et al，and LI et al.not only had reached a consensus of cyclotrimerization，ring-opening polymerization of BOZ，and co-reaction hydroxyl of polybenzoxazine and CE，but also had some divergences about further reaction.SANTHOSH-KUMAR et alpostulated a cross-linked structure comprised of triazine as a part of the polybenzoxazine structure formed via iminocar‑bonate intermediate.LI et al.concluded that BOZ could insert into the triazine ring and then be isomer‑ized to alkyl isocyanurates.An associated work was carried out to produce a ring-opened BOZ model with‑out hydroxyl.The incorporation via blending BOZ and CE significantly concluded that the mechanism might be the nucleophilic addition reaction of the oxy‑gen anion on the ring-opened BOZ and the positively charged carbon atom on the cyano groups.Recently，LIN et al.and WANG et al.eloquently revealed a step forward with the development of the cure reac‑tion，i.e.，the cyclotrimerization of CE occurred and the co-reaction of the BOZ and triazine ring generated alkyl isocyanurate and diphenyl ether by one step in‑stead of two processes as LI et al.suggested.They observed that gelation occurred at 30 ℃after 24 h in a solution of the blends while ring-opening hardly oc‑curred at this temperature.It must be BOZ itself that catalyzed the trimerization of CE.They speculated that the tertiary amine of BOZ catalyzed the trimeriza‑tion of CE.

Owing to the total solubility of oligomer prepolymers which is the intermediates on the way to full cure，it is difficult to determine the structural relation to the reactions and fully characterize the resultant.In order to solve this problem，two products of sufficient purity，i.e.，BOZ and dicyanate of bisphenol A（BADCY），are prepared in an acceptable yield.The reactive properties of the blends of BOZ and BADCY are studied by exposure to 80 ℃and thermal cycling，respectively.The analyses are dominated by Fourier transform infrared radiation（FT-IR），nuclear magnetic resonance（NMR）spectroscopy，and differential scanning calorimetry（DSC）to understand the evolution of the molecular structure in the network-forming reaction.

1 Experimental

1.1 Materials

Aniline，phenol，toluene，and hexane are purchased from Titan.Paraformaldehyde is pur‑chased from Macklin.Dicyanate of bisphenol A（BADCY，mp：79 ℃）and granular crystal are pur‑chased from Shanghai Hui Feng trade Co.，Ltd.

1.2 Synthesis and characterization of 3-phenyl-3,4-dihydro-2H-1,3-BOZ

The one pot method with solvent-free is taken to synthesize the BOZ monomer（see Fig.1）.A general synthesis procedure is adopted.First，9.4 g phenol，9.3 g aniline，and 6.0 g paraformaldehyde are mixtured in a 500 mL three-necked flask equipped with thermometer，condenser，and stirrer.The mixture is stirred for 10 min at room tempera‑ture，and then the temperature is raised to 80 ℃slowly.Keep stirred for 2 h at 80 ℃，and then dis‑solved in 200 mL toluene.The solvent is washed by 3 mol/L NaOH solution to eliminate the phenolic structures.Evaporate the toluene，and obtain a vis‑cous fluid at 35 ℃.The viscous fluid crystallizes in hexane to afford a light yellow crystal.The melt point is 56 ℃.The purity is 96.5%.H NMR（400 MHz，CDCl，10）：4.63（s，2H，C―CH―N），5.36（s，2H，O―CH―N），6.79-7.28（Aro‑maticH）；FT-IR（KBr），cm：1 498（stretching of substituted benzenering），1 225（asymmetric stretching of C―O―C），1 081（asymmetric stretch‑ing of C―N―C），838（symmetric stretching of C―N―C），and 937（out-of-plane C―H of benzene ring of BOZ）.EI analysis：N，6.50%；C，80.12%；H，5.62%；O，7.76%.（C，79.59；H，6.20；N，6.63；O，7.57，theoretically.）

Fig.1 Structures of BADCY and BOZ

1.3 Characterization

The DSC scans are obtained with TAQ2000 in a nitrogen atmosphere at a heating rate of 10 ℃/min.The FT-IR spectra are obtained from at least 32 scans in the standard wavenumber range from 400 cmto 4 000 cmwith a Thermo Fisher Nico‑let 6 700 infrared spectrophotometer.The in-situ FT-IR spectra are obtained at a heating rate of 10 ℃/min from room temperature to 260 ℃.The Bruker AVANCE 400 MHz（NMR）（H andC）is used with CDClas the solvent，tetramethylsilane as the internal standard for the nuclear magnetic res‑onance spectrum analysis.The elemental analyzer Vario EL III is used for the elastic impedance（EI）analysis.The purity is measured by the high perfor‑mance liquid chromatography Agilent 1260SL.

1.4 Sample preparation

BOZ and BADCY are mixed by various mole ra‑tios of function groups in the agate mortar to pulver‑ize as the blend powders.The blend in which the mo‑lar ratio of oxazine-ring and cyanate groups is 1∶1 is named as BOZ-CY.The blends are put in oven to cure at 80 ℃for 96 h.We take a sample every 4 h in the first 24 h，and then take it every 24 h.

2 Results and discussion

The DSC thermograms of BOZ，BADCY，and BOZ-CY are shown in Fig.2.It can be seen that the BOZ，BADCY，and BOZ-CY are cured at the heat rate of 10 ℃/min from the room temperature to 350 ℃.The DSC thermograms provide a very useful assessment of the purity and stability of the two monomers.For the BOZ-CY sample，two distinct but somewhat overlapping exothermic peaks are ob‑served.The exothermic peak temperatures are re‑duced by 30 ℃or more，the heat of polymerization re‑duces clearly，and the effectiveness of the BOZ in the chemical reactivity of BADCY is examined，which is also possible.However，a more detailed analysis is required to determine the effect certainly.The main object of this work is to investigate the native of the reaction at the BOZ and CE with the FT-IR.

Fig.2 DSC thermograms of BOZ，BADCY，and BOZ-CY

The FT-IR spectra of BOZ，BADCY，and BOZ-CY cured at 80 ℃for 24 h and more are shown in Fig.3.It can be seen that the two monomers，i.e.，BOZ and BADCY，have almost no change，which are deemed to have no reaction at 80 ℃by the mono‑mers themselves.The absorbance FT-IR spectra of BOZ-CY subjected to isothermal polymerization at 80 ℃ for various time are shown in Fig.3（c）.Evidently，the spectra for the BOZ-CY sample（see Fig.3（c））decrease in the bands corresponding to the cyanate groups and oxazine-ring groups at 2 237 cm，2 270 cm，and 943 cm，respective‑ly.The cyanate groups are no longer evident，and are almost completely consumed when the curing time approaches 20 h，owing to the reactions of the CE groups and oxazine-ring groups.Moreover，the intensity of peaks increases with the N―H stretch（3 400 cmand 1 635 cm）or C=N groups（1 690 cmand 1 407 cm）.This is useful for the detection of iminocarbonate formation.The triazinering peak in the spectra around 1 550 cmsuggests that cyclotrimerization does not occur at this stage.

Fig.3 FT-IR spectra of（a）BOZ，（b）BADCY，and（c）BOZ-CY cured at 80 ℃

Since the possible reaction routes involving the consumption of cyanate groups and oxazine-ring groups could be used to monitor the degree of cure.The methyl（―CH）stretch at 2 969 cmis used as an internal standard band.The reaction is men‑tioned at 80 ℃over the time range from 4 h-96 h.The intensities are illustrated in Fig.4.The results of the plots change indicating that the reaction extent increases with the cure time ranging from 0 h-20 h.The absorption of oxazine-ring（943 cm）mainly decreases to half of it at the beginning of the reac‑tion，and the cyanate groups are completely con‑sumed after 20 h.This readily indicates that the structure of the cyanate groups changes，owing to the reactions with BOZ.In addition，the new absorp‑tion is hopefully assigned to C=N and N―H，in‑creases with the cure time，and becomes invariant af‑ter 20 h.After 20 h，all the absorption intensities re‑main unchanged till to 96 h.However，it is difficult to determine the termination time of the reaction.As the reaction time prolongs，e.g.，two weeks，the in‑termediate undergoes cyclotrimerization in the cross‑linking process.Moreover，it is clear that lower tem‑perature（80 ℃）necessitates very long reaction time to drive the reaction to completion.With the in‑crease in the reaction time，the intensities of N―H，oxazine-ring，and C=N decrease.Meanwhile，the new absorbance of triazine-ring arises，owing to the crosslinking at 1 569 cmand 1 370 cm（see Fig.5）.

Fig.4 Variation curves of the functional groups of the absorption peak intensity

Fig.5 FT-IR spectra of BOZ-CY cured at 80 ℃

The carbon atom of the ―OCN group is strong‑ly electrophilic，and thus is lightly susceptible to nu‑cleophilic reagents owing to the unpaired electrons of the oxygen atoms of BOZ.In turn，it reacts with cya‑nate groups during the thermal curing reaction.Then，the electrophilic carbon atom of ―O―C―N― of oxazine-ring will be attacked by the carbon of the ben‑zene-ring near the oxygen of oxazine-ring to form imi‑nocarbonates with the BOZ structure.

The reaction of these iminocarbonates with cya‑nate will continue until the cyanate groups consume definitely，leading to the formation of the intermedi‑ate.Moreover，the iminocarbonate formation is al‑ways more pronounced in less cured CE.The ther‑mal curing is depicted in Fig.6.

Fig.6 Probable reaction of BOZ and CE

The FT-IR spectra of CE and BOZ could not furnish conclusion proof for the traces of reaction.In order to get more information about the structure of the copolymerization sample，the NMR spectra of BOZ-CY cured at 80 ℃for various hours are shown in Fig.7.The peaks at 4.64×10，5.36×10（see Fig.7（a））and 50.4×10，79.5×10（see Fig.7（d））of the BOZ sample before curing can be assigned to the oxazine-ring hydrogens and the carbon resonances，respectively.The peaks of hydrogens broadened with the increase in the curing hour（see Fig.7（c））.The N―H bond，however，shows new resonances at 8.63×10and 8.25×10（see Fig.7（c））.These results can be best understood by assuming that part of oxazine-ring structure has been changed after curing for 8 h at 80 ℃.A smaller additional signal（160.5×10，see Fig.7（f））is observed after the sample has cured for 8 h at 80 ℃which shows that the C=N is in fact confirmed.This peak would come from the conformational distribution of the iminocarbonate（position carbon，see Fig.6）products.The signal（153.4×10，see Fig.7（f））corresponded to carbon（position carbon，see Fig.6）of benzene-ring directly attached oxygen was observed.Form these results，it was suggested that the iminocarbonate would be formed and that the ―OCN group would be changed into the ―O―C=N― bond by curing.These results of NMR analysis agreed well with the characteristic feature observed in the FT-IR results.

Fig.7 NMR spectra of BOZ-CY cured at 80 ℃

Owing to the disappearance of cyanate groups of BOZ-CY at 20 h，the function groups of blends with various molar ratios are cured at 80 ℃for 20 h.The results are shown in Fig.8.From Fig.8，it can be seen that when the cyanate groups are excessive，absorbance appears at 1 569 cmand thus triazinering forms.Therefore，it can be concluded that the excessive cyanate groups could react with the inter‑mediate to generate triazine-ring.The relative intensi‑ty of triazine-ring diminishes with the increase in the cyanate groups.When the molar ratio is 1∶16，the tri‑azine-ring could hardly be observed because of the less intermediate formed owing to the less BOZ.Thus，a proper molar ratio of oxazine-ring and cya‑nate groups can be determined according to the formation degree of triazine-ring.The relative intensi‑ty of cyanate groups and triazine-ring is shown in Fig.9.The formation rate of triazine-ring first rises with the increase in the percentage of cyanate groups，and then declines after the molar ratio reach 1∶2.The conversion rate of the cyanate groups has a similar trend except that the molar ratio is 3∶4，which has the maximum conversion rate of cyanate groups but no such formation rate of triazine-ring.It implies that the reaction to the formed intermediate is mainly in this blend ratio.The best molar ratio of function groups is 1∶2，which maximizes the forma‑tion rate of triazine-ring.

Fig.8 FT-IR spectra of the function groups of blends cured for 20 h with various molar ratios

Fig.9 Relative intensity of the cyanate groups and triazine-ring of blends cured for 20 h

3 Conclusions

The BOZ monomer is synthesized by the one pot method with solvent-free to investigate the ther‑mal polymerization of BOZ-CE.A soluble intermedi‑ate is obtained after the BOZ-CE has been cured for 20 h at 80 ℃.The two model compound and the blends are analyzed with the FT-IR，NMR，and DSC.The results show that an intermediate of imi‑nocarbonate and BOZ is formed by the ring-open BOZ through reactions with cyanate groups and ring-unopened BOZ.The triazine-ring is formed when the intermediate reacts at 80 ℃for a long time or the excessive cyanate groups react with the inter‑mediate.The molar ratio of the optimum function groups is 1∶2，which is beneficial to the formation of triazine-ring.