Preformed Aluminum Triisobutyl-Titanium Tetrachloride Catalysts for Isoprene Polymerization

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Abstract
Variables in the preparation of mixed aluminum triisobutyl-titanium tetra-chloride catalysts for isoprene polymerization were studied. All the preformed catalysts were tested unaged in solution polymerizations at 50° C using various catalyst levels. The yields of solid rubber and low-molecular-weight oily material in the polymerization products were determined. The solid rubber was analyzed for cis 1,4 content, inherent viscosity, and per cent gel. The most active catalysts for the synthesis of cis 1,4-polyisoprene were those prepared at low temperatures, below 20° C. Superior catalytic performance and improved polymer properties were obtained with catalysts preformed by either gradually adding the aluminum alkyl to titanium tetrachloride or by mixing the two cocatalysts at a constant Al/Ti ratio in the range of 0.9 to 1.0. By comparison, the gradual addition of titanium halide to the alkyl resulted in catalysts of lower activity and inferior polymer properties; it also required higher Al/Ti ratios for the preparation of all cis 1,4 rubber. An increase in catalyst preparation temperature led to gradually lower activity to where species preformed at 125° C showed little or no activity. High preparation temperatures, further, caused a shift in the Al/Ti ratio—optimum for cis 1,4 synthesis—to lower values when the aluminum to titanium order of addition was employed, and to higher ratios for the reversed type of addition. Al/Ti ratios above and below the optimum produced a cis 1,4 rubber which was increasingly contaminated with by-products of oily and leathery type polymeric material, respectively. Maximum yields of oily polymers were obtained with low temperature catalysts of Al/Ti ratios within 1.5 and 2.0. Catalysts preformed by the aluminum to titanium order of addition at 90° C produced cis 1,4-polyisoprene at Al/Ti ratios of 0.5 to 0.6. Preformed catalysts are preferred over those prepared in situ in the presence of isoprene because they show higher activity, more reproducible polymerizations and improved polymer products.

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