Influence of Monomer Concentration on the Stereospecificity of 1-Alkene Polymerization Promoted by C2-symmetric ansa-Metallocene Catalysts

An influence of monomer concn. on the stereospecificity of Group 4 ansa-metallocene catalysts for the polymn. of 1-alkenes is well documented for the Cs-sym. syndiotactic-specific metallocenes, which has been rationalized in the framework of a "chain migratory" reaction mechanism. No such influence is predicted when applying the same mechanism to the C2-sym. isotactic-specific metallocenes. The isotacticity of propene polymers prepd. with these latter catalysts also decreases significantly with decreasing propene concn., possibly due to a slow epimerization of the last-inserted monomer unit competing with chain propagation. Such an effect should always be considered when studying the mechanistic behavior of this class of homogeneous Ziegler-Natta catalysts.