PHOTOCHEMICAL CONTROL OF MOLECULAR-WEIGHT DURING FREE-RADICAL POLYMERIZATION

Photolysis of 1,3-(di-1-naphthyl)propan-2-one (DNP) during the free-ra dical polymerization of vinyl monomers such as styrene and methyl meth acrylate provides naphthylmethyl radicals which appear to terminate th e growing chains in a nearly quantitative manner. The number-average m olecular 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 inde pendent of other polymerization conditions. Furthermore, since each ch ain appears to be terminated with a fluorescent group, the number-aver age molecular weight of the polymer can be determined independently fr om a simple measurement of the fluorescence intensity per unit mass.