| Popis: |
Ethylene polymerization was carried out in a slurry micro reactor (100 cm3) under mild reaction conditions (25°C, P(C2H4) = 1 atm) to slow the polymerization rate during the early stages of reaction on a 5 wt% Cr/SiO2 support catalyst. Polymer yield (g polymer/g catalyst) and scanning electron micrographs (from SEM) were obtained as a function of time. Polymerization seems to occur by addition of ethylene at the active sites, first on the catalyst surface primarily within pores and then on such sites buried within the mass of growing polymer. The evidence suggests that the rate decreases because of pore-blocking by the polymer, and then gradually increases as additional sites become exposed from fracturing of the catalyst particles. “Cauliflower-like” polymer structures grow about the exterior of both fragmented and clustered smaller particles, and these, in turn, also fracture visibly. The particle exteriors were covered by a fibrous network of polymer strands and major fissures whose texture should present little resistance to monomer transport to the active sites. Finally, the rate of polymerization increases exponentially, likely from the growing number of active sites and not from the exothermic heat release within the polymer/catalyst mass. |
