The radical polymerization of methyl methacrylate has been studied in toluene and benzonitrile at 30, 45 and 60 degrees C using 2-2＇ azobisisobutyronitrile as initiator. Under the conditions of our experiments both the rates of polymerization and the molecular weights of the polymer obtained are dependent on the nature of the solvent. Determination of the effective initiation rate and the coupled parameter k(p)/k(t)(1/2) (where k(p) and k(t) are the constants for propagation and termination reactions, respectively) show that the dependence of the rate of polymerization on the nature of solvent arises in part from the effects of viscosity, polymer chain extension and polymer chain lengths on the termination coefficient and in part from a dependence of the effective initiation rate on the nature of the solvent.