Controlling the reactivity of bonds along polymer chains enables both functionalization and deconstruction with relevance to chemical recycling and circularity. Because the substrate is a macromolecule, however, understanding the effects of chain conformation on the reactivity of polymer bonds emerges as important yet underexplored. Here, we show how oxy-functionalization of chemically recyclable condensation polymers affects acidolysis to monomers through control over distortion and interaction energies in the rate-limiting transition states. Oxy-functionalization of polydiketoenamines at specific sites on either the amine or triketone monomer segments increased acidolysis rates by more than three orders of magnitude, opening the door to efficient deconstruction of linear chain architectures. These insights substantially broaden the scope of applications for polydiketoenamines in a circular manufacturing economy, including chemically recyclable adhesives for a diverse range of surfaces.

Circular plastics thrive on the ability to chemically recycle polymers into reusable monomers, ideally closing the loop from manufacturing to end of life. Mechanisms for heterogeneous polymer deconstruction are complex, involving diffusion and transport of reagents to reactive sites in a material continuously undergoing chemical transformations. A deeper understanding of deconstruction phenomena would better inform the molecular basis of circularity in plastics. Here, we show how nuclear magnetic resonance (NMR) spectroscopy, relaxometry, and diffusometry enable monitoring of heterogeneous deconstruction of a model elastomer with acid-cleavable diketoenamine bonds. In chaotropic aqueous HBr, polydiketoenamine (PDK) deconstruction is fast, enabled by macro- and micro-scale swelling in early stages, which facilitates acid penetration and protonation of reaction sites deep within the polymer sample. We observe a previously unrecognized hydrogen-bond stabilized protonated amine intermediate that is persistent throughout deconstruction, and found a strong correlation of its reactivity with swelling and chain kinetics. In kosmotropic aqueous H2SO4, PDK deconstruction is notably slower. Swelling occurred at a more gradual pace, creating a porous polymer matrix, yet polymer chain mobility remained low. Most notably, the kosmotropic character of aqueous H2SO4 resulted in a reaction site that was comparably less active in advancing hydrolysis and deconstruction to shorter oligomer chains, instead trapping acid in matrix pores and modifying the activity of the reaction medium under confinement in the process.

Footwear, carpet, soft furnishings, automotive interiors, and multi-layer packaging are examples of products manufactured from several types of polymers whose inextricability poses significant challenges for recycling at end-of-life. Here, we show that chemical circularity in mixed-polymer recycling becomes possible by controlling the rates of depolymerization of polydiketoenamines (PDKs) 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 in 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 PDKs in managing thermal and materials entropy points to new opportunities for pairing circular design with sustainable manufacturing.

Here, we show that oxy-functionalization at specific sites along polydiketoenamine (PDK) backbones affects depolymerization rates by over three orders of magnitude, due to differences in distortion energies associated with reactive chain conformations in transition states for acidolysis. Site-specific oxy-functionalization, resulting in the fastest rates of acidolysis, opens the door to deconstructing linear PDK chain topologies for the first time, broadening the scope of applications for PDK plastics in a circular manufacturing economy, including chemically recyclable adhesives for a diverse range of surfaces.

Plastics have played a crucial role in shaping the modern era. However, they are produced almost exclusively from non-renewable feedstocks and face considerable challenges in recycling, especially crosslinked polymers used in consumer electronics, building materials, automotive parts, and aerospace composites1–3. Here, we integrate computational materials design, polymer chemistry, synthetic biology, and systems analysis into a workflow to accelerate the design of biorenewable polydiketoenamines (bioPDKs) with tailored properties and circularity through molecular engineering of their monomers, which are derived from β-keto-δ-lactones (BKDLs). Our approach leverages the modular assembly of hybrid polyketide synthases (PKSs) to enable the biosynthesis of BKDLs with diverse substituents at the γ- and δ-positions with defined stereochemistry. We exploit these features to tailor bioPDK hydrophobicity and depolymerization temperature in aqueous acid, which would be nearly impossible to do at scale using synthetic chemistry. In specific designs, we observe increases in bioPDK acidolysis above thresholds for comparable PDK materials derived from petrochemicals, which has implications for mixed-plastic and composite recycling, solvent and chemical resistance, and degradation.

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.

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 outstanding creep resistance at high temperature. By appending polytopic crosslinking 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.

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.

Hydrolysis reactions are ubiquitous in biological, environmental, and industrial chemistry. Density functional theory (DFT) is commonly employed to study the kinetics and reaction mechanisms of hydrolysis processes. Here, we present a new data set, Barrier Heights for HydrOlysis - 36 (BH2O-36), to enable the design of density functional approximations (DFAs) and the rational selection of DFAs for applications in aqueous chemistry. BH2O-36 consists of 36 diverse organic and inorganic forward and reverse hydrolysis reactions with reference energy barriers ΔE‡ calculated at the CCSD(T)/CBS level. Using BH2O-36, we evaluate 63 DFAs. In terms of mean absolute error (MAE) and mean relative absolute error (MRAE), ωB97M-V is the best-performing DFA tested, while MN12-L-D3(BJ) is the best-performing pure (nonhybrid) DFA. Broadly, we find that range-separated hybrid DFAs are necessary to approach chemical accuracy (0.043 eV). Although the best-performing DFAs include a dispersion correction to account for long-range interactions, we find that dispersion corrections do not generally improve MAE or MRAE for this data set.

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.

This work demonstrates 3D printed soft actuators with complex shapes and remote actuation using an external magnetic field. Instead of embedding magnetic particles in a polymeric matrix, we fabricated a novel ferrofluid-based actuator, in which the fluid can be moved to different locations in the actuator to affect actuator response. We studied the effect of both the ferrofluid and the 3D printed material on the motion of simple actuators using 3D printed tubes. In addition, we 3D printed more complex actuators mimicking a human hand and a worm to demonstrate more complex motion.

Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique.