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Chain Epimerization during Propylene Polymerization with Metallocene Catalysts: Mechanistic Studies Using a Doubly Labeled Propylene

Yoder, Jeffrey C. and Bercaw, John E. (2002) Chain Epimerization during Propylene Polymerization with Metallocene Catalysts: Mechanistic Studies Using a Doubly Labeled Propylene. Journal of the American Chemical Society, 124 (11). pp. 2548-2555. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20170515-152616804

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Abstract

The mechanisms of chain epimerization during propylene polymerization with methylaluminoxane-activated rac-(EBTHI)ZrCl_2 and rac-(EBI)ZrCl_2 catalysts (EBTHI = ethylenebis(η^5-tetrahydroindenyl); EBI = ethylenebis(η^5-indenyl)) have been studied using specifically isotopically labeled propylene: CH_2═CD^(13)CH_3. These isospecific catalysts provide predominantly the expected [mmmm] pentads with [−CH_2CD^(13)CH_3-] repeating units (^(13)C NMR). Under relatively low propylene concentrations at 50 and 75 °C, where stereoerrors attributable to chain epimerization are prevalent, ^(13)C NMR spectra reveal ^(13)C-labeled methylene groups along the polymer main chain, together with [CD^(13)CH_3] units in [mmmr], [mmrr], and [mrrm] pentads and [CH^(13)CH_3] units in [mmmmmm] and [mmmmmr] heptads, as well as [mrrm] pentads. The isotopomeric regiomisplacements and stereoerrors are consistent with a mechanism involving β-D elimination, olefin rotation and enantiofacial interconversions, and insertion to a tertiary alkyl intermediate [Zr−C(CH_2D)(^(13)CH_3)P] (P = polymer chain), followed by the reverse steps to yield two stereoisomers of [Zr−CHDCH(^(13)CH_3)P] and [Zr−^(13)CH_2CH(CH_2D)P], as well as unrearranged [Zr−CH_2CD(^(13)CH_3)P]. The absence of observable [−CH2CH13CH2D−] in the [mrrm] pentad region of the ^(13)C NMR spectra provides evidence that an allyl/dihydrogen complex does not mediate chain epimerization.


Item Type:Article
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http://dx.doi.org/10.1021/ja0123296DOIArticle
http://pubs.acs.org/doi/abs/10.1021/ja0123296PublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/ja0123296PublisherSupporting Information
Additional Information:© 2002 American Chemical Society. Received 10 October 2001. Published online 20 February 2002. Published in print 1 March 2002. This work has been funded by the USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-88ER13431). We wish to gratefully acknowledge the expert assistance of Dr. Peter Green and Dr. Nathan Daleska with the GC-MS experiments.
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Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-88ER13431
Issue or Number:11
Record Number:CaltechAUTHORS:20170515-152616804
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170515-152616804
Official Citation:Chain Epimerization during Propylene Polymerization with Metallocene Catalysts:  Mechanistic Studies Using a Doubly Labeled Propylene Jeffrey C. Yoder and John E. Bercaw Journal of the American Chemical Society 2002 124 (11), 2548-2555 DOI: 10.1021/ja0123296
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77469
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:16 May 2017 16:22
Last Modified:03 Oct 2019 17:58

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