Vanadium complexes with tetradentate salen-type ligands were first time explored in ethylene polymerizations. The effects of the vanadium complex structure, the alkyl aluminum cocatalysts type (EtAlCl2, Et2AlCl, Et3Al, and MAO), and the polymerization conditions (Al/V molar ratio, temperature) on polyethylene yield were explored. It was found that EtAlCl2 in conjunction with investigated vanadium complexes produced the most efficient catalytic systems. It was shown, moreover, that the structural changes of the tetradentate salen ligand (type of bridge which bond donor nitrogen atoms and type of substituent on aryl rings) affected activity of the catalytic system. The complexes containing ligands with cyclohexylene bridges were more active than those with ethylene bridges. Furthermore, the presence of electron-withdrawing groups at the para position and electron-donating substituents at the ortho position on the aryl rings of the ligands resulted in improved activity in relation to the systems with no substituents (with the exception of bulky t-Bu group). The results presented also revealed that all vanadium complexes activated by common organoaluminum compounds gave linear polyethylenes with high melting points (134.8-137.6 degrees C), high molecular weights, and broad molecular weight distribution. The polymer produced in the presence of MAO possesses clearly lower melting point (131.4 degrees C) and some side groups (around 9/1000 C). (c) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6940-6949, 2008