Evidence for Methylaluminoxane (MAO) Molecular Structure and Reactivity from Ultrahigh Magnetic Field 27Al MAS NMR Spectroscopy Combined with DFT Calculations

Abstract

The structure and reactivity of methylaluminoxane (MAO), a reagent of major interest for olefin polymerization, both industrially and academically, has been probed using ultrahigh magnetic field solid-state NMR (28.2 T, 1200 MHz for 1H Larmor frequency). High resolution methods combined with density functional calculations allowed for the identification and quantification of five major aluminum sites, providing precise information on the structure of MAO at the molecular level. Based on reactivity studies with THF and [ZrCp2Me2], the main reactive centers are identified as bismethyl aluminum species stabilized via a bridging methyl group from a neighboring Al center, featuring both high chemical shift and quadrupolar coupling constants (162 ppm and 27.4 MHz, respectively). This approach demonstrates the ability to monitor the chemistry of MAO with unprecedented precision, enabling a state-of-the-art understanding of its structure and reactivity.