Since its discovery, methylalumoxane (MAO) has become of great importance as a cocatalyst in homogeneous olefin polymerization. The working principles of single-site polymerization catalysts are well-understood, but those of the cocatalyst MAO itself are not. Thus far structural and functional investigations have yielded limited insights and often give contradicting results. MAO's complex nature is due to multiple equilibria between undefined oligomers and "free" trimethylaluminum. Fundamental studies do not clearly portray the [a]38 Scheme 16. Structure of a MgCl 2 /Me 3 Al model compound.

In the presence of the weak donor octamethyltrisiloxane (OMTS), methylalumoxane (MAO) undergoes ionization through [Me 2 Al] + abstraction. The [Me 2 Al•OMTS] + [(MeAlO) x (Me 3 Al) y Me] − ion pairs formed can conveniently be studied by electrospray ionization mass spectrometry. The anion distribution changes with OMTS:Al ratio, with low molecular weight (MW) anions forming at high ratios due to oligomer degradation. Monitoring of room-temperature aging shows that MAO oligomers of low MW are slowly and systematically converted to species with higher MW through addition of (MeAlO) units, and intermediate oligomers are observed and assigned. This process occurs at room temperature but does not occur significantly over a period of months at low temperatures. Computational investigations of these species show large cagelike clusters with four-coordinate Al and threecoordinate O. All neutral structures have sites that could readily give up [Me 2 Al] + to form the observed ions, though sites that can lose [Me 2 Al] + without leaving a two-coordinate oxygen site are favored. These studies provide concrete insights into the absolute MW of MAO oligomers and their exchange over time.

A range of symmetric amidinate ligands RAmAr (R is backbone substituent, Ar is N substituent) have been investigated for their ability to stabilize calcium hydride complexes of the type RAmArCaH. It was found that the precursors of the type RAmArCaN­(SiMe3)2 are only stable toward ligand exchange for Ar = DIPP (2,6-diisopropylphenyl). The size of the backbone substituent R determines aggregation and solvation. The following complexes could be obtained: [RAmDIPPCaN­(SiMe3)2]2 (R = Me, p-Tol), RAmDIPPCaN­(SiMe3)2·Et2O (R = Np, tBu), AdAmDIPPCaN­(SiMe3)2·THF, and AdAmDIPPCaN­(SiMe3)2. Reaction of these heteroleptic calcium amide complexes with PhSiH3 gave only for larger backbone substituents (R = tBu, Ad) access to the dimeric calcium hydride complexes (RAmArCaH)2. (N,aryl)-coordination of the amidinate ligand seems crucial for the stability of these complexes, and the aryl···Ca interaction is found to be strong (17 kcal/mol). Addition of polar solvents led to a new type of trimeric calcium hydride complex exemplified by the crystal structures of (tBuAmDIPPCaH)3·2Et2O and (AdAmDIPPCaH)3·2THF. The overall conclusion of this work is that minor changes in sterics (tBu vs Ad) or coordinated solvent (THF vs Et2O) can have large consequences for product formation and stability.

Activation of Cp2ZrCl2 and Cp2ZrMe2 by methylaluminoxane (MAO) in toluene is largely complete at Al:Zr ratios of 100:1 to 200:1 as revealed by electrospray ionization mass spectrometry (ESI MS). The anions present undergo chlorination in the case of Cp2ZrCl2. DFT calculations reveal that chlorination of MAO is favorable and involves dissociation of Me3Al, followed by association of Me2AlCl. Ethylene polymerizations were conducted using these catalyst precursors in toluene. The activity vs [Zr] data are essentially identical, while higher MW polyethylene is formed with a narrower MWD at lower Al:Zr ratios in the case of Cp2ZrMe2. The activity vs [Zr] data could be fit to a model which invokes bimolecular deactivation of growing chains. ESI MS reveals that a dinuclear Zr2 cation with m/z 557 is formed on exposure of [Cp2Zr­(μ-Me)2AlMe2]+ to ethylene, in addition to other cations that are dinuclear with respect to Zr. Labeling experiments using ethylene-d 4 indicate that these dinuclear cations are derived from ethylene, either through direct incorporation in the case of m/z 557, or indirectly through incorporation of deuterium following e.g. β-H elimination. These experiments shed light on the need for high Al:Zr ratios for ethylene polymerization using soluble metallocene catalysts. The active catalyst [Cp2ZrR]­[MAO­(Me)] (R = H, Et or a higher homologue) suffers a second order deactivation, and thus activity improves upon dilution of the catalyst precursor at constant [Al].

The cover picture shows the birth of methylalumoxane MeAlO (MAO), or at least one of the possible MAO species, by reaction of trimethylaluminum with water. MAO is the cocatalyst of choice in Ziegler–Natta type olefin polymerization catalysis and is therefore produced in bulk quantities. Interestingly, it is also a compound shrouded in mystery and subject to significant controversy. This is the first comprehensive review article dealing with all aspects of MAO. Details are presented in the Microreview by H. S. Zijlstra and S. Harder on . For more on the story behind the cover research, see the .