The nature of the initiation reaction in the polymerization of styrene sensitized by benzoyl peroxide

Benzoyl peroxide labelled with carbon-14 has been used to sensitize the polymerization of styrene at 60°C. Experiments have been performed with bulk styrene and with various solutions of styrene in benzene. The rate of initiation by benzoyloxy radicals has been measured by using peroxide labelled in the carboxyl groups only; the total rate of initiation, by phenyl as well as by benzoyloxy radicals, has been measured by using peroxide labelled in the benzene rings only. It has been shown that the fraction of the initiating radicals which are benzoyloxy radicals decreases as the concentration of monomer is reduced; it is possible to compare the velocity constants for the alternative reactions of the benzoyloxy radicals, namely, loss of carbon dioxide to give phenyl radicals and capture by molecules of styrene.