The present application provides salen indium catalysts of the following general structure wherein R is a coordinating alkoxide comprising at least one coordinating atom that forms a dative bond with In. The mononuclear salen indium catalysts are particularly useful in catalyzing ring-opening polymerizations of cyclic ester monomers, such as lactides. Also provided herein are methods of using the mononuclear salen indium complexes to catalyze polymerization of cyclic ester monomers. Of particular interest is the successful polymerization of lactides using the mononuclear salen indium catalysts to produce poly(lactic acid) having high isotacticity.