Stereomicrostructure engineering of biodegradable poly(3-hydroxybutyrate) towards mono-material products

Abstract

This dissertation describes advancements in the synthesis, characterization, and applications of biodegradable poly(3-hydroxybutyrate) (P3HB). These advancements were facilitated through the modulation of P3HB's stereomicrostructure, the relation of stereocenters to one another in the polymer chain, and the installation of stereoerrors through engineering its stereomicrostructure. An extensive literature review is also included on (bio)degradable and chemically recyclable polyesters, methods used in the modulation of polyhydroxyalkanoates properties, and mono-material product design based on bio-based, biodegradable, and chemically recyclable plastics. Biologically, P3HB is synthesized as stereoperfect (sp) isotactic (it) P3HB, meaning all of the stereocenters are in the absolute (R) configuration, which leads to high strength and high melting transition temperature but brittleness and opacity which limits its commercial utility. P3HB can be chemocatalytically synthesized as it, syndiotactic (st), stereocenters alternate between (R) and (S); atactic, having no regularity in stereochemistry; as well as iso- (ir) or syndio-rich (sr) having character of it or st but also atactic regions. To create these stereodiverse P3HBs, two monomers are used: β-butyrolactone and the eight-membered dimethyl diolide (both rac and meso diastereomers and mixtures of them), and many organic and organometallic catalysts can be employed. In this dissertation, the chemocatalytic route is focused on for its fast kinetics, scalability, and its ability to access a wide-range of stereomicrostructures. The commercial implementation of P3HB has interested many as it can biodegrade in both managed (commercial composting) and unmanaged (fresh water and soil) environments, making it a viable option for reducing the massive amounts of plastics that are leaked into the environment. However, the lack of material performance and diversity in biologically produced P3HB has led to a limited range of applications. Herein, P3HB's stereomicrostructure was engineered to create biodegradable tough, optically clear thermoplastics for packaging applications in the form of sr- and ir-P3HB with installed stereoerrors, strong adhesives that outcompete commercial super glues in the form of sr-P3HB, and to in-between stereomicrostructures that act as thermoplastics, elastomers, and pressure sensitive adhesives. Further, these stereodiverse P3HBs were blended with sp-P3HB, and homologous P3HB blends were created with advanced synergistic material properties. Combining these advanced materials, mono-material products were created in the form of all-P3HB tapes. These mono-material products are biodegradable and greatly reduce the complexity of potential recycling routes that typically plagues multi-material products. Topological effects on P3HBs material properties were also explored by creating stereodiverse cyclic P3HB through judicious monomer and catalyst selection that showed varied material properties to their linear counterparts. Overall, this dissertation demonstrates that tuning P3HB's stereomicrostructure leads to useful biodegradable materials that can be employed in the fabrication of mono-material products.