A thermocouple method of following the non-stationary state of chemical reactions V. The evaluation of velocity coefficients for the propagation and termination reactions for the heterogeneous polymerization of acrylonitrile

Abstract
An attempt has been made to evaluate the velocity coefficients for the propagation and termination reactions in the bulk polymerization of acrylonitrile using the thermocouple non-stationary-state method. The reaction has been studied at 25 °C, and 1. 1'-azo-bis- cyclo -hexane carbonitrile has been employed as a photosensitizer. In the very early stages of the reaction (< 0.1 % conversion) when there is very little dead polymer present, the intensity exponent is 0.5, which indicates that the termination reaction proceeds by the interaction of growing polymer chains. The values obtained for the propagation and termination velocity coefficients (namely, 52 and 5.0 × 10 6 1. mole -1 s- 1, respectively) are lower than those reported in the literature for the polymerization of acrylonitrile in dimethylformamide solution. The difference has been attributed to environmental factors. Measurements of rates and lifetimes of the kinetic chain have also been made in the region of 1 to 7% conversion. By means of a suitably designed dilatometer which could be centrifuged, the effects of polyacrylonitrile on the polymerization reaction was studied. It was shown that the presence of polymer produced an enormous increase in the lifetime of the kinetic chain, while the rate decreased a little probably due to the scattering of initiating irradiation by the precipitated polymer. The intensity exponent appeared to increase to a value of 0.7, but this may not have been a true effect, as errors in the determination of the rate may have arisen due to difficulties in maintaining adiabatic conditions throughout the non-stationary state. The results in general are in keeping with the occlusion theory suggested by previous workers to explain some peculiar characteristics of the bulk polymerization of acrylonitrile.