ZR-ALKYL ISOMERIZATION IN ANSA-ZIRCONOCENE-CATALYZED OLEFIN POLYMERIZATIONS - CONTRIBUTIONS TO STEREOERROR FORMATION AND CHAIN TERMINATION

In polymers made from (E) or (Z)-[1-D]propene with methyl alumoxane (M AO)-activated ansa-zirconocene catalysts, signals of deuterium-labeled mrrm pentads document that stereoerrors arise mainly from an isomeriz ation 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) approxi mate to 3 for the beta-H/D transfer associated with the isomerization reaction. In all poly([1- and [2-D]propenes) studied, D-labeled mmmm p entads occur with a probability similar to that of D-labeled mrrm pent ads; this observation requires further mechanistic clarification. Olef inic chain ends of polymers obtained with C2H4(thind)(3)ZrCl2-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 cataly st Me(2)Si(2-Me-4-tBu-C5H2)(2)ZrCl2-MAO, almost all chain terminations appear to occur vir chain-end isomerization, which does not lead to s tereoerrors here. With the high-performance catalyst Me(2)Si(2-Me-benz [e]indmyl)(2)ZrCl2-MAO, finally, no D-label is found in the mrrm and m mmm pentad signals here, Zr-alkyl isomerization can apparently not com pete with the high rate of olefin insertion.