The polymerizations of N,N'-bismaleimide-4,4'-diphenylmethane (BMI) initiated by barbituric acid (BTA) carried out in a variety of solvents at 130 degrees C were studied. The nitrogen-containing cyclic solvents such as N-methyl-2-pyrrolidinone acted as a catalyst to promote the formation of the three-dimensional crosslinked network structure. By contrast, the polymerization in a cyclic solvent that did not contain nitrogen such as gamma-butyrolactone resulted in nil gel content. The higher the solvent basicity, the larger the amount of insoluble polymer species formed. The molar ratio of BTA to BMI also played an important role in the polymerizations. The resultant polymers, presumably having a hyper-branched structure, exhibited much narrower molecular weight distributions than those prepared by conventional free radical polymerizations. The BMI polymerizations using BTA as the initiator could not be adequately described by conventional free radical polymerization mechanisms. A polymerization mechanism that took into account the generation of a ketone radical pair between BTA and BMI and the subsequent initiation, propagation and termination reactions was proposed. It was concluded that the nitrogen-containing cyclic solvents were capable of participating in the ketone radical pair formation process, thereby increasing the extent of polymer crosslinking reactions. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 596-603, 2010