Investigating the Behaviour of a Bi-supported SiO2/TiCl4/THF/MgCl2 Catalyst in Slurry Ethylene Polymerization: Activity and Molecular Weight

Bi-supported catalyst SiO2/TiCl4/THF/MgCl2 was prepared via four steps including, (i) heat activation of silica at 600ºC, (ii) preparation of precursor composition, (iii) impregnation of catalyst components on silica, and (iv) catalyst prereduction with alkyl-aluminium. Homopolymerization was carried out at 3-15 bar pressure and 65-85ºC temperature. The GPC data was analyzed using the concept of multincentre catalysis and four types of active centres detected that differ in molecular weight. The constancy of molecular weight of each active centre with respect to monomer concentration showed that a bimolecular chain transfer mechanism exists. An increase in the ethylene concentration led to a gradual decrease of polymer fractions produced by centres I, II and III and a progressive increase in the fraction produced by centre IV. The dependency of molecular weight and activity on some other parameters such as temperature, Al/Ti ratio, polymerization time and hydrogen concentration were examined. Activity as a function of polymerization temperature showed a maximum at 70ºC. The polymerization rate was first order with respect to monomer concentration. A correlation was obtained between the average molecular weight and hydrogen concentration. The molecular weight decreased th increasing polymerization temperature, H2 concentration and Al/Ti ratio.