Controlled propagation in associated monomer aggregates

Abstract

The polymerization of acrylic acid in bulk proceeds with a very high rate and gives rise to a syndiotactic polymer. This result is attributed to the association of monomer molecules by hydrogen bonds into linear aggregates in which a stereospecific propagation is favoured. In order to verify this assumption the polymerization of acrylic acid was investigated in various solvents. It was found that the addition of methanol, dioxane and water did not significantly affect the reaction rates nor the tacticity of the polymer. In these solvents the viscosity of the monomer remains high, suggesting that the linear aggregates are not dissociated. In contrast, the addition of toluene or n-hexane sharply reduces the polymerization rate and the fraction of syndiotactic polymer. The viscosity of acrylic acid also drops in the presence of these solvents. These results are in agreement with the assumption that the polymerization of acrylic acid is strongly controlled by linear monomer aggregates. A similar situation is believed to apply to acrylamide. The propagation rate constant of this monomer was found to vary widely depending on the solvent used. This could indicate that the propagation rate is governed by monomer aggregates, the extent of aggregation being a function of the nature of the solvent. Methacrylic acid behaves in a different manner. This monomer forms the same types of associations as acrylic acid but the polymerization kinetics do not reflect any peculiarities related to such aggregates. A very strict control of chain propagation is met when 4-vinylpyridine is polymerized in the presence of polycarboxylic acids. A considerable rate increase was observed when vinylpyridine was grafted into polytetrafluoroethylene films which contained poly(acrylic acid) branches. This effect is accounted for by assuming that the pyridine substituents associate with the carboxylic groups, thereby providing a very favourable orientation of the vinyl groups for chain propagation. © 1972, Walter de Gruyter. All rights reserved.