Photolysis of arene chromium tricarbonyls and photoinitiation of polymerization

Abstract

The photolyses of bzCr(CO)₃ and tolCr(CO)₃(bz = benzene, tol = toluene) have been studied in hydrocarbon and methyl methacrylate solutions. Scission of carbon monoxide is found to be the main route of photodecomposition in hydrocarbon media (as proposed by some earlier workers), and the resulting fragment arCr(CO)₂ decomposes, possibly by a complex mechanism, to yield arene. Addition of CCl₄ enhances the rate of arene generation by reaction with arCr(CO)₂ and arCr(CO)^*₃, while in the absence of CCl₄, CO suppresses reaction by recombination with arCr(CO)₂. In methyl methacrylate solution the rate of arene formation is higher than in hydrocarbon media and is little affected by carbon monoxide, even in the absence of CCl₄. We propose that the exciplex [M ·· arCr(CO)₃]^* is formed in methyl methacrylate (M) solution and may subsequently decompose into ar + MCr(CO)₃ or arCr(CO)₂ M + CO.

Systems containing arCr(CO)₃ and halides such as CCl₄ are active photoinitiators of free-radical polymerization; the kinetics of the polymerization with ar = benzene and methyl methacrylate as monomer have been studied. Carbon monoxide has a strong inhibitory effect on the reaction. It turns out that at “high”[CCl₄](∼0.1 mol dm^–3) each molecule of bzCr(CO)₃ photolysed yields one initiating radical, the quantum yields of photodecomposition and photoinitiation (λ= 365 nm) being 0.41.

A reaction scheme (A) is propsed which is consistent with the available photochemical and polymerization data.