Theoretical studies of the structure and function of MAO (methylaluminoxane)

Single-site homogeneous catalysts need to be activated by a co-catalyst or counterion. The high activity imparted by methylaluminoxane (MAO) has caused it to be one of the most important co-catalysts. In fact it can be argued that the success of the metallocenes is largely due to the discovery of MAO. However, despite intensive studies MAO has remained a 'black box'. The presence of multiple equilibria between different (AlOMe)_n oligomers coupled with the interaction between MAO and TMA has hindered experimental structural assignment of MAO. This has made it nearly impossible to characterize the dormant and active species present in olefin polymerization and therefore to theoretically investigate the mechanism of this process. Moreover, the binding of MAO with porous inorganic oxides such as silica, alumina and MgCl ₂ is currently not understood. Perhaps, even more puzzling is the fact that a large excess of MAO is necessary in order for polymerization to occur (Al:catalyst ratios of ∼10,000:1), whereas in the case of supported MAO this ratio is greatly decreased (ratios of ∼100:1-500:1). Despite the fact that the co-catalytic ability of MAO was discovered nearly 25 years ago, its exact structure and function is still unknown. In recent years, theoretical studies of MAO aiming to give further insight into the aforementioned issues have emerged. In this article, we give a quick overview of experimental studies of MAO and an in-depth review of recent theoretical investigations.