The ring-opening polymerization (ROP) of cyclic esters such as s-caprolactone (epsilon-CL), L- and rac-lactide, promoted by yttrium silylamido complexes bearing binaphthyl-bridged salen (1 and 2) and diamine bisphenolate salan ligands (3 and 4) is described. The yttrium silylamido complexes 1-4 are effective initiators for the ROP of s-caprolactone, showing extremely high turnover frequencies (TOF up to 18000 h(-1)) under mild reaction conditions. All complexes promote the ROP of rac-lactide in toluene solution at room temperature, providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (M-w/M-n = 1.73-1.99). Interestingly, in THF solution the same catalysts produce heterotactic polylactides with P-r between 0.58 and 0.91 via a chain-end stereocontrol mechanism. The salan complexes 3 and 4 are more active than the binaphthyl salen complexes 1 and 2, reasonably due to the presence of the more electron donating amino groups. On the other hand, the former resulted in a lower stereoselectivity than the latter. The rigidity of the "bridge" between the two nitrogen atoms seems to have a predominant role in governing the selectivity of the corresponding complexes, while the effect of the steric hindrance of the ortho substituents at the phenoxy rings appears less significant. (C) 2013 Elsevier B.V. All rights reserved.