Kinetics and Mechanism Comparison between Cr/Ti-Based Bimetallic and Ti-Based Monometallic Catalysts for Ethylene Polymerization

Two (SiO 2 /MgR 2 /MgCl 2 )·TiCl x model catalysts are made by refluxing TiCl 4 with 0.35 wt% Cr modified silica gel/alkyl Mg adducts or silica gel/alkyl Mg adducts, which are named as Cr/Ti-based bimetallic Cat-1 and Ti-based monometallic Cat-2, respectively. The kinetics, active center counting, morphology, and polymer characterizations are studied to disclose the effect of low loading Cr active sites on the Cr/Ti-based bimetallic Cat-1 polymerization under mild conditions. The activity of Cat-1 is 120.4% higher than that of Cat-2, with a 114.1% higher [ C *]/[ M ] value. Morphology results show the Cat-1 fragmentation in the first 3 min is highly accelerated, which helps to release buried clustered Ti sites. Differential scanning calorimetry results show that low-temperature heat absorbing shoulder of polyethylene (PE) from Cat-2 demonstrates the signal of low crystallinity polymer made by Cat-2 during the first 60 s, verifying the fluffy polymer in morphology results. GPC results show PE from Cat-1 has a higher M w in the first 3 min while a lower M w in the end. The Cat-1, which release active sites faster, has a high M w in the early time. Lower M w in the 900 s attributes to the effect of relative lower M w polymer made by Cr sites, compared with Cat-2.