Benzocyclobutene in polymer synthesis. III. Heat-resistant thermosets based on Diels–Alder polymerization of a bisbenzocyclobutene and a bismaleimide

Several compatible mixtures of 2,2-bis[4-(N-4-benzocyclobutenyl) phthalimid-4-phenyl]hexafluoropropane (BCB) and 1,1′-(methylene di-4,1-phenylene)bismaleimide (BMI) were prepared according to the molar ratios (BCB : BMI): 1 : 1; 1 : 1.5; 1 : 3; 1.5 : 1. Complete compatibility of the mixtures was evidenced by a single initial T_g. All mixtures showed relatively low initial T_g's (61–70°C) and characteristic polymerization exotherms of benzocyclobutene-based systems (onset: 221–225°C; maximum: 257–259°C), providing an excellent processing window (ca. 155°C). The cured sample of the mixtures, pure BCB and BMI (250°C; N₂; 8 h) were subjected to comparative isothermal gravimetric analysis (ITGA). After 200 h at 650°F (343°C) in circulating air, the cured BMI sample retained only 3% of its original weight, whereas the mixtures of BCB and BMI exhibited thermo-oxidative stabilities similar to BCB (13–15% weight loss). A model compound was synthesized from the intimate mixture of N-phenylmaleimide and N-benzocyclobutenyl phthalimide in 63% yield. The ITGA results and isolation of the model Diels–Alder adduct render strong support to the conviction that Diels–Alder polymerization is indeed the predominant curing process in the BCB/BMI system.