Photoinitiation of free-radical polymerization by rhenium carbonyl in the presence of tetrafluoroethylene

Abstract

Irradiation (λ= 365 nm) of Re₂(CO)₁₀ in methyl methacrylate (mma), ethyl acetate or toluene solution leads to an increase in optical density at λ < 450 nm which is attributed to the formation of a product Q, probably [(mma)Re(CO)₄]₂ in the case of mma. At sufficiently long periods of irradiation a photostationary state between Re₂(CO)₁₀ and Q is established. In the presence of C₂F₄ similar results are obtained, although the development of Q in methyl methacrylate occurs more rapidly.

The system Re₂(CO)₁₀+ C₂F₄ is an effective photoinitiator (λ= 365 nm) for the free-radical polymerization of methyl methacrylate; the rate of polymerization ω is proportional to I^1/2₀, [Re₂(CO)₁₀]^1/2, [M] and is slightly increased by the presence of carbon monoxide. Under the conditions studied, chain transfer, retardation and primary termination are insignificant. The dependence of ω on [C₂F₄] has the familiar form showing a plateau value, similar to that found with Mn₂(CO)₁₀+ C₂F₄, although the maximum rate is effectively reached at much lower [C₂F₄]–3 × 10^–2 mol l^–1, approximately. The quantum yield of initiation at λ= 365 nm is unity. The product Q is unreactive towards C₂F₄ in the dark and there is no observable after-effect; however, Q photoinitiates polymerization of methyl methacrylate in the presence of C₂F₄, but with low quantum yield.

Determination of the Re content of the polymers suggests that the initiating radicals are (CO)₅ReCF₂ĊF₂. Infra-red observations reveal the presence of terminal groups with this structure.

Mechanisms for photoinitiation by Re₂(CO)₁₀+ C₂F₄ are discussed. Although the data do not lead unambiguously to one mechanism, a primary photolytic step leading, in the presence of methyl methacrylate, to two dissimilar fragments is favoured.