Biometal Catalyzed Ring-Opening Polymerization of Cyclic Esters: Ligand Design, Catalyst Stereoselectivity, and Copolymer Production

Biodegradable polyesters represent a class of extremely useful polymeric materials for many applications. Among these polyesters, the biodegradable and biocompatible, polylactide is very promising for many applications in both medical and industrial areas. Other biodegradable polymers such as polytrimethylene carbonate, polybutyrolactone, polyvalerolactone, and polycaprolactone can be blended or copolymerized with polylactide to fine tune the properties to fit the needs for their applications. The properties of these polymers and copolymers depend upon the tacticity of the polymers which can be directly controlled by the catalysts used for polymer production. Therefore, it has been of great interest to develop new selective catalytic systems for the ring-opening polymerization of lactide and other cyclic monomers. This dissertation focuses on developing new zinc and aluminum complexes and studying their selectivity and reactivity of these complexes for the ring-opening polymerization of lactide and other cyclic monomers, i.e. trimethylene carbonate, beta-butyrolactone, delta-valerolactone, and epsilon-caprolactone. Herein, aspects of the ring-opening polymerization of lactide and other cyclic monomers utilizing novel zinc and aluminum complexes will be discussed in detail. In the process for the ring-opening polymerization of lactide, chiral zinc half-salen complexes derived from natural amino acids have shown to be very active catalysts for producing polymers with high molecular weight and narrow polydispersities at ambient temperature. The chiral zinc complexes were found to catalyze rac-lactide to heterotactic polylactides with Pr values ranging from 0.68-0.89, depending on the catalyst and reaction temperature employed during the polymerization process. The reactivities of the various catalysts were greatly affected by substituents on the Schiff base ligands, with sterically bulky substituents being rate-enhancing. Furthermore, a series of both chiral and achiral aluminium half-salen complexes have been synthesized and characterized. These aluminum complexes all showed moderate selectivity to the ring-opening polymerization of rac-lactide to produce isotactic polylactide with Pm value up to 0.82 in toluene at 70 degrees C. Moreover, some of the studied aluminum complexes displayed epimerization of rac-lactide to meso-lactide during the polymerization process. Kinetic studies for the ring-opening polymerization of lactide utilizing these zinc and aluminum complexes are included in this dissertation. Along with these studies, the copolymerization of lactide with epsilon-caprolactone and delta-valerolactone will also be presented