Initiation of polymerization by bis(triphenyl phosphine) nickel dicarbonyl

Abstract

The initiation of the free-radical polymerization of methyl methacrylate by bis(triphenyl phosphine) nickel dicarbonyl in association with carbon tetrachloride has been studied in detail at 25°C. A few experiments have also been carried out with ethyl trichloracetate as the halide component. These systems are considerably more active than those involving nickel tetracarbonyl, and, unlike other carbonyl systems studied, do not inhibit at high concentrations.

In benzene solution the rate-determining step of the initiation reaction is first order in monomer concentration. The primary process involves the S_N2 displacement of a comparatively labile triphenylphosphine ligand by a monomer molecule to form a complex (I), which subsequently reacts with the organic halide giving a free radical capable of initiating polymerization. Addition of triphenylphosphine to the reaction mixture reverses the primary step, thus reducing the rate of initiation. Carbon monoxide also causes inhibition by the deactivation of complex (I). The nature of the initiation process has been confirmed by experiments designed to measure the evolution of carbon monoxide under a variety of experimental conditions. The activation energy for the primary step is 21.2 kcal mole^–1 and the frequency factor has a value close to the normal for a bimolecular reaction. Measurement of polymer yields has shown that a maximum of one free radical is formed for each initiator molecule which decomposes, corresponding to the oxidation Ni⁰→Ni^I. Kinetic parameters have been evaluated from equations derived for steady-state relationships.