Reaction of [V(X)(OR)(3)] (X = O, Np-tolyl, R= nPr, tBu) with 5,11,17,23-tBu-25,27-dihydroxycalix[4]arene (LH2) led to the formation of [V(X)(OR)L](2) X = O, R = nPr (1); X = Np-tolyl, R = nPr (2); X = Np-tolyl, R = tBu (3) as the major product. In the case of X = O, the minor hydrolysis product {[VO(OnPr)](2)(mu-O)L}(2) (4) has also been characterized. Complexes (1)-(4), in the presence of the co-catalyst dimethylaluminum chloride and the reactivator ethyltrichloroacetate, are highly active (<= 16,400 g/mmol h bar), thermally stable pro-catalysts for the polymerization of ethylene. The use of silica supports with (1) and (2) under slurry conditions yielded polymer with activities <= 30 g/mmol h. Complexes (1)-(3) have also been screened as pro-catalysts for the ring-opening polymerization of epsilon-caprolactone; the conversion rate order (1) (94%) > (2) (46%) > (3) (20%) was observed at 80 degrees C over 72 h.