Zr-alkyl isomerization in ansa-zirconocene-catalyzed olefin polymerizations. Contributions to stereoerror formation and chain termination

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1996
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Leclerc, Margarete K.
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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 kH/kD ≈ 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 C2H4(thind)2ZrCl2-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 Me2Si(2-Me-4-tBu-C5H2)2ZrCl2-MAO, almost all chain terminations appear to occur via chain-end isomerization, which does not lead to stereoerrors here. With the high-performance catalyst Me2Si(2-Me-benz[e]indenyl)2ZrCl2-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.

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ISO 690LECLERC, Margarete K., Hans-Herbert BRINTZINGER, 1996. Zr-alkyl isomerization in ansa-zirconocene-catalyzed olefin polymerizations. Contributions to stereoerror formation and chain termination. In: Journal of the American Chemical Society. 1996, 118(38), pp. 9024-9032. ISSN 0002-7863. eISSN 1520-5126. Available under: doi: 10.1021/ja961157n
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@article{Leclerc1996Zralk-23759,
  year={1996},
  doi={10.1021/ja961157n},
  title={Zr-alkyl isomerization in ansa-zirconocene-catalyzed olefin polymerizations. Contributions to stereoerror formation and chain termination},
  number={38},
  volume={118},
  issn={0002-7863},
  journal={Journal of the American Chemical Society},
  pages={9024--9032},
  author={Leclerc, Margarete K. and Brintzinger, Hans-Herbert}
}
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    <dcterms:abstract xml:lang="eng">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&lt;sub&gt;H&lt;/sub&gt;/k&lt;sub&gt;D&lt;/sub&gt; ≈ 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&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;4&lt;/sub&gt;(thind)&lt;sub&gt;2&lt;/sub&gt;ZrCl&lt;sub&gt;2&lt;/sub&gt;-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&lt;sub&gt;2&lt;/sub&gt;Si(2-Me-4-tBu-C&lt;sub&gt;5&lt;/sub&gt;H&lt;sub&gt;2&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;ZrCl&lt;sub&gt;2&lt;/sub&gt;-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&lt;sub&gt;2&lt;/sub&gt;Si(2-Me-benz[e]indenyl)&lt;sub&gt;2&lt;/sub&gt;ZrCl&lt;sub&gt;2&lt;/sub&gt;-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.</dcterms:abstract>
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