A Density Functional Theory (DFT) Quantum-Chemical Approach to the Real Structure of Poly(methylaluminoxane)

Abstract

DFT quantum chemical calculations have been performed in order to optimize the geometric and electronic cage structure of poly(methylaluminoxane) (MAO) with oligomerization degree n = 9–15, and to find such structures that fit most closely the existing experimental data on the MAO composition and structure. The following peculiarities of the MAO structure were found: i) In “classic” MAO (n = 9, 12, 15; Al : CH3 : O = 1 : 1 : 1), which has a triple‐layer cage structure, the inner layer contains highly reactive Al‐O bonds. ii) The reaction between “classic MAO” and trimethylaluminium (TMA) proceeds 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 calculated geometric and electronic structures of “true” MAO with n = 6, 9, 12 are presented. iii) “True” MAO and “classic” MAO exist in equilibrium. The driving force for the formation of “true” MAO is the decrease in enthalpy, and of “classic” MAO the increase in entropy, in the equilibrium reaction between “classic” MAO and TMA.

A DFT Quantum-Chemical Study of Ion-Pair Formation for the Catalyst Cp2ZrMe2 /MAO

“…Molecular-orbital analysis of "classic" MAO shows that the LUMO (lowest unoccupied molecular orbital) has the most contribution from the anti-bonding r*-orbitals of Al 2 1O 2 bonds. [16] Basing on the above data, it seems reasonable to suggest that the Al 2 1O 2 bond is the most reactive, and exactly this bond seems to be responsible for the formation of the Zr1O complex as a result of the reaction between "classic" MAO (AlMeO) 6 and (g 5 -C 5 H 5 ) 2 ZrMe 2 . The calculated data presented in Table 1 and Figure 1b show that the Zr1O complex represents a stable molecular compound with the length of the Zr1O bond equal to 2.15 .…”

“…[21] Models of "true" MAO structures with the composition [(AlMeO) 3m N mAlMe 3 ] (m = 2, 3, 4) have been suggested. [16] A common principle for the design of molecular MAO structures characterized by the ratio Al/ Me/O = 1 : 1.5 : 0.75 has been proposed, [13,16] in which it is suggested that the (AlMeO) 3m N mAlMe 3 structures ("true" MAO) are formed by bonding the mAlMe 3 molecules to the cage structure (AlMeO) 3m ("classic" MAO), with m = 2, 3, 4. Ziegler et al [17] have considered the different MAO cage structures with the general formula (AlMeO) n (n = 4-30).…”

“…In "classic" MAO the reactive centers are represented by the highly reactive bonds Al 2 1O 2 . [16] High reactivity of the Al 2 1O 2 bonds in the reactions of aluminoxanes with organometallic compounds was stated for the first time by Barron et al [9] for tert-butylaluminoxane of the composition [Al(t-Bu)O)] 6 . They used the definition "latent Lewis acidity" to stress the ability of the Al1O bonds of aluminoxane to react with organometallic compounds via the cleavage of these bonds.…”

“…[13] Molecular models of three-dimensional oxo-bridged structures of MAO calculated by DFT quantum-chemical methods have been presented in the literature. [13, 16 -20] The results of the DFT quantum-chemical calculations presented in our work [16] suggest an existence of MAO whith three-dimensional structures of two types : "classic" MAO with the ratio Al/Me/O = 1 : 1 : 1 and "true" MAO with the ratio Al/Me/O = 1 : 1.5 : 0.75. These forms of MAO are in equilibrium, as illustrated by Reaction 1:…”

“…The reactive centers in "true" MAO are the tri-coordinated aluminum atoms in the OAlMe 2 groups. [16] In the present work we have performed the quantum-chemical modeling of the reactions between Cp 2 ZrMe 2 and reactive centers of "classic" MAO with the composition (AlMeO) 6 , "true" MAO with the composition (AlMeO) 6 N 2AlMe 3 , and with the more simple molecules of AlMe 3 and Al 4 O 2 Me 8 .…”