Transition metal catalysts. VIII. The role of oxygen in ethylene polymerizations with the AlBr3‐VXn‐Sn(C6H5)4 catalyst

Abstract

AbstractEthylene polymerizations with the aged AlBr3–VXn–Sn(C6H5)4 catalyst required the presence of about 0.02% oxygen in the ethylene feed. When the catalyst components were combined in the presence of ethylene, polymerization ceased in less than 1 min. with the production of about one polymer molecule per vanadium atom. The reaction was repeatedly reactivated by the addition of milligram amounts of oxygen, ultimately resulting in a higher yield of polyethylene with a molecular weight of 230,000. In the concentration range from 0.003 to 6% oxygen in the ethylene, there was a sharp maximum in the amount of polymer produced, before polymerization ceased, at about 0.05–0.1% oxygen. However, the molecular weight of the polyethylene decreased mono‐tonically with increasing oxygen concentration from 230,000 at 0.003% to 12,500 at 6.1% oxygen. It was demonstrated that oxygen enters into an oxidation‐reduction cycle with the vanadium species and that this reaction induces molecular termination and reactivates the vanadium center. In the absence of oxygen, the polyethylene was found to contain one phenyl and one vinyl endgroup per polymer molecule. These data are interpreted in light of a previously proposed propagation mechanism. Polymer end group analyses suggest that termination occurs by transfer of the polymer from the active vanadium center to an inactive aluminum center when oxygen is absent and by oxidation of the vanadium active site to a catalytically inactive compound when oxygen is added.