The synthesis and characterization of a zirconium complex, having two alkoxide functionalized N-heterocyclic carbene ligands, and its behavior as catalyst in the polymerization of ethylene and propylene, have been reported. NMR analysis showed that more than one species was obtained during synthesis. These data were confirmed by ethylene polymerization that gave rise to a linear polyethylene having a high Molecular weight and a polydispersity index (MDI) > 2 and often bimodal. The same catalytic system was able to produce highly isotactic polypropylene together with an atactic fraction. OFT studies on the complex stereoisomer stability gave indications on the species possibly involved in the polymerizations. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 862-870, 2011