چكيده لاتين :
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 prereduction
with alkyl-aluminium. Homopolymerization was carried out at 3-15 bar pressure
and 65-85°C 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. AIITi ratio, polymerization time and hydrogen concentration
were examined. Activity as a function of polymerization temperature
showed a maximum at 70°C. The polymerization rate was first order with respect to
monomer concentration . A correlation was obta ined between the average molecular
weight and hydrogen concentration. The molecular weight decreased with increasing
polymerization temperature, H2 concentration and AI/Ti ratio.
