Vinyl polymerization. 93. Polar effects in radical polymerization of <i>p</i>‐substituted styrenes

Abstract

Abstract The radical polymerization of styrene and of the p ‐substituted styrenes p ‐methoxy‐, p ‐methyl‐, p ‐fluoro‐, p ‐chloro‐, p ‐bromo‐, and p ‐cyanostyrene, was kinetically studied. The styrenes with more electron‐attracting substituents show a greater overall rate of polymerization. At 30,0°C., the individual rate constants of propagation (k p ) and termination (k t ) were determined by a rotating sector and by usual inhibitor method using diphenylpicrylhydrazyl (DPPH) or p ‐benzoquinone. It was found that the following H AMMETT relationship is established in the homopolymerization of p ‐substituted styrenes: where σ is the H AMMETT constant, and ρ′ is a reaction constant which was calculated as +0.6. This relation is interpreted from a participation of ionic structures in the transition state of the propagation reaction. For confirmation, M ULLIKEN 's electronegativity of the p ‐substituted styrene and the p ‐substituted benzyl radical was calculated by simple LCAO MO method. The relationship between the rate constants of termination, k t , and the σ‐values is expressed by two lines, crossing at p ‐fluorostyrene which is at the maximum point. These two lines are explained by two ways; one is the diffusion controlled termination and the other is the recombination of the two polarized radical ends.