In situ FTIR spectroscopy during addition of trimethylaluminium (TMA) to methylaluminoxane (MAO) shows no formation of MAO-TMA compounds

Commercial methylaluminoxane (MAO) solutions were dried under reduced pressure to remove trimethylaluminium (TMA) and solvent. TMA has been added to a solution of such dried MAO, and the response was followed by a special in situ FTIR spectroscopic technique. No reaction was seen between TMA and MAO in toluene solution at room temperature within a time span of up to 4 hours. TMA and MAO remained in the solution as separate components, as verified by the spectra of the mixtures being superpositions of the spectra of the two components, linear dependence of the main TMA dimer peaks to the added amount, and appearance of isosbestic points. Experiments at 80°C gave the same results. A further important observation is the identification of bridging methyl groups in the TMA-depleted MAO, clearly seen as an IR band at 1257 cm-1. This, in addition to the documented lack of reaction between MAO and TMA, forces a rethinking of the hitherto proposed models of the MAO structure.