Mechanistic studies of cyclic ester polymerizations by discrete metal-alkoxide catalysts

Metal alkoxide complexes have been used extensively to catalyze the ring-opening polymerization of cyclic ester monomers, however a deep mechanistic understanding of the relationship between the structural features of the catalyst and its polymerization efficiency, selectivity, and mechanism has not been achieved. Our constant research objective has been to develop a deeper mechanistic understanding of the metal-alkoxide-promoted cyclic ester polymerization process as a means toward rational catalyst design of more effective and versatile catalysts that exert control over polymer molecular weight and exhibit high reactivity.; Complexes of the form LzMx(OR), where L denotes an ancillary supporting ligand, were targeted as catalysts. By design, the metal center possesses only a single alkoxide-initiating-group in order to simplify the kinetic behavior of the polymerization reaction. Using this basic structure we were able to create a diverse array of catalysts that are active for the polymerization of lactide and epsilon-caprolactone. The kinetic information that was obtained with each catalyst was used to help form mechanistic hypotheses, and improve the design of subsequent catalysts.; Additional mechanistic questions were answered by observing the polymerization kinetics for a series of catalysts that differed in the electronic nature of the metal center. This was accomplished by two different approaches; in one case the electronics were varied by changing substituents on the ancillary ligand, while in another case different metal centers were employed. In each case the kinetic data provided crucial information, allowing us to better understand the relationship between the catalyst structure and polymerization behavior.