The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes, LMX n (M ) Fe, X ) Cl, n ) 2, 3, X ) Br, n ) 2; M ) Co, X ) Cl, n ) 2), bearing chelating 2,6-bis(imino)pyridyl ligands L [L ) 2,6-(ArNCR 1 ) 2 C 5 H 3 N] is reported. X-ray diffraction studies show the geometry at the metal centers to be either distorted square pyramidal or distorted trigonal bipyramidal. Treatment of the complexes LMX n with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts converting ethylene to highly linear polyethylene (PE). LFeX 2 precatalysts with ketimine ligands (R 1 ) Me) are approximately an order of magnitude more active than precatalysts with aldimine ligands (R 1 ) H). Catalyst productivities in the range 3750-20600 g/mmol‚h‚bar are observed for Fe-based ketimine catalysts, while Co ketimine systems display activities of 450-1740 g/mmol‚h‚bar. Molecular weights (M w ) of the polymers produced are in the range 14000-611000. Changing reaction conditions also affects productivity and molecular weight; in some systems, a bimodal molecular weight distribution is observed. On the basis of evidence gathered to date, the lower molecular weight fraction is a result of chain transfer to aluminum while the higher molecular weight fraction is produced by a combination of mainly -H transfer and some chain transfer to aluminum.

Polymerization of propylene with bis(2-phenylindenyl)zirconium dichloride and bis(2-[3,5-bistrifluoromethyl)indenyl]zirconium dichloride produces elastomeric polypropylene. The elastomeric properties of these polymers have been interpreted in terms of a stereoblock microstructure. Analysis of the microstructure by 13C NMR reveals isotactic pentad contents [mmmm] ranging from 6 to 74%. The hafnium derivatives were investigated to probe the influence of the transition metal on the polymerization behavior. The hafnium-based catalysts yield polypropylenes that are significantly less isotactic than the corresponding zirconium compounds, although molecular weights and productivities were similar for hafnium and zirconium derivatives. The X-ray crystal studies of these catalysts show nearly identical structures for corresponding zirconium and hafnium compounds. Variable temperature NMR of the metallocene dibenzyl analogues showed behavior consistent with rotation of indenyl ligands, where rotation of the indenyl ligand of bis(2-phenylindenyl)zirconium dibenzyl was 6800 Hz and that of hafnium was 6700 Hz at 20 °C. Based on our proposed mechanism of polymerization, the origin of the different microstructures can be ascribed to a faster propagation of the isospecific zirconium site relative to that of the isospecific hafnium site.

H 3 ] with AlMe 3 at elevated temperature gives, via migration of a methyl group to the ligand backbone, the pseudo-five coordinate dimethylaluminium species {2-[ArNCR(Me)],6-(ArNCR)C 5 H 3 N}-AlMe 2 (1a-c); upon treatment with B(C 6 F 5 ) 3 , 1a-c cleanly afford the cationic methyl complexes [{2-[ArNCR(Me)],6-(ArNCR)C 5 H 3 N}AlMe] + [MeB(C 6 F 5 ) 3 ] 2 (2a-c) which are active for ethylene polymerization.

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