Living/controlled polymerization of hex-1-ene initiated by nickel diimine catalysts of general form [ArN=C (1,8-naphthalenediyl)C=NAr]NiBr2 activated by simple organoaluminium compounds was investigated. Polyhexenes with a very narrow molar mass distribution and molar mass controlled by the monomer/initiator ratio were prepared using diethylaluminiumchloride, ethylaluminiumdichloride and methylaluminiumdichloride as cocatalysts for nickel complexes 1 (Ar = 2,6-iPr(2)C(6)H(3)) and 2 (Ar = 2-tBuC(6)H(4)). Nickel complexes with smaller aryl substituents (3, Ar = 2,6-Me2C6H3: 4 Ar = 2,4,6-Me3C6H2; 5, Ar = 2,6-Et2C6H3) undergo transfer reactions significantly. For the most bulky complex 1 activated by ethylaluminiumdichloride living hex-1-ene polymerization was achieved, proved by the reinitiation of polymer growth upon the addition of fresh monomer. The effect of Al/Ni ratio on the activity of the catalytic systems was studied by dilatometry. The catalytic systems were investigated by UV-vis spectrometry and a new interpretation of their absorption spectra was suggested. (C) 2010 Elsevier Ltd. All rights reserved.