Methylaluminoxane (MAO) is the most widely used cocatalyst in single‐site olefin polymerization, yet its molecular structures have decisively resisted characterization. Computational chemistry has played an indispensable role throughout the characterization efforts, from the earliest cage‐based structural proposals inspired by tert‐butylaluminoxane analogs through systematic hydrolysis‐based modeling to the identification of two‐dimensional sheet structures as the thermodynamically preferred motif. This review traces that computational journey, with particular attention to the methodological advances and failures that shaped it. The discovery that widely used density functional theory methods for MAO carry systematic errors for four‐coordinate aluminum–oxygen environments, and that vibrational entropy rather than electronic energy is the decisive thermodynamic quantity in the experimentally relevant size domain, fundamentally redirected the field. The resulting ovalene‐based sheet model for the dominant [(AlOMe)16(AlMe3)6Me]− anion is simultaneously consistent with electrospray ionization mass spectrometry, solid‐state NMR, X‐ray crystallography, and synchrotron pair distribution function data, and yields catalyst activation barriers in reasonable agreement with experiment. The structural foundation now established for the dominant anionic component of fresh MAO opens a realistic path toward understanding the full complexity of the MAO mixture and toward rational design of next‐generation aluminum‐based cocatalysts.
Mikko Linnolahti, Perttu Hanhisalo, Aleksi Vähäkangas· ChemPlusChem· 0 citations
Methylaluminoxane (MAO) is the most widely used cocatalyst for olefin polymerization, yet its molecular structure remains incompletely understood. Computational methods are essential for elucidating the structure and reactivity of MAO, but their accuracy has been difficult to assess. Benchmarking based on trimethylaluminum (TMA) dimerization has proven insufficient because it omits the oxygen environments central to MAO chemistry. Here, we benchmark 25 density functional methods against DLPNO-CCSD(T) and RI-MP2 reference calculations for a set of MAO oligomers, evaluating geometry, electronic energy, vibrational frequencies, and thermodynamic properties. We identify energetic biases toward specific structural motifs (µ4-O and µ-Me) in ten functionals, with errors ranging from under 5 kJ mol-1 to over 45 kJ mol-1. ωB97X-D4 and MN15 show the largest errors and should be avoided for pure MAO systems. Two functionals, ωB97X-V and ωB97M-D4, accurately reproduce reference electronic energies without structural bias. For vibrational properties, quasi-harmonic treatment effectively reduces method-dependent entropy variations, and thermochemical corrections prove robust across functionals. However, extending these methods to metallocene-MAO ion pairs reveals distinct system-dependent behavior that warrants further investigation. Together, these findings highlight that functional performance is system-dependent, with significant implications for computational studies of catalyst activation mechanisms.
