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Methylaluminoxane (MAO) is used as a precatalyst activator for million ton2scale production of commercial polyolefins, but its precise structure and associated activation mechanisms have been a fundamental research puzzle for more than 40 years. We report here the crystallographic characterization of an active MAO component, which reveals a discrete two-dimensional sheet cluster [Al33O26(CH3)47][Al(CH3)3]2 featuring two trimethylaluminum units, Al(CH3)3, coordinated to two unsaturated aluminum sites. Our structural data are consistent with the hypothesis that the active sites bear coordinated Al(CH3)3 and provide [Al(CH3)2]+ for precatalyst activation. Quantum chemical calculations revealed the most preferred sites for [Al(CH3)2]+ abstraction (a change in Gibbs free energy of 0.0 kilocalories per mole). Olefin polymerization tests on metallocenes activated with the crystallized MAO show higher activities than with the commercial MAO. [ABSTRACT FROM AUTHOR]

The coordinative/insertive stereoregular homopolymerizations and copolymerizations of styrene and methyl methacrylate mediated by a highly active single-site organotitanium catalyst are described. A new mechanism involving a sequential conjugate addition steps to explain the copolymeric results are suggested.

A synthetic, molecular structure, thermochemical and ab initio study of bonding and bonding energetics in the group 6 metallocene oxo series Cp2Mo/(MeCp)2MO is presented. Among findings are the efficient and high-yield syntheses of the pairs Cp2MH2/(MeCp)2MH2, Cp2MCl2/(MeCp)2MCl2 and Cp2MO/(MeCp)2MO and a 'bent sandwich' geometry in the molecular structure of (MeCp)2WO. These findings lead to increased understanding of the electronic structure, molecular geometry, bond strengths and chemical reaction patterns of metallocene oxo complexes.

The synthesis of ruthenium tetraazannulene complex, (Me8taa)Ru(PPh3)2, is described. Analysis showed that the PPh3 adduct is labile toward ligand substitution. The complex has been observed to act as a catalyst precursor in the hydrogenation and isomerization of 1-hexene under mild reaction conditions.

The solution thermochemistry of group 8 complexes is determined in relation to metal-ligand bond enthalpies for mono- and bidentate ligands in the iron tricarbonyl system. In addition, the chelate ring size effect of bidentate ligands and their effect on organoiron complex stability is reported. An overall relative order of stability was determined as follows: for monodentate ligands, AsPh3 < AsEt3 < tertiary phosphine; for bidentate phosphine and arsine ligands, dppm < arphos < dmpm < dppb < dppv < dppp < dppe < dmpe < dcpe < depe; for dienes and enones, benzylideneacetone < chalcone < cyclooctatetraene < cyclohexadiene.

Early transition metal alkylidene compounds have a rich and diverse chemistry, including a central catalytic part in olefin metahesis processes. The first experimental analysis of metal alkylidene bonding in a Ta(V), revealing a large Ta=C bond enthalpy is reported. The results show that early transition metal aklylidene-forming alpha-elimination processes are fuelled by strong metal-alkylidene bonds.