Transition Metal Catalysts. IX. Random Ethylene-Propylene Copolymers with a Low Pressure Polymerization Catalyst

Abstract

Ethylene–propylene copolymers were prepared at atmospheric pressure with the VCl4–(C6H5)4Sn–AlBr3, hydrocarbon-soluble, polymerization catalyst. A controlled rate of initiation and thus a constant comonomer composition were obtained by the introduction of a constant amount of oxygen into the monomer stream. Four copolymers, containing 2.6, 3.8, 10, and 27 mole-% propylene, were found to have uniform comonomer compositions when fractionated by successive extraction with boiling solvents, in contrast to copolymers prepared with other reported Zieglertype catalysts. An elution fractionation of the 10 mole-% propylene copolymer was conducted with cellosolve-xylene mixtures above the copolymer melting point. The molecular weight distribution was shown to approximate the “most probable” distribution. This is in agreement with a previously proposed polymerization mechanism. The propylene contents of each of the fractions from the 10% copolymer were within ±1% propylene of the value of the whole copolymer. This narrow composition distribution is in accord with the statistical distribution predicated theoretically for random copolymers and is in accord with the previous finding that r1r2 ≅ 1. These data suggest that Ziegler-type catalysts containing a single active species are amendable to the same statistical and kinetic treatment that has been used for the classic polymerization catalysts.11