Lignin-derived thermosets and vitrimers: Bridging the gap between renewable and recyclable materials

Naturally occurring in wood, agricultural residues, and perennial plants, lignin is the second largest source of renewable carbon produced on Earth. Despite its million tons scale production as industrial wastes from paper pulping or as biorefinery processes, its complex polyphenolic structure limits its use for high value-added applications. One strategy to overcome structural heterogeneity, reduced processability, and reactivity issues relies on refining and functionalization techniques. The past decades witnessed the numerous chemistries deployed to develop thermosetting materials such as polyurethane, phenolic, epoxy, or polyester resins from lignin biopolymer. More recently, the potential to create covalent-adaptable networks from lignin envisions the sustainable design of polymeric materials with a closed-loop lifecycle. This review covers both the topics of ligninderived thermosets and lignin-derived vitrimers with the aim to bridge the gap between renewable and recyclable materials. After an overview introducing the foundation of lignin background (origin, structure, extraction, and refining techniques), the synthetic pathways explored to produce thermosets from technical, refined, or chemically modified lignin are described in a polymer chemistry perspective. The review concludes with a description of the performance of the few reported lignin-derived vitrimers thus far and the remaining challenges to incorporate new dynamic covalent bonds.