Origin of the Rapid Trimerization of Cyanate Ester in a Benzoxazine

Apr 15, 2015 - blend after 24 h at 30 °C, in which P-oda is a 4,4′-oxyaniline/ phenol-based benzoxazine and BACY is a dicyanate ester of bisphenol ...
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Origin of the Rapid Trimerization of Cyanate Ester in a Benzoxazine/ Cyanate Ester Blend Meng Wei Wang,† Ru Jong Jeng,*,† and Ching Hsuan Lin*,‡ †

Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan

‡

S Supporting Information *

ABSTRACT: Blends of cyanate ester and benzoxazine have been independently studied by several researchers, and different reaction mechanisms were reported. Recently, we unexpectedly observe that gelation occurred in a 50 wt % methyl ethyl ketone solution of P-oda/BACY (1/1 mol/mol) blend after 24 h at 30 °C, in which P-oda is a 4,4′-oxyaniline/ phenol-based benzoxazine and BACY is a dicyanate ester of bisphenol A. Previous studies suggest that the rapid trimerization of cyanate ester in the blend is related to the ring-opened structure of benzoxazine. However, the possibility of ring-opening polymerization for benzoxazine at 30 °C is rare. Therefore, it is highly likely that the catalytic effect results from the benzoxazine itself, not from the ring-opened structure of benzoxazine. Through IR and DSC analyses, we conclude that the tertiary amine of benzoxazine catalyzes the trimerization of cyanate ester, and we propose a three-step catalytic mechanism of benzoxazine for the trimerization of cyanate ester.

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INTRODUCTION Benzoxazines are generally synthesized from the Mannich condensation of phenolic compound/primary amine/formaldehyde, and can be polymerized to thermosets by thermally activated ring-opening polymerization.1−7 With the strong intra- and intermolecular hydrogen bonding interactions, these thermosets exhibit many advantages, such as low surface energy,3,8−10 and superior thermal11,12 and electrical properties.13 On the other hand, cyanate esters are known for their high glass transition temperature, low dielectric properties, and high thermal stability after being fully cured to form a triazine network (or polycyanurate).14−22 Blends of cyanate ester and benzoxazine have been independently studied by Nair et al.,23 Kimura et al.,24 Gu et al.,25 and Lin et al.26 Different reaction mechanisms have been reported, but they all found that the blends are miscible, and the trimerization of cyanate ester is accelerated in the presence of benzoxazine. Hamerton et al. has reported that the trimerization of cyanate ester can be catalyzed by the phenol group.27 Therefore, several researchers indicated that the phenol resulting from the ring-opening polymerization of benzoxazine catalyzes the trimerization.23,24 More recently, Gu et al. prepared a phenol/aniline-based benzoxazine (PA) and 2,4-dimethylphenol/2,4,6-trimethyl aniline-based benzoxazine (s-PA).28 Free ortho positions to the O of oxazine are available for PA, but the ortho and para positions to O or N of oxazine of s-PA are blocked by methyl groups (see Figure 4 of the above study28). Therefore, a phenolic OH structure was formed for ring-opened PA, yet only the phenolic arylether structure was formed for the ring-opened s-PA. Given the fact that both P-a and s-PA catalyzed the trimerization of cyanate ester in the benzoxazine/cyanate ester blend, it is concluded that the fundamental catalyst for the trimerization of cyanate © 2015 American Chemical Society

ester is not the phenolic hydroxyl but the oxygen anion, which is the same intermediate in the ring-opening processes of both benzoxazines. Recently, we unexpectedly observe that gelation occurred in a 50 wt % methyl ethyl ketone solution of P-oda/BACY (1/1 mol/mol) blend after 24 h at 30 °C, in which P-oda is a 4,4′oxyaniline/phenol-based benzoxazine and BACY is a dicyanate ester of bisphenol A (Scheme 1). In addition, gelation also occurred in a 50 wt % methyl ethyl ketone solution of PBz-6M/ BACY (1/1 mol/mol) blend after 24 h 30 °C, in which PBz-6 M is an ortho-dimethyl bisphenol A/ortho-tetramethyldiaminodiphenylmethane-based polybenzoxazine precursor (Scheme Scheme 1. Structures of P-oda, BACY, and PBz-6M

Received: February 16, 2015 Revised: April 2, 2015 Published: April 15, 2015 2417

DOI: 10.1021/acs.macromol.5b00334 Macromolecules 2015, 48, 2417−2421

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Macromolecules

Figure 1. Pictures of a 50 wt % MEK solution of P-oda/BACY (1/1, mol/mol): (a) freshly prepared solution, and (b−d) the thermally treated solutions, (b) 30 °C for 24 h, (c) 50 °C for 4 h, and (d) 100 °C for 2 h.

1).29 The possibility of ring-opening polymerization for benzoxazine at 30 °C is rare. Therefore, it is highly likely that the catalytic effect results from the benzoxazine itself, not from the ring-opened structure of benzoxazine. From IR and DSC analyses, we conclude that the tertiary amine of benzoxazine catalyzes the trimerization of cyanate ester. Detailed analyses and a proposed mechanism are reported in this work.

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EXPERIMENTAL SECTION

Materials. 4,4′-Oxydianiline/phenol-based benzoxazine (P-oda) with a melting point at 128 °C was prepared, according to a previously reported procedure.26 The shape melting and correct 1H NMR spectrum support the purity of P-oda. The diicyanate ester of bisphenol A (BACY) was purchased from TCI. N,N-Dimethylaniline and anisole were purchased from Acros. The o-dimethylbisphenol A/otetramethyldiaminodiphenylmethane-based polybenzoxazine precursor (PBz-6M) was prepared in our previous work.6 All solvents were HPLC grade and were purchased from TEDIA. Characterization. Differential scanning calorimetry (DSC) scans were obtained using a PerkinElmer DSC 7 in a nitrogen atmosphere at a heating rate of 10 °C/min. IR spectra were obtained from at least 32 scans in the standard wavenumber range of 650−4000 cm−1 using a PerkinElmer RX1 infrared spectrophotometer. For IR measurement, a few drops of MEK solution of benzoxazine/BACY were dropped onto a KBr pellet, and dried at various temperatures for various periods of time in a vacuum oven.

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Figure 2. FTIR spectra of (a) BACY (b) P-oda, and P-oda/BACY blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C for 4 h, and (e) 100 °C for 2 h.

solution is homogeneous, whereas the solution turns into gel after thermal treatment. Figure 4 shows the IR spectra of the (a) BACY, PBz-6M, and PBz-6M/BACY blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C for 4 h, and (e) 100 °C for 2 h. The cyanate ester absorption peaks at 2273 and 2237 cm−1 disappeared after 4 h at 50 °C (Figure 4d). The triazine absorptions at 1573 and 1366 cm−1 emerged gradually as thermal treatment proceeded. This also suggests the rapid trimerization of cyanate ester. According to the IR spectrum of MEK (Figure S1), there are strong absorption at 1714 cm−1 and medium absorption at 1364 cm−1. Therefore, the absorption at around 1710 cm−1 in Figure 4 comes from the residual MEK in the gel. The higher molecular weight of PBz-6 M (a main-chain type polybenzoxazine precursor) leads to chain entanglement and hinders the evaporation of MEK in the gel, so the amount of residual MEK is more obvious in PBz-6M/BACY system (Figure 4) than that in P-oda/BACY system (Figure 2). Therefore, some absorption intensity at 1366 cm−1 in Figure 4 comes from the absorption of MEK. However, no absorption at around 1500−1600 cm−1 was observed for MEK, so the absorption intensity of 1573 cm−1 was not influenced by the residual MEK. This explains the reason why the intensity increase of the 1573 and 1366 cm−1 bands do not coincide in Figure 4. It is highly unlikely that ring-opening polymerization for Poda would occur at 30 °C. In addition, the ortho positions to O or N of oxazine of PBz-6 M are all blocked by the methyl group. As a result, the ring-opening polymerization for PBz-6 M is very difficult to proceed at 30 °C. This can be supported by the intact signal of the out-of-plane mode of the benzene to

RESULTS AND DISCUSSION

Figure 1 shows the pictures of a 50 wt % methyl ethyl ketone solution of P-oda/BACY before and after thermal treatment at 30−100 °C. The freshly prepared solution is homogeneous, while the solution turns into gel after thermal treatment. The gel is also insoluble in tetrahydrofuran and dimethyl sulfoxide, suggesting that a cross-linking structure is present. Figure 2 shows IR spectra of (a) BACY, (b) P-oda, and Poda/BACY blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C for 4 h, and (e) 100 °C for 2 h. The cyanate ester absorption peaks at 2273 and 2237 cm−1 disappeared after 24 h at 30 °C (Figure 2c). The triazine absorptions at 1576 and 1368 cm−1 appeared gradually with increasing annealing temperature (Figure 2c−e). The results indicate the rapid trimerization of cyanate ester in the blend. To further investigate the origin of rapid trimerization of cyante ester, a 50 wt % methyl ethyl ketone solution of PBz6M/BACY (1/1 mol/mol) was prepared, in which PBz-6 M is an o-dimethyl bisphenol A/o-tetramethyldiaminodiphenylmethane-based polybenzoxazine precursor (Scheme 1).29 Figure 3 shows pictures of a 50 wt % solution of PBz-6M/BACY before and after thermal treatments at 30−100 °C. The results are the same as those in Figure 1. Likewise, the freshly prepared 2418

DOI: 10.1021/acs.macromol.5b00334 Macromolecules 2015, 48, 2417−2421

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Figure 3. Pictures of a 50 wt % MEK solution of PBz-6M/BACY (1/1, mol/mol): (a) freshly prepared solution and (b−d) the thermally treated solutions, (b) 30 °C for 24 h, (c) 50 °C for 4 h, and (d) 100 °C for 2 h.

Figure 4. FTIR spectra of (a) BACY, (b) PBz-6M, and PBz-6M/ BACY blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C for 4 h, and (e) 100 °C for 2 h.

which oxazine is attached at 943 and 936 cm−1 for P-oda and PBz-6M. Therefore, the information from IR analysis in Figures 2 and 4 suggests that the catalytic effect results from the benzoxazine itself, not from the ring-opened structure of benzoxazine. Furthermore, Figures S2 and S3 show pictures of a 50 wt % MEK solution of phenol/BACY and bisphenol A/ BACY (2/1 and 1/1 mol/mol), respectively, before and after thermal treatments. Homogeneous solutions were observed for all the phenol/BACY system after thermal treatment. This results exclude the catalytic effect from phenol structure. We divided the benzoxazine structure of P-oda into two parts to discuss the catalytic mechanism. The first one is an N,Ndimethylaniline-containing section (shown as red color in Scheme 1), whereas the second one is the anisole-containing section (shown as blue color in Scheme 1). Figure S4 shows pictures of a 50 wt % solution of (a) anisole/BACY and (b) N,N-dimethylaniline/BACY (2/1, mol/mol) before and after thermal treatments. A homogeneous solution was observed for the anisole/BACY system after thermal treatment, while gelation occurred for the N,N-dimethylaniline/BACY system after thermal treatment at 80 °C for 48 h. Figure 5 shows IR spectra of (a) anisole/BACY and (b) N,N-dimethylaniline/ BACY blend after thermal treatment at 80 °C for various periods of time. No obvious change in the cyanate ester absorption peaks at 2273 and 2237 cm−1 was found after 60 h of thermal treatment at 80 °C for the anisole/BACY blend

Figure 5. FTIR spectra of (a) anisole/BACY and (b) N,Ndimethylaniline/BACY blend after thermal treatment at 80 °C for various periods of time.

(Figure 5a). However, the cyanate ester absorptions decreased gradually when the thermal treatment started and disappeared after 60 h of thermal treatment for the N,N-dimethylaniline/ BACY blend (Figure 5b). In addition, the triazine absorptions at 1563 and 1364 cm−1 appeared gradually. These results suggest that the tertiary amine linkage, not the ether linkage, catalyzes the trimerization of cyanate ester. The catalytic effect of N,N-dimethylaniline on the trimerization of cyanate ester can also be confirmed by DSC thermograms. Figure 6 shows the DSC heating scans of an N,N-dimethylaniline/BACY blend with molar ratios of 0.2/1, 0.4/1, and 0.6/1. The exothermic peak temperature decreased with increasing amount of N,Ndimethylaniline, while the exothermic enthalpy increased with 2419

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Figure 6. DSC thermogram of (a) BACY, and N,N-dimethylaniline/BACY blend with molar ratios of (b) 0.2/1, (c) 0.4/1, and (d) 0.6/1.

Scheme 2. Proposed Catalytic Mechanism of Benzoxazine to the Trimerization of Cyanate Ester

2. Indeed, the phenolic OH23,24 and the oxygen anion28 also catalyze the trimerization of cyanate ester, but the catalysis occurs after the ring-opening polymerization of benzoxazine.

increasing amount of N,N-dimethylaniline, further pointing to the catalytic effect of N,N-dimethylaniline on the trimerization of cyanate ester. More recently, Endo et al. reported that triphenylphosphine reacts with cyanate ester, and forms phosphonium salt according to 1H NMR analysis.30 The reaction occurs through the nucleophilic addition of unpaired electrons of triphenylphosphine on the electron-deficient cyanate ester (see Scheme 3 of the above study30). Based on Endo’s result along with the IR and DSC analyses mentioned above, we propose a threestep catalytic mechanism of benzoxazine for the trimerization of cyanate ester in Scheme 2. First, a nitrogen anion is formed as the unpaired electron of nitrogen attacks the electron-deficient carbon of cyanate ester. Second, the trimerization reaction occurs through the attack of the nitrogen anion on the other two cyanate esters. Third, triazine structure is formed as the benzoxazine is released.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4 showing the IR spectrum of MEKand pictures of a 50 wt % MEK solution with various compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(R.J.J.) E-mail: [email protected]. *(C.H.L.) E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Gelation occurred in a 50 wt % methyl ethyl ketone solution of P-oda/BACY and PBz-6M/BACY blend after 24 h at 30 °C. IR spectra revealed the rapid trimerization of cyanate ester in both blends at 30 °C. Since the possibility of ring-opening polymerization for benzoxazine at 30 °C is rare, we think that the catalytic effect results from the benzoxazine itself, not from the ring-opened structure of benzoxazine. Through IR and DSC analyses, we conclude that the tertiary amine of benzoxazine catalyzes the trimerization of cyanate ester. Based on Endo’s result30 along with the IR and DSC analyses mentioned above, we propose a three-step catalytic mechanism of benzoxazine for the trimerization of cyanate ester in Scheme

ACKNOWLEDGMENTS Financial support of this work from the Ministry of Science and Technology (MOST 103-2622-E-007-025), Taiwan is highly appreciated.

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REFERENCES

(1) Ghosh, N. N.; Kiskan, B.; Yagci, Y. Prog. Polym. Sci. 2007, 32 (11), 1344−1391. (2) Ning, X.; Ishida, H. J. Polym. Sci., Part A: Polym. Chem. 1994, 32 (6), 1121−1129. (3) Ning, X.; Ishida, H. J. Polym. Sci., Part B: Polym. Phys. 1994, 32 (5), 921−927. 2420

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Macromolecules (4) Kim, S.-K.; Choi, S.-W.; Jeon, W. S.; Park, J. O.; Ko, T.; Chang, H.; Lee, J.-C. Macromolecules 2012, 45 (3), 1438−1446. (5) Liu, C.; Shen, D.; Sebastián, R. M. a.; Marquet, J.; Schönfeld, R. Macromolecules 2011, 44 (12), 4616−4622. (6) Wang, M. W.; Jeng, R. J.; Lin, C. H. Macromolecules 2015, 48 (3), 530−535. (7) Taskin, O. S.; Kiskan, B.; Yagci, Y. Macromolecules 2013, 46 (22), 8773−8778. (8) Wang, C.-F.; Su, Y.-C.; Kuo, S.-W.; Huang, C.-F.; Sheen, Y.-C.; Chang, F.-C. Angew. Chem., Int. Ed. 2006, 45 (14), 2248−2251. (9) Lin, C. H.; Shih, Y. S.; Wang, M. W.; Tseng, C. Y.; Juang, T. Y.; Wang, C. F. RSC Adv. 2014, 4 (17), 8692−8698. (10) Mohamed, M. G.; Hsiao, C.-H.; Hsu, K.-C.; Lu, F.-H.; Shih, H.K.; Kuo, S.-W. RSC Adv. 2015, 5 (17), 12763−12772. (11) Chernykh, A.; Agag, T.; Ishida, H. Macromolecules 2009, 42 (14), 5121−5127. (12) Dogan Demir, K.; Kiskan, B.; Yagci, Y. Macromolecules 2011, 44 (7), 1801−1807. (13) Zhang, K.; Zhuang, Q.; Liu, X.; Cai, R.; Yang, G.; Han, Z. RSC Adv. 2013, 3 (15), 5261−5270. (14) Fang, T.; Shimp, D. A. Prog. Polym. Sci. 1995, 20 (1), 61−118. (15) Hamerton, I.; Hay, J. N. Polym. Int. 1998, 47 (4), 465−473. (16) Chaplin, A.; Hamerton, I.; Howlin, B. J.; Barton, J. M. Macromolecules 1994, 27 (18), 4927−4935. (17) Guenthner, A. J.; Yandek, G. R.; Wright, M. E.; Petteys, B. J.; Quintana, R.; Connor, D.; Gilardi, R. D.; Marchant, D. Macromolecules 2006, 39 (18), 6046−6053. (18) Maya, E. M.; Snow, A. W.; Buckley, L. J. Macromolecules 2001, 35 (2), 460−466. (19) Snow, A. W.; Buckley, L. J. Macromolecules 1997, 30 (3), 394− 405. (20) Lin, C. H.; Hsiao, C. N.; Li, C. H.; Wang, C. S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (16), 3986−3995. (21) Ryu, B.-Y.; Emrick, T. Macromolecules 2011, 44 (14), 5693− 5700. (22) Cash, J. J.; Davis, M. C.; Ford, M. D.; Groshens, T. J.; Guenthner, A. J.; Harvey, B. G.; Lamison, K. R.; Mabry, J. M.; Meylemans, H. A.; Reams, J. T.; Sahagun, C. M. Polym. Chem. 2013, 4 (13), 3859−3865. (23) Sudo, A.; Kudoh, R.; Nakayama, H.; Arima, K.; Endo, T. Macromolecules 2008, 41 (23), 9030−9034. (24) Kimura, H.; Ohtsuka, K.; Matsumoto, A. Expr. Polym. Lett. 2011, 5 (12), 1113−1122. (25) Li, X.; Gu, Y. Polym. Chem. 2011, 2 (12), 2778−2781. (26) Lin, C. H.; Huang, S. J.; Wang, P. J.; Lin, H. T.; Dai, S. A. Macromolecules 2012, 45 (18), 7461−7466. (27) Hamerton, I. Chemistry and Technology of Cyanate Ester Resins. Springer: Berlin, 1994. (28) Li, X.; Luo, X.; Liu, M.; Ran, Q.; Gu, Y. Mater. Chem. Phys. 2014, 148 (1−2), 328−334. (29) Chernykh, A.; Liu, J.; Ishida, H. Polymer 2006, 47 (22), 7664− 7669. (30) Tsuchiya, K.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2014, 52 (5), 699−706.

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DOI: 10.1021/acs.macromol.5b00334 Macromolecules 2015, 48, 2417−2421