Four -diimine nickel complexes [(ArNC(R) C(R)NAr)NiBr2; RH, CH3, cyclohexane-1,2-diyl, naphthalene-1,8-diyl, Ar2,6-i-Pr-2-C6H3-) were investigated in propene and hex-1-ene polymerization to identify the limits of backbone substituent R size needed to provide living/controlled -olefins polymerization by the sufficient suppression of -H elimination transfer. Propagation kinetics measurements, molar mass on monomer conversion dependencies and reinitiation tests were used to evaluate the livingness of hex-1-ene polymerization. Interestingly, living/controlled hex-1-ene polymerization was observed in the case of all diimine derivatives including the one bearing only hydrogen atom in backbone positions. Unexpectedly, in the case of catalysts bearing H and CH3 backbone substituents, we observed the unusual isomerization of hex-1-ene into internal hexenes in parallel with its polymerization. Nevertheless, by subtracting the amount of monomer consumed in isomerization side reaction, polymerization still keeps the features of living/controlled process. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3193-3202