Various vanadium complexes containing N-heterocyclic carbenes, VOCl₃[1,3-R₂(NCH)₂C:] (V1, R = 2,6-Me₂C₆H₃; V2, R = 2,6-Et₂C₆H₃; V3, R = 2,6-ⁱPr₂C₆H₃; V4, R = 2,4,6-Me₃C₆H₂), have been synthesized and employed as catalyst precursors for ethylene/propylene copolymerization after activation by Et₃Al₂Cl₃. Complex V4 showed higher catalytic activity of ca. 38 kg copolymer per (mol of V) per h and an ethylene/propylene copolymer with random monomer distribution could be prepared. Complex V3 consumed more cocatalyst than its analogues to reach higher catalytic activity. The obtained copolymers exhibit relatively narrow polydispersity and contain more randomly distributed monomer units than that the copolymers prepared by using the traditional vanadium catalytic system.

Various half-titanocenes containing monodentate anionic nitrogen ligands, CpTiCl₂[3-C₆H₅(CH₂N)(CH₂O)CN] (T1), CpTiCl₂[(C₆H₅O)(NⁱPr₂)CN] (T2), CpTiCl₂[1,3-(4-MeC₆H₄)(CH₂N)₂CN] (T3), CpTiCl₂[1,3-(2,4,6-Me₃C₆H₂)(CH₂N)₂CN] (T4), CpTiCl₂[1,3-(2,6-ⁱPr₂C₆H₃)(CH₂N)₂CN] (T5) and CpTiCl[1,3-(2,6-ⁱPr₂C₆H₃)(CH₂N)₂CN][CH₃(CH₂)₃CH(C₂H₅)COO] (T6), were synthesized and characterized. The molecular structures of T1, T2 and T5 were determined by X-ray crystallography. All the above half-titanocenes were employed as precatalysts for ethylene/propylene copolymerization after activation by methylaluminoxane (MAO) or modified MAO (MMAO). The catalytic activity and propylene incorporation into copolymers increased in the order of T1 < T2 < T3 < T4 < T5, which is attributed to the electron donating nature of the ligands. The half-titanocene containing the 2-ethylhexanoate ligand (T6) has good solubility in hexanes. An ethylene–propylene copolymer with high molecular weight (M_w = 530 kg mol⁻¹) could be obtained by the T6/MMAO catalytic system. The ethylene/propylene copolymerization catalyzed by the T6/TIBA/MMAO catalytic system was more controllable than that by the T5/MAO catalytic system. Importantly, both the T5/MAO and T6/TIBA/MMAO catalytic systems displayed a high catalytic activity of 2.4 × 10⁶ and 2.2 × 10⁶ g polymer per mol of Ti per h, respectively, even at a low Al/Ti molar ratio (Al/Ti = 300). All the values of r_E × r_P (r_E: reactivity ratio for ethylene; r_P: reactivity ratio for propylene) were around 0.9 in the copolymerization catalyzed by T2, T4, T5 and T6, indicating that nearly random ethylene–propylene copolymers could be prepared. Compared to the T5/MAO catalytic system, ethylene–propylene copolymers with higher molecular weight (M_w = 381 kg mol⁻¹) and higher propylene incorporation (43.6 mol%) could be produced by using the T6/TIBA/MMAO (TIBA/Ti = 50) system with a similar catalytic activity.

Highly efficient terpolymerization of ethylene, propylene and 5-ethylidene-2-norbornene using a half-titanocene containing iminoimidazolidine with methylaluminoxane/Al(iBu)₃/2,6-ditertbutyl-4-methyl-phenol was achieved.

A series of imidazolidin-2-iminato vanadium complexes VOCl₂[1,3-R₂-(CH₂N)₂CN], Vi1, R = 2,6-Me₂C₆H₃; Vi2, 2,4,6-Me₃C₆H₂; Vi3, R = 2,6-ⁱPr₂C₆H₃; Vi4, R = cyclohexanyl, were synthesized and fully characterized by NMR (¹H, ¹³C and ⁵¹V) and elemental analysis. The copolymerizations of ethylene with propylene using the imidazolidin-2-iminato vanadium complexes were conducted in the presence of Et₃Al₂Cl₃. The catalytic activity increased in the order of Vi4 < Vi1 < Vi2 < Vi3, probably due to the different electron donating natures of the imidazolidin-2-iminato ligands. Both high catalytic activity of 2.44 × 10⁶ g copolymer per mol of V per h and incorporation ratio of propylene (up to 32.5 mol%) were afforded by using the Vi3/Et₃Al₂Cl₃ catalytic system in the copolymerization of ethylene with propylene. An ethylene–propylene copolymer with ultra-high molecular weight (M_w = 2660 kg mol⁻¹) could be also synthesized by using the Vi3/Et₃Al₂Cl₃ catalytic system. Importantly, the highly efficient terpolymerizations of ethylene, propylene with 5-ethylidene-2-norbornene (ENB) were also achieved by using this catalytic system, affording ethylene–propylene–ENB terpolymers (EPDMs) with high contents of ENB units up to 9.8 mol% (29.8 wt%) and ultra-high molecular weight (M_w = 1521–2537 kg mol⁻¹). This work offers a unique catalytic system of vanadium complexes for the synthesis of EPDM with both high content of ENB units and ultra-high molecular weight.