Metallocene ion pairs: A direct insight into the reaction equilibria and polymerization by 13C NMR spectroscopy

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Abstract
The reaction equilibria of Cp2Ti13CH3Cl and Cp2Ti(CH3)2 with AlMe3 (TMA) and/or methylaluminoxane (MAO) have been investigated by 13C NMR. Several adducts have been identified. A study of the 13C 90% enriched ethylene polymerization in an NMR tube in the presence of the above catalytic systems, in the most experimentally significant conditions, and a comparison of the NMR data with the catalytic activity have been made as well. It has been shown that: i) some species are side products, inactive for addition ethylene polymerization; ii) active cation‐like species such as Cp2TiMe+Cl·[AlMeO]n‐ and Cp2TiMe+Me·[AlMeO]n‐ are formed in titanocene‐MAO systems. Concerning the role of AlMe3, contained in MAO solutions, it has been shown that: a) AlMe3 is mainly bound to MAO; b) if some “free” AlMe3 exists in solution it is not the actual cocatalyst in the metallocene‐MAO based catalytic systems; c) the amount of AlMe3 influences either active or inactive species.