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

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.