COPOLYMERIZATION OF STYRENE AND METHYL-METHACRYLATE IN THE PRESENCE OF A CATALYTIC CHAIN TRANSFER AGENT

Copolymerizations of styrene and methyl methacrylate have been perform ed using different monomer feed compositions in the presence of a cata lytic chain transfer agent at 40 degrees C. Average chain transfer con stants 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 p rocedure using both high (Lambda(H)) and peak (Lambda(P)) molecular we ight slopes. It is found that the average chain transfer constants det ermined from M-w and Lambda(P) are generally very similar, with those obtained from M-n and Lambda(H) being larger and smaller, respectively . The average chain transfer constants obtained from M-w and Lambda(P) are compared with model predictions based upon both the terminal and penultimate unit models of free-radical copolymerization and are in sa tisfactory agreement. These two models are used to predict the fractio n of propagating radicals with a terminal styrene unit, and it is foun d, similar to earlier studies reported in the literature, that this pa rameter is very sensitive to the penultimate model s values (in contra st to the average propagation rate coefficient), and a satisfactory ag reement between model and experiment is obtained for s(M) = 2. This fi nding possibly suggests a complimentary route to measuring average pro pagation rate coefficients, for the determination of penultimate model s values.