Methyaluminoxane (MAO) Polymerization Mechanism and Kinetic Model from Ab Initio Molecular Dynamics and Electronic Structure Calculations

摘要

MAO is the cocatalyst in metallocene catalytic systems,which are widely used in single-site olefin polymerization due to their high stereoselectivity.To date,the structures of the catalytically active compound or compounds in MAO have eluded researchers.Although many structural models have been proposed,none are generally accepted.In this study,aspects of the formation mechanism of MAO are addressed.Molecular dynamics simulations at the MP2 level of theory were carried out for presumed elementary steps in MAO formation via hydrolysis of trimethylaluminum (TMA).Methane production was observed,in agreement with experiment,as well as intermediate species that are consistent with the known structural features of MAO and similar to isolated and structurally characterized aluminoxanes.A (CH_3)_3-AI-OH_2 species,which we denote as TMA-OH_2,containing a stable Al-O single bond emerged as the building block molecule.From this species,a hexameric cage was formed and activation barriers for the various reactions were calculated.Three distinct channels were identified for growth beyond the hexameric cage.It was concluded that MAO formation is a step polymerization through a bifunctional monomer,with [(CH_3)AI-O] as the structural unit and a kinetic model was proposed.The structures that emerged were in agreement with the crystallographic evidence for aluminoxanes and support the experimental data regarding the MAO chemical composition.