Gottfried Lichti

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

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 text-decoration:overlinen 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 text-decoration:overlinen < 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.

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

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.

Molecular Weight Distributions of Polymer Formed in Emulsion Polymerizations

A theory for the molecular weight distributions (MEDs) of linear polymers made by emulsion polymerization is presented. Full account is taken of bimolecular termination, of exit from the part icles and of chain transfer. Analytic solutions are obtained for the full MWD, which are accurate if the average number of free radicals per latex particles is less than 0.7. Compartmentalistation of free radicals broadens the MWD and may change the shape of the MWD significantly.