Highly Processible Maleimide and Nitrile Functionalized Benzoxazines for Advanced Composites Applications Thanyalak Chaisuwan, Hatsuo Ishida Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106 Received 19 March 2009; accepted 29 June 2009 DOI 10.1002/app.31131 Published online 22 April 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Maleimide and 2-aminobenzonitrile (MIan)- based benzoxazine has been synthesized and character- ized. MIan contains imide, oxazine, and nitrile functional groups that can react almost simultaneously, leading to complicated reaction mechanisms. For understanding the fundamental polymerization mechanism, the model ben- zoxazine compound is synthesized. The ortho-nitrile group in the model compound undergoes cyclization reaction, producing the thermally stable six-membered ring species resulting in the excellent thermal properties of the mate- rial. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2559– 2565, 2010 Key words: polybenzoxazine; polymerization mechanism; maleimide; nitrile; high performance; high char yield INTRODUCTION Recently, the development of the benzoxazine-based family of phenolic resins has received significant attention. Various phenols and primary amines have been used to synthesize benzoxazine resins. For example, polybenzoxazines with mechanical proper- ties comparable to epoxy 1 and polyimide 2 were obtained with the combination of certain phenols and amines. Polybenzoxazines are derived via a thermally activated ring-opening reaction to form a phenolic structure characterized by a Mannich base bridge as shown in Scheme 1, instead of the methyl- ene bridge structure associated with traditional phe- nolic resins. 3 The attractive characteristics of benzox- azine polymers include low-melt viscosity, no release of volatiles during polymeriztion or need of harsh catalysts, high thermal stability, good mechan- ical properties, excellent electrical properties, and rich molecular design flexibility. 3–7 These character- istics enable benzoxazine polymers to be excellent candidates for high-performance composites. By taking advantage of the rich molecular design flexibility of the benzoxazine chemistry, thermal and thermo-oxidative stabilities of polybenzoxazine have been improved by altering the functional group on the amine and/or phenol. Low and Ishida studied the thermal and thermo-oxidative degradation of polybenzoxazines and concluded that there are three stages in the thermal degradation of bisphenol-based polybenzoxazines. Evaporation of amine moiety first occurs below 300  C followed by degradation of the Schiff base between 300  C and 400  C. Finally, above 400  C, the evaporation of phenolic moiety occurs. This observation gives rise to the postulation that if amine evaporation is reduced, the char yield can be greatly increased. 8 Kim et al. synthesized acetylene-functional ben- zoxazines, in which acetylene-functionalized side chain could further crosslink upon thermal activa- tion. 9 In a separate study, phthalonitrile-functional polybenzoxazines, which had low flammability due to their high-char yield, were synthesized. 10,11 Maleimide-based polymers generally have high T g s and are thermally stable. Recently, Ishida and Ohba synthesized monofunctional benzoxazine with maleimide and aniline to develop low-viscosity ben- zoxazine monomers with a glass transition tempera- ture above 200  C. Incorporation of the maleimide functionality into the monofunctional benzoxazine resulted in an increased char yield and glass transi- tion temperature. 12 In our previous work, a monofunctional benzoxa- zine with maleimide and 2-aminobenzonitrile (MIan) has been developed. This newly synthesized poly- benzoxazine exhibits the improvement in thermal and mechanical properties without significantly increasing the viscosity of the monomer. Further- more, by using Lewis acid catalysts, thermal proper- ties have been improved. 13 However, there has been no report related to the polymerization mechanism Correspondence to: H. Ishida (hxi3@cwru.edu). Contract grant sponsors: Air Force Office of Scientific Research, Royal Thai Government. Journal of Applied Polymer Science, Vol. 117, 2559–2565 (2010) V C 2010 Wiley Periodicals, Inc.