Kinetic studies with a Ziegler catalyst system in the gaseous phase

Abstract

The kinetics of the Ziegler-catalyzed polymerization of ethylene have been studied in a static system in the complete absence of any hydrocarbon vehicle. The apparatus which was developed consisted essentially of two mixing vessels, each of which was connected to the reaction vessel and to various manometric devices. Two mixtures were prepared: one of ethylene and aluminium trimethyl vapour and one of ethylene and titanium tetrachloride vapour. The fall in pressure due to the polymerization reaction was recorded. The dependence of the rate of polymerization on catalyst concentration and composition has been established. An optimum catalyst ratio was found at an Al/Ti mole ratio of 2:1. The rate of polymerization was concluded to be independent of the amount of aluminium trimethyl present over a large range of alkyl concentration. The kinetic order with respect to total pressure was found to be non-integral. It has been shown, moreover, that the effective catalyst concentration decreases during any given run. By employing a kinetic technique the effect of changing catalyst concentration has been removed and it has thus been demonstrated that the rate of polymerization is a linear function of catalyst and monomer concentration. It has been proved that diffusion of monomer through a polymer layer is not the rate-controlling process. The effect of added oxygen on the reaction rate has been investigated and the reaction was found to have a small positive temperature coefficient. The molecular weights of the polymers as a function of reaction conditions were determined using a viscometric procedure. The results are interpreted in terms of a physical mechanism of catalyst coverage by polymer and by a kinetic scheme. A titanium species is shown to participate in an important termination process; no evidence for chain transfer processes was forthcoming. A number of mechanisms which are compatible with the observed kinetics are tentatively proposed.