Zr−Alkyl Isomerization in ansa-Zirconocene-Catalyzed Olefin Polymerizations. Contributions to Stereoerror Formation and Chain Termination¹

In polymers made from ( E )- or ( Z )-[1-D]propene with methyl alumoxane (MAO)-activated ansa-zirconocene catalysts, signals of deuterium-labeled mrrm pentads document that stereoerrors arise mainly from an isomerization of the Zr-bound chain end. Reduced D-atom redistribution in poly([2-D]propene) indicates a kinetic isotope effect of k H / k D ≈ 3 for the β-H/D transfer associated with the isomerization reaction. In all poly([1- and [2-D]propenes) studied, D-labeled mmmm pentads occur with a probability similar to that of D-labeled mrrm pentads; this observation requires further mechanistic clarification. Olefinic chain ends of polymers obtained with C 2 H 4 (thind) 2 ZrCl 2 -MAO from ( E )- or ( Z )-[1-D]propene deviate in their D-label distributions from the expected stereochemistry. Isomerization of the Zr-bound chain end, probably via a Zr-bound tertiary alkyl intermediate, thus contributes also to chain-growth termination. With the sterically hindered catalyst Me 2 Si(2-Me-4- t Bu-C 5 H 2 ) 2 ZrCl 2 -MAO, almost all chain terminations appear to occur via chain-end isomerization, which does not lead to stereoerrors here. With the high-performance catalyst Me 2 Si(2-Me-benz[ e ]indenyl) 2 ZrCl 2 -MAO, finally, no D-label is found in the mrrm and mmmm pentad signals; here, Zr-alkyl isomerization can apparently not compete with the high rate of olefin insertion.