Studies in polymerization XVIII. Initiation by metal chelates

Abstract

The initiation of vinyl polymerization by manganic acetylacetonate and four related manganic chelates (compounds (II) to (V)) has been studied. In all cases the polymerization involves free-radical intermediates. With Mn(acac) 3 the initiation process is essentially the scission of a ligand as a free-radical, with reduction of the metal to the Mn II state. The change in oxidation number of the metal has been established by magnetic susceptibility measurements. Monomers, as well as solvents capable of coordination, can participate in the rate-determining step. The activation energy and frequency factor for initiation are 26.2 kcal mole -1 and 6.7 x 10 11 s -1 , respectively. Initiation by manganic 1, 1, 1-trifluoroacetylacetonate (V) shows novel features. It is highly selective; the chelate rapidly initiates polymerization of those monomers which polymerize readily anionically, e. g. methyl methacrylate and acrylonitrile, but is very much less effective with other monomers such as styrene and vinyl acetate. A mechanism is proposed in which heterolytic fission of an Mn— O bond is followed by addition of the monomer to the anion; the resulting monomer anion then participates in electron transfer with the manganese III atom forming Mn II and a free-radical which is the initiating species. This mechanism is consistent with the low frequency factor of initiation, 2.9 x 10 6 s -1 for methyl methacrylate. Manganic 1, 1, 1-trifluoroacetylacetonate has been shown to be a strong retarder of the styrene polymerization, the retardation constant at 80°C being 7.3 x 10 4 mole -1 l. s -1 . Retardation is attributable to the additional positive charge on the metal atom in this compound; as expected, it is not significant with methyl methacrylate. Kinetic evidence is presented for ligand transfer reactions in mixtures of Mn(acac) 3 and Mn(facac) 3 .