•A series of novel [ONNO]-type oxovanadium(V) complexes were synthesized and characterized.•These complexes contain two isomers, and trans configuration proved to be the main isomer.•These catalysts displayed efficient catalytic behavior toward ethylene (co)polymerization.•Catalytic property was significantly influenced by catalyst structure and reaction conditions.A series of oxovanadium(V) complexes bearing dianionic [ONNO] chelate ligands 2-[bis(3-R1-5-R2-2 -hydroxybenzyl) aminomethyl]pyridine (2a: R1 = tBu, R2 = H; 2b: R1 = CF3, R2 = H; 2c: R1 = OCH3, R2 = H; 2d: R1 = R2 = tBu) and 2-[bis(3-R1-5-R2-2 -hydroxybenzyl) aminoethyl]pyridine (2e: R1 = R2 = tBu) have been synthesized by reacting VO(OnPr)3 with 1.0 equiv. of the ligands in CH2Cl2. All these complexes were characterized by 1H, 13C, 51V NMR spectra and elemental analysis. X-ray structural analysis for 2d revealed a six-coordinate distorted octahedral geometry around the vanadium center in the solid state. It was observed that these complexes existed as a mixture of two isomers, and the main isomer had the oxo moiety in trans configuration to the tripodal nitrogen atom. In the presence of Et2AlCl and CCl3COOEt, these complexes displayed high catalytic activities for ethylene polymerization even at elevated reaction temperature, depending on ligand structures. The resultant polymers possessed high molecular weights and unimodal molecular weight distributions, indicative of a single active site nature. In addition, copolymerizations of ethylene and norbornene using precatalysts 2a–e were also investigated, and the observed catalytic activity was nearly comparable with that for ethylene homopolymerization. When the concentration of comonomer in the feed amounted to 3.0 mol/L, a NBE incorporation up to 41.5% could be achieved. Other reaction parameters that influenced the polymerization behavior, such as reaction temperature and Al/V (molar ratio), are also examined in detail.

A series of oxovanadium(V) complexes containing amine pyridine(s) phenolate ligands [ONN] (2a–f) have been synthesized in high yields (68–83%) by reacting VO(OnPr)3 with 1.0 equiv. of the ligands in CH2Cl2. These complexes were characterized by 1H, 13C and 51V NMR spectroscopy and elemental analysis. X-ray structural analysis for 2a, 2c and 2d revealed that these complexes adopt a six-coordinate distorted octahedral geometry around the vanadium center in the solid state. Upon treatment with Et2AlCl and CCl3COOEt, these complexes displayed high catalytic activities for ethylene polymerization even at elevated reaction temperatures, depending on ligand structures. The resulting polymers possessed high molecular weight and unimodal molecular weight distributions, indicative of the formation of a single catalytically active species during the polymerization catalysis. Excitingly, these vanadium(V) complexes could efficiently promote ethylene/norbornene copolymerization. The observed catalytic activity for the copolymerization was higher than that for ethylene homopolymerization. Moreover, the molecular weights of the resulting copolymers increased upon increasing the norbornene feed. These results indicated that introducing a suitable amount of norbornene into the system not only could accelerate the polymerization rate but also could restrain chain transfer reactions to some extent.

A series of half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands, Cp′[2,5-C4H2N(CHNAr)2]CrCl [Cp′ = C5H5, Ar = C6H5 (2a), 2,6-Me2C6H3 (2b), 2,6-iPr2C6H3 (2c), C6F5 (2d); Cp′ = C5Me5, Ar = C6H5 (3a), 2,6-iPr2C6H3 (3c)] were synthesized with good yields. The complexes were characterized by FTIR and mass spectrometry in addition to elemental analyses. X-ray structural analyses for 2a–c showed that the Cr complexes have a pseudo-octahedral coordination environment with a three-legged “piano stool” geometry. One of the imino nitrogen atoms is coordinated with the Cr metal. On activation with methylaluminoxane, the Cp-based complexes showed high catalytic activities for ethylene polymerization. High molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the nature of the polymerization was single site. The copolymerization of ethylene with norbornene by the pre-catalysts 2a–d was also explored in the presence of methylaluminoxane. The catalytic activity, co-monomer incorporation and the properties of the resultant polymers can be controlled over a wide range by tuning the catalyst structures and reaction parameters.

Cyclopolymerization of unsymmetric and symmetric Si-containing α,ω-diolefins by dimethylpyridylamidohafnium/organoboron catalytic system with high cyclization selectivity and stereoselectivity was first reported here. Compared with the constrained geometry catalyst [Me2Si(η5-Me4C5)(NtBu)]TiCl2 and the typical metallocene catalyst rac-Et(Ind)2ZrCl2, the Hf complex has proved to be one more promising catalyst for the cyclopolymerization of unsymmetric 3,3-dimethyl-3-sila-1,5-hexadiene (DMSHD). The poly(DMSHD) with high molecular weight (up to 953 kg/mol), high cyclization selectivity (100%), high cis selectivity (>90%), and high isotactic selectivity (>90%) could be easily obtained by the Hf catalyst under mild conditions. Moreover, the poly(DMSHD) also possessed a high glass transition temperature (up to 99.8 °C) and high melting temperature (Tm >200 °C). The insertion reaction of the vinyl group into Hf–CAr bond was further investigated, and a possible cyclization mechanism of DMSHD was proposed according to density functional theory calculation. Meanwhile, the Hf catalyst also showed notable catalytic activity for the cyclopolymerization of the symmetric Si-containing α,ω-diolefins including 4,4-dimethyl-4-sila-1,6-heptadiene, 4-methyl-4-phenyl-4-sila-1,6-heptadiene and 4,4-diphenyl-4-sila-1,6-heptadiene. Although the cyclization selectivity is somewhat low, the copolymerization by Hf catalyst still shows excellent cis/trans selectivity and isotactic selectivity.

A series of oxovanadium(V) complexes containing amine pyridine(s) phenolate ligands [ONN] (2a–f) have been synthesized in high yields (68–83%) by reacting VO(OnPr)3 with 1.0 equiv. of the ligands in CH2Cl2. These complexes were characterized by 1H, 13C and 51V NMR spectroscopy and elemental analysis. X-ray structural analysis for 2a, 2c and 2d revealed that these complexes adopt a six-coordinate distorted octahedral geometry around the vanadium center in the solid state. Upon treatment with Et2AlCl and CCl3COOEt, these complexes displayed high catalytic activities for ethylene polymerization even at elevated reaction temperatures, depending on ligand structures. The resulting polymers possessed high molecular weight and unimodal molecular weight distributions, indicative of the formation of a single catalytically active species during the polymerization catalysis. Excitingly, these vanadium(V) complexes could efficiently promote ethylene/norbornene copolymerization. The observed catalytic activity for the copolymerization was higher than that for ethylene homopolymerization. Moreover, the molecular weights of the resulting copolymers increased upon increasing the norbornene feed. These results indicated that introducing a suitable amount of norbornene into the system not only could accelerate the polymerization rate but also could restrain chain transfer reactions to some extent.