Polymerization of hex-1-ene and propene initiated by several methylalumoxane-activated diimine complexes was critically investigated. Effect of bulkiness of ortho aryl diimine substituents on extent of transfer reactions was examined. All of the complexes allowed us to prepare poly(hex-1-ene) with a very narrow molar mass distribution, molar mass being controlled by stoichiometry in a broad range of reaction conditions. Poly(hex-1-ene)s with molar mass between 15 and 220 kg mol(-1) and dispersity (M-w/M-n) between 1.01 and 1.20 were prepared by varying the catalyst, temperature and monomer concentration. Livingness of hex-1-ene polymerization was demonstrated for the first time for nickel complex bearing ethyl ortho aryl substituents by reinitiation of chain growth upon addition of a new portion of monomer. Complexes with ortho methyl substituents did not allow complete reinitiation of chain growth and despite its good control over molar mass cannot be classified as a living polymerization catalyst. Chain branching can effectively be controlled by the choice of the ligand structure due to the chain-walking mechanism. Transfer reactions were more pronounced in propene polymerization. Polypropylene with narrow molar mass distribution could not be prepared using complexes with methyl substituents. (C) 2009 Elsevier Ltd. All rights reserved.