Characterization of modified methylaluminoxane by ion mobility spectrometry mass spectrometry and ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry

The analysis of methylaluminoxanes (MAOs) is strictly hampered by their high reactivity, in particular, toward oxygen and water. Commercially available modified MAO (MMAO) has received major interest as a cocatalyst in olefin polymerization. The cocatalytic activity of MMAO is mainly linked to its chemical composition, and a complete understanding of the precise structure of these complex molecules is still lacking. To this aim, we have used in this study our recently developed piASAP technique to analyze air-sensitive MMAO-12 by mass spectrometry. The analysis of MMAO-12 was performed by ion mobility-mass spectrometry and Fourier transform mass spectrometry. The molecular analysis unveiled ten main families of MMAO clusters, whose isotopic patterns are separated by m/z 57.999 corresponding to the MAO repeat unit (CH3AlO). The collision cross-section (CCS) values of these MMAO clusters were determined by ion mobility spectrometry (IMS). It was found that the detected lower molecular weight (MW) oligomers are probably representative segments of MMAO-12 that could be derived from pyrolysis and hydrolysis, as it has been typically observed in the pyrolysis of polymers by atmospheric solid analysis probe (ASAP). High resolution mass spectrometry and ion mobility spectrometry with paraffin inert ASAP provide valuable information on the structure of modified methylaluminoxane (MMAO).