Kiesewetter, Matthew, K
ring-opening polymerization, catalysis, polymers, organocatalysis
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Well-defined and functionalized polymers have a wide variety of applications in biomedical studies. Advancements in the fields of drug delivery, microelectronics, and medical imaging, among other applications, are predicated upon the development of methods for the synthesis of precision polymers. Organocatalytic ring-opening polymerization (ROP) of cyclic monomers is an effective route to the production of polymeric materials, namely polyesters and polycarbonates, whose biodegradable properties make them a viable option in biomedical research. Arguably, H-bond mediated catalysts are among the most selective catalysts for ROP, giving rise to highly functionalized polycarbonates, well-controlled polymerizations, and other custom tailored applications. With respect to drug delivery, such polymers can be used as transporters to carry cargo molecules to cells, release their cargo, and then degrade safely. These capabilities, along with many others, give polymers derived through organocatalytic ROP great potential in biomaterials applications.
Despite the applicability of its products, there is a major problem associated with organocatalytic ROP, and this is the dearth of efficient methods for the synthesis of well-defined polymers. One highly-selective system for polymer synthesis employs an H-bond donating cocatalyst (typically a thiourea) and an H-bond accepting base. Despite their selectivity, these catalysts are inefficient, requiring both high quantities of the cocatalysts and long reaction periods. However, my development of systems for the ROP of cyclic ester monomers, namely δ-valerolactone and ε-caprolactone, involving various H-bond donating bis- and tris- (thio)urea cocatalysts has yielded significant reaction rate enhancement in comparison with that of the monomeric analogue. Polymerizations catalyzed by the tris-urea cocatalyst exhibited reaction rates up to 100 times those of ROPs employing the previously disclosed mono-thiourea. Despite this significant rate enhancement, these polymerizations retain the characteristics of ‘living’ polymerizations: low polydispersity, predictable molecular weight, and linear evolution of molecular weight with conversion. A mechanism for the rate acceleration with both bis- and tris- catalysts is proposed.