Seeded emulsion polymerizations of styrene. The fate of exited free radicals

Abstract

The kinetics of the seeded emulsion polymerization of styrene at different particle number concentrations and different initiator concentrations have been studied. The results were analysed using a theoretical treatment that incorporates the possibilities of both the re-entry of exited free radicals into the latex particles and the cross-termination in the aqueous phase of exited free radicals by free radicals generated through initiator decomposition. The results exclude the possibility of complete re-entry of exited free radicals into the latex particles for the initiator concentrations studied. They strongly support the occurrence of significant cross-termination in the aqueous phase of the exited free radicals with free radicals generated by initiator decomposition. This is in accordance with the known rapidity of cross-termination reactions compared with the corresponding self-termination reactions. It was also shown that the rate of entry of free radicals into each latex particle increases with decreasing particle number at constant initiator concentration. The radical capture efficiency was, however, relatively insensitive to the particle number concentration. The overall polymerization rate was found to be a complex function of the number concentration of latex particles; this is because this overall rate is itself a complicated function of the rate coefficients for entry, exit, etc. each of which may be individually a simple function of number concentration. Such behaviour reflects significant deviations from Smith–Ewart case 2 kinetics that occur in these systems, rather than deviations from the general Smith–Ewart kinetic scheme.