Room-temperature free-radical-induced polymerization of 1,1 '-(methylenedi-1,4-phenylene)bismaleimide via a novel diphenylquinoxaline-containing hyperbranched aromatic polyamide

Abstract

Two new diphenylquinoxaline-containing AB2 monomers, 2,3-bis(4-aminophenyl)quinoxaline-6-carboxylic acid, 5 and 2,3-bis[4-(4-aminophenoxy)phenyl]quinoxaline-6-carboxylic acid, 9 were prepared and polymerized via the Yamazaki reaction to form the hyperbranched aromatic polyamides (designated as II and III, respectively) with -NH2 as the reactive chain-end groups. Although these AB2 monomers and their respective hyperbranched polymers are structurally similar except for the presence of a p-phenyloxy spacer between the quinoxaline and p-aminophenyl segments in 9 and III, the physical and chemical properties of both monomers and hyperbranched polymers are distinctly different. It is believed that the tautomerism in 5 and II is likely the basis for these differences. Since III was only marginally soluble in polar aprotic solvents in which II readily dissolved, a known, soluble hyperbranched polyamide (I) was prepared from 3,5-bis(4-aminophenyloxy)benzoic acid for comparison purposes in a subsequent blends study. The curing behaviors and thermal properties of the hyperbranched polyamides I and II blended in 0.75-3.75 wt % with a common bismaleimide [1,1′-(methylenedi-4, 1-phenylene)bismaleimide, BMI] resin were studied with differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy. Whereas the DSC results indicated that I reacted normally with BMI in a Michael-addition fashion, followed by homopolymerization of the excess BMI, II appeared to be able to initiate free-radical polymerization of BMI at room temperature after co-dissolution with BMI in N-methyl-2-pyrrolidinone. The DSC results of the BMI/II blends indicated that, at ≥1.5 wt % of II, no exotherm attributable to the thermal curing of BMI was detected. Electron spin resonance (ESR) experiments confirmed that the parmagnetic species present in II were more reactive toward BMI in solution at room temperature than the radical detected in I. This unique property of II to initiate room-temperature radical polymerization of BMI makes it important as a prototype for the development of low-temperature, thermally curable thermosetting resin systems for high-temperature applications.