Chromacene

Chromium trioxide based catalysts supported on silica were developed by Phillips Petroleum at the same time as the original work of Ziegler and Natta. These catalysts polymerize non-polar olefins by mechanisms which are similar to those involved in Ziegler-Natta polymerization but do not give such good stereochemical control and are used principally for the preparation of linear polyethylene. More recently, supported catalysts of very high activity for the polymerization of ethylene have been prepared from chromates and also from chromacene. 

In Union Carbide processes, ethylene is polymerized in the gas phase. Pressures of 0.7-2 MPa (7-20 atmospheres) and temperatures of about 100°C are used. Polymerization is effected by proprietary catalysts, which consist of supported organochromium compounds such as chromacene ((C5H5)2Cr). The processes use a fluidized bed and are inherently simple in that ethylene serves as the ffuidizing gas (as well as reactant) and polyethylene is the bed material. The polymer is produced as granules (from which catalyst is not removed), which can be used directly. Since no solvent is involved, gas phase processes are simpler to operate and have lower energy consumption than other processes for high density polyethylene. 

---