How do hyperbranched polyimides form in polymerizations of N,N’-bismaleimide-4,4′-diphenylmethane with barbituric acid?

The reactivity of the >CH2 and >NH groups of barbituric acid (BTA) toward the two terminal -C=C- groups of N,N’-bismaleimide-4,4′-diphenylmethane (BMI) was studied with the aid of three model compounds (N-phenylmaleimide (PMI) containg one -C=C- group, 1,3-dimethylbarbituric acid (1,3DMBTA) containing 1 >CH2 group and 5,5-dimethylbarbituric acid (5,5DMBTA) containing 2 >NH groups per molecule) and a molecular probe (hydroquinone (HQ)) that was capable of detecting free radical polymerization. Kinetics studies (DSC) and molecular structure characterization (1H-NMR) showed that it was the >CH2 group of BTA that predominated in the isothermal polymerization of BMI with BTA in N-methyl-2-pyrrolidone in the temperature range 373–403 K. On the contrary, the 2 >NH groups of BTA did not contribute to polymerization of BMI with BTA to an appreciate extent.