Studies in polymerization VI. Acrylonitrile: the behaviour of free radicals in heterogeneous systems

Abstract A kinetic study of the polymerization of acrylonitrile has been made. During polymerization of the pure monomer the polymer is precipitated. The reaction shows kinetic features which are different from those of homogeneous vinyl polymerizations. Among these are the following: (a) the thermal catalyzed and photopolymerizations show an initial increase in rate with time; (b) the photochemical after-effect depends on the duration of irradiation; (c) the molecular weights of polymers prepared in the thermal catalyzed reaction at constant catalyst concentration at different temperatures show a maximum near 60° C; (d) the photopolymer prepared near room temperatures is able at higher temperatures to act as an initiator for the polymerization of acrylonitrile and other vinyl monomers. This initiation can be very rapid, and give rise to a striking ‘fast’ reaction. It is shown that these and other observations can be interpreted on the assumption that the propagating free radicals become occluded in polymer aggregates during polymerization. This occlusion may affect all the rate coefficients; while the effect of mild degrees of occlusion is chiefly to reduce the termination coefficient, severe occlusion can prevent both termination and propagation. The factors influencing the degree of occlusion are discussed. Similar considerations probably apply to other vinyl polymerizations which produce insoluble polymers; in these reactions it is unlikely that a true stationary state is achieved.