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

Bi-supported catalyst Si02lTiCI4ITHF/MgCI2 was prepared via four steps including,(i) heat activation of silica at 600°C , (ii) preparation of precursor composition, (iii) impregnation of catalyst components on silica , and (iv) catalyst prereductionwith alkyl-aluminium. Homopolymerization was carried out at 3-15 bar pressureand 65-85°C temperature. The GPC data was analyzed using the concept ofmultincentre catalysis and four types of active centres detected that differ in molecularweight. The constancy of molecular weight of each active centre with respect tomonomer concentration showed that a bimolecular chain transfer mechanism exists.An increase in the ethylene concentration led to a gradual decrease of polymer fractionsproduced by centres I, II and III and a progressive increase in the fraction producedby centre IV. The dependency of molecular weight and activity on some otherparameters such as temperature. AIITi ratio, polymerization time and hydrogen concentrationwere examined. Activity as a function of polymerization temperatureshowed a maximum at 70°C. The polymerization rate was first order with respect tomonomer concentration . A correlation was obta ined between the average molecularweight and hydrogen concentration. The molecular weight decreased with increasingpolymerization temperature, H2 concentration and AI/Ti ratio.