Photolysis of 1,3-(di-1-naphthyl)propan-2-one (DNP) during the free-radical polymerization of vinyl monomers such as styrene and methyl methacrylate provides naphthylmethyl radicals which appear to terminate the growing chains in a nearly quantitative manner. The number-average molecular weight decreases with the amount of DNP used and is inversely proportional to the square root of the light intensity. This provides a means of controlling the molecular weight of polymers which is independent of other polymerization conditions. Furthermore, since each chain appears to be terminated with a fluorescent group, the number-average molecular weight of the polymer can be determined independently from a simple measurement of the fluorescence intensity per unit mass.