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

Two (SiO₂/MgR₂/MgCl₂)·TiCl_x model catalysts are made by refluxing TiCl₄ 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.