Mysterious MAO: Towards a better structural understanding of Methylalumoxane and the development of well-defined Alumoxanes

Since its discovery in the early eighties, methylalumoxane (MAO) has grown from an academic serendipity to an industrial commodity. Today it is produced on a large scale and widely used as a cocatalyst for various olefin oligomerization and polymerization processes. Despite years of investigation, not much is known about the exact nature of MAO. We have investigated MAO and modified its structure in order to gain a better understanding of its configuration and working principles and to develop more stable, well-defined cocatalysts. TEM studies into the large MAO particles show a highly regular material consisting spheres connected in a chain like fashion. These spheres grow over time through sintering of the particles. Replacement of the O2− for in MAO for RO− and (RO2)− moieties led to isolatable complexes that give insights into the possible hydrolysis pathways of trimethylaluminum during MAO synthesis. Similarly the O2− could be replaced for a RN2− fragment again leading to well-defined structures based on the size of the R substituent. Unfortunately these clusters do not functions as cocatalysts. Alternative nitrogen based ligand frameworks allow for the isolation of complexes with a masked three-coordinate Al site. These are highly reactive towards small molecules but are inert towards polymerization precatalysts. Replacement of part of the (MeAlO)n units with ArB(OH)2 or HB(NRH)2 fragments gave again well-defined results. Easy ligand exchange between Al and B gave for unprecedented complexes whereas the introduction of bulky substituents led to the isolation of various boralumoxanes. These were found to be mildly active cocatalysts