Mechanistic Pathways for the Polymerization of Methylol-FunctionalBenzoxazine MonomersMohamed Baqar,†,§ Tarek Agag,‡,⊥ Rongzhi Huang,‡ Joao Maia,‡ Syed Qutubuddin,*,†,‡

and Hatsuo Ishida‡

†Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States‡Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States

ABSTRACT: The polymerization mechanism of methylol-functional benzoxazine monomers is reported using a series ofmonofunctional benzoxazine monomers synthesized via acondensation reaction of ortho-, meta-, or para-methylol−phenol, aniline, and paraformaldehyde following the traditionalroute of benzoxazine synthesis. A phenol/aniline-typemonofunctional benzoxazine monomer has been synthesizedas a control. The structures of the synthesized monomers havebeen confirmed by 1H NMR and FT-IR. The polymerizationbehavior of methylol monomers is studied by DSC and showsan exothermic peak associated with condensation reaction of methylol groups and ring-opening polymerization of benzoxazine ata lower temperature range than the control monomer. The presence of methylol group accelerates the ring-openingpolymerization to give the ascending order of para-, meta-, and ortho-positions in comparison to the unfunctionalized monomer.Furthermore, rheological measurements show that the position of methylol group relative to benzoxazine structure plays asignificant role in accelerating the polymerization.

■ INTRODUCTION

Benzoxazine resins are a new class of thermosetting polymersthat are characterized by several unique properties such as near-zero shrinkage or volumetric expansion upon polymerization,considerable molecule-design flexibility, low water absorptionand flammability, low surface energy, and high glass transitiontemperature.1 Additionally, the polymerization of benzoxazinemonomers takes place through thermally accelerated ring-opening mechanism without any added initiator or catalyst.2

These numerous unusual properties of benzoxazine resins areassociated with the existence of inter- and intramolecularhydrogen bonds in the network structure.3a−d Therefore, inorder to understand the structure−property relationships of thepolymer, it is of great importance to know the mechanisticpathways of polymerization.Although the polymerization chemistry of benzoxazine

monomers still remains rather poorly understood, the highbasicity of benzoxazine monomers, which is attributed to boththe oxygen and the nitrogen of the oxazine ring by Lewisdefinition, suggests that the ring-opening polymerization ofbenzoxazine monomers proceeds through a cationic mecha-nism.4a−c During the polymerization, the monomer comes toequilibrium with the corresponding zwitterionic intermediatewhich eventually forms the polymer as depicted in Scheme 1.Several fundamental studies for polymerization mechanismswere conducted to understand the polymerization pathways ofbenzoxazine monomers.5a−c These investigations suggest that,as the polymerization of benzoxazine monomers progresses,

more phenolic groups are formed, which promote anautocatalytic polymerization process.6a,b

Wang and Ishida7 showed that monomers synthesized usingunsubstituted phenol exhibited variations of the ring-openingpolymerization temperature. A more detailed study by Andreuet al.,8 who synthesized several substituted 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine monomers using substituentphenol and aromatic amine, showed that increasing theelectron-withdrawing in the para-position of phenol exhibiteda noticeable decrease in the polymerization temperature. This isattributed to the acidic phenol species that are formed due tothe electron-withdrawing groups. On the other hand, increasingthe electron-withdrawing in the para-position of the phenylsubstituent monomers showed an increase of the polymer-ization temperature due to the destabilization of thepropagating iminium intermediates as proposed by McDonaghand Smith.9a,b However, for the electron-donating substituentsno notable effect on the polymerization was observed regardlessof the position of the substituent.The effect of functionalization and copolymerization of

carboxylic group into benzoxazine structure has beenstudied10a−c and a significant decrease in polymerizationtemperature was observed. Polybenzoxazine chains preferen-tially form intramolecular 6-membered hydrogen bond,11a−i

which tends to slow the propagation reaction.12a,b This

Received: September 18, 2012Revised: September 21, 2012Published: October 1, 2012

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hydrogen bonding by COOH group to the newly formedphenolic OH groups competes with the intramolecular 6-membered ring formation between the phenolic and aminegroups,13 hence the retarding effect on chain propagation bythe intramolecular hydrogen bonding.Kiskan et al.14a,b synthesized hydroxyethyl terminated ether

chain-functional benzoxazine monomers and reported thereduced polymerization temperature of these monomers incomparison to ordinary unfunctional benzoxazine monomers.Kudoh et al.15 subsequently studied the mechanistic aspects ofthe polymerization and the role of hydroxylethyl group inactivating the ring-opening of hydroxyethyl functional benzox-azine monomers. They found that the hydroxyethyl monomerpolymerized much faster than a similar structure of other N-alkylbenzoxazines, which is attributed to the intramolecularreaction of hydroxyl group with cationic moieties of thezwitterionic intermediates formed by the ring-opening reactionof benzoxazine to afford a 5-membered cyclic N,O-acetal.Sudo et al.16 showed that increasing the bulkiness of the

substituent N-alkyl group on to benzoxazine monomers leadsto a decrease in the polymerization rate due to the release of alarger amount of N-alkylimine as a volatile compound. Oie etal.17 reported that N-allylbenzoxazine monomers exhibit a fasterrate of polymerization than N-(n-propyl)benzoxazines due tothe presence of allyl group on nitrogen atom. Consequently,the intramolecular interaction between the cationic moietiesand electron-rich carbon−carbon double bond of this groupprompted the formation of the zwitterionic intermediates.In continuation of our observed reduction of the ring-

opening polymerization temperature of methylol benzoxazinemonomers compared to the unfunctionalized monomers,18a−c

this paper is aimed at studying the role of incorporatingmethylol groups into benzoxazine structure. Furthermore, thepolymerization mechanism of benzoxazine monomers withvarious isomeric methylol positions is proposed.

■ RESULTS AND DISCUSSIONPreparation of Monofunctional Benzoxazine Mono-

mers. A successful synthesis of methylol monofunctionalbenzoxazine monomers has been achieved using aniline,paraformaldehyde, and hydroxybenzyl alcohols (ortho-, meta-and para-isomers) as well as phenol (P) for comparison asindicated in Scheme 2. The monomers are designated as

(oHBA-a, mHBA-a, and pHBA-a) in which the smaller italicletters represent the position of methylol group intobenzoxazine structure, either in ortho, meta or para,respectively. P-a represents a phenol/aniline type monofunc-tional benzoxazine monomer synthesized as a control with nomethylol group in the structure.The structures of the monomers were confirmed using 1H

NMR and FT-IR spectra. In the 1H NMR spectra, as depictedin Figure 1, the typical resonances attributed to the benzoxazinestructure, Ar−CH2−N− and −O−CH2−N−, for pHBA-a,mHBA-a, oHBA-a and P-a are observed at 4.62, 5.36; 4.64, 5.37;4.66, 5.41; and 4.74, 5.48 ppm, respectively. Also, the 1H NMRspectra confirm the presence of methylol group (−CH2OH)from the resonance of −CH2−O of methylol group at 4.55,4.56, and 4.64 ppm for pHBA-a, mHBA-a, and oHBA-a,respectively.There are a number of infrared absorption peaks, highlighted

in Figure 2, that are used to verify the formation of oxazinerings in each monomer. For example, the characteristic out-of-plane absorption modes of benzene with an attached oxazinering are located at 947, 950, 945, and 946 cm−1 for the pHBA-a,mHBA-a, oHBA-a, and P-a, respectively.19 Furthermore, thepresence of the benzoxazine ring aromatic ether in themonomers is indicated by an absorbance peak centered inthe range of 1035−1032 cm−1 due to the C−O−C symmetricstretching mode. Also, the peak between 1242 and 1226 cm−1

for the asymmetric stretching modes confirms the presence ofthe oxazine ring in the monomer structure.20a,b Peakscharacteristic of asymmetric trisubstituted benzene that appearin the methylol monomers between 1505 and 1500 cm−1

confirm the incorporation of methylol group into benzoxazinemonomers. Furthermore, the presence of methylol group in thesynthesized monomers is confirmed through the very broad

Scheme 1. Mechanism of Ring-Opening Polymerization ofBenzoxazine Monomers

Scheme 2. Preparation of Monofunctional BenzoxazineMonomers

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absorption peak between 3340 and 3320 cm−1 due to the OHstretching mode.21a,b

Effect of Methylol Group on the PolymerizationBehavior of Benzoxazine Monomers. The polymerizationprofiles of methylol benzoxazine monomers were studied byDSC as depicted in Figure 3 and the results are summarized inTable 1. The thermograms show that the onset of the ring-opening polymerization of unfunctionalized benzoxazinemonomer started at 237 °C with its maximum centered at255 °C. However, the onset of polymerization is shifted in thepresence of methylol group to as low as 151, 183, and 205 °Cfor oHBA-a, mHBA-a, and pHBA-a, respectively. In addition,the peaks are significantly broadened and the maxima areshifted to temperatures of 196, 214, and 231 °C for oHBA-a,mHBA-a, and pHBA-a, respectively. Moreover, the methylol-functional benzoxazine monomers show lower values for theheat of polymerization compared to the control. For example,the heat of polymerization values were 201, 203, and 241 J/gfor oHBA-a, mHBA-a, and pHBA-a, respectively. However, P-a,which has no methylol groups, exhibits the higher exotherm of336 J/g.

From the DSC results, the values of the heat of polymer-ization for the functionalized monomers were lower than thecontrol, showing clearly that another thermal event is involvedin the ring-opening reaction of methylol benzoxazines. Sincethe presence of methylol groups is the main feature of thesemonomers, one possible thermal event may be attributed to therelease of water as a byproduct from the polymerization ofmethylol groups similar to traditional phenolics.22a,b The

Figure 1. 1H NMR spectra of benzoxazine monomers.

Figure 2. FT-IR spectra of benzoxazine monomers.

Figure 3. DSC thermograms of benzoxazine monomers.

Table 1. Results of the DSC Analysis of BenzoxazineMonomers

monomer onset temp (°C) max temp (°C)heat of polymerization

(J/g)

P-a 237 255 336oHBA-a 151 196 201mHBA-a 183 214 203pHBA-a 205 231 241

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occurrence of methylol condensation reaction together withring-opening polymerization of methylol benzoxazine mono-mers has been confirmed elsewhere.18a

Chemorheological Studies of Monomers. Viscoelasticproperties during the polymerization of each monomer wereevaluated during isothermal polymerization at 140 °C. Aqualitative comparison of the rates of polymerization for themonomers is determined by the increase of the viscoelasticmoduli as shown in Figure 4. The figure shows that methylol-

functional benzoxazine monomers react faster than the control(P-a) because of the lack of initiating species other than theimpurity level phenolic structures. The concentration of thephenolic structures builds gradually and autocatalytic processstarts, accelerating the rate of polymerization. In the polymer-ization of P-a, G′ did not increase for 45 min, and thenincreased suddenly. However, in the case of methylolmonomers, the onset of polymerization occurred at muchshorter times: oHBA-a exhibits the quickest onset ofapproximately 6 min, followed by mHBA-a at 20 min andpHBA-a at 32 min. This is because of the catalytic effect of themethylol groups, in addition to the aforementioned autocata-lytic effect of the phenolic structure produced by the ring-opening reaction of the oxazine rings.The DSC technique is often used to study the polymer-

ization behavior of benzoxazine monomers through ring-opening polymerization. The observed values of the curingpeak of the DSC thermogram indicate the local reactivity of themonomers toward ring-opening. The chemorheological resultsare interpreted with emphasis on the onset of the increase instorage modulus which also represents the reactivity of themonomer. Therefore, the DSC peaks and G′ results areconsistent in their trend: (ortho, meta, para). However, the G′and G″ crossover represents not the reactivity but theformation of the global infinite network. Hence the networkformation does not have to follow the trend observed inexothermic peak. In other words, the local reactions detected byDSC do not necessarily have to agree with the global networkformation. This is the case if the local structure tends to form

closed molecular structure in comparison to open structure thattends to extend.Since both the storage and loss moduli eventually attain

maximum values and remain at the plateaus, the followingrheological fractional conversion is defined assuming reaction iscomplete:23

αα

= ′′

G tG

( )( ) (1)

where G′(t) is the storage modulus at time t and G′(α) is themaximum modulus at the end of polymerization. The resultanttime conversion plots are shown in Figure 5. The fractionalconversion increases rapidly immediately after the reaction isprompted, due to the double catalytic effects of methylol andphenol as stated above.

Figure 6 shows a number of highlighted infrared bands,which can be used to verify the ring-opening reaction and thesubsequent polymerization of oxazine rings in each monomer.For example, the monomers, pHBA-a, mHBA-a, oHBA-a, andP-a show a decrease in the characteristic absorption bands ofthe benzene ring to which oxazine is attached that are located at947, 950, 945, and 946 cm−1, respectively. In addition, theabsorption bands located in the range of 1242−1226 cm−1 dueto the symmetric stretching of C−O−C are decreasedconfirming the ring-opening polymerization. Furthermore,methylol monomers show a decrease in the peaks characteristicof asymmetric trisubstituted benzene between 1505 and 1500cm−1 while the appearance of new peaks around 1490 cm−1 dueto the formation of tetra-substituted benzene confirms thepolymerization of methylol benzoxazine monomers.Both DSC and rheological analysis show that the methylol

group accelerates the autocatalytic ring-opening polymerizationof benzoxazine. The remarkable difference in reactivity ofmethylol monomers over the unfunctionalized benzoxazinemonomer suggests a mechanism where the methylol group hasan important role to promote the ring-opening polymerization.In Scheme 3, only the o-methylol mechanisms for the cross-linking reaction are shown as an example. In this proposal, the

Figure 4. Storage (G′) and loss (G″) modulus as a function of time at140 °C for oHBA-a (□, ○),mHBA-a (◊, ◁), pHBA-a (△, ▽), and P-a (▷, ○) monomers.

Figure 5. Isothermal fractional conversion as a function of time formethylol monomers.

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monomer reaches equilibrium with the corresponding zwitter-ionic intermediate (A) which has a phenoxide and an iminium

moiety.4b,9a,14−16 Another type of zwitterionic intermediate (B)is a tautomer of the intermediate (A) which is considered as acarbocationic. Since the methylol monomers have a higherpossibility for hydrogen bonding, and the intermediate (A + B)would be unstable because of the high concentration of OHions. Therefore, by the ionization of phenol and as nucleophilicphenolic nuclei are strengthened, the methylene bridgeformation is favored by means of the formation of phenoxideions. The zwitterionic intermediate (A and B) then proceedthrough different pathways where water is a byproduct of themethylol condensation toward the formation of methylenelinkages. In addition, the resulting structure has an iminium andphenoxide moieties, of which reactions can contribute toincrease number of cross-linking points and eventually anetwork structure of polybenzoxazine. The proposed mecha-nims in the current study is fundamentally different from themechanisms proposed by Kudoh et al. for hydroxyethylfunctional benzoxazine since methylol group is incapable offorming the intermediate structure proposed by thoseauthors.15

Moreover, the DSC thermograms show that the ring-openingpolymerization is shifted to the lower curing temperature as themethylol group is closer to the oxygen atom in the cyclicbenzoxazine. The results support the proposed mechanismwhere, in the case of o-methylol, the resonance of thebenzoxazine ring is affected by the methylol to form thehydrogen bonding. This ring activates the oxazine ring to openat lower temperature. However, the mechanisms propose that

Figure 6. FT-IR spectra of unpolymerized and 140 °C polymerizedmonomers.

Scheme 3. Proposed Polymerization Mechanism of Methylol Benzoxazine Monomers

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benzoxazine ring is less affected by methylol in the case of meta-and para-monomers as supported by the DSC thermograms.

■ CONCLUSIONS

A series of methylol functional benzoxazine monomers withdifferent hydroxybenzyl alcohol isomers were successfullysynthesized. The DSC results show that exothermic peaksdue to condensation reaction of methylol groups and ring-opening polymerization of benzoxazine are 231, 214, and 196°C for monomers that methylol group placed on para-, meta-,and ortho-position, respectively. However, the exothermic peakof unfunctionalized monomer shows a higher value of 255 °C.The rheological study indicates that the onset of polymerizationoccurs at much shorter times of 6, 20, and 32 min for oHBA-a,mHBA-a, and pHBA-a compared to the unfunctionalizedmonomer that takes 45 min. The highest reactivity of themethylol monomers is attributed to the catalytic effect of themethylol group on the ring-opening due to intramolecularhydrogen bonding between the methylol and the oxygen in thebenzoxazine ring as proposed in the polymerization mecha-nism.

■ EXPERIMENTAL SECTIONMaterials. 2-Hydroxybenzyl alcohol (oHBA) (99%), 3-hydrox-

ybenzyl alcohol (mHBA) (99%), 4-hydroxybenzyl alcohol (pHBA)(98%), phenol (98%), and aniline (99%) were obtained from Sigma−Aldrich and used as-received. Paraformaldehyde (96%) was obtainedfrom Acros Organics, USA. Toluene, ethyl acetate, chloroform,hexanes (a mixture of isomers), and 1,4 dioxane were obtained fromFisher and used as received.Preparation of 3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-

oxazine [abbreviated as P-a]. P-a was prepared from phenol,aniline and paraformaldehyde following the reported method.8 IRspectra (KBr, cm−1): 1230 (asymmetric stretching of Ar−O−C), 1035(symmetric stretching of C−O−C), 946 (out-of-plane vibration,benzene ring to which oxazine is attached).

1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.74 (s, C−CH2−N), 5.48 (s, N−CH2−O−). 6.99−7.44 (m, Ar).Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-

oxazin-8-yl)methanol [abbreviated as oHBA-a]. Into a 100 mLbottom-rounded flask were mixed oHBA (3.73 g, 30 mmol), aniline(2.8 g, 30 mmol), and paraformaldehyde (1.9 g, 60 mmol) and themixture refluxed in 1,4-dioxane (35 mL) for 4 days. The product wasfiltered and then concentrated using a rotary evaporator. The productwas then redissolved in ethyl acetate and base-washed followed byonce with water. The organic layer was then dried over sodium sulfateanhydrous, followed by vacuum evaporation to afford viscous oilyproduct (yield: 3.71 g, 51%).IR spectra (KBr, cm−1): 3320 (stretching of OH of methylol), 1505

(stretching of trisubstituted benzene ring), 1226 (asymmetricstretching of Ar−O−C), 1032 (symmetric stretching of C−O−C),945 (out-of-plane vibration, benzene ring to which oxazine isattached).

1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.64 (s,−CH2−OH), 4.66 (s, C−CH2−N), 5.41 (s, N−CH2−O−), 6.90−7.29 (m, Ar).Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-

oxazin-7-yl)methanol [abbreviated as mHBA-a]. In a 100 mLbottom-rounded flask, mHBA (7.52 g, 60 mmol), aniline (5.64 g, 60mmol), and paraformaldehyde (3.61 g, 120 mmol) were mixedtogether and refluxed for 48 h in 1,4 dioxane (65 mL). The productwas concentrated using a vacuum evaporator and redissolved in ethylacetate followed by base-washed then once with water. The productwas then dried over sodium sulfate anhydrous, followed by vacuumevaporation to afford a viscous product (yield: 9.12 g, 63%).IR spectra (KBr, cm−1): 3325 (stretching of OH of methylol), 1500

(stretching of trisubstituted benzene ring), 1242 (asymmetric

stretching of Ar−O−C), 1033 (symmetric stretching of C−O−C),950 (out-of-plane vibration, benzene ring to which oxazine isattached).

1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.56 (s,−CH2−O), 4.64 (s, C−CH2−N), 5.37 (s, N−CH2−O−), 6.79−7.30(m, Ar).

Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-oxazin-6-yl)methanol [abbreviated as pHBA-a]. In a 500 mLbottom-rounded flask, pHBA (14.95 g, 120 mmol), aniline (11.18 g,120 mmol), and paraformaldehyde (7.52 g, 250 mmol) were mixedtogether and refluxed for 6 h in toluene (135 mL). The product wasconcentrated using a vacuum evaporator and redissolved in chloroformfollowed by base-washed then once with water. The product was thendried over anhydrous sodium sulfate and recrystallized fromchloroform to yield a white product (yield: 21.15 g, 73%).

IR spectra (KBr, cm−1): 3340 (stretching of OH of methylol), 1500(stretching of trisubstituted benzene ring), 1228 (asymmetricstretching of Ar−O−C), 1032 (asymmetric stretching of C−O−C),947 (out-of-plane vibration, benzene ring to which oxazine isattached).

1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.55 (s,−CH2−OH), 4.62 (s, C−CH2−N), 5.36 (s, N−CH2−O−), 6.80−7.28 (m, Ar).

Characterization and Measurements. Proton nuclear magneticresonance (1H NMR) spectra were acquired on a Varian OxfordAS300 at a proton frequency of 300 MHz using an average number oftransients of 64. A relaxation time of 10 s was used for the integratedintensity determination of 1H NMR spectra. Deuterated chloroform(CDCl3) was used to obtain the spectra with tetramethylsilane (TMS)as an internal standard. Fourier transform infrared (FT-IR) spectrawere acquired on a Bomem Michelson MB100 which was equippedwith a deuterated triglycine sulfate (DTGS) detector at a resolution of4 cm−1 with 32 coadditions. The spectra were taken by casting a thinfilm onto a KBr plate. Thermal analysis of the samples was performedvia differential scanning calorimetry (DSC) using TA InstrumentsDSC model 2920 with temperature ramped at 10 °C/min and anitrogen flow rate of 65 mL/min. All samples were crimped inhermetic aluminum pans with lids. The evolution and comparison ofrheological properties during polymerization for each of the differentbenzoxazine monomers were performed utilizing an Anton PaarRheometer (Model Physica MCR 501) with disposable parallel upperand lower plates, measuring 25 mm and 50 mm in diameter,respectively. Small amplitude oscillatory shear time sweep experimentsover temperatures at 140 °C were performed using a constantfrequency of 10 rad/s for all experiments, but continuously increasingstress between a minimum of 1 Pa and a maximum of 400 Pa.Although the actual stress ramp varied per experiment, it was alwaysset to a range of values that kept the fluids’ response in the linearviscoelastic regime, while providing a high enough strain to providereproducible results.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: sxq@case.edu.

NotesThe authors declare no competing financial interest.§ On leave from Azzaytuna University, Libya.⊥ On leave from Tanta University, Tanta, Egypt.

■ ACKNOWLEDGMENTSM. Baqar acknowledges the Ministry of Higher Education andScientific Research of Libya and Azzaytuna University-Libya fora scholarship.

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