The termination reaction in radical polymerizations. II. Polymerizations of styrene at 60° and of methyl methacrylate at 0 and 60°, and the copolymerization of these monomers at 60°

Styrene has been polymerized at 60° and methyl methacrylate at 0 and 60° using 2,2′‐azo‐bis‐isobutyronitrile labeled with C 14 as initiator. The average number of labeled end groups per molecule has been determined for the various polymers and so the relative importances of combination and disproportionation have been assessed. It has been found that polystyrene radicals undergo combination at 60°. In the case of methyl methacrylate, disproportionation occurs about 6 times as frequently as combination at 60° and about 1.5 times as frequently at 0° the activation energy for disproportionation is about 4 kcal. per mole greater than that for combination. The copolymerization of styrene and methyl methacrylate at 60° has been studied by this technique. It is found that when polystyrene radicals interact with polymethyl methacrylate radicals at this temperature combination occurs. The rates of initiation in the copolymerizations have been determined by a new method; the rate of this step is found to be virtually independent of the composition of the monomer mixture.