The effect of high pressure on the individual steps of free radical polymerization

A study of the effect of pressure on the emulsion polymerization of styrene gives a value of ΔV‡ for chain propagation of −11.5 cc./mole, close to the value for the peroxideinitiated bulk reaction. Chain initiation and termination are both somewhat pressureretarded, the latter presumably because the reaction becomes diffusion‐controlled. Pressure has little effect upon the chain‐transfer constant of styrene–CCl4 or upon transfer with monomer in allyl acetate polymerization. However, the overall rate of allyl acetate polymerization is greatly accelerated, indicating that transfer is no longer degradative. Pressure retards the unimolecular decomposition of both di‐tert‐butyl peroxide and benzoyl peroxides, and some complications in these reactions are discussed as well as the principles governing the effect of pressure on radical reactions in general.