Organochromium Chemistry - Ethylene Polymerization and Oligomerization

Ethylene Polymerization and Oligomerization

Chromonium catalysts are important in ethylene polymerization. The Phillips catalyst are prepared by impregnating chromium(VI) oxide on silica followed activation in dry air at high temperatures. The bright yellow catalyst becomes reduced by the ethylene to afford a probable Cr(II) species that is catalytically active. A related catalytic systems developed by Union Carbide and DSM are also based on silica with chromocene and other chromium complexes. How these catalysts work is unclear. One model system describes it as coordination polymerization:

With two THF ligands the catalyst is stable but in dichloromethane one ligand is lost to form a 13 electron chromium intermediate. This enables side-on addition of an ethylene unit and a polymer chain can grow by migratory insertion.

Chromium compounds also catalyse the trimerization of ethylene to produce the monomer 1-hexene.