Comparison of PGSE NMR and ESI-MS Measurements on Methylaluminoxane

Abstract

PGSE NMR and ESI-MS studies of different grades of hydrolytic MAO demonstrate that the average dimensions and anion distribution of MAO are correlated with one another, as revealed through studies of aging and gelation of a commercial formulation of MAO. Formulations featuring an anion distribution skewed to higher m/z ratios have significantly higher activator contents as measured by 1H NMR spectroscopy in otherwise very similar formulations. PSGE NMR studies in toluene vs. chlorobenzene media demonstrate that the average hydrodynamic dimensions of MAO are largely unaffected by solvent polarity, though 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 for MAO featuring a variety of architectures, molar mass and Me3Al content reveal that a linear correlation between Dt, as measured by PGSE NMR, can be related to molar mass using simple relationships and estimates of molecular volume if suitable high molecular weight standards are available. There is a reasonable agreement in molar mass with available ESI-MS data, recognizing that MAO is not monodisperse.