Studies in polymerization XIV. The Photosensitized initiation of vinyl polymerization by the carbonyls of group VII metals

Abstract

The photoinitiation of vinyl polymerization, sensitized by rhenium carbonyl in the presence of carbon tetrachloride, has been studied at A = 3650 Å. Although in some ways the course of reaction is similar to that observed with manganese carbonyl as photosensitizer (Bamford, Crowe & Wayne 1965 a ), yet one important new feature is found with rhenium carbonyl. After irradiation a relatively high ‘dark’ rate of polymerization persists for several hours, and leads ultimately to high conversions. Studies made to elucidate the mechanism of the dark reaction are presented in this paper. It is shown that the kinetic results are consistent with the formation of two photolytic fragments, only one of which reacts rapidly with carbon tetrachloride to generate free radicals; the other reacts more slowly, so that its concentration builds up gradually during irradiation and decays over several hours after the light has been turned off. The quantum yield for initiation in the light is only slightly greater than unity, the result expected if the second fragment reacts but slowly with carbon tetrachloride. A comparison of the initiation photosensitized by manganese and rhenium carbonyls makes possible certain generalizations about the photochemistry of the group VII metal carbonyls, and suggestions are made about the primary and secondary photolytic processes in these systems.