Studies in polymerization XVII. The initiation of vinyl polymerization by tetrakis (triphenyl phosphite) nickel (0)

Abstract

It has been found that tetrakis (triphenyl phosphite) nickel (0), in the presence of a suitable organic halide, is a very active initiator of free-radical polymerization. This paper describes a detailed kinetic study of the initiation of the polymerization of methyl methacrylate at 25 °C by the system Ni{P(O Ph ) 3 } 4 + CCl 4 , together with some observations on the systems in which ethyl trichloracetate and methylene chloride are the halide components. The reaction has been studied in benzene, ethyl acetate, dioxan and NN -dimethylformamide solutions, as well as in bulk monomer. In general, the mechanism of initiation resembles that previously reported for metal carbonyls. The primary step, which becomes rate-determining at sufficiently high halide concentrations, is an S N 2 process in which a triphenyl phosphite ligand is replaced by monomer or a solvent with electron-donating properties, with formation of a complex (I). Reaction of this with the halide produces a second complex (II) which yields a free radical on decomposition. Addition of triphenyl phosphite reverses the primary step and thus reduces the concentration of complex (I) and the rate of radical formation. Carbon monoxide is a powerful inhibitor, and appears to function by deactivation of complex (II). This initiating system differs from most metal carbonyl systems in that it is entirely free from inhibition at high concentrations of initiator. Measurements of the yield of polymer show that two free radicals are formed by each molecule of the nickel derivative which decomposes, presumably corresponding to the oxidation Ni 0 → Ni II . Although Ni{P(O Ph ) 3 } 4 is stable in the solid state, its half-life in bulk monomer at 25 °C is only 1 h. The primary step has an activation energy of 25∙2 kcal/mole and a high frequency factor, which may be the result of steric overcrowding in the molecule of the nickel derivative.