Sustainability and Polyesters: Beyond Metals and Monomers to Function and Fate.

Poor waste management and unchecked consumption underpin our current paradigm of plastics use, which is demonstrably unsustainable in the long term. Nonetheless, the utility and versatility of plastics suggest that the notion of a plastic-free society is also unsustainable. Responses to this conundrum are increasing, and among these are research efforts focused on the development of more sustainable plastics. This Account, written by trained chemists, reflects an academic research journey culminating in an appreciation of the importance of improving and enabling the overarching systems that plastics exist within. Our primary initial focus was on catalyst development because catalysts are key drivers of sustainability by improving the efficiency and ease of polymerization. Metal catalysts ranging in ligand structure and the incorporated metal(s) were developed for the preparation of traditional polyesters such as poly(lactic acid) and polycaprolactone. The central themes in these works were stereocontrol (tacticity), efficiency (polymerization rate), and versatility (monomer scope). Alongside insights gained by systematically varying catalyst structure came impressive results gained through collaboration, including the remarkably high activity of novel heterometallic zinc catalysts toward various cyclic esters.This catalysis work was complemented by and slowly transitioned to a focus on polymer functionality and monomer design. Several fundamental studies focus on polymer topology, specifically star-shaped polyesters, tuned arm number, length, and tacticity. These reports feature emphases on the end of life (solvolysis) and physical properties of polymers, which were increasingly important themes as work shifted toward new methods of incorporating functionality in polymers produced by ring-opening polymerization. Three key highlights demonstrate this shift: the first two rely upon the exploitation of olefin metathesis (cross- and ring-closing) to functionalize polyesters or polyethers, and the third involves the manipulation of ring-opening polymerization equilibrium to enable selective monomer recovery from a polyester. Our foundational work on 1,3-dioxolan-4-one (DOX) monomers is then discussed because this emerging class of molecules offers a distinct synthetic pathway toward functional polyesters, both conventional and novel. With this DOX framework, polyesters that are usually challenging to synthesize (e.g., poly(mandelic acid)) are accessible because polymerization is driven by the concomitant, controlled extrusion of small molecules (acetone or formaldehyde).After these polyester-focused highlights, the foundation of our ongoing work is presented, namely, that polymer sustainability must be viewed from a systems-level perspective, including economic and social components alongside the environmental considerations. Material design must be driven by practice, and we have to involve key players in academia, industry, and government in a concerted effort to enable positive and robust change. The key goal is to develop sustainable systems that retain plastics in their highest value state for as long as possible by designing materials and products for a particular (and assured) end-of-life fate, whether that be reuse, recycling, (bio)degradation, or energy recovery.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport.

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Thiol-Ene Networks from Sequence-Defined Polyurethane Macromers.

To date, scalability limitations have hindered the exploration and application of sequence-defined polymers in areas such as synthetic plastics, fibers, rubbers, coatings, and composites. Additionally, the impact of sequence on the properties of cross-linked networks remains largely unknown. To address the need for synthetic methods to generate sequence-defined materials in gram quantities, we developed a strategy involving inexpensive and readily functional vanillin-based monomers to assemble sequence-defined polyurethane oligomers via sequential reductive amination and carbamation. Three oligomers were synthesized with monomer sequence precisely dictated by the placement of reactive side chains during the reductive amination reaction. Avoiding excessive chromatographic purification and solid- or liquid-phase supports enabled synthesis of sequence-defined oligomers on the gram-scale. Remarkably, sequence was shown to influence network topology upon cross-linking, as evidenced by sequence-dependent rubbery moduli values. This work provides one of the first examples of a scalable synthetic route toward sequence-defined thermosets that exhibit sequence-dependent properties.

Photochemical Transformation of Poly(butylene adipate- co-terephthalate) and Its Effects on Enzymatic Hydrolyzability.

Biodegradable polyesters are being increasingly used to replace conventional, nondegradable polymers in agricultural applications such as plastic film for mulching. For many of these applications, poly(butylene adipate- co-terephthalate) (PBAT) is a promising biodegradable material. However, PBAT is also susceptible to photochemical transformations. To better understand how photochemistry affects the biodegradability of PBAT, we irradiated blown, nonstabilized, transparent PBAT films and studied their enzymatic hydrolysis, which is considered the rate-limiting step in polyester biodegradation. In parallel, we characterized the irradiated PBAT films by dynamic mechanical thermal analysis. The rate of enzymatic PBAT hydrolysis decreased when the density of light-induced cross-links within PBAT exceeded a certain threshold. Mass-spectrometric analysis of the enzymatic hydrolysis products of irradiated PBAT films provided evidence for radical-based cross-linking of two terephthalate units that resulted in the formation of benzophenone-like molecules. In a proof-of-principle experiment, we demonstrated that the addition of photostabilizers to PBAT films mitigated the negative effect of UV irradiation on the enzymatic hydrolyzability of PBAT. This work advances the understanding of light-induced changes on the enzyme-mediated hydrolysis of aliphatic-aromatic polyesters and will therefore have important implications for the development of biodegradable plastics.

Sustainable Polyester Elastomers from Lactones: Synthesis, Properties, and Enzymatic Hydrolyzability.

Chemically cross-linked elastomers are an important class of polymeric materials with excellent temperature and solvent resistance. However, nearly all elastomers are petroleum-derived and persist in the environment or in landfills long after they are discarded; this work strives to address these issues by demonstrating the synthesis of renewable, enzymatically hydrolyzable, and mechanically competitive polyester elastomers. The elastomers described were synthesized using a novel bis(ß-lactone) cross-linker and star-shaped, hydroxyl-terminated poly(γ-methyl-ε-caprolactone). Using model compounds, we determined that the bis(ß-lactone) cross-linker undergoes acyl bond cleavage to afford ß-hydroxyesters at the junctions. The mechanical properties of the cross-linked materials were tunable and competitive with a commodity rubber band. Furthermore, the elastomers demonstrated high thermal stability and a low glass transition (-50 °C), indicating a wide range of use temperatures. The polyester networks were also subjected to enzymatic hydrolysis experiments to investigate the potential for these materials to biodegrade in natural environments. We found that they readily hydrolyzed at neutral pH and environmentally relevant temperatures (2-40 °C); complete hydrolysis was achieved in all cases at temperature-dependent rates. The results presented in this work exemplify the development of high performance yet sustainable alternatives to conventional elastomers.

Circularizing PET-G Multimaterials: Life Cycle Assessment and Techno-Economic Analysis.

The recycling of multimaterials such as payment or access cards poses significant challenges. Building on previous experimental work demonstrating the feasibility of chemically recyclable payment cards made from glycol-modified poly(ethylene terephthalate) (PET-G), we use life cycle assessment and techno-economic analysis to investigate two chemical recycling scenarios and evaluate their potential environmental and economic benefits. Recovering all components from the depolymerized products (Scenario 1) achieves substantial environmental benefits across most categories, reducing global warming by up to 67% compared to only recovering major components (Scenario 2). However, the environmental benefits in Scenario 1 incur 69% higher total annualized costs, causing its profitability to be dependent on a minimum selling price of £13.4/kg for cyclohexanedimethanol and less than a 10% discount rate. In contrast, Scenario 2 is less sensitive to discount rate variation and thus a lower risk and more economically feasible option, albeit less environmentally sustainable.

4-Carboalkoxylated Polyvalerolactones from Malic Acid: Tough and Degradable Polyesters.

Eight 4-carboalkoxyvalerolactones (CRVLs), varying in the composition of their alkyl (R) side chains, were synthesized from malic acid and subjected to ring-opening transesterification polymerization (ROTEP) using diphenyl phosphate [DPP, (PhO)2PO2H] as a catalyst. Each CRVL produced a semicrystalline poly(4-carboalkoxyvalerolactone) (PCRVL), and the nature of the R group impacted the thermal transitions of these polyesters. Bulk polymerizations at 70 °C allowed for preparation of high molar mass samples that contained small amounts of branching, as evidenced by 1H NMR spectroscopy, MALDI spectrometry, size-exclusion chromatography, and eliminative degradation. Tensile testing of these lightly branched, high molar mass samples revealed that these polyesters are tough (tensile toughness values up to 88 ± 33 MJ•m-3) and have Young's moduli (E) up to 186 ± 13 MPa. The acid- and base-catalyzed hydrolytic degradation of the PCRVLs was quantitatively monitored using total organic carbon analysis, and effect of the alkyl chain length on PCRVL hydrolysis rate was determined. Finally, the methyl ester variant of these malic acid-derived thermoplastics is known to be chemically recyclable.

Mechanistic Study of Stress Relaxation in Urethane-Containing Polymer Networks.

Cross-linked polymers are used in many commercial products and are traditionally incapable of recycling via melt reprocessing. Recently, tough and reprocessable cross-linked polymers have been realized by incorporating cross-links that undergo associative exchange reactions, such as transesterification, at elevated temperatures. Here we investigate how cross-linked polymers containing urethane linkages relax stress under similar conditions, which enables their reprocessing. Materials based on hydroxyl-terminated star-shaped poly(ethylene oxide) and poly((±)-lactide) were cross-linked with methylene diphenyldiisocyanate in the presence of stannous octoate catalyst. Polymers with lower plateau moduli exhibit faster rates of relaxation. Reactions of model urethanes suggest that exchange occurs through the tin-mediated exchange of the urethanes that does not require free hydroxyl groups. Furthermore, samples were incapable of elevated-temperature dissolution in a low-polarity solvent (1,2,4-trichlorobenzene) but readily dissolved in a high-polarity aprotic solvent (DMSO, 24 to 48 h). These findings indicate that urethane linkages, which are straightforward to incorporate, impart dynamic character to polymer networks of diverse chemical composition, likely through a urethane reversion mechanism.