Retardation of radical polymerization by phenylacetylene and its p‐substituted derivatives

Abstract

AbstractRadical polymerizations of styrene and methyl methacrylate in the presence of phenylacetylene and five of its p‐substituted derivatives were carried out with the use of 2,2′‐azobisisobutyronitrile as the initiator at 60°C. The initial overall rates of the polymerizations of styrene and methyl methacrylate in the presence of phenylacetylene were not proportional to the square root of the initiator concentration under the experimental conditions employed. The relationship between the overall polymerization rate and the concentration of the phenylacetylenes could be expressed by the Kice equation for the rate of a radical polymerization in the presence of a terminator. From this relationship the rate constant (ks) of the reaction of a growing polymer radical with the phenylacetylenes and the constant Cs = (ks/kp), where kp is the propagation rate constant of vinyl monomers, were determined. The Cs value thus obtained agree well with that derived from the relationship between the number‐average degree of polymerization and the molar ratio of the phenylacetylenes to the vinyl monomer. Therefore the mechanism of the reaction may be considered as being one in which the growing radical reacts with the ethynyl group of the phenylacetylenes to yield a comparatively stable radical which terminates mainly by reaction with the growing radical, and so apparently the phenylacetylenes retard the vinyl polymerization. The substituent effects on the reaction were discussed on the basis of the following modified Hammett equation proposed by Yamamoto and Otsu: log [Cs(p‐sub. PA)/Cs(PA)] = ρσ + γER where PA represents phenylacetylene, σ and ER are the Hammett polar substituent constant and resonance substituent constant, respectively, and both ρ and γ are reaction constants. The γ value for the polymerization of both styrene and methyl methacrylate was 1.7. The ρ value was 1.0 for the polymerization of styrene and approximately zero for that of methyl methacrylate. These results demonstrate that the reactivity of the phenylacetylenes with the growing chain is influenced by both polar and resonance effects of their p‐substituents in the degradative copolymerization of styrene and only by the resonance effect in that of methyl methacrylate.