LIGAND EFFECTS AT TRANSITION-METAL CENTERS FOR ETHYLENE POLYMERIZATION

Ethylene polymerization catalysts based on titanium, vanadium, chromiu m, and zirconium display dramatic changes when the ligand environment at the active site is altered. Catalysts with a ligand environment inv olving chloride, alkoxide, electron-donor compounds, alkyl, cyclopenta dienyl, or indenyl groups show distinctive polymerization characterist ics. These characteristics include polymer molecular weight and molecu lar weight distribution. Catalyst response parameters such as comonome r incorporation, hydrogen response, and catalytic activity are influen ced by ligand environment at the active site. Changes in the level of unsaturation at active metal centers can also have a profound effect o n polymerization kinetics and the nature of the final reaction product s. Alpha-olefins play a subtle role in altering the structure of the a ctive centers. Copolymerization studies provide a powerful tool for pr obing the steric and electronic features of olefin polymerization cata lysts.