A thermocouple method of following the non-stationary state of chemical reactions - II. The evaluation of velocity coefficients and energies of activation for the propagation and termination reactions for the initial and later stages of the polymerization of vinyl acetate

Abstract
A study has been made of the changes which occur in the velocity coefficients and energies of activation for the propagation and termination reactions in the polymerization of vinyl acetate, photosensitized with 0$\cdot $0009 mole/l. 1. 1$^{\prime}$-azo-bis-cyclohexanecarbonitrile. The rate of polymerization increases from the early stages of the reaction until about 50% conversion and subsequently decreases rapidly to an extremely low value at about 75% conversion, while the overall energy of activation decreases from 4$\cdot $2 kcal/mole initially to 1$\cdot $9 kcal/mole at 50% conversion and then increases rapidly as the polymerization continues towards completion. This decrease in the overall energy of activation is due to a continuous increase in the activation energy for the termination reaction as the polymerization progresses, and this is believed to be due to an increase in the activation energy for the diffusion of polymer radicals in the increasingly viscous medium. Beyond 50% conversion the activation energy for the propagation reaction begins to increase, presumably due to the propagation step becoming diffusion controlled as the system becomes a solid polymer gel. The energies of activation for the propagation and termination reactions have been shown to vary from less than 5 and 1 kcal/mole respectively initially, to values of greater than 14 kcal/mole at about 70% conversion.