Solvent effects on the rate constant of chain propagation in free radical polymerization [Lösungsmitteleinfluß auf die geschwindigkeitskonstante des kettenwachstums in der radikalischen polymerisation]

Using pseudostationary techniques (pulsed laser polymerization followed by an analysis of the chain-length distribution), the rate constant of chain propagation kp was directly determined for 1:1 mixtures of the monomer and a number of solvents for styrene and methyl methacrylate as well as for a 1:1 comonomer system of these two monomers, thus reducing the bulk monomer concentration to half of its bulk value in all cases. The presence of solvent emerged to be of moderate influence on kp only, the effects never exceeding 20% in either direction. Depressions of kp were more frequent than elevations. The results did not favour one of the existing theories (EDA-complex theory, hot radical theory) over the other. In case of a bad solvent, kp may rather reflect changes in the local monomer concentration at the site of reaction caused by preferential solvation by the monomer which constitutes the better "solvent".