Interaction of Chromocene with the Silica Surface, and Structure of the Active Species for Ethene Polymerization

The anchoring process of Cr(Cp)2 on silica hydroxyl groups occurs by elimination of C5H6 and formation of Si—O—CrCp mononuclear species. These anchored species are then able to adsorb incoming Cr(Cp)2 to give catalytically inactive dimeric species. These reactions are heavily diffusion-controlled. Interaction of CO and NO with mononuclear anchored species gives well defined dicarbonylic and dinitrosylic compounds, while the reaction of CO with dinuclear (catalytically inactive) species gives much more complex polycarbonylic compounds. The Si—O—CrCp mononuclear surface species are the active sites for ethene polymerization. The chain-initiation mechanism probably consists of the formation of a metallocyclic structure.