CROSS-LINKING REACTIONS IN MALEIMIDE AND BIS(MALEIMIDE) POLYMERS - AN ESR STUDY

The results of electron spin resonance (ESR) measurements on the homopolymerization reactions of two bis(maleimides) and several maleimide model compounds containing different amounts of radical initiators (0-10 wt %) are reported. Two types of radicals were identified: the propagating radical, which is an intermediate in the chain growth polymerization, and species such as the bis(maleimidophenyl) methyl radical and a substituted vinyl radical, both of which are the result of hydrogen atom abstraction reactions. The concentrations of the radicals observed in the hot-melt homopolymerization of one particular bis(maleimide) were determined as a function of cure time from the integrated ESR intensities. The similarity of the ESR results obtained in air-curing and in vacuum-curing suggests that hydroperoxides, which might be formed by reactions with ambient oxygen during the cure process, play no role as radical initiators. On the other hand, differential scanning calorimetry, along with ESR, suggests that trace amounts of impurities act as radical initiators; but, even in the absence of impurities, the bis(maleimides) will still homopolymerize by direct thermal homolysis. There is also evidence for thermal as well as thermooxidative degradation. © 1990, American Chemical Society. All rights reserved.