Maleimide homopolymerization can be initiated by the maleimide monomer without the need for added initiator. This self-initiation is believed to occur through a donor-acceptor complex formed between maleimide groups. An in-situ electron spin resonance (e.s.r.) spectroscopic technique was used to study the free-radical homopolymerization of 1,1＇-(methylenedi-4,1-phenylene)bismaleimide (MDP-BMI) over a range of temperatures. Total radical concentrations were measured as a function of time at polymerization temperatures from 180 to 230 degrees C. By analysis of the e.s.r. spectra from different maleimide monomers, MDP-BMI, N-phenylmaleimide (NPM) and 1,1＇-[methylenebis-(2,6-diethyl-4,1-phenylene)] bismaleimide (MDEP-BMI), with and without free-radical initiators, it was possible to identify the e.s.r. signals due to various radical species. Unreactive diphenylmethylene radicals were observed on the polymer chains formed from MDP-BMI. Additional spectral features are present that include the signal due to the propagating radical. Maleimides containing free-radical initiator also exhibited a broad doublet signal assigned to a vinyl radical formed by hydrogen abstraction from the maleimide double bond. From comparison of the spectra of NPM and MDEP-BMI monomers, both with and without free-radical initiators, it was shown that the maleimide propagating radical gives an unusually narrow, singlet e.s.r, peak quite different from the four-line signal previously reported.