POLYMERIZATION OF ETHYLENE AT HIGH-TEMPERATURE BY VANADIUM-BASED HETEROGENEOUS ZIEGLER-NATTA CATALYSTS .1. STUDY OF THE DEACTIVATION PROCESS

Kinetics of the polymerization of ethylene initiated by heterogeneous vanadium-based Ziegler-Natta catalysts (VCl3-1/3 AlCl3) have been studied at high temperature (160-degrees-C, 5 bars) and compared with a titanium-based system. For the V catalyst, the dependence of the polymerization activity versus time, with the nature and the concentration of the associated aluminum alkyl, has been investigated. Kinetic results have also been correlated with the oxidation state of vanadium in the polymerization conditions. Despite the relatively high initial activity a low productivity is obtained; it can be attributed to a very fast deactivation of the active sites due to the reduction of vanadium III into vanadium II. The effect of the nature of the alkyl aluminum component of the catalytic system on the reduction process is shown. A kinetic model for the polymerization is proposed.