The radical copolymerization of N-(2,6-dimethylphenyl)maleimide (DMPhMI) and 2,4,4-trimethylpentene (TP) was investigated in several solvents at 60 degrees C. The copolymerization rate and the molecular weight of the resulting copolymers were dependent on the kind of solvent used. It was also revealed that the monomer reactivity ratios depended on the solvent; r(1) = 0.086 and r(2) = 0 in chloroform and r(1) = 0.25 and r(2) = 0 in benzene, where DMPhMI and TP are M-1 and M-2, respectively. The propagation rate constants were determined for the homopolymerization and copolymerization in chloroform and benzene using electron spin resonance spectroscopy. The homo- and crosspropagation rate constants (k(11) and k(12), respectively) were revealed to depend on the solvent : k(11) is 20 and 37 L/mol.s and k(12) is 230 and 150 L/mol.s in chloroform and in benzene, respectively. The interaction between the maleimide moiety and the solvent molecules was discussed based on the acceptivity of the solvents.