Sustainable Design of Dynamic Poly(disulfide)s

ConspectusChemical solutions to enable sustainable polymers have become a central topic in modern society due to the rising issues caused by traditional plastics. Replacing the inert covalent bonds in classical plastics with dynamic chemical bonds allows one to balance robustness and recyclability and provides a versatile strategy to engineer the dynamics and properties of polymeric materials. The molecular toolbox of dynamic chemistry has been well established with the development of supramolecular chemistry and dynamic covalent chemistry. The former features weak and highly reversible noncovalent bonds that support the formation of supramolecular polymers and materials from small-molecule building blocks, while the latter allows exchangeable covalent linkages enabling malleable and adaptable networks. Combining supramolecular noncovalent bonds and dynamic covalent bonds within a single molecular system enables the complementary advantages of these properties and results in synthetic materials with intrinsic dynamic functions without compromising the loss of mechanical performances. However, such "dual dynamic" building blocks with accessible feedstocks and structural versatility remain very rare and mostly lack chemical space for further molecular engineering. In this Account, we present the discovery, design, and development of poly(disulfide)s-based sustainable materials featuring dual dynamic chemical bonds in the polymeric backbone. Focusing on a commercially available biobased building block, thioctic acid, we show that this natural small molecule elegantly integrates two types of dynamic chemistry within its simple structure, i.e., disulfide-mediated dynamic covalent ring-opening polymerization and supramolecular side-chain cross-linking. A series of polymerization methodologies of thioctic acid derivatives were developed that allow scalable, solvent-free, and efficient preparation under mild conditions. Taking advantage of the readily modifiable carboxylic group, we explore the supramolecular engineering control of the side chain, allowing fine-tuning of the polymeric architectures, mechanical properties, and material functions of the resulting poly(disulfide)s. For example, by introducing strong and multivalent noncovalent cross-links (e.g., iron-carboxylate complexes, reticular H-bonds), the resulting poly(disulfide)s materials could be strengthened compared to commercial engineering plastics without compromising dynamic properties. In addition, we prove the concept that the dual dynamic poly(disulfide)s are chemically recyclable as a result of the reversibility of the supramolecular side-chain and dynamic covalent main-chain, to allow polymer-to-monomer closed-loop recycling, which enables high yield recovery of virgin-quality monomers resulting in an "One-Monomer-Two-Materials" circular loop. We further explore the potential applications of the poly(disulfide)s including self-healing elastomers, supramolecular adhesives, shape-memorable materials, dynamic emissive materials, and degradable sulfur-rich thermoplastics. Finally, personal perspectives on the future opportunities and key challenges of sustainable poly(disulfide)s are presented, thus picturing a blueprint by compiling the next necessary steps toward future practical applicable sustainable polymeric materials.