Mechanistic Pathways for the Polymerization of Methylol-Functional

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Mechanistic Pathways for the Polymerization of Methylol-Functional Benzoxazine Monomers Mohamed Baqar,†,§ Tarek Agag,‡,⊥ Rongzhi Huang,‡ Joaõ 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 methylolfunctional benzoxazine monomers is reported using a series of monofunctional benzoxazine monomers synthesized via a condensation reaction of ortho-, meta-, or para-methylol− phenol, aniline, and paraformaldehyde following the traditional route of benzoxazine synthesis. A phenol/aniline-type monofunctional benzoxazine monomer has been synthesized as a control. The structures of the synthesized monomers have been confirmed by 1H NMR and FT-IR. The polymerization behavior of methylol monomers is studied by DSC and shows an exothermic peak associated with condensation reaction of methylol groups and ring-opening polymerization of benzoxazine at a lower temperature range than the control monomer. The presence of methylol group accelerates the ring-opening polymerization 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 a significant role in accelerating the polymerization.

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INTRODUCTION

more phenolic groups are formed, which promote an autocatalytic polymerization process.6a,b Wang and Ishida7 showed that monomers synthesized using unsubstituted phenol exhibited variations of the ring-opening polymerization temperature. A more detailed study by Andreu et al.,8 who synthesized several substituted 3-phenyl-3,4dihydro-2H-1,3-benzoxazine monomers using substituent phenol and aromatic amine, showed that increasing the electron-withdrawing in the para-position of phenol exhibited a noticeable decrease in the polymerization temperature. This is attributed to the acidic phenol species that are formed due to the electron-withdrawing groups. On the other hand, increasing the electron-withdrawing in the para-position of the phenyl substituent monomers showed an increase of the polymerization temperature due to the destabilization of the propagating iminium intermediates as proposed by McDonagh and Smith.9a,b However, for the electron-donating substituents no notable effect on the polymerization was observed regardless of the position of the substituent. The effect of functionalization and copolymerization of carboxylic group into benzoxazine structure has been studied10a−c and a significant decrease in polymerization temperature was observed. Polybenzoxazine chains preferentially form intramolecular 6-membered hydrogen bond,11a−i which tends to slow the propagation reaction.12a,b This

Benzoxazine resins are a new class of thermosetting polymers that are characterized by several unique properties such as nearzero shrinkage or volumetric expansion upon polymerization, considerable molecule-design flexibility, low water absorption and flammability, low surface energy, and high glass transition temperature.1 Additionally, the polymerization of benzoxazine monomers takes place through thermally accelerated ringopening mechanism without any added initiator or catalyst.2 These numerous unusual properties of benzoxazine resins are associated with the existence of inter- and intramolecular hydrogen bonds in the network structure.3a−d Therefore, in order to understand the structure−property relationships of the polymer, it is of great importance to know the mechanistic pathways of polymerization. Although the polymerization chemistry of benzoxazine monomers still remains rather poorly understood, the high basicity of benzoxazine monomers, which is attributed to both the oxygen and the nitrogen of the oxazine ring by Lewis definition, suggests that the ring-opening polymerization of benzoxazine monomers proceeds through a cationic mechanism.4a−c During the polymerization, the monomer comes to equilibrium with the corresponding zwitterionic intermediate which eventually forms the polymer as depicted in Scheme 1. Several fundamental studies for polymerization mechanisms were conducted to understand the polymerization pathways of benzoxazine monomers.5a−c These investigations suggest that, as the polymerization of benzoxazine monomers progresses, © 2012 American Chemical Society

Received: September 18, 2012 Revised: September 21, 2012 Published: October 1, 2012 8119

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RESULTS AND DISCUSSION Preparation of Monofunctional Benzoxazine Monomers. A successful synthesis of methylol monofunctional benzoxazine monomers has been achieved using aniline, paraformaldehyde, and hydroxybenzyl alcohols (ortho-, metaand para-isomers) as well as phenol (P) for comparison as indicated in Scheme 2. The monomers are designated as

Scheme 1. Mechanism of Ring-Opening Polymerization of Benzoxazine Monomers

Scheme 2. Preparation of Monofunctional Benzoxazine Monomers

hydrogen bonding by COOH group to the newly formed phenolic OH groups competes with the intramolecular 6membered ring formation between the phenolic and amine groups,13 hence the retarding effect on chain propagation by the intramolecular hydrogen bonding. Kiskan et al.14a,b synthesized hydroxyethyl terminated ether chain-functional benzoxazine monomers and reported the reduced polymerization temperature of these monomers in comparison to ordinary unfunctional benzoxazine monomers. Kudoh et al.15 subsequently studied the mechanistic aspects of the polymerization and the role of hydroxylethyl group in activating the ring-opening of hydroxyethyl functional benzoxazine monomers. They found that the hydroxyethyl monomer polymerized much faster than a similar structure of other Nalkylbenzoxazines, which is attributed to the intramolecular reaction of hydroxyl group with cationic moieties of the zwitterionic intermediates formed by the ring-opening reaction of 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 leads to a decrease in the polymerization rate due to the release of a larger amount of N-alkylimine as a volatile compound. Oie et al.17 reported that N-allylbenzoxazine monomers exhibit a faster rate of polymerization than N-(n-propyl)benzoxazines due to the presence of allyl group on nitrogen atom. Consequently, the intramolecular interaction between the cationic moieties and electron-rich carbon−carbon double bond of this group prompted the formation of the zwitterionic intermediates. In continuation of our observed reduction of the ringopening polymerization temperature of methylol benzoxazine monomers compared to the unfunctionalized monomers,18a−c this paper is aimed at studying the role of incorporating methylol groups into benzoxazine structure. Furthermore, the polymerization mechanism of benzoxazine monomers with various isomeric methylol positions is proposed.

(oHBA-a, mHBA-a, and pHBA-a) in which the smaller italic letters represent the position of methylol group into benzoxazine structure, either in ortho, meta or para, respectively. P-a represents a phenol/aniline type monofunctional benzoxazine monomer synthesized as a control with no methylol group in the structure. The structures of the monomers were confirmed using 1H NMR and FT-IR spectra. In the 1H NMR spectra, as depicted in Figure 1, the typical resonances attributed to the benzoxazine structure, 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 NMR spectra 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 oxazine rings in each monomer. For example, the characteristic out-ofplane absorption modes of benzene with an attached oxazine ring are located at 947, 950, 945, and 946 cm−1 for the pHBA-a, mHBA-a, oHBA-a, and P-a, respectively.19 Furthermore, the presence of the benzoxazine ring aromatic ether in the monomers is indicated by an absorbance peak centered in the range of 1035−1032 cm−1 due to the C−O−C symmetric stretching mode. Also, the peak between 1242 and 1226 cm−1 for the asymmetric stretching modes confirms the presence of the oxazine ring in the monomer structure.20a,b Peaks characteristic of asymmetric trisubstituted benzene that appear in the methylol monomers between 1505 and 1500 cm−1 confirm the incorporation of methylol group into benzoxazine monomers. Furthermore, the presence of methylol group in the synthesized monomers is confirmed through the very broad 8120

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Figure 1. 1H NMR spectra of benzoxazine monomers.

Figure 2. FT-IR spectra of benzoxazine monomers.

absorption peak between 3340 and 3320 cm−1 due to the OH stretching mode.21a,b Effect of Methylol Group on the Polymerization Behavior of Benzoxazine Monomers. The polymerization profiles of methylol benzoxazine monomers were studied by DSC as depicted in Figure 3 and the results are summarized in Table 1. The thermograms show that the onset of the ringopening polymerization of unfunctionalized benzoxazine monomer started at 237 °C with its maximum centered at 255 °C. However, the onset of polymerization is shifted in the presence of methylol group to as low as 151, 183, and 205 °C for oHBA-a, mHBA-a, and pHBA-a, respectively. In addition, the peaks are significantly broadened and the maxima are shifted to temperatures of 196, 214, and 231 °C for oHBA-a, mHBA-a, and pHBA-a, respectively. Moreover, the methylolfunctional benzoxazine monomers show lower values for the heat of polymerization compared to the control. For example, the heat of polymerization values were 201, 203, and 241 J/g for oHBA-a, mHBA-a, and pHBA-a, respectively. However, P-a, which has no methylol groups, exhibits the higher exotherm of 336 J/g.

Figure 3. DSC thermograms of benzoxazine monomers.

Table 1. Results of the DSC Analysis of Benzoxazine Monomers monomer

onset temp (°C)

max temp (°C)

heat of polymerization (J/g)

P-a oHBA-a mHBA-a pHBA-a

237 151 183 205

255 196 214 231

336 201 203 241

From the DSC results, the values of the heat of polymerization for the functionalized monomers were lower than the control, showing clearly that another thermal event is involved in the ring-opening reaction of methylol benzoxazines. Since the presence of methylol groups is the main feature of these monomers, one possible thermal event may be attributed to the release of water as a byproduct from the polymerization of methylol groups similar to traditional phenolics.22a,b The 8121

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closed molecular structure in comparison to open structure that tends to extend. Since both the storage and loss moduli eventually attain maximum values and remain at the plateaus, the following rheological fractional conversion is defined assuming reaction is complete:23

occurrence of methylol condensation reaction together with ring-opening polymerization of methylol benzoxazine monomers has been confirmed elsewhere.18a Chemorheological Studies of Monomers. Viscoelastic properties during the polymerization of each monomer were evaluated during isothermal polymerization at 140 °C. A qualitative comparison of the rates of polymerization for the monomers is determined by the increase of the viscoelastic moduli as shown in Figure 4. The figure shows that methylol-

α=

G′(t ) G′(α)

(1)

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

Figure 4. Storage (G′) and loss (G″) modulus as a function of time at 140 °C for oHBA-a (□, ○),mHBA-a (◊, ◁), pHBA-a (△, ▽), and Pa (▷, ○) monomers.

functional benzoxazine monomers react faster than the control (P-a) because of the lack of initiating species other than the impurity level phenolic structures. The concentration of the phenolic structures builds gradually and autocatalytic process starts, accelerating the rate of polymerization. In the polymerization of P-a, G′ did not increase for 45 min, and then increased suddenly. However, in the case of methylol monomers, the onset of polymerization occurred at much shorter times: oHBA-a exhibits the quickest onset of approximately 6 min, followed by mHBA-a at 20 min and pHBA-a at 32 min. This is because of the catalytic effect of the methylol groups, in addition to the aforementioned autocatalytic effect of the phenolic structure produced by the ringopening reaction of the oxazine rings. The DSC technique is often used to study the polymerization behavior of benzoxazine monomers through ringopening polymerization. The observed values of the curing peak of the DSC thermogram indicate the local reactivity of the monomers toward ring-opening. The chemorheological results are interpreted with emphasis on the onset of the increase in storage modulus which also represents the reactivity of the monomer. Therefore, the DSC peaks and G′ results are consistent in their trend: (ortho, meta, para). However, the G′ and G″ crossover represents not the reactivity but the formation of the global infinite network. Hence the network formation does not have to follow the trend observed in exothermic peak. In other words, the local reactions detected by DSC do not necessarily have to agree with the global network formation. This is the case if the local structure tends to form

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

Figure 6 shows a number of highlighted infrared bands, which can be used to verify the ring-opening reaction and the subsequent polymerization of oxazine rings in each monomer. For example, the monomers, pHBA-a, mHBA-a, oHBA-a, and P-a show a decrease in the characteristic absorption bands of the benzene ring to which oxazine is attached that are located at 947, 950, 945, and 946 cm−1, respectively. In addition, the absorption bands located in the range of 1242−1226 cm−1 due to the symmetric stretching of C−O−C are decreased confirming the ring-opening polymerization. Furthermore, methylol monomers show a decrease in the peaks characteristic of asymmetric trisubstituted benzene between 1505 and 1500 cm−1 while the appearance of new peaks around 1490 cm−1 due to the formation of tetra-substituted benzene confirms the polymerization of methylol benzoxazine monomers. Both DSC and rheological analysis show that the methylol group accelerates the autocatalytic ring-opening polymerization of benzoxazine. The remarkable difference in reactivity of methylol monomers over the unfunctionalized benzoxazine monomer suggests a mechanism where the methylol group has an important role to promote the ring-opening polymerization. In Scheme 3, only the o-methylol mechanisms for the crosslinking reaction are shown as an example. In this proposal, the 8122

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moiety.4b,9a,14−16 Another type of zwitterionic intermediate (B) is a tautomer of the intermediate (A) which is considered as a carbocationic. Since the methylol monomers have a higher possibility for hydrogen bonding, and the intermediate (A + B) would be unstable because of the high concentration of OH ions. Therefore, by the ionization of phenol and as nucleophilic phenolic nuclei are strengthened, the methylene bridge formation is favored by means of the formation of phenoxide ions. The zwitterionic intermediate (A and B) then proceed through different pathways where water is a byproduct of the methylol condensation toward the formation of methylene linkages. In addition, the resulting structure has an iminium and phenoxide moieties, of which reactions can contribute to increase number of cross-linking points and eventually a network structure of polybenzoxazine. The proposed mechanims in the current study is fundamentally different from the mechanisms proposed by Kudoh et al. for hydroxyethyl functional benzoxazine since methylol group is incapable of forming the intermediate structure proposed by those authors.15 Moreover, the DSC thermograms show that the ring-opening polymerization is shifted to the lower curing temperature as the methylol group is closer to the oxygen atom in the cyclic benzoxazine. The results support the proposed mechanism where, in the case of o-methylol, the resonance of the benzoxazine ring is affected by the methylol to form the hydrogen bonding. This ring activates the oxazine ring to open at lower temperature. However, the mechanisms propose that

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

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

Scheme 3. Proposed Polymerization Mechanism of Methylol Benzoxazine Monomers

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stretching of Ar−O−C), 1033 (symmetric stretching of C−O−C), 950 (out-of-plane vibration, benzene ring to which oxazine is attached). 1 H 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 mL bottom-rounded flask, pHBA (14.95 g, 120 mmol), aniline (11.18 g, 120 mmol), and paraformaldehyde (7.52 g, 250 mmol) were mixed together and refluxed for 6 h in toluene (135 mL). The product was concentrated using a vacuum evaporator and redissolved in chloroform followed by base-washed then once with water. The product was then dried over anhydrous sodium sulfate and recrystallized from chloroform 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 (asymmetric stretching of Ar−O−C), 1032 (asymmetric stretching of C−O−C), 947 (out-of-plane vibration, benzene ring to which oxazine is attached). 1 H 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 magnetic resonance (1H NMR) spectra were acquired on a Varian Oxford AS300 at a proton frequency of 300 MHz using an average number of transients of 64. A relaxation time of 10 s was used for the integrated intensity 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) spectra were acquired on a Bomem Michelson MB100 which was equipped with a deuterated triglycine sulfate (DTGS) detector at a resolution of 4 cm−1 with 32 coadditions. The spectra were taken by casting a thin film onto a KBr plate. Thermal analysis of the samples was performed via differential scanning calorimetry (DSC) using TA Instruments DSC model 2920 with temperature ramped at 10 °C/min and a nitrogen flow rate of 65 mL/min. All samples were crimped in hermetic aluminum pans with lids. The evolution and comparison of rheological properties during polymerization for each of the different benzoxazine monomers were performed utilizing an Anton Paar Rheometer (Model Physica MCR 501) with disposable parallel upper and lower plates, measuring 25 mm and 50 mm in diameter, respectively. Small amplitude oscillatory shear time sweep experiments over temperatures at 140 °C were performed using a constant frequency of 10 rad/s for all experiments, but continuously increasing stress between a minimum of 1 Pa and a maximum of 400 Pa. Although the actual stress ramp varied per experiment, it was always set to a range of values that kept the fluids’ response in the linear viscoelastic regime, while providing a high enough strain to provide reproducible results.

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

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CONCLUSIONS A series of methylol functional benzoxazine monomers with different hydroxybenzyl alcohol isomers were successfully synthesized. The DSC results show that exothermic peaks due to condensation reaction of methylol groups and ringopening 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 peak of unfunctionalized monomer shows a higher value of 255 °C. The rheological study indicates that the onset of polymerization occurs at much shorter times of 6, 20, and 32 min for oHBA-a, mHBA-a, and pHBA-a compared to the unfunctionalized monomer that takes 45 min. The highest reactivity of the methylol monomers is attributed to the catalytic effect of the methylol group on the ring-opening due to intramolecular hydrogen bonding between the methylol and the oxygen in the benzoxazine ring as proposed in the polymerization mechanism.

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EXPERIMENTAL SECTION

Materials. 2-Hydroxybenzyl alcohol (oHBA) (99%), 3-hydroxybenzyl 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 obtained from Acros Organics, USA. Toluene, ethyl acetate, chloroform, hexanes (a mixture of isomers), and 1,4 dioxane were obtained from Fisher 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 IR spectra (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). 1 H 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 mL bottom-rounded flask were mixed oHBA (3.73 g, 30 mmol), aniline (2.8 g, 30 mmol), and paraformaldehyde (1.9 g, 60 mmol) and the mixture refluxed in 1,4-dioxane (35 mL) for 4 days. The product was filtered and then concentrated using a rotary evaporator. The product was then redissolved in ethyl acetate and base-washed followed by once with water. The organic layer was then dried over sodium sulfate anhydrous, followed by vacuum evaporation to afford viscous oily product (yield: 3.71 g, 51%). IR spectra (KBr, cm−1): 3320 (stretching of OH of methylol), 1505 (stretching of trisubstituted benzene ring), 1226 (asymmetric stretching of Ar−O−C), 1032 (symmetric stretching of C−O−C), 945 (out-of-plane vibration, benzene ring to which oxazine is attached). 1 H 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 mL bottom-rounded flask, mHBA (7.52 g, 60 mmol), aniline (5.64 g, 60 mmol), and paraformaldehyde (3.61 g, 120 mmol) were mixed together and refluxed for 48 h in 1,4 dioxane (65 mL). The product was concentrated using a vacuum evaporator and redissolved in ethyl acetate followed by base-washed then once with water. The product was then dried over sodium sulfate anhydrous, followed by vacuum evaporation 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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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

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ACKNOWLEDGMENTS M. Baqar acknowledges the Ministry of Higher Education and Scientific Research of Libya and Azzaytuna University-Libya for a scholarship.

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REFERENCES

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