Ring-Opening Polymerization of Lactones and Lactides by Aluminum and Yttrium Initiators

During the past decade strong political, environmental, and economical issues spurred much research effort to develop green, possibly degradable, alternatives to polyolefins. In this context, aliphatic polyesters, i.e. poly(lactide), poly(caprolactone), poly(hydroxybutyrate) and their copolymers emerged as promising materials, and have been applied for packaging and agricultural products, and for special use in medicine. The synthesis can be achieved via ring-opening polymerization (ROP) of cyclic esters, a process initiated by a variety of species, ranging from metal complexes to organocatalysts, to enzymes. Our current interest has been devoted to the design of well-defined ROP initiators, with the aim to control the molecular weight and the polymer microstructure, enhance the catalytic activity, and limit deleterious transesterification reactions. Recent studies, concerning ROP initiators based on aluminum and rare-earth metal complexes bearing bidentate or tetradentate Schiff-base ligands, or dianionic NNN anilidopyridyl-pyrrolide, will be presented. Emphasis will be given to the mechanistic studies, in particular those concerning the achievement of a controlled and living polymerization, the synthesis of block and random copolymers of -caprolactone and lactides, the stereoselective polymerization of rac--butyrolactone to syndiotactic enriched poly(3-hydroxybutyrate).