A study of the copolymerization of methyl methacrylate and styrene in monosubstituted benzenes such as benzene, benzonitrile, benzyl alcohol, and phenol was made at 60°C. It was found that the copolymerization parameters changed significantly with the polarity of solvents, and the relative reactivity of methyl methacrylate toward the polystyryl radical was correlated with the ETvalues of solvents. These results and spectroscopic data suggest that in the transition state of this copolymerization the polarized structureof methyl methacrylate monomer became important, enhancing its reactivity, as the solvents became more protic.