Methyl Methacrylate Polymerization at High Conversion. II. Factors Determining the Onset of the Gel Effect

Abstract

The free volume at the onset of the gel effect depends both on chain length and temperature. For high molecular weights, the conversion at which the gel effect appears can be related to the critical entanglement. Balke and Hamielec have proposed that the onset of the gel effect in the bulk polymerization of methyl methacrylate occurs at a characteristic free-volume value [1]. Cardenas and O'Driscoll have introduced the concept of polymer entanglement to describe quantitatively the main factors of the bulk polymerization carried out to high conversion, but retained some elements of the free-volume theory to characterize the onset of the gel effect [2]. Nevertheless, there seems to be some incompatibility between the two theories, since the amount of entanglement, for a given conversion, depends upon the molecular weight of the polymer while the free volume is almost independent of it. If a critical amount of entanglement must be reached to produce the gel effect, the conversion at which the effect appears must be sensitive to the molecular weight of the polymer and cannot then be determined exclusively by the free volume which, for high molecular weights, is determined exclusively by the conversion. In order to investigate which factors determine the onset of the gel effect, we have carried out measurements of the rate of methyl methacrylate polymerization over a wide range of experimental conditions. A similar study was carried out by O'Driscoll, Wurtz, and Husar employing styrene [3]. These authors concluded that, although the gel effect appears before than the critical entanglement determined from viscosimetric measurements, both effects obey approximately the same equations.