Homogeneous metallocene catalysts, e.g., derived from or related to zirconocene dichloride introduced a level of microstructural control that was unavailable with heterogeneous systems.
Initially ligand modifications focused on various cyclopentadienyl derivatives, but great diversity was uncovered through high throughput screening.
[3] Efforts to copolymerize polar comonomers led to catalysts based upon nickel and palladium, inspired by the success of the Shell Higher Olefin Process.
[6] These complexes catalyze the homopolymerize ethylene to a variety of structures that range from high density polyethylene through hydrocarbon plastomers and elastomers by a mechanism referred to as “chain-walking”.
By modifying the bulk of the alpha-diimine, the product distribution of these systems can be 'tuned' to consist of hydrocarbon oils (alpha-olefins), similar to those produced by more tradition nickel(II) oligo/polymerization catalysts.
[3] A salicylimine catalyst system based on zirconium exhibits high activity for ethylene polymerization.