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. doi:10.1021/ja0123296. 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.
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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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Issue or Number: | 11 | ||||||||||||
DOI: | 10.1021/ja0123296 | ||||||||||||
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: | 15 Nov 2021 17:31 |
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