Titanium trichloride alkyl halides

The most widely used transition metal salts have been the relatively cheap titanium or vanadium halides, reduced with aluminum alkyls. In Ziegler s work, titanium tetrachloride was reduced to brown yS-titanium trichloride, which was able to polymerize ethylene. However, when yS-titanium trichloride catalyst was used in the polymerization of propylene, the product contained a high proportion of the gum like atactic polymer, which was not viable for commercial use. In contrast, Natta, in his work on the polymerization of propylene and other a-olefins, showed that violet a-titanium trichloride could polymerize propylene to the useful, crystalline, isotactic polymer. Nevertheless, a relatively large quantity of atactic polypropylene still had to be separated from the commercial isotactic product. 

An exaniple of the monometallic mechanisms is one originally proposed by Cossee and Arlman. This mechanism assumes that the reaction occurs at a transition metal ion on the surface layer of the metal trichloride (or perhaps dichloride) lattice. Here, the halide is replaced by an alkyl group (R). The adjacent chloride site is vacant and accommodates the incoming monomer molecule. Consider titanium chloride as an illustration ... 

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