Toluene-d, solutions of methylaluminoxane (MAO) after the addition of different amounts of trimethylaluminium (TMA) have been studied by means of 'H and I3C NMR at a temperature as low as -78°C to freeze out some of the fluxional equilibria of the molecule. A comparison of the half height width of the NMR signal W,,, and of the mole ratio of the methyl signals before and after addition of known amounts of TMA allowed us to reach the following conclusions about the TMA contained in MA0 solutions: a) TMA is mainly bound to MAO; b) TMA participates in disproportionation reactions.

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A study of the reactions of Cp2Ti13CH3CI and Cp2Ti(CH3)2 with AIMe3 (TMA) and/or methylaluminoxane (MAO) and of the 13C 90% enriched ethylene polymerization in situ in the presence of the same catalytic systems has been made by 13C NMR spectroscopy. The NMR data have been compared with the catalytic activity. Active cation‐like species such as Cp2TiMe+CI·[AIMeO]n− and Cp2TiMe+Me·[AIMeO]n− and side products have been identified. The role of “free” and “associated” AIMe3, contained in MAO solutions, has been elucidated.