Organometallic Polymerization Catalysts

Abstract

A study has been made of the polymerization of styrene initiated by vanadium-based Ziegler-Natta catalysts, the most active of which was formed from VOC1₃/A1Et1.5Cl1.5 and was the most active also for the copolymerization of ethylene and propylene. A dilatometric study of the polymerization, using a device that could be filled rapidly, showed that there were three stages during the polymerization, two of which involved polymerization and the other depolymerization. It is likely that each stage was catalysed by a different vanadium complex. The decay in the rate of polymerization observed during the initial polymerization stage was correlated with a decrease in the rate of polymerization that occurred with increasing pre-mixing time when the vanadium and aluminium compounds were pre-mixed. The depolymerization that occurred during the second stage of the reaction was correlated with the ability of catalysts formed by reacting  VOC1₃ with A1Et1.5Cl1.5  to depolymerize polystyrene. No plausible thermodynamic reasons could be proposed for this depolymerization process. The kinetic study of the polymerization showed that increasing the concentration of  VOC1₃/A1Et1.5Cl1.5  at constant VOCl₃ concentration caused an increase in the rate of polymerization. It is likely that increasing the concentration of A1Et1.5Cl1.5  did not lead to an increase in the number of active sites, but the increased rate of polymerization was caused by activation of the monomer towards polymerization by the formation of a π complex between the monomer and the aluminium compound. At constant concentration of A1Et1.5Cl1.5 , the rate of polymerization decreased with increasing concentration of VOCl₃. UV/visible spectroscopy showed that a number of active species were present during the polymerization.