A new generation of iron and cobalt catalysts for the polymerization of olefins

Recent advances in the polymerization of olefins by late transition metal complexes have dramatically altered the traditional views of Ziegler-Natta chemistry. Once primarily the province of Group IV metals in high oxidation states, an array of chain growth polymerization catalysts spanning many transition metals and ligand sets has begun to attract considerable academic and industrial interest. For many years, the most well-known late metal catalysts used in industry were the neutral nickel systems employed in the Shell Higher Olefin Process (SHOP) for making linear a-olefins. Of late, however, cationic palladium catalysts also developed by Shell have been utilized to make perfectly alternating copolymers of olefins, and carbon monoxide. Cationic nickel and palladium systems have also been developed at The University of North Carolina at Chapel Hill and DuPont for the polymerization of ethylene and alpha-olefins. Building on the observations that beta-H elimination is competitive with chain growth in late metal complexes and that ligand exchange is often associative, bulky alpha-diimine ligands were used to retard chain transfer following beta-H elimination steps. These catalysts can produce highly branched or chain-straightened materials, depending on the nature of the olefin, metal, ligand, and reaction conditions.; Chapter Two describes the synthesis of a new alpha-diimine ligand, and the formation of cationic Ni(II) and Pd(II) complexes of this ligand. Ethylene polymerization data for these complexes are presented, and the ligand is compared to a similar species derived from theoretical calculations.; Chapter Three presents a significant extension of the ideas used to develop the Ni(II) and Pd(II) catalysts. Bulky, tridentate pyridine-bis-imine ligands complexes of iron and cobalt salts, upon activation with MMAO, are found to be extremely active for the polymerization of ethylene to strictly linear polymer (Tm's 133--139°C). The iron catalysts have turn-over frequencies (TOF's) exceeding 107/hr, and the cobalt catalyst TOF's approach 106/hr.; Chapter Four relates how reductions in the steric bulk of the pyridine-bis-imine ligands result in iron catalysts that are extremely active and selective for the preparation of linear alpha-olefins. The TOF's of these systems can exceed 108/hr, making them at least as active as the most active polymerization systems known. Even when running the reactions in alpha-olefms as the solvent, these catalysts produce >97% linear products and >99% a-olefin products. The Schulz-Flory oligomer distributions are easily manipulated in the desired range by modifications in temperature, ligand structure, solvent, or cocatalyst.; Chapter Five details the use of a series of iron catalysts for the polymerization of propylene. Using known metallocene complexes as a guide, a variety of tridentate ligands possessing different symmetries are prepared. The corresponding iron complexes are used to polymerize propylene, in all cases leading to isotactic polymer. The TOF's for propylene polymerization approach 4.0 x 10 5, representing a significant decrease from the rates for ethylene polymerization. Propagation proceeds via a secondary (2,1) chain-end control mechanism.