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. Here 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-8DLR; R denotes the two substituents on the ring). Depending on the composition of the soft B block, originated from rac-8DLR (R = Et, nBu), and its ratio to the semicrystalline, high-melting hard A block, derived from rac-8DLMe, the resulting all-PHA tri-BCPs with high molar mass (Mn up to 238 kg mol-1) and low dispersity (D = 1.07) exhibit tunable mechanical properties characteristic of a strong and tough thermoplastic, elastomer, or a semicrystalline thermoplastic elastomer.

Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity.

Stereo-defects present in stereo-regular polymers often diminish thermal and mechanical properties, and hence suppressing or eliminating them is a major aspirational goal for achieving polymers with optimal or enhanced properties. Here, we accomplish the opposite by introducing controlled stereo-defects to semicrystalline biodegradable poly(3-hydroxybutyrate) (P3HB), which offers an attractive biodegradable alternative to semicrystalline isotactic polypropylene but is brittle and opaque. We enhance the specific properties and mechanical performance of P3HB by drastically toughening it and also rendering it with the desired optical clarity while maintaining its biodegradability and crystallinity. This toughening strategy of stereo-microstructural engineering without changing the chemical compositions also departs from the conventional approach of toughening P3HB through copolymerization that increases chemical complexity, suppresses crystallization in the resulting copolymers, and is thus undesirable in the context of polymer recycling and performance. More specifically, syndio-rich P3HB (sr-P3HB), readily synthesized from the eight-membered meso-dimethyl diolide, has a unique set of stereo-microstructures comprising enriched syndiotactic [rr] and no isotactic [mm] triads but abundant stereo-defects randomly distributed along the chain. This sr-P3HB material is characterized by high toughness (UT = 96 MJ/m3) as a result of its high elongation at break (>400%) and tensile strength (34 MPa), crystallinity (Tm = 114 °C), optical clarity (due to its submicron spherulites), and good barrier properties, while it still biodegrades in freshwater and soil.

Effect of Chain Stereoconfiguration on Poly(3-hydroxybutyrate) Crystallization Kinetics.

Poly(3-hydroxybutyrate) (PHB) is naturally accumulated by bacteria but can also be synthesized chemically. Its processability is limited, as it tends to degrade at temperatures above its melting temperature; hence, investigation into crystallization kinetics and morphology of PHB materials of both natural and synthetic origins is of great need and interest to get a better understanding of structure-property relationship. Accordingly, this contribution reports a first study of the crystallization and morphology of synthetic PHB materials of different molecular weights. These synthetic PHBs are racemic mixtures (50/50 mol %) of R and S chain configurations and are compared with an enantiopure bacterial R-PHB. Nonisothermal and isothermal crystallization studies show that R and S chains of PHB can cocrystallize in the same unit cell as the R-PHB. Most significantly, the results show that the presence of S chains decreases the overall crystallization rate, which could enhance the processability and industrialization of PHB-based materials.

Polyolefin Innovations toward Circularity and Sustainable Alternatives.

The unprecedented growth and socioeconomic impacts of polyolefins clearly outline a major success story in the world of polymer science. Polyolefins revolutionizes industries such as health care, construction, and food packaging. Despite the benefits of polyolefins, there is a rising concern for the environment due to high production volume (i.e., fossil fuel consumption), often short usage time, and problems related to waste management and accumulation in the natural environment. Creating a circular economy for polyolefins through effective recycling technologies has the potential to decrease the environmental impact of these materials. This perspective discusses polyolefins and their impact, existing and emerging recycling/upcycling solutions, and recycle-by-design alternatives that are challenging the status quo.

Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin-Like Properties.

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DLR , R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DLMe with rac-8DLR (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties.

Stereosequenced crystalline polyhydroxyalkanoates from diastereomeric monomer mixtures.

Stereoselective polymerization of chiral or prochiral monomers is a powerful method to produce high-performance stereoregular crystalline polymeric materials. However, for monomers with two stereogenic centers, it is generally necessary to separate diastereomers before polymerization, resulting in substantial material loss and added energy cost associated with the separation and purification process. Here we report a diastereoselective polymerization methodology enabled by catalysts that directly polymerize mixtures of eight-membered diolide (8DL) monomers with varying starting ratios of chiral racemic (rac) and achiral meso diastereomers into stereosequenced crystalline polyhydroxyalkanoates with isotactic and syndiotactic stereodiblock or stereotapered block microstructures. These polymers show enhanced ductility and toughness relative to polymers of pure rac-8DL, subject to tuning by variation of the diastereomeric ratio and structure of the 8DL monomers.