Tuning the topology and stereomicrostructure of sustainable poly(3-hydroxybutyrate) materials

Abstract

Poly(3-hydroxybutyrate) (P3HB), a member of the polyhydroxyalkanoate (PHA) family, is a biodegradable polyester with the potential to serve as a sustainable alternative to conventional, petrochemical-derived polymers. This dissertation explores chemocatalytic strategies to precisely tune the topology and stereomicrostructure of synthetic P3HB, enabling access to an increased range of thermal and mechanical properties. Chapter 1 provides and overview of recent advances in PHA chemosynthesis seeking to improve their performance, processability, and functionality, using strategies including stereocontrolled ring-opening polymerization (ROP) of 4- and 8-membered lactone monomers, fundamental redesign efforts to stabilize the PHA backbone against thermal and hydrolytic degradation, and the introduction of unusual substituents that increase the commercial utility of PHAs especially for biomedical applications. Providing one of the latest examples of innovation in this space, Chapter 2 showcases cyclic stereoregular P3HBs, selectively synthesized through a catalyst-enabled end-to-end cyclization strategy, which exhibit superior thermal and mechanical performance compared to their linear counterparts. Cyclization was made possible by increasing the ionic radius of the catalyst metal center (La > Y) and through utilizing the precatalyst's existing substituent, silylamide (-NHSiHMe2) as an initiator, providing a good leaving group able to readily undergo cyclization. Therefore this polymerization strategy is more simplified compared to linear ROP, as it does not require addition of a pro-tic alcohol for initiation. Chapter 3 provides another example of a simplified polymerization strategy that advances the scope of accessible materials. Using a single set of polymerization conditions (catalyst, solvent, initiator), a stereochemical spectrum of P3HBs ranging from tough thermoplastics to plastomers and adhesives can be accessed simply by modulating the diastereomeric monomer feed ratio (rac and meso-8DLMe). A selection of these materials were then incorporated into an all-P3HB tape, exemplifying their potential to re-place traditionally hard-to-recycle multi-material products. Using these simple chemosynthetic strategies, it is possible to tune the topology and stereomicrostructure of P3HB enabling access to a platform for sustainable materials innovation. To conclude this work, Chapter 4 provides an outlook on the future of development in this space according to the limitations that remain to be overcome, and the new opportunities made accessible by the discoveries described herein. Continued refinement of P3HB synthesis will enable their utility across a greater application space, advancing the market of sustainable plastics and benefitting society through the reduction of plastic waste accumulation.