Reaction of the zirconium tetrachloride with one equivalent of the sodium salt of the diamine bis (phenolate) ligand, (LH2)-H-1 (Me2NCH2CH2N-(CH2-2-OH-3,5-tBu-C6H2)(2)) or (LH2)-H-2 (Me2NCH2CH2N-(CH2-2-OH-3,5-tBu-C6H2)(CH2-2-OH-C6H4)) in the presence of air led to formation of [Lig(1)ZrCl](2)(mu-0) and [Lig(2)ZrCl](2)(mu-O), respectively. These novel oxo-bridged dinuclear zirconium complexes were characterized by elemental analysis, H-1 NMR spectroscopy and by single-crystal X-ray diffraction. Their reactivities in polymerization of ethylene and 1-octene, upon activation with Al(iBu)(3)/Ph3CB(C6F5)(4) and MAO, were examined. It was found that lack of t-Bu substituents on one phenolate ring cause a significant decrease in catalytic activity but leads to polymers with improving properties, i.e., to poly(1-olefin)s with significantly higher stereoregularity and polyethylenes with higher molecular weight. In addition, the interaction of the [Lig(1)ZrCl](2)(mu-O) precursors with MAO and with the Me3Al activator was investigated by H-1 NMR spectroscopy giving an insight into the catalyst transformations in the course of the polymerization reaction. (C) 2015 Elsevier B.V. All rights reserved.