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

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Abstract
The mechanisms of chain epimerization during propylene polymerization with methylaluminoxane-activated rac-(EBTHI)ZrCl2 and rac-(EBI)ZrCl2 catalysts (EBTHI = ethylenebis(η5-tetrahydroindenyl); EBI = ethylenebis(η5-indenyl)) have been studied using specifically isotopically labeled propylene: CH2CD13CH3. These isospecific catalysts provide predominantly the expected [mmmm] pentads with [−CH2CD13CH3-] repeating units (13C NMR). Under relatively low propylene concentrations at 50 and 75 °C, where stereoerrors attributable to chain epimerization are prevalent, 13C NMR spectra reveal 13C-labeled methylene groups along the polymer main chain, together with [CD13CH3] units in [mmmr], [mmrr], and [mrrm] pentads and [CH13CH3] 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(CH2D)(13CH3)P] (P = polymer chain), followed by the reverse steps to yield two stereoisomers of [Zr−CHDCH(13CH3)P] and [Zr−13CH2CH(CH2D)P], as well as unrearranged [Zr−CH2CD(13CH3)P]. The absence of observable [−CH2CH13CH2D−] in the [mrrm] pentad region of the 13C NMR spectra provides evidence that an allyl/dihydrogen complex does not mediate chain epimerization.

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