The nature of active sites and stereospecificity of their action in diene polymerization catalysed by chromium-containing systems

Abstract

A study has been made of the nature of active sites, stereospecificity of their action and the regularities of diene polymerization catalysed by chromium-containing systems. All possible polymer structures with high stereospecificity can be produced for butadiene and isoprene with π-allyl chromium compounds. Tris-π-allyl chromium produces polybutadiene predominantly of 1,2-units. Cis-polybutadiene is formed when the electronegative group (Cl −, CCl 3COO −) is substituted for one or two π-allyl groups in Tris-allyl chromium or in the catalytic system (π-C 3H 5) 3CrAl 2O 3. A catalyst obtained through interaction of (π-C 3H 5) 3Cr with silica-alumina or silica gel produces 1,4- trans-polybutadiene and 1,4- trans-polyisoprene. The rate of butadiene polymerization in the presence of Tris-π-allyl chromium is given by k[Cr] 2, and in polymerization of isoprene with the catalytic system (π-C 3H 5) 3Cr-silica-alumina, by k[Cr].[M] 2. Polymerization of dienes catalysed by (π-C 3H 5) 3Cr-silica-alumina system or supported chromium oxide catalyst proceeds according to a type of living system. A study was made of copolymerization of butadiene and isoprene in the presence of supported chromium oxide catalyst and with that produced by the reaction of (π-C 3H 5) 3Cr with silica-alumina. The constants of copolymerization for the systems were equal. A conclusion has been drawn regarding the similar mechanisms for diene polymerization under the action of supported chromium oxide catalyst or of catalyst formed in the reaction of (π-C 3H 5) 3Cr with silica-alumina or silica gel.