Effects of salts of metals on vinyl polymerization. Part 2.—Ferric chloride as photo-initiator and terminator in the polymerization of acrylonitrile in dimethylformamide

Abstract

A study has been made of the kinetics of the polymerization of acrylonitrile in dimethylformamide between 25 and 50°C. At ferric chloride concentrations below 5 × 10–4 M, the rate of polymerization is proportional to the square root of the ferric chloride concentration, whereas at concentrations above 2 × 10–3 M, the rate varies inversely with the ferric chloride concentration. The rate varies directly with the monomer concentration over the range 1.5–4.5 M. At higher monomer concentrations the system becomes heterogeneous thus complicating the kinetics of the reaction. The overall activation energy is about 5.5 kcal/mole at low ferric chloride concentrations and in the region of 2 kcal/mole when the ferric chloride concentration is above 2 × 10–3 M. These results indicate that, at low ferric chloride concentrations, the termination involves the mutual interaction of growing polymer radicals, while, at higher concentrations, termination proceeds by reaction between the growing polymer radicals and ferric chloride. Again, at low ferric chloride concentrations, where a relatively small fraction of the initiating light is absorbed by the ferric chloride, the rate of initiation is directly proportional to the ferric chloride concentration, whilst at higher concentrations all the initiating irradiation is absorbed, and the rate of initiation becomes independent of the ferric chloride concentration. The intensity exponent at high ferric chloride concentrations is 0.6–0.7 and not unity as expected. This difference is attributed to solvent participation in the initiation reaction. This view is supported by the results of preliminary work using radioactive ferric chloride, which also show that chlorine is incorporated in the polymer in the termination reaction but not in the initiation step. The value of the velocity coefficient for the reaction between a growing polymer radical and ferric chloride agrees well with the results in the literature, if the efficiency of initiation is about 0.23, a value supported by kinetic and tracer measurements.