A DFT quantum-chemical study on the structures and active sites of polymethylaluminoxane

The electronic structure and geometry of polymethylaluminoxane (MAO)[-Al(CH 3)O-], with different size (n = 4- 12) have been studied using quantum-chem ical DFT (density functional theory) calculations. It has been found: 1) St arting from n = 6, the three-dimensional oxo-bridged (cage) structure of MA O is more stable than the cyclic structure. 2) Both for cage structure and for cyclic structure the Lewis acidity of Al atoms characterized by their n et positive charge amplifies with increasing size of MBO (n). 3) Trimethyla luminium (AlMe3) reacts with the cage structure of MAO with cleavage of an Al-O dative bond and formation of acidic tri-coordinated Al-v and basic di- coordinated O-v atoms in the MAO molecule. Two molecules AlMe3 are associat ed with acidic Al-v and basic O-v centers. As the MAO increases in size, th e acidity of Al-v centers amplifies and the distance Al-v-(AlMe3) shortens; on the contrary, interaction of AlMe3 with O-v centers weakens and the dis tance O-v-(AlMe3) increases with increasing n value. The total heat of Al2M eo interaction with MAO (sum interaction of Al-v-(AlMe3) and O-v-(AlMe3)) n oticeably decreases as the size of MAO increases (from 50.9 kcal/mol for n = 4 to 20.2 kcal/mol for n = 12). It is proposed that acidic Al-v and basic O-v centers formed in the cage structure of MAO interact with zirconocene yielding 'cation-like' zirconium active centers.