Eva Zurek, Tom Woo and Dr. Tom Ziegler, Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N-1N4
Density Functional Theory (DFT) has
been used to calculate the energies of over 30 different methylaluminoxane
(MAO) caged structures with the general formula (MeAlO)n, where
n ranges from 4 to 16.
Via the use of a least-squares fit
a formula has been devised which predicts the relative energies of the
MAO structures reasonably well. These energies in conjunction with frequency
calculations based on molecular mechanics have been used to estimate the
enthalpies, free energies and equilibrium constants for the general equilibrium
:
(MeAlO)n1+ (MeAlO)n2 Æ (MeAlO)n1+n2
This procedure makes it possible to estimate the relative abundence of (MeAlO)n as a function of n.
Similar procedures have been used to study the equilibrium between Al2(CH3)6 and MAO in the equilibrium :
(AlCH3)m(MeAlO)n Æ (AlCH3)m-2x(MeAlO)n +xAl2(CH3)6
and the relative abundence of (AlCH3)m(MeAlO)n for different m.
We have further shown from topological arguments that the MAO cage structure contains a limited number of squares compared to hexagons and octagons. It is futher suggested that the limited number of squares with their strained and reactive Al-O bonds can explain the high molar Al:(Catalyst) ratio required for activation. The mode of activation as well as the way in which AlCH3 is coordinated to MAO is also discussed.