Copolymerization of Styrene and Methyl Methacrylate in the Presence of a Catalytic Chain Transfer Agent

Copolymerizations of styrene and methyl methacrylate have been performed using different monomer feed compositions in the presence of a catalytic chain transfer agent at 40 °C. Average chain transfer constants as a function of monomer feed composition were determined with the conventional Mayo procedure using both number ( M n ) and weight ( M w ) average molecular weights, and with the chain length distribution procedure using both high (Λ H ) and peak (Λ P ) molecular weight slopes. It is found that the average chain transfer constants determined from M w and Λ P are generally very similar, with those obtained from M n and Λ H being larger and smaller, respectively. The average chain transfer constants obtained from M w and Λ P are compared with model predictions based upon both the terminal and penultimate unit models of free-radical copolymerization and are in satisfactory agreement. These two models are used to predict the fraction of propagating radicals with a terminal styrene unit, and it is found, similar to earlier studies reported in the literature, that this parameter is very sensitive to the penultimate model s values (in contrast to the average propagation rate coefficient), and a satisfactory agreement between model and experiment is obtained for s M = 2. This finding possibly suggests a complimentary route to measuring average propagation rate coefficients, for the determination of penultimate model s values.