A density functional theory (DFT) quantum-chemical approach to the real structure of poly(methylaluminoxane)

Full Paper: DFT quantum chemical calculations have been performed in order to optimize the geometric and electronic cage structure of poly(methylalumi noxane) (MAO) with oligomerization degree n = 9-15 and to find such structu res that fit most closely the existing experimental data on the MAO composi tion and structure. The following peculiarities of the MAO structure were f ound i) in "classic" MAO (n = 9, 12, 15: Al:CH3:O = 1:1:1), which has a tri ple layer cage structure, the inner layer contains highly reactive Al-O bon ds. ii) The reaction between "classic MAO" and trimethylaluminium (TMA) pro ceeds by the concerted mechanism, with the insertion of Al-CH3 groups into these Al-O bonds producing "true" MAO (Al:CH3:O = 1:1.5:0.75). The calculat ed geometric and electronic structures of "true" MAO with n = 6, 9, 12 are presented. iii) "True" MAO and "classic" MAO exist in equilibrium. The driv ing force for the formation of "true" MAO is the decrease in enthalpy, and of "classic" MAO the increase in entropy, in the equilibrium reaction betwe en "classic" MAO and TMA.