Free-radical formation in nickel carbonyl+halide systems. Comparison with carbon monoxide exchange

Abstract

Initiation of the free-radical polymerization of methyl methacrylate and styrene by nickel carbonyl in association with carbon tetrachloride and carbon tetrabromide has been studied at 25°C. A few experiments have also been carried out with ethyl trichloracetate as the halide component. 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_N2 process in which a carbon monoxide ligand is replaced by a monomer or solvent molecule with electron-donating properties to form complex (I). Reaction of (I) with a halide molecule produces a complex (II) by a reversible reaction. Complex (II) decomposes to give a free-radical. Carbon monoxide is a powerful inhibitor, and appears to function by deactivating complex (II). The activation energy for the primary step is 52.6 kJ mol^–1.

Measurements of the yield of polymer show that two free radicals are formed by each molecule of nickel carbonyl which decomposes, corresponding to the oxidation Ni⁰→ Ni^II. Kinetic parameters have been evaluated from equations derived from steady-state relationships, and various alternative mechanisms have been considered. The rate of initiation of polymerization and the rate of exchange of carbon monoxide with nickel carbonyl have been compared.