Polymerization of ethylene with a preactivated aluminum‐titanium(IV) chloride catalyst and a statistical study of the polymerization variables

Abstract

AbstractConversion of ethylene to a high‐density polymer was readily effected at low pressures and temperatures with the use of a catalyst consisting initially of a mixture of aluminum and titanium(IV) chloride. The principal factors influencing the polymerization and properties of the resulting polyethylene were investigated. These reactions at preferred catalyst concentrations were always preceded by a prolonged induction period; consequently, a means of reducing this period was developed. Factors influencing the polymerization included the nature of the aluminum in the catalyst, type of solvent, and molecular weight regulators. The effects of pressure, temperature, aluminum concentration, and Al/TiCl4 mole ratio on induction period, reaction rate, and polymer properties were determined from a factorial experiment in which each factor was investigated at two levels. The polymerization could be initiated at low catalyst concentrations and temperatures by preactivating the Al‐TiCl4 mixture, that is, partially converting it to aluminum chloride and titanium(III) chloride. Optimum catalyst efficiency was obtained at an Al/TiCl4 mole ratio of 0.5 to 1.0; the aluminum concentration was 0.0028 to 0.04% based on weight of solvent. Polyethylene yields as great as 1873 parts/part total catalyst were obtained at aluminum concentrations of 0.0028 to 0.008% and ethylene pressures of 150 to 280 psi. Liquid aliphatic hydrocarbons were more effective than aromatic hydrocarbons as media for the polymerization. Low molecular weight polymers and high‐density waxes were produced when hydrogen was used in the polymerization. The statistical study showed that the induction period was reduced by increasing the ethylene pressure and by decreasing the Al/TiCl4 mole ratio. The average reaction rate was increased by increasing the pressure, temperature, and aluminum concentration. A kinetic analysis of the data showed that the length of the induction period was inversely proportional to the first power of the pressure and titanium(IV) chloride concentration. The rate equation for the polymerization was found to be where P is the ethylene pressure. Formation of high molecular weight polymers was favored by high monomer concentrations and low titanium(IV) chloride concentrations. High‐density polyethylene (0.950–0.960 g./cc.) can be produced conveniently, safely, and economically at low pressures (40 to 280 psi) and low temperatures (50 to 85°C.) with the use of preactivated Al‐TiCl4 catalyst.