Photosensitization of free-radical polymerisation by Mn2(CO)10+C2F4

Abstract

A study has been made of the free-radical polymerization of methyl methacrylate at 25°C photo-initiated (λ= 435.8 nm) by Mn2(CO)10+ C2F4. The manganese and fluorine contents of the polymers are consistent with the view that initiation leads to the incorporation of one manganese atom and one C2F4 unit in each growing chain.In the absence of C2F4 no initiation and no significant decomposition of manganese carbonyl occur. With increasing [C2F4] the rate of initiation i…ƒ at first increases and then reaches a plateau value at high [C2F4]. Decomposition of Mn2(CO)10 in the presence of C2F4 follows a first-order law, the rate coefficient k being proportional to the incident intensity; k, like i…ƒ, reaches a plateau value at high [C2F4]. At low incident intensity i…ƒ=k[Mn2(CO)10], within the limits of experimental error, each manganese carbonyl molecule decomposing giving rise to one growing chain. However, at high intensity i…ƒk[Mn2(CO)10] for low [C2F4]; the difference between the two quantities decreases as [C2F4] increases. At high [C2F4] the quantum yields for initiation at low intensity and carbonyl decomposition are close to unity. The rate coefficient k is independent of methyl methacrylate concentration so long as this exceeds 2 mol l.–1; it falls off at lower concentrations.Two possible types of initiation mechanism are discussed. In the first, photodissociation (assisted by the monomer M) gives rise to two fragments M ·· Mn(CO)4, Mn(CO)6 of which the former reacts with C2F4 to produce an initiating radical M ·· Mn(CO)4—CF2ĊF2 or (CO)5Mn—CF2ĊF2. The second mechanism assumes formation of an exciplex [C2F4·· Mn2(CO)10] which ultimately yields (CO)5Mn—CF2ĊF2. The low values of i…ƒ at high intensity and low [C2F4] are ascribed in both cases to recombination between the initiating radicals and other manganese species e.g. Mn(CO)6.The polymerizations of styrene and acrylonitrile may also be photoinitiated by Mn2(CO)10+ C2F4.The activity of C2F4 in these initiating systems is attributed to the relatively high energy of the Mn—CF2CF2 bond.