Photolysis of arene chromium tricarbonyls and photoinitiation of polymerization. Part 3.—Benzene chromium tricarbonyl in styrene

Abstract

Irradiation of a styrene solution of benzene chromium tricarbonyl with light of λ= 365 nm produces strong absorption in the region 327–450 nm, with a maximum near 360 nm. This spectral change is enhanced by the presence of carbon monoxide. Reasons are given for believing that photolysis involves an exciplex intermediate and yields a complex (II), bzCr(CO)₂ st, and perhaps a second complex (III), Cr(CO)₅st, in both of which styrene is coordinated to chromium through the vinyl group. Both complexes react with carbon tetrachloride, the reactions being accelerated by illumination.

During irradiation in the presence of CCl₄ polymerization of styrene occurs; the dependence of the rate on [CCl₄] is of the familiar type and the polymerization follows conventional kinetics for a simple free-radical reaction. A very unusual feature, not hitherto observed in a metal carbonyl/halide system, is an increase in the rate of polymerization brought about by the presence of carbon monoxide. It is shown that there is correspondence between rates of initiation and concentrations of complexes (II) and (III); the influence of CO is attributed to its effect in increasing these concentrations. On turning off the light there is a prolonged after-effect. It is concluded that, except in the earliest stages, a major portion of the radical generation arises from photochemical and thermal reactions of (II) and (III) with carbon tetrachloride. The quantum yields of photoinitiation (ca. 5 × 10^–2) are low.

The major differences between the systems having methyl methacrylate and styrene as solvent appear to arise from the formation of much more strongly absorbing, and more stable, complexes in the latter.