Circular Polydiketoenamine Elastomers with Exceptional Creep Resistance via Multivalent Cross-Linker Design

Elastomers are widely used in textiles, foam, and rubber, yet they are rarely recycled due to the difficulty in deconstructing polymer chains to reusable monomers. Introducing reversible bonds in these materials offers prospects for improving their circularity; however, concomitant bond exchange permits creep, which is undesirable. Here, we show how to architect dynamic covalent polydiketoenamine (PDK) elastomers prepared from polyetheramine and triketone monomers, not only for energy-efficient circularity, but also for outstanding creep resistance at high temperature. By appending polytopic cross-linking functionality at the chain ends of flexible polyetheramines, we reduced creep from >200% to less than 1%, relative to monotopic controls, producing mechanically robust and stable elastomers and carbon-reinforced rubbers that are readily depolymerized to pure monomer in high yield. We also found that the multivalent chain end was essential for ensuring complete PDK deconstruction. Mapping reaction coordinates in energy and space across a range of potential conformations reveals the underpinnings of this behavior, which involves preorganization of the transition state for diketoenamine bond acidolysis when a tertiary amine is also nearby.

Variable Amine Spacing Determines Depolymerization Rate in Polydiketoenamines

The design of circular polymers has emerged as a necessity due to the lack of efficient recycling methods for many commodity plastics, particularly those used in durable products. Among the promising circular polymers, polydiketoenamines (PDKs) stand out for their ability to undergo highly selective depolymerization in strong acid, allowing monomers to be recovered from additives and fillers. Varying the triketone monomer in PDK variants is known to strongly affect the depolymerization rate; however, it remains unclear how the chemistry of the cross-linker, far from the reaction center, affects the depolymerization rate. Notably, we found that a proximal amine in the cross-linker dramatically accelerates PDK depolymerization when compared to cross-linkers obviating this functionality. Moreover, the spacing between this amine and the diketoenamine bond offers a previously unexplored opportunity to tune PDK depolymerization rates. In this way, the molecular basis for PDK circularity is revealed and further suggests new targets for the amine monomer design to diversify PDK properties, while ensuring circularity in chemical recycling.

Circularity in mixed-plastic chemical recycling enabled by variable rates of polydiketoenamine hydrolysis

Footwear, carpet, automotive interiors, and multilayer packaging are examples of products manufactured from several types of polymers whose inextricability poses substantial challenges for recycling at the end of life. Here, we show that chemical circularity in mixed-polymer recycling becomes possible by controlling the rates of depolymerization of polydiketoenamines (PDK) over several orders of magnitude through molecular engineering. Stepwise deconstruction of mixed-PDK composites, laminates, and assemblies is chemospecific, allowing a prescribed subset of monomers, fillers, and additives to be recovered under pristine condition at each stage of the recycling process. We provide a theoretical framework to understand PDK depolymerization via acid-catalyzed hydrolysis and experimentally validate trends predicted for the rate-limiting step. The control achieved by PDK resins in managing chemical and material entropy points to wide-ranging opportunities for pairing circular design with sustainable manufacturing.