Novel Imido-Cr/Silica Ethylene Polymerization Catalysts Modified from the Phillips Catalyst

In this work, novel imido-Cr/silica catalysts are synthesized by modifying the Phillips catalyst CrOx/silica with isocyanates. The imido-Cr/silica, which has moderate activities of 200-300 kgPE molCr(-1) bar(-1), shows lower efficiency than CrOx/silica for C2H4 polymerization. On the other hand, regarding to the modified catalysts, the productivity can be improved almost linearly with increasing C2H4 pressure, and very stable activities are observed during the 1 h run of C2H4 polymerization. When compared with CrOx/silica, the modified catalysts can produce polyethylene (PE) with much higher molecular weight (MW) and broader molecular weight distribution (MWD), which is featured by a distinct ultrahigh MW shoulder. Meanwhile, the MW and MWD of the PE products can be reduced obviously by adding a small amount of H-2 before C2H4 polymerization. In addition, it is found that more 1-hexene can be copolymerized by CrOx/silica, and it is preferentially incorporated into a portion of the PE chains. While such a preference is much less significant for the modified catalysts.