Parker, Celine R. | Search NASA

DOE OSTI2023

All-Polyhydroxyalkanoate Triblock Copolymers via a Stereoselective-Chemocatalytic Route

Biodegradable polyhydroxyalkanoate (PHA) homopolymers and statistical copolymers are ubiquitous in microbially produced PHAs, but the step-growth polycondensation mechanism the biosynthesis operates on presents a challenge to access well-defined block copolymers (BCPs), especially higher-order tri-BCP PHAs. We report a stereoselective-chemocatalytic route to produce discrete hard–soft–hard ABA all-PHA tri-BCPs based on the living chain-growth ring-opening polymerization of racemic (rac) 8-membered diolides (rac-8DL R ; R denotes the two substituents on the ring). Depending on the composition of the soft B block, originated from rac-8DL R (R = Et, n Bu), and its ratio to the semicrystalline, high-melting hard A block, derived from rac-8DL Me , the resulting all-PHA tri-BCPs with high molar mass (M n up to 238 kg mol –1 ) and low dispersity (&#208: = 1.07) exhibit tunable mechanical properties characteristic of a strong and tough thermoplastic, elastomer, or a semicrystalline thermoplastic elastomer.

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DOE OSTI2022

Synthetic biodegradable polyhydroxyalkanoates (PHAs): Recent advances and future challenges

This article reviews the advances made over the past five decades of research in developing effective chemocatalytic pathways to synthesize polyhydroxyalkanoates (PHAs), a prominent class of biodegradable polyesters found in nature and considered as sustainable alternatives to petroleum-based non-degradable plastics. Focused in this review are recent efforts that seek to address the key challenges facing the biosynthetic routes by taking advantage of precision in synthesis, expedient tunability in polymer stereomicrostructures and structures of monomers and molecular catalysts, as well as scalability and speed in polymer production that chemical catalysis can offer. This article is organized by poly(3-hydroxybutyrate) (P3HB) stereomicrostructures (tacticities), from isotactic to syndiotactic to atactic P3HB materials, followed by other PHA homopolymers and copolymers. Under each type of stereochemically defined PHAs, monomers, catalysts, and polymerizations employed for the synthesis, as well as mechanistic aspects when possible, are described. Next, recent advances in expanding the PHA scope and developing functionalized, uncommon or unnatural PHAs, inaccessible by biological methods, especially block and stereoblock or stereosequenced PHAs, are highlighted in their synthetic methods and advanced materials properties. As a result, four key remaining challenges, and thus corresponding future directions directed at addressing those challenges, are discussed.

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