Effects of chain-transfer agents on the kinetics of the seeded emulsion polymerization of styrene

Abstract

The kinetics of the seeded emulsion polymerization of styrene have been studied in the presence and absence of the chain-transfer agents carbon tetrachloride and carbon tetrabromide. Initiation was achieved by both a chemical initiator (potassium peroxydisulphate) and irradiation with γ-rays. The latter permitted relaxation studies to be performed.

A combination of γ-ray initiation, relaxation and particle size distribution studies allowed the fate of the exited free radicals generated in the presence of carbon tetrabromide to be determined. Cross-termination in the aqueous phase was found to be operative in γ-ray initiated systems when the free radical concentration in the aqueous phase was relatively high. In contrast, re-entry of the exited free radicals into the latex particles was found to be important in relaxation studies when the free radical concentration in the aqueous phase was comparatively low. These results show that the exited free radical fate parameter can vary between –1 and +1.

The exit rate coefficient was found from relaxation measurements to increase linearly with increasing concentration of chain-transfer agent; this result is consistent with a diffusion/transfer mechanism for exit. The increase in the exit rate coefficient paralleled the increase in the chain-transfer constant for the additives: CBr₄ > CCl₄ > styrene. On the other hand, the efficiency of exit from the latex particles of free radicals formed by chain transfer follows the inverse order: CBr₄ < CCl₄ < styrene. This order may well reflect the relative reactivities with monomer of the low-molecular-weight free radicals formed by atom abstraction.

As expected from the increase in exit rate coefficient, the presence of carbon tetrabromide reduced the rate of polymerization of chemically initiated systems. At high initiator concentrations, for which the average number of free radicals per particle n was ca. 0.5, the rate reduction was small but increased monotonically with increasing concentration of carbon tetrabromide. This shows that any effect of carbon tetrabromide on the propagation rate constant was small in these studies. At lower initiator concentrations, however, a much larger reduction in rate was observed, as expected theoretically for values of n < 0.5. The rate in this case did not decrease monotonically with increasing concentration of carbon tetrabromide but passed through a minimum. This minimum was caused by the enhanced rate of entry of free radicals into the latex particles counterbalancing the rate reduction arising from the increased exit rate. The increase in the entry rate in the presence of carbon tetrabromide was explained by the production of hydrophobic free radicals by chain transfer in the aqueous phase and/or a colloidal contribution to the measured entry rate.