Catalysts for the polymerization of butadiene consisting of an alkylaluminum compound with a titanium(IV) compound show variations in reactivity with Al/Ti ratio. The ratio at which maximum reactivity is obtained increases as the electronegativity of the ligands bound to titanium decreases. This is observed on replacement of the chloride ligands of titanium tetrachloride by either iodide or isopropoxy ligands. Catalysts prepared from titanium dichlorodiiodide or titanium chlorotriiodide behave like those from titanium tetraiodide, as would be expected from the previous observation that titanium is the preferred site of attachment for iodide in the reduction products of mixed salts. The cis content of the polymers obtained with the iodide‐containing salts can be 90% or more, but decreases with increasing titanium concentration, whereas the structure of polymers obtained with titanium tetrachloride is independent of catalyst concentration. Polymerization is not induced by either the soluble alkylaluminum compounds or the insoluble reduced titanium iodides which are produced by the reaction of triisobutylaluminum with titanium tetraiodide. The insoluble material can be activated by triisobutylaluminum or, less readily, by diisobutylaluminum iodide.

Titanium tetraiodide and titanium dichlorodiiodide are reduced by triisobutylaluminum or diisobutylaluminum iodide to form products analogous to those obtained with titanium tetrachloride. In the reaction products, the preferred site for iodide ligands is titanium, that for chloride ligands, aluminum.

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Replacement of up to two of the chloride ligands on titanium tetrachloride with isopropoxy or butoxy groups results in an increase in the rate of reduction by alkylaluminum chlorides. The final product is still a titanium(III) compound. A decrease in rate results from replacement of more than two of the chloride ligands by alkoxy, or of the chloride ligands on aluminum by alkoxy. Exchange of alkoxy between titanium and aluminum occurs concurrently with reduction. These observations are explained in terms of the relative ease of replacement of a ligand on titanium by alkyl and the relative stability of the alkyltitanium compounds formed.

The structure of polybutadiene obtained with a n‐BuLi–TiX4 (X = Cl, I) catalyst depends on the Li/Ti ratio. At Li/Ti < 2.5 the result is similar to that with (i‐Bu)3Al–TiX4, predominantly, cis‐1,4‐polybutadiene. On the other hand, in the range Li/Ti = 3–4, the chloride catalyst produces largely 1,2‐polybutadiene, and at Li/Ti = 3 it is unaffected by ether in contrast to other ratios investigated. At Li/Ti ≥ 5 in both halide systems, the structure is essentially a mixture of isomers characteristic of the n‐BuLi catalyst alone.