Photo-initiation of Polymerization by Manganese(III) Chelates

Abstract

Manganese(lII) acetylacctonatc (Mn(acac)3) and manganese(III) tris(l,l,l-trifluoroacetyl- acetonate) (Mn(facac)3) photosensitize (A = 365 nm) the free-radical polymerization of methyl methacrylate and styrene. Photolysis of the chelates in benzene and cyclohexane solutions is shown to yield acac* or facac* radicals, which undergo hydrogen abstraction reactions. Initiation of polymerization occurs with low quantum yields, and, in bulk monomer, does not depend on the nature of the monomer. In the latter respect it therefore differs markedly from thermal initiation by Mn(facac)3, which is strongly monomer-selective. The rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation, the difference being proportional to the monomer concentration. On the other hand, photo-decomposition and initiation proceed at similar rates with Mn(facac)3. Photo-initiation of the polymerization of methyl methacrylate by Mn(facac)3 is subject to marked solvent effects, being less efficient in benzene than in ethyl acetate solutions, particularly at low monomer concentrations. No similar behaviour is encountered with Mn(acac)3. A mechanism involving exciplex formation between Mn(facac)3 and methyl methacrylate or ethyl acetate is proposed.