Amino-alkoxy-bis (phenolate) yttrium complexes act in the presence of an excess of alcohol (up to 50 equiv. vs. Y) as highly active and stereoselective catalysts for rac-lactide and rac-beta-butyrolactone polymerizations. These versatile systems enable the production of large quantities of polymer with small amounts of catalyst, optimizing productivity, and also allow the preparation of polymers with functional end groups, which may be employed as intermediates for macro-molecular engineering applications.