THE CROSS-LINKING OF EUTECTIC MIXTURES OF BISMALEIMIDES

The high reactivity of the maleimide groups in bismaleimide resins is attributable to the electron withdrawing effect of the carbonyl groups causing the carbon carbon double bonds to become electron deficient. Consequently, bismaleimides homopolymerize readily to form network polymers without the aid of catalyst at temperatures between 150 and 300-degrees. This paper is concerned with the effect on the reaction rate of the bridging group between the maleimide end groups. Observations of the crosslinking of a eutectic mixture of three oligomers containing different bridging groups suggest that the reactivity is similar for all three. Nevertheless when one of the three oligomers was crosslinked by itself, it reacted much more rapidly than the mixture. These observations are believed to reflect the steric hindrance effect of one or both of the other bismaleimide oligomers on the rate of reaction of the eutectic. Similar observations were found with rubber-toughened bismaleimide systems. The probability of steric hindrance from the aliphatic and the aromatic oligomers is discussed.