Mechanistic Aspects of Olefin Polymerization with Metallocene Catalysts: Evidence from NMR Investigations

Abstract

Isotopically 13C enriched Cp2M13CH3X metallocenes (M = Ti, Zr; X = 13CH3 , Cl) have been used as a probe for the reactivity of metallocenemethylaluminoxane catalysts for olefin polymerization. 1H and 13C NMR studies of the reactions between metallocenes and Lewis acids such as methylaluminoxane (MAO), AlMe3 , and the well-defined B(C6F5)3 have been performed in order to clarify the role of MAO in the activation of metallocenes. The isotopic enrichment has permitted us to study these Systems in conditions as close as possible to usual polymerization conditions. Evidence of the formation in Solution of “cationic” metallocene species having [X MAO] as counterion has been provided. In situ-polymerization of 13C enriched ethylene permitted us to gain an insight into the role of the observed products in the catalytic activity and to make the first direct Observation of methylenes of polymeryls of Cp2Zr-polymeryl species. A survey of these results is presented. Recent results of the study of the reactivity of two s-alkyl substituted aluminoxanes with two metallocenes Cp2ZrCl2 and Cp2 *ZrCl2 are also reported. It was found that the Cp2 *Zr-i- BuCl and Cp2 *Zr-i-OctCl stability is greater than that of Cp2Zr-i-BuCl and Cp2Zr-i-OctCl. The polymerization activities exhibited by different cocatalysts could be explained.