TRANSITION-METAL CATALYSTS FOR OLEFIN POL YMERIZATION .1. LOW-PRESSURE SYNTHESIS OF POLYETHENE

The preparation of a variety of polyethene products can be accomplishe d either by radical mechanisms or by transition metal catalyzed polyin sertions of ethene monomers into metal-carbon bonds. The first attempt requires rigorous conditions, like high pressure and high temperature and leads to branched polyolefin products. The catalyzed process can be performed at low pressure and at ambient temperature. Variations in the catalyst composition allow an effective control of the polymer pa rameters, such as molecular weight, molecular weight distribution, and comonomer content. The present review focuses on reaction mechanisms and on the development of high-mileage transition metal catalysts, bas ed on Ti-III, Zr-IV, and Cr-II, as active species. The usage of these catalysts in different polymerization processes is discussed with resp ect to the properties of the resulting polyethene commodities.