Progress in vanadium complex catalysts and their catalytic behavior toward copolymerization of ethylene with α-olefins

Abstract

<p indent=0mm>Ethylene-propylene rubber, linear low density polyethylene and polyolefin elastomer, which are prepared by copolymerization of ethylene with α-olefins, have an important market share of the industry due to their excellent properties. Catalyst plays important roles in the copolymerization of ethylene with α-olefins. Many vanadium complexes in the oxidation states of +3, +4 and +5 have been developed as precatalyst for the copolymerization. The progress of vanadium complex catalysts and their use in copolymerization of ethylene with α-olefins were reviewed. The effects of chelated ligands on the catalytic activity of catalysts, composition, molecular weight, molecular weight distribution and sequence of the resulting copolymers were discussed. Vanadium(III) complexes are widely used since vanadium(III) species are usually considered as active species in olefin coordination polymerization. Vanadium(III) complexes bearing monodentate <italic>N</italic>-heterocyclic carbene (NHC) ligand or bidentate β-diketonate ligands have been used as prcatalyst in copolymerization of ethylene with propylene, affording ethylene-propylene copolymers with high propylene content. Ethylene-propylene copolymers with ultra-high molecular weight were prepared by using vanadium(III) complexes containing NHC ligand. Vanadium(III) complexes bearing chelated bidentate N^N or N^O, etc. ligands show high catalytic activity up to <sc>8820 kg mol^–1 h^–1</sc> toward copolymerization of ethylene with 1-hexene. The introduction of bulky substitutes on the ligand results in an increase in the reactive ratio of ethylene and a decrease in the reactive ratio of 1-hexene, and thus a change in the sequence of monomer in the resulting copolymer. Vanadium(III) complexes containing tridentate and tetradentate ligands display relatively lower catalytic activity than their analogues containing similar bidentate ligand. Most vanadium(IV) complexes display lower catalytic activity than vanadium(III) complexes bearing similar ligands. Ethylene-propylene copolymer with high propylene content could be prepared by copolymerization of ethylene and propylene catalyzed by vanadium(IV) complexes containing monodentate amide ligands. Vanadium(V) complexes bearing chelating aryloxides have been shown to be highly active (up to <sc>144400 kg mol^–1 h^–1)</sc> in copolymerization of ethylene with propylene, affording ethylene-propylene copolymers with low propylene content (less than 15%(mol)). Ethylene-propylene copolymers with high propylene content (more than 30%(mol)) could be prepared by vanadium complexes containing monodentate or bedentate ligands. It is interesting to note that quasi-living copolymerization of ethylene with propylene was realized by using vanadium(V) complexes containing NHC ligand and ethylene-propylene copolymers with ultrahigh molecular weight and narrow molecular weight distribution were obtained. In conclusion, the vanadium complex catalysts have displayed good catalytic behavior towards the copolymerization of ethylene with α-olefins. The vanadium complexes discussed here may be an inspiration for future work in the development of vanadium complex catalyst with high catalytic activity and high comonomer incorporation for copolymerization of ethylene with α-olefins.