Studies in polymerization. I. A method for determining the velocity constants in polymerization reactions and its application to styrene

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Abstract
A general method for determining the velocity constants of initiation, propagation, transfer and termination in polymerization reactions is described. The method depends essentially on the measurement of the rate of increase of viscosity for various rates of chain initiation. In the case of radical polymerizations photochemical initiation is most convenient. Measurement of the photochemical after-effect then gives four independent relations between the four velocity constants. The method has been tested by a detailed study of the thermal and photochemical polymerization of styrene. Velocity constants have been determined at 0 and 25° C, and the activation energies and frequency factors calculated. Results are given in tables 5 and 6. It is shown for the first time that chain transfer is important in the thermal polymerization of pure styrene, occurring 11 times per kinetic chain at 25° C. This result invalidates conclusions which previous workers have reached. Other rather unexpected results are that the initiation reaction has a much higher activation energy than has been generally supposed, and a normal frequency factor, while termination is activated. A detailed kinetic scheme is proposed which removes certain unjustifiable assumptions present in earlier ones. This is shown to be consistent with all the available evidence.

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