Probing the Structure of Methylalumoxane (MAO) by a Combined Chemical, Spectroscopic, Neutron Scattering, and Computational Approach

Abstract

The composition of methylalumoxane (MAO) and its interaction with trimethylaluminum (TMA) have been investigated by a combination of chemical, spectroscopic, neutron scattering, and computational methods. The interactions of MAO with donor molecules such as THF, pyridine, and PPh 3 as a means of quantifying the content of "free" and "bound" TMA have been evaluated, as well as the ability of MAO to produce [Me 2 AlL 2 ] + cations, a measure of the electrophilic component likely to be involved in the activation of single-site catalysts. THF, pyridine, and diphenylphosphinopropane (dppp) give the corresponding TMA−donor ligand complexes accompanied by the formation of [Me 2 AlL 2 ] + cations. The results suggest that MAO contains not only Lewis acid sites but also structures capable of acting as sources of [AlMe 2 ] + cations. Another unique, but still unresolved, structural aspect of MAO is the nature of "bound" and "free" TMA. The addition of the donors OPPh 3 , PMe 3 , and PCy 3 leads to the precipitation of polymeric MAO and shows that about one-fourth of the total TMA content is bound to the MAO polymers. This conclusion was independently confirmed by pulsed field gradient spin echo (PFG-SE) NMR measurements, which show fast and slow diffusion processes resulting from free and MAO-bound TMA, respectively. The hydrodynamic radius R h of polymeric MAO in toluene solutions was found to be 12 ± 0.3 Å, leading to an estimate for the average size of MAO polymers of about 50−60 Al atoms. Small-angle neutron scattering (SANS) resulted in the radius R S = 12.0 ± 0.3 Å for the MAO polymer, in excellent agreement with PFG-SE NMR experiments, a molecular weight of 1800 ± 100, and about 30 Al atoms per MAO polymer. The MAO structures capable of releasing [AlMe 2 ] + on reaction with a base were studied by quantum chemical calculations on the MAO models (OAlMe) n (TMA) m for up to n = 8 and m = 5. Both −O−AlMe 2 −O− and −O−AlMe 2 −μ-Me− four-membered rings are about equally likely to lead to dissociation of [AlMe 2 ] + cations. The resulting MAO anions rearrange, with structures containing separated Al 2 O 2 4-rings being particularly favorable. The results support the notion that catalyst activation by MAO can occur by both Lewis acidic cluster sites and [AlMe 2 ] + cation formation.

The Dynamic Equilibrium Between (AlOMe)n Cages and (AlOMe)n·(AlMe3)m Nanotubes in Methylaluminoxane (MAO): A First-Principles Investigation

“…23 Some experiments suggest that the size of an average MAO oligomer, (AlOMe) n , is temperature dependent ranging from n = 9−30, 24 while others have proposed that up to 50−60 Al atoms comprise a typical species in MAO. 25 These hypothetical (AlOMe) n are reactive and exhibit latent Lewis acidity (LLA) because of the ring strain present in some of the Al−O bonds. 26,27 As a result, the residual TMA that is present in all MAO solutions interacts with the MAO oligomers such that the average C:Al:O ratio in MAO is estimated as being 1.5:1:0.75.…”

NMR and EPR trapping of the active species in the ethylene polymerization and trimerization catalyst systems

“…This tendency supports stable interest to the detailed mechanistic studies of metallocene and post-metallocene catalyzed polymerization [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, quite often the available information on the reaction mechanism remains insufficient.…”

Metallocene Catalysts

“…As the aluminum atoms in the unit structures of MAO are coordinatively unsaturated, the units join together forming clusters and cages. These have molecular weights from 1000 to 1800 measured by cryoscopy in benzene or by mass spectroscopy and are soluble in hydrocarbons especially in aromatic solvents (34). Figure 3 shows a probable unit structure and a cage structure of four of three [Al 4 O 3 Me 6 ] units.…”