KINETICS OF CHAIN TRANSFER DURING ETHYLENE POLYMERIZATION WITH CHROMIUM-OXIDE SILICA CATALYST

Homopolymerizations of ethylene have been performed with different partial pressures of ethylene (3.9-13.2 bar) and at different temperatures (334-371 K) with a prereduced chromium oxide on silica catalyst. Unlike Ziegler catalyst, the molecular weight of the polymer formed is not dependent on the ethylene pressure used during polymerization. An interpretation of this result is that chain termination by spontaneous beta-elimination is not frequent at the chosen polymerization conditions, and that the predominant chain termination reaction is beta-hydrogen transfer to monomer. On the other hand, the molecular weight is strongly dependent on the polymerization temperature. From experiments performed at different temperatures activation parameters have been estimated. From copolymerization experiments an estimate for the reactivity ratio between 1-hexene and ethylene has been calculated. The number of both vinyl and vinylidene fragments in the polymer increase with increased 1-hexene content in the reactor. The number of transvinylene fragments in the polymer is low, but this number also increases with increased 1-hexene content in the reactor. These results are discussed from the reaction scheme (Scheme 1).