Propene polymerization promoted by C-2-symmetric metallocene catalysts: From atactic to isotactic polypropene in consequence of an isotope effect

We have recently shown that a reaction of epimerization of the growing polymer chain competes with that of isotactic 1-alkene polyinsertion promoted by C2-sym. metallocene catalysts. As a matter of fact, part of the stereoerrors in poly(1-alkene) samples produced with these catalysts arise from this "parasitic" process, rather than from monomer insertions with the "wrong" enantioface, particularly when the polymn. is operated at low monomer concn. and moderately high temp. Understanding the mechanism of this epimerization can be the first step for the synthesis of less "vulnerable" catalysts. In this context, investigating the behavior of selectively deuterated monomers may provide important information. In this communication, we report the outcome of a study on propene-2-d polymn. in the presence of the prototypical catalyst system rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2/MAO, documenting a large isotope effect on the balance between polyinsertion and epimerization, and therefore on the stereospecificity. The results prove that the epimerization of a growing polypropene chain involves a stereospecific migration of the methine hydrogen in a last-inserted monomeric unit (most likely induced by a β-agostic interaction with the Zr atom) and that the process is significantly slower when it requires the cleavage of a C-D bond instead of a C-H one. Indeed, under the same reaction conditions, an atactic polypropene and a fairly isotactic poly(propene-2-d) were obtained. The tentative mechanistic proposal presented by two of us in a previous communication is shown able to account for the new results on the polymn. of propene-2-d as well and on the recently published ones on that of propene-1-d.