Chain Length Dependent Termination in Pulsed-Laser Polymerization. 9. The Influence of Solvent on the Rate Coefficient of Bimolecular Termination in the Polymerization of Styrene

Abstract

The chain length dependence of bimolecular termination in the solution polymerization of styrene was examined at 25 °C with the solvents (50% per volume) toluene, cyclohexane, bis(3,5,5-trimethylhexyl)phthalate, and ethyl acetate. Irrespective of which method was applied, the good solvent systems with toluene and ethyl acetate exhibited a chain length dependence comparable to or even greater than that observed for the bulk polymerization of styrene, while a markedly weaker chain length dependence was observed with the bad solvents cyclohexane and bis(3,5,5-trimethylhexyl)phthalate. This is documented by the exponent b in the power law k t = A-v' -b connecting the average termination coefficient k t and the chain length v' of the radical chains at the moment of undergoing bimolecular termination: b 0.2 for styrene (in bulk) and in toluene solution, b 0.3 in ethyl acetate solution and 0.10 to 0.14 in cyclohexane and bis(3,5,5-trimethylhexyl)phthalate solution, respectively. Only in the case of the bad solvents the bulk viscosity and k t or the prefactor A are inversely correlated. In the case of the good solvents, the bulk viscosities were too close to detect such a relationship. The observed values of the parameter b (with the exception of the ethyl acetate system) are consistent with the idea of a chain length dependent shielding exercised by the rest of the chains on the two radical chain ends (some sort of kinetic excluded volume effect) which is much weaker in bad solvents.