Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators
Anuj Joshi, Scott Collins, Mikko Linnolahti, Harmen S. Zijlstra, Elena Liles, J. Scott McIndoe
Abstract
Abstract Hydrolysis of trimethylaluminum (Me 3 Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI‐MS) using the donor additive octamethyltrisiloxane [(Me 3 SiO) 2 SiMe 2 , OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h‐MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non‐hydrolytic MAO was prepared using trimethylboroxine. The properties of this material, which contains incorporated boron, differ significantly from h‐MAO. In the case of MAO prepared by direct hydrolysis, oligomeric anions are observed to rapidly form, and then more slowly evolve into a mixture dominated by an anion with m/z 1375 with formula [(MeAlO) 16 (Me 3 Al) 6 Me] − . Theoretical calculations predict that sheet structures with composition (MeAlO) n (Me 3 Al) m are favoured over other motifs for MAO in the size range suggested by the ESI‐MS experiments. A possible precursor to the m/z 1375 anion is a local minimum based on the free energy released upon hydrolysis of Me 3 Al.