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1. Recent developments towards performance-enhancing lignin-based polymers

   https://doi.org/10.1039/d1py00694k  

   Bass, Garrett F.; Epps, Thomas H. (July 2021, Polymer Chemistry)

   null (Ed.)

   Fossil fuels are a cheap and abundant feedstock for polymeric materials that have enabled innumerable quality-of-life improvements. Yet, their declining supply and non-renewable nature have driven the pursuit of bio-based alternatives. Lignin represents the largest natural source of aromatic carbon on the planet, and thus, lignin-derived products have emerged as critical elements in the next generation of polymers. The relative abundance, large concentration of functional handles, and thermal stability of lignin make it an attractive target for bio-based polymers. However, the valorization of lignin to high-performance and cost-competitive materials remains a challenge. In this review, developments in the translation of lignin into value-added macromolecular components are discussed. Strategies to incorporate bulk lignin in polymer blends and composites are introduced with a focus on applications. Furthermore, recent advances in the preparation of higher-value thermoplastics, thermosets, and vitrimers from deconstructed lignin products are highlighted from a synthetic perspective. Finally, key hurdles and future opportunities in lignin valorization are explored. 

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2. Lignin-derivable alternatives to petroleum-derived non-isocyanate polyurethane thermosets with enhanced toughness

   https://doi.org/10.1039/d2ma00895e  

   Mhatre, Sampanna V.; Mahajan, Jignesh S.; Epps, Thomas H.; Korley, LaShanda T. (January 2023, Materials Advances)

   The structural similarities between lignin-derivable bisguaiacols and petroleum-derived bisphenol A/F (BPA/BPF) suggest that bisguaiacols could be ideal biobased alternatives to BPA/BPF in non-isocyanate polyurethane (NIPU) thermosets. Herein, bisguaiacol/bisphenol-derived cyclic carbonates with variations in methoxy content and bridging-carbon substitution were cured with two triamines of different chain lengths, and the impact of these differences on the thermomechanical properties of NIPU networks was examined. The methoxy groups present in the lignin-derivable cyclic carbonates led to thermosets with significantly improved toughness (∼49–59 MJ m −3 ) and elongation at break ( ε b ∼195–278%) vs. the BPA/BPF-based benchmarks (toughness ∼ 26–35 MJ m −3 , ε b ∼ 86–166%). Furthermore, the addition of dimethyl substitution on the bridging carbon resulted in increased yield strength ( σ y ) – from ∼28 MPa for networks with unsubstituted bridging carbons to ∼45 MPa for the dimethyl-substituted materials. These enhancements to mechanical properties were achieved while retaining essential thermoset properties, such as application-relevant moduli and thermal stabilities. Finally, the triamine crosslinkers provided substantial tunability of thermomechanical properties and produced NIPUs that ranged from rigid materials with a high yield strength ( σ y ∼ 65–88 MPa) to flexible and tough networks. Overall, the structure-property relationships presented highlight a promising framework for the design of versatile, bio-derivable, NIPU thermosets. 

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3. Hydrolyzable Poly( β ‐thioether ester ketal) Thermosets via Acyclic Ketal Monomers

   https://doi.org/10.1002/marc.202200028  

   Alameda, Benjamin M.; Murphy, Joseph Scott; Barea‐López, Bernardo L.; Knox, Karly D.; Sisemore, Jonathan D.; Patton, Derek L. (February 2022, Macromolecular Rapid Communications)

   Abstract Hydrolytically degradable poly(β‐thioether ester ketal) thermosets are synthesized via radical‐mediated thiol‐ene photopolymerization using three novel dialkene acyclic ketal monomers and a mercaptopropionate based tetrafunctional thiol. For all thermoset compositions investigated, degradation behavior is highly tunable based on the structure of the incorporated ketal and pH. Complete degradation of the thermosets is observed upon exposure to acidic and neutral pH, and under high humidity conditions. Polymer networks composed of cross‐link junctions based on acyclic dimethyl ketals degrade the quickest, whereas networks containing acyclic cyclohexyl ketals undergo hydrolytic degradation on a longer timescale. Thermomechanical analysis reveals low glass transition temperatures and moduli typical of thioether‐based thermosets. 

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4. Lignin‐Based Solid Polymer Electrolytes: Lignin‐Graft‐Poly(ethylene glycol)

   https://doi.org/10.1002/marc.202000428  

   Liu, Hailing; Mulderrig, Logan; Hallinan, Jr., Daniel; Chung, Hoyong (October 2020, Macromolecular Rapid Communications)

   Abstract Lignin is an aromatic‐rich biomass polymer that is cheap, abundant, and sustainable. However, its application in the solid electrolyte field is rare due to challenges in well‐defined polymer synthesis. Herein, the synthesis of lignin‐graft‐poly(ethylene glycol) (PEG) and its conductivity test for a solid electrolyte application are demonstrated. The main steps of synthesis include functionalization of natural lignin's hydroxyl to alkene, followed by graft‐copolymerization of PEG thiol to the lignin via photoredox thiol‐ene reaction. Two lignin‐graft‐PEGs are prepared having 22 wt% lignin (lignin‐graft‐PEG 550) and 34 wt% lignin (lignin‐graft‐PEG 2000). Then, new polymer electrolytes for conductivity tests are prepared via addition of lithium bis‐trifluoromethanesulfonimide. The polymer graft electrolytes exhibit ionic conductivity up to 1.4 × 10⁻⁴ S cm⁻¹ at 35 °C. The presence of lignin moderately impacts conductivity at elevated temperature compared to homopolymer PEG. Furthermore, the ionic conductivity of lignin‐graft‐PEG at ambient temperature is significantly higher than homopolymer PEG precedents. 

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5. Characterizing natural variability of lignin abundance and composition in fine roots across temperate trees: a comparison of analytical methods

   https://doi.org/10.1111/nph.18515  

   Xia, Mengxue; Valverde‐Barrantes, Oscar J.; Suseela, Vidya; Blackwood, Christopher B.; Tharayil, Nishanth (October 2022, New Phytologist)

   Summary Lignin is an important root chemical component that is widely used in biogeochemical models to predict root decomposition. Across ecological studies, lignin abundance has been characterized using both proximate and lignin‐specific methods, without much understanding of their comparability. This uncertainty in estimating lignin limits our ability to comprehend the mechanisms regulating root decomposition and to integrate lignin data for large‐scale syntheses.We compared five methods of estimating lignin abundance and composition in fine roots across 34 phylogenetically diverse tree species. We also assessed the feasibility of high‐throughput techniques for fast‐screening of root lignin.Although acid‐insoluble fraction (AIF) has been used to infer root lignin and decomposition, AIF‐defined lignin content was disconnected from the lignin abundance estimated by techniques that specifically measure lignin‐derived monomers. While lignin‐specific techniques indicated lignin contents of 2–10% (w/w) in roots, AIF‐defined lignin contents werec.5–10‐fold higher, and their interspecific variation was found to be largely unrelated to that determined using lignin‐specific techniques. High‐throughput pyrolysis–gas chromatography–mass spectrometry, when combined with quantitative modeling, accurately predicted lignin abundance and composition, highlighting its feasibility for quicker assessment of lignin in roots.We demonstrate that AIF should be interpreted separately from lignin in fine roots as its abundance is unrelated to that of lignin polymers. This study provides the basis for informed decision‐making with respect to lignin methodology in ecology. 

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