Polymerization of styrene with a magnesium-reduced titanium chloride-based catalyst system

Abstract

The number of sites active for polymerization in a ZieglerNatta catalyst system is a quantity whose evident importance has led to repeated attempts to measure it. Since the activity is normally ascribed to the transition metal component it is usual to express the number of active sites as a fraction of the number of transition metal atoms. There have been a number of methods employed for such measurements: they have been divided into kinetic methods and methods based upon labelling studies, and there are numerous publications, including a recent authoritative review ~, concerned with this area. Unfortunately, until fairly recently the methods available have been laborious (but see ref. 1) or unreliable, and so there have been few attempts either to cover a wide spectrum of catalysts using a consistent methodology or to study the reactivity of a single catalyst with a series of monomers. A notable exception to this is the study 2 of Schnecko et al. We 3 and others 4 have previously reported the high activity of Grignard reagent-reduced titanium tetrachloride-based catalysts in the polymerization of ethylene. In the course of our study, the fraction of titanium atoms active as polymerization centres at 60 C was estimated to be about 60o;, and the conclusion was drawn that the high activity was due to this large fraction of active transition metal atoms. Subsequently, we reported work with propylene 5, where a similar estimation yielded a figure of 3'I,, as the fraction of titanium atoms active at 40 C again this is a fraction greater than commonly found. We now report our studies on the polymerization of styrene with this catalyst system, emphasizing in particular the estimation of sites active for styrene polymerization. After an initial trial polymerization using aluminium triethyl as cocatalyst had established that styrene does react appreciably with this catalyst system, a conclusion reinforced by the isolation of an isotactic fraction 7 via solvent extraction, kinetic runs were carried out in magnetically stirred dilatometers, adopting the usual precautions against oxygen and water. Polymers were isolated to check conversions and to give samples for viscometry and gel permeation chromatography, since the active site determination depends upon the variation of molecular weight with conversion.