Polymer studies by gel permeation chromatography

Abstract

In order to determine if quantitative molecular weight distributions could be obtained from gel permeation chromatography (gpc), experimental distribution curves obtained by using gpc were compared with calculated distributions based on kinetic data and Flory's "most probable" distribution. Samples of polystyrene and poly(methyl methacrylate) were prepared under conditions previously investigated in the literature. The agreement between the experimental and the calculated distributions, as well as the agreement between molecular weights obtained by solution viscometry and gpc, indicated the utility of the gpc technique to study the details of vinyl polymerizations. Until recently; it has usually been easier to determine the appropriate rate terms and to calculate molecular weight distributions rather than to suffer the difficulties and uncertainties of an experimental measurement of a molecular weight distribution. The advent of gpc offered a rapid and potentially accurate method for determining molecular weight distributions. Furthermore, it became apparent that calculations made from molecular weight distributions allowed the determination of the following kinetic parameters: chain transfer to initiator constant (C_I), chain transfer to monomer constant (C_M), and the ratio k_t/k_p^2, where k_t and k_p are the rate constants for termination and propagation respectively. The values of C_I, C_M and k_t/k_p^2 were determined for the polymerization of styrene at 60º by using several concentrations of azobisisobutyronitrile and several concentrations of benzoyl peroxide as the initiators. Two systems which reportedly gave abnormal multipeaked, shouldered, or "tailed" molecular weight distributions were investigated. There has been considerable dispute over the question of whether or not multipeaked distributions are observed in high-conversion poly(methyl methacrylate) that underwent autoacceleration. It was found that there are abnormal shoulders which appear on the gpc curves for high-conversion poly(methyl methacrylate), but there are no multipeaked distributions. There have also been reports that low-conversion samples of polystyrene and poly(methyl methacrylate) prepared with high initiator concentrations of azobisisobutyronitrile have a molecular weight distribution possessing an abnormal high molecular weight "tail. It was a result of polymerization that occurred at room temperature, rather than at 60º, during solution preparation.