Comparison of PGSE NMR and ESI-MS measurements on methylaluminoxane†

Abstract

PGSE NMR and ESI-MS studies of different grades of hydrolytic methylaluminoxane (MAO) demonstrate a correlation between the average dimensions and anion distribution of MAO. This correlation is revealed through aging and gelation studies of a commercial formulation of MAO. Formulations featuring an anion distribution skewed to higher m/z ratios have been observed to contain significantly higher activator contents as measured by ¹H NMR spectroscopy, despite being otherwise very similar. PGSE NMR studies in toluene vs. chlorobenzene media demonstrate that the average hydrodynamic dimensions of MAO are largely unaffected by solvent polarity, although aggregation behavior is somewhat sensitive to solvent polarity. As for catalyst activation and ion-pair speciation, the situation in polar solvents is complicated by the formation of solvated cations (and anions) in chlorobenzene which dramatically lower dimensions. DFT studies of model aluminoxane structures in the size range of MAO featuring a variety of architectures, molar masses and Me₃Al contents reveal a linear correlation between D_t, as measured by PGSE NMR, and molar mass using simple relationships and estimates of molecular volume if suitable high molecular weight standards are available. There is reasonable agreement in molar mass with available ESI-MS data, recognizing that MAO is not monodisperse.