Observation of zirconium allyl species formed during zirconocene-catalyzed propene polymerization and mechanistic insights

Abstract

Oligomeric Cp2Zr+–allyl species were detected in reaction mixtures of either [Cp2ZrMe][MeB(C6F5)3] or [Cp2ZrMe][B(C6F5)4] with propylene by a combination of 1H NMR spectroscopy and electrospray ionization (tandem) mass spectrometry techniques. Conjointly, the data imply that formation of Cp2Zr+–allyl species occurs via (re)coordination of alkene (propylene and vinylidene end groups of unsaturated polymer chains, respectively) to Cp2Zr+R (R=CH3, H, polymeryl), followed by intramolecular proton transfer from Cγ to R and release of RH. Analysis of the olefinic region of the 1H NMR spectra of the reaction mixture obtained from [Cp2ZrMe][MeB(C6F5)3] and propylene reveals the presence of a triplet resonance at δ∼5.2, which was attributed to the unprecedented 3-propenyl end group. A plausible mechanism for the origin of the 3-propenyl end groups is discussed. Additionally, a mechanism for incorporation of stereodefects into stereoregular polymers is also discussed. The cationic Cp2Zr+–allyl intermediates formed during polymerization may contribute to catalyst deactivation.