The quasi-living copolymerization of ethylene with propylene was achieved by using N-heterocyclic carbene (NHC) ligated vanadium complex (V3, VOCl3[1,3-(2,6-(Pr2C6H3)-Pr-i)(2)(NCH(sic))(2)C:]) due to the stabilization of active center by the introduction of bulky and electron rich NHC ligand with bulky isopropyl substituents at the ortho positions of the phenyl rings. The weight-average molecular weight (M-w) of the resulting copolymer increases linearly with its weight in 20 min. The ultra-high-molecular-weight (UHMW) ethylene-propylene copolymer (M-w = 1612 kg mol(-1)) can be synthesized with V3/Et3Al2Cl3 catalytic system. The novel complex V4 ' (VCl3[1,3-(2,4,6-Me3C6H2)(2)(NCH(sic))(2)C:]center dot 2THF) was constructed by the introduction of two coordinated tetrahydrofuran molecules and decrease in steric hindrance at the ortho positions of phenyl rings. The UHMW ethylene-propylene copolymer (M-w = 1167 kg mol(-1)) can also be synthesized by using V4 '/Et3Al2Cl3 catalytic system. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 553-561