Crystalline titanium dichloride—an active catalyst in ethylene polymerization. I. Catalyst activation

Abstract

AbstractCrystalline titanium dichloride, in the absence of organometallic cocatalyst, is a very poor catalyst for the polymerization of ethylene. It is transformed into a very active catalyst through mechanical activation (ball‐milling). This catalyst is active in the absence not only of organometallic cocatalysts, but also metals and compounds (such as aluminium and AlCl3) capable of forming organometallic compounds in situ (i.e., with ethylene, before polymerization starts). Ball‐milling causes not only the expected increase in surface area but also disproportionation of Ti++ to Ti+++ and metallic titanium, as well as a crystal phase change to a structure not previously identified with those of TiCl2 or TiCl3. Catalyst activity (polymerization rate) is shown to be proportional to surface area and a direct function of Ti++ content of the catalyst; an empirical equation relates catalyst activity to surface area and to Ti++ lost through disproportionation. Titanium trichloride was found to be inactive in the absence of organometallic cocatalyst, even after ball‐milling. The difference in structure of the catalytically active species in the conventional Ziegler (organometallic cocatalyst) and in the titanium dichloride catalyst are discussed. The mechanism of polymerization is compared with that of the supported (CrO3 on SiO2/Al2O3 and MoO3 on Al2O3) catalyst systems.