Abstract
Polylactide (PLA) is a biodegradable polyester that has found a wide range of applications in the biomedical field including resorbable sutures, artificial tissue scaffolding, and drug delivery substrates. In addition, PLA and PLA copolymers have recently been introduced as engineering materials with properties competitive with those of well-established thermoplastic materials. The unique combination of physical properties and environmental 'friendliness' found in PLA have made this polymer very attractive in the marketplace, and has stimulated much interest in improving methods for its preparation, in controlling its structural properties in well-defined ways, and in discovering new PLA copolymers. A powerful method for the synthesis of polyesters is the ring-opening of cyclic esters or lactones, which has many advantages over traditional step condensation polymerizations. Traditional anionic and cationic ring-opening lactide polymerizations are plagued by side reactions (e.g., racemization, transesterification) due to the highly reactive nature of the propagating species. On the other hand, metal-based polymerization systems are far more versatile, and current research in this area is focused on the use of Sn and Al alkoxides and carboxylates such as tin 2-ethyl-hexanoate and aluminum trisisopropoxide. We have uncovered a new family of discrete yttrium(III) complexes active for the polymerization of lactide and caprolactone. Our focus has been on ligands with amine, alkoxide, amidinate and amide functionalitites.