Dynamic decrosslinking enables self-healing, reprocessability, and upcycling in polyurethane networks

Domain: chemistry
Published: 2026-01-06
Editorial outcome: minor_revision

Abstract
Thermosetting polymers are widely used for their high mechanical performance and long-term structural reliability, but their permanent crosslinked networks prevent reprocessing and recycling, leading to persistent environmental challenges. Existing dynamic covalent and supramolecular strategies offer partial solutions, yet often suffer from performance loss after repeated recycling due to incomplete reversibility and the accumulation of defects. Here, we present a strategy for the decrosslinking and upcycling of post-service thermosets based on a dynamic polyurethane network containing hindered urea bonds (HUBs). The elastic network exhibits high mechanical strength, self-healing, and thermal reprocessability. Upon exposure to small-molecule amines, the HUBs undergo exchange reactions that enable controlled network disassembly, allowing the thermosets to be transformed into reprocessable thermoplastics. The decrosslinked derivatives facilitate high-value composites—toughening glass fiber matrices by 229% through interfacial reinforcement and achieving commercial-grade electromagnetic shielding (28.1 dB) via optimized carbon nanotube integration. This molecularly engineered decrosslinking-upcycling paradigm establishes a viable pathway for circular thermoset utilization, advancing sustainable polymer technologies.

Introduction
With the mounting pressure of global environmental challenges and the unsustainable consumption of resources, the development of polymer materials that combine high performance with environmental responsibility has become a pressing imperative1–3. Structurally, polymeric materials are generally classified into thermoplastics and thermosets4,5. Thermoplastics have linear or lightly branched molecular chain structures that allow them to be extremely flexible in the manufacture of new product shapes, supporting a variety of molding techniques, diverse injection molding machines, and various mold designs, which makes them widely used in packaging, textiles, and consumer goods6–9. Crucially, any scrap generated during in-factory processing can be effectively recycled internally, contributing to material efficiency and reduced waste10. Nevertheless, their relatively poor solvent resistance and thermal stability severely limit their performance in demanding environments. In contrast, traditional thermosetting polymers form permanent covalent crosslinked networks that confer outstanding mechanical strength, chemical resistance, and thermal stability11–13. These features make thermosets indispensable in structural, electronic, and transportation applications14. However, processing thermosetting polymers typically requires specialized equipment, including injection molding machines and molds dedicated to a specific curing process15. Creating new product geometries often necessitates extensive retooling and reconfiguration of the manufacturing line, leading to significant capital expenditure, protracted development cycles, and limited options for recycling process scrap16. In addition, the same permanent crosslinking that provides durability also precludes repair, reshaping, or recycling—leading to persistent waste accumulation and resource loss.

Currently available recycling methods for thermosets—including mechanical grinding17–19, pyrolysis20,21, chemical depolymerization22–24, and landfill disposal25–27—are either inefficient, energy-intensive, or environmentally detrimental17. Alarmingly, thermosets account for approximately 20% of global polymer production, amounting to over 65 million tons annually—a figure projected to increase in the coming decades28–30. This creates an urgent need for sustainable recycling solutions tailored to the unique challenges of thermoset materials.

In recent years, the design of dynamic polymer networks has garnered significant attention as a promising pathway to address the recyclability of crosslinked materials4,11,16,31. Incorporating supramolecular interactions into covalently crosslinked polymer networks imparts dynamic behavior through reversible, non-covalent motifs3,32,33. These interactions promote self-healing and reprocessability without compromising the structural integrity of the covalent backbone. However, the limited mechanical robustness of supramolecular interactions has restricted their broader applicability, particularly in load-bearing or harsh-environment scenarios. To overcome these limitations, the incorporation of reversible covalent bonds into polymer networks has emerged as a powerful strategy34. These dynamic covalent chemistries, which include transesterification35,36, disulfide exchange37,38, imine metathesis39,40, and others, can undergo bond exchange reactions under external stimuli such as heat41,42, light43, or pH changes44. As a result, they endow thermoset polymers with reprocessability, self-healing, and shape adaptability30,45. Nonetheless, repeated recycling often leads to incomplete reversibility, network fatigue, and the accumulation of defects, compromising long-term mechanical performance and structural integrity. Thus, developing methods to rejuvenate aged or over-serviced dynamic networks remains a central challenge in the field.

In this study, we present a sustainable materials strategy that enables the decrosslinking and upcycling of post-service thermoset polymers through the design of a dynamic polyurethane network (PUN) incorporating hindered urea bonds (HUBs) as reversible covalent crosslinks. This elastic PUN exhibits not only excellent mechanical performance, healing ability, and thermal reprocessability during service, but also allows for controlled decrosslinking into reprocessable thermoplastics, which can be further upcycled into high-value functional composites. This strategy offers a practical and generalizable approach to extend the utility of high-performance thermosets beyond their initial service life. By integrating dynamic covalent chemistry with scalable polymer processing techniques, it provides a promising route to mitigate end-of-life challenges in thermoset materials and contributes meaningfully to the broader pursuit of circular, resource-efficient polymer technologies.

Results
Design and synthesis of sustainable PUNs
Dynamic polymer networks offer designable functionality and intrinsic recyclability46; however, their gradual performance degradation over repeated recycling cycles remains a significant barrier to practical application47,48. To mitigate the environmental risks associated with material overutilization, we propose strategies involving decrosslinking and upcycling, aiming to enhance the added value and promote the sustainable development of thermosetting materials (Fig. 1a and Supplementary Table 1). The exceptional dynamic properties of HUB-based PUNs have garnered widespread interest. Notably, systematic studies by Cheng and co-workers on HUBs have paved the way for the development of dynamic networks with exceptional reprocessability, recyclability, and self-healing performance49–51. PUNs containing dynamic HUBs are employed as a model system to investigate the decrosslinking and upcycling processes mediated by small molecules (Fig. 1b). Considering that the bulky tert-butyl group provides stronger steric hindrance, which enhances the dynamics and reversibility of the polymer network, its application may be limited by temperature and environmental constraints. In this work, we designed a dynamic network utilizing HUBs with lower steric hindrance, which provides enhanced stability and sustainable performance. The sustainability of the PUN arises from two key mechanisms: (1) the network undergoes HUB exchange reactions under specific external stimuli; and (2) the introduction of monofunctional small molecules facilitates the decrosslinking and linearization of the polymer network. This work not only retains the advantageous properties of dynamic HUB-based polymer networks but also introduces an innovative dynamic competition reaction strategy. The strategy efficiently converts chemically crosslinked elastomers into reprocessable thermoplastics, which can then be upgraded into high-performance composites. This approach therefore integrates self-healing, reprocessability, and upcycling within a single polymer platform—an achievement not reported in previous HUB-based systems.

Specifically, polytetramethylene ether glycol with a number-average molecular weight (Mn) of 2000 g mol−1 (PTMEG2000) and dicyclohexylmethane 4,4’-diisocyanate (HMDI) monomers were prepolymerized in N,N-dimethylformamide (DMF) at 80°C for 1 h, followed by the addition of the crosslinking agent diethanolamine (DEA) for another hour to obtain the dynamic PUN, denoted as PTMEG2000-HMDI-DEA (PHD) (Fig. 1b). PHD can be synthesized in either a 250 mL flask or a 100 L double-layer glass reactor (Fig. 2a). Considering environmental and health factors, N, N’-dimethylacetamide (DMAc) with low toxicity and high stability can also be used instead of DMF as the solvent for polymer synthesis to improve safety during the synthesis process (Supplementary Table 2). The product can be isolated by vacuum drying or via a water-dropping method previously developed by our group3,52. The resulting PUN exhibits excellent optical transparency, with a transmittance of 90.2% at 650 nm (Fig. 2b and Supplementary Fig. 1). Fourier transform infrared (FTIR) spectroscopy confirmed the successful formation of PHD (Supplementary Fig. 2a). X-ray diffraction (XRD) analysis revealed the amorphous nature of the material (Supplementary Fig. 2b). Furthermore, small-angle X-ray scattering (SAXS) provided insight into the internal structure, with the two-dimensional (2D) SAXS diffraction pattern displaying a concentric ring, indicative of a uniformly dispersed phase-separated morphology (Fig. 2b). The calculated long period (L) of the phase separation domain was 9.8 nm.

The mechanical properties of the PHD elastomer were systematically evaluated through tensile testing. As shown in the stress–strain curve (Fig. 2c), the PHD exhibited a high tensile strength of 31.4 MPa and an impressive tensile toughness of 273.5 MJ m−3. Furthermore, replacing DMF with DMAc as the solvent has virtually no effect on the mechanical properties of the synthesized material (Supplementary Fig. 3). A visual demonstration further highlights its outstanding mechanical performance: a rectangular PHD sample weighing only ~0.25 g was capable of effortlessly lifting a 7.3 kg dumbbell—approximately 29,000 times its own weight. Upon undergoing 10 loading–unloading cycles at 500% strain, the elastomer displayed a first-cycle rebound efficiency of 85% (Fig. 2d). After being stretched to 500% strain and subsequently released, the sample recovered completely within 10 min, underscoring its excellent elasticity. Incremental cyclic tensile tests (100–1500% strain) revealed a progressive increase in hysteresis loop area with strain, indicating enhanced energy dissipation at higher deformations (Supplementary Fig. 4). To assess its damage tolerance, a single-edge notched tensile test was performed, yielding a fracture energy of 103.1 kJ m−2 as calculated by the Greensmith method (Fig. 2e). In a puncture resistance test, the elastomer demonstrated a puncture energy of 381.6 mJ, confirming its outstanding resistance to mechanical damage (Fig. 2f). The dynamic crosslinked network also endowed the PHD elastomer with excellent thermal and solvent stability. The crosslinking density, calculated using the rubber elasticity theory3, was determined to be 2.1 k mol m−3 (Fig. 2g). Thermogravimetric analysis (TGA) showed that the maximum decomposition temperature was approximately 414.8°C, indicating good thermal stability (Supplementary Fig. 5). As shown in Fig. 2h, PHD samples retained over 92% of their original mass after 24 h of immersion in various corrosive media, demonstrating exceptional solvent resistance.

Dynamic behaviors of sustainable PUNs
To investigate the molecular structure and intermolecular interactions within the elastomer, temperature-dependent FTIR spectra of PHD were recorded (Fig. 3a and Supplementary Fig. 6). Peak assignments for both synchronous and asynchronous spectra, along with cross-peak notations, are summarized in Supplementary Table 3. According to Noda’s rule53, the sequential response of characteristic groups followed the order: 1498 → 1551 → 1728 → 1696 cm−1. In addition, two prominent auto-peaks were observed at 3310 and 3487 cm−1, corresponding to N–H stretching vibrations. The sequential response from 3310 to 3487 cm−1 indicates temperature-dependent disruption of hydrogen bonding, confirming the dynamic nature of the material.

To further evaluate the dynamic behavior of the PHD network, stress relaxation tests were performed at various temperatures. As the temperature increased from 110 to 140°C, the relaxation time decreased markedly from 398 to 10.2 s, indicating a significant acceleration in the relaxation rate (Fig. 3b). The stress relaxation temperature, which reflects the onset of local segmental motion without causing macroscopic flow, is generally lower than the temperature required for macroscopic viscous flow. This enhancement is attributed to the thermally activated exchange reactions between HUBs. The temperature dependence of the relaxation time (τ*) follows the Arrhenius equation54,55:

ln(τ*) = Ea/RT + ln(τ0) (1)

where Ea is the activation energy, R is the universal gas constant, T is the absolute temperature, and τ0 is the relaxation time at infinite temperature. As shown in Fig. 3c, the relaxation time exhibited a linear correlation with 1/T, consistent with Arrhenius behavior. The calculated activation energy for the dynamic bond exchange was 157.8 kJ mol−1. The presence of reversible covalent bonds within the dynamic polymer network enables both self-healing and recyclability. With increasing temperature, the self-healing performance of the damaged samples significantly improves (Supplementary Fig. 7). The hydrogen bonds between elastomer chains, being much weaker than covalent bonds, break first under thermal agitation. As the temperature rises further, the dissociation and recombination rates of dynamic covalent bonds in polymers accelerate, thereby enhancing the activity of chain segments and the ability of network reconstruction, and significantly improving the dynamic performance of the material. As shown in Fig. 3d and Supplementary Fig. 8, visible surface scratches on the damaged samples nearly disappeared after healing for 2 h at 120°C, demonstrating excellent self-healing capability. Moreover, the material could be fully reprocessed through hot pressing, with the regenerated chemical structure remaining nearly identical to that of the original sample (Fig. 3e and Supplementary Fig. 9). Mechanical testing before and after recycling (Fig. 3f) showed that the reprocessed sample retained comparable mechanical performance, confirming its outstanding reprocessability.

Subsequently, a linear polymer (PTMEG2000-HMDI-PDO, PHP) and a covalently crosslinked PUN (PTMEG2000-HMDI-TEA, PHT) were synthesized as control samples (Supplementary Fig. 10), with their molecular structures illustrated in Fig. 3g, h. Compared to PHP and PHT, the dynamic PHD network demonstrated superior comprehensive performance, including high transparency (90.2%), a crosslinking density of 2.1 k mol m−3, excellent fracture stress (31.4 MPa), and remarkable tensile toughness (273.5 MJ m−3), as shown in Fig. 3i and Supplementary Figs. 11–13. PHD also exhibited a gel fraction comparable to that of the permanently covalent network PHT, while maintaining self-healing and recyclability on par with the linear polymer PHP (Supplementary Figs. 14–18). Control experiments reveal that the PHT sample, despite undergoing hydrogen bond dissociation upon heating, cannot be recycled because it lacks dynamic covalent bond exchange. This result underscores that the HUBs in the elastomer are essential for granting the polymer its dynamic properties. In addition, compared with other materials containing dynamic HUBs reported in previous studies56–61, the PHD sample shows reasonably good recyclability and healing performance (Supplementary Fig. 19). The reversible nature of HUBs enables the network to rearrange under suitable conditions, facilitating efficient repair and recycling without compromising mechanical integrity, thereby achieving a balance between high performance and sustainability. This dynamic PUN thus combines the structural stability of thermoset polymers with the reprocessability of linear polymers, offering a promising pathway toward sustainable material development.

Decrosslinking of sustainable PUNs
To investigate the bond exchange behavior of the PHD network, a small-molecule model system was employed to simulate and elucidate the kinetic mechanism of reversible covalent bond exchange62. Initially, a sterically hindered urea compound—3-cyclohexyl-1,1-diethylurea—was synthesized via the reaction of cyclohexyl isocyanate with diethylamine (Supplementary Fig. 20). The successful synthesis of the target molecule was confirmed by high-resolution mass spectrometry (HR-MS) and proton nuclear magnetic resonance (1H NMR) spectroscopy. Subsequently, the obtained 3-cyclohexyl-1,1-diethylurea was reacted with n-propylamine in DMSO-d6 at 120°C for 12 h (Fig. 4a). 1H NMR analysis revealed the appearance of a new set of peaks that increased in intensity over time (Fig. 4b and Supplementary Figs. 21–23), indicating that an exchange reaction had occurred between the urea and n-propylamine, forming 1-cyclohexyl-3-propylurea. As shown in Fig. 4c, the reaction approached equilibrium within 12 h, yielding the target product at approximately 85%. Correspondingly, a new HR-MS absorption peak at m/z = 185.2 emerged and plateaued over time (Fig. 4d), further confirming the dynamic exchange behavior of the HUB. For comparison, the dynamic exchangeability of carbamate bonds was also examined. Propyl cyclohexylcarbamate was synthesized by reacting cyclohexyl isocyanate with n-propanol, and its structure was verified by HR-MS and 1H NMR spectroscopy (Supplementary Fig. 24). The resulting carbamate compound was then reacted with n-propanolamine in DMSO-d6 at 120°C for 12 h (Supplementary Fig. 25a). However, 1H NMR spectra showed no noticeable new peaks during the reaction (Supplementary Fig. 25b), suggesting that under the same conditions, the carbamate bond did not undergo exchange, in contrast to the urea system.

To further explain the dynamic behavior of HUBs, density functional theory (DFT) calculations were performed at the B3LYP/6–31+G(d) level63,64. Specifically, 4-methylpiperidine and methyl isocyanate (MeNCO) were employed as model reactants to explain the mechanism of this reaction. The reaction mechanism governing the formation of 1-cyclohexyl-3-propylurea from 3-cyclohexyl-1,1-diethylurea and n-propylamine is depicted in Fig. 4e. Theoretical studies indicated that n-propylamine gradually approached the carbonyl group of the HUB during the reaction, forming a transition state (TS) with an activation enthalpy (ΔHsol) and Gibbs free energy (ΔGsol298K) of 23.4 and 37.6 kcal mol−1, respectively. Upon detachment of diethylamine, the exchange reaction was successfully completed. To further investigate the dynamics of the HUB, we calculated the resonance energy (ER) to explain the dynamic differences at the B3LYP/6–31+G(d) level. The results showed that the ER of the HUB (10.0 kcal mol−1) was lower than that of a conventional urea bond (12.6 kcal mol−1), indicating enhanced dynamic behavior of the HUB due to reduced resonance stabilization.

Based on the above studies, small primary amine molecules were introduced into the PHD dynamic network to undergo exchange reactions with HUBs, thereby inducing decrosslinking and polymer network reconstruction. This strategy led to the formation of a thermoplastic polyurethane with excellent recyclability (Fig. 5a). The PHD network could be decrosslinked and reconfigured through solvent treatment, hot pressing, or screw extrusion, all mediated by n-hexylamine (HA), yielding PHD-HA elastomers (Fig. 5b). The small molecule-assisted solvent reconstruction process is illustrated in Fig. 5c. Specifically, the PHD sample was cut into pieces and dissolved in a 0.1 mol L−1 HA solution in DMF under heating and stirring. After complete dissolution, the solvent was evaporated to obtain the regenerated product, PHD-HA. The resulting thermoplastic polymer could be redissolved and cast into molds with desired geometries, enabling flexible sample patterning. Gel permeation chromatography (GPC) analysis revealed that PHD-HA had a Mn of 28.4 kg mol−1 and a polydispersity index (PDI) of 2.8 (Fig. 5d). As shown in Fig. 5e, PHD-HA exhibited a tensile strength of 24.7 MPa and a tensile toughness of 209.8 MJ m−3, slightly lower than that of the original PHD material.

PHD could also be decrosslinked and reprocessed through hot pressing. Specifically, PHD and HA were uniformly blended and hot-pressed at 120°C under 10 MPa for 2 h to obtain the decrosslinked and reconstructed material, denoted as PHD-HA (Fig. 5f). The successful synthesis of PHD-HA was confirmed by 1H NMR spectroscopy and in situ FTIR spectroscopy, and GPC analysis showed a Mn of 34.8 kg mol−1 and a PDI of 3.3 (Supplementary Figs. 26–28). Additionally, thermal analysis revealed an increase in the glass transition temperature (Tg) from −56.4 to −50.2°C and a decrease in the flow temperature (Tf) from 162.7 to 103.7°C after decrosslinking, indicating that an exchange reaction occurred between HA and the PHD network (Supplementary Figs. 29 and 30). Under the action of HA, the original HUBs in the dynamic network of PHD samples are transformed into ordinary urea bonds, and the intermolecular forces are enhanced, inhibiting the movement of chain segments. Meanwhile, the disruption of dynamic crosslinking enhances the regularity of the chain, increases non-covalent interactions and reduces the free volume, thereby raising the Tg of the elastomer. The PHD-HA solution could be cast into various patterns, such as a cat shape (Supplementary Fig. 31). Moreover, PHD-HA exhibited excellent self-healing and recovery capabilities (Supplementary Fig. 32). The hot-pressed recycled PHD-HA showed a tensile strength of 23.7 MPa and a tensile toughness of 201.1 MJ m−3 (Supplementary Fig. 33). The PHD-HA samples were capable of solvent recycling, and their mechanical properties showed negligible changes after multiple recycling cycles (Supplementary Fig. 34), confirming their excellent recyclability.

Furthermore, PHD could also be reconstituted and regenerated using 1,6-hexanediamine (HDA), following a recovery process similar to that of the PHD-HA hot-press reconstruction (Supplementary Figs. 35−37). As shown in Supplementary Fig. 38, after the PHD sample was depolymerized into the PHD-HDA sample, the Tg increased from −56.4 to −47.1°C. Notably, after the introduction of HDA, the Mn of the resulting PHD-HDA polymer reached 68.3 kg mol−1 (Supplementary Fig. 39a), nearly twice that of PHD-HA. During depolymerization, the difunctional HDA converts the HUBs in the reversible network into normal urea bonds, but it struggles to form new crosslinking structures with the difunctional isocyanate, serving only to effectively extend the polymer molecular chains. Moreover, PHD-HDA could be regenerated via solvent dissolution without significant loss of mechanical properties (Supplementary Fig. 39b), demonstrating excellent recyclability. Based on these findings, the incorporation of other functional small molecules into the dynamic polymer network is anticipated to enable the design of a diverse range of sustainable materials with tunable and targeted functionalities.

Finally, PHD was decrosslinked and reconstructed using screw extrusion technology. Specifically, HA was uniformly mixed with PHD and subsequently cut into small particle-like fragments (Fig. 5g). These particles were then loaded into the feed port of the single-screw extruder, where a chain exchange reaction between HA and PHD occurred during processing, enabling the decrosslinking and reconstruction of the network to yield the thermoplastic polymer PHD-HA. Upon extrusion (inlet temperature: 60°C; barrel temperature: 150°C), the PHD-HA was cooled in a water bath, forming uniform, circular filaments. Microscopic examination of the cross-section and surface of the PHD-HA filaments revealed a flat and smooth morphology (Fig. 5h). In contrast, the original PHD network exhibited white, granular textures after extrusion under the same conditions (Supplementary Fig. 40). This solvent-free screw extrusion approach effectively drives the reconstruction of the PUN, enabling the transformation of thermosetting materials into thermoplastic ones. This strategy holds great promise for advancing the recyclability and sustainable utilization of polyurethane-based materials.

Upcycling of decrosslinked PUNs
The thermoplasticization of thermosetting materials mediated by small molecules not only facilitates material recycling but also enables the development of high-performance composites, thereby enhancing the added value of the materials. To validate this concept, the hot-compressed decrosslinked product PHD-HA was compounded with glass fiber (GF) fabrics. Specifically, the PHD-HA elastomer was dissolved in DMF and uniformly applied onto GF fabrics—pretreated with acetone and ethanol—by brush coating to ensure complete fiber impregnation. The elastomer content was controlled at 30 wt% of the total composite mass (Supplementary Fig. 41). The impregnated fabric was dried at 60°C for 12 h, followed by heat treatment at 80°C for 24 h to obtain the final composite (PHD-HA/GF). The pristine GF fabrics exhibited smooth, bundled arrangements (Fig. 6a). After compounding, the fiber surfaces were uniformly coated with the PHD-HA elastomer, with no discernible interfacial gaps, indicating excellent interfacial adhesion between the matrix and the fiber. As shown in Fig. 6b, the stress–strain curves demonstrated that the fracture stress and tensile toughness of the PHD-HA/GF composite reached 355.1 MPa and 63.8 MJ m−3, representing enhancements of 136% and 229% over those of pure GF, respectively. Additionally, the impact strength of the composite was significantly improved, reaching 2.6 MJ m−2—2.2 times higher than that of GF alone (Fig. 6c). These improvements demonstrate that the mechanical performance of the fiber was substantially enhanced, broadening its potential application scope. Notably, the PHD-HA matrix in the composite could be redissolved in DMF, enabling the recovery of intact GF fabrics (Supplementary Fig. 42), which maintained their original smooth surface morphology, highlighting the recyclability of the composite system.

The decrosslinked PHD products also demonstrated potential for application in electromagnetic interference (EMI) shielding, thereby further enhancing the material’s overall value. Specifically, PHD-HA was dissolved in DMF, followed by the addition of carbon nanotubes (CNTs) at concentrations of 5 wt%, 10 wt%, and 15 wt%, respectively, to prepare a series of PHD-HA/CNT composites (x=5%, 10%, 15%). Scanning electron microscopy (SEM) images revealed that the pristine PHD-HA elastomer exhibited a smooth and flat surface, while surface roughness increased with higher CNTs content (Fig. 6d, e), reflecting CNTs aggregation and structural changes. The observed shift in the carbonyl absorption peak from 1713 to 1716 cm−1, along with the increased intensity at 1698 cm−1 in the FTIR spectra, suggests strong supramolecular interactions between the CNTs and the PHD-HA matrix (Supplementary Fig. 43). Additionally, TGA showed a gradual decrease in mass loss with increasing CNTs content, suggesting enhanced thermal stability of the composites (Supplementary Fig. 44). Samples with a high CNT content may exhibit a lower decomposition temperature, likely because the highly thermally conductive CNTs disrupt the ordered structure of the polyurethane chains and induce localized overheating, thereby accelerating the dissociation of the polymer network.

As shown in Fig. 6f, the PHD-HA/CNT composites exhibited excellent EMI shielding performance. Notably, the composite with 15 wt% CNTs achieved an average shielding effectiveness (SE) of 28.1 dB, effectively attenuating approximately 99.8% of incident electromagnetic radiation, which is higher than the 20 dB EMI shielding requirement for standard commercial applications65 (consumer electronics, office and industrial equipment, etc.). With increasing CNTs loading, the overall shielding performance improved steadily (Fig. 6g), indicating that higher CNT content enhances the material’s ability to block electromagnetic waves, making it suitable for use in high-performance EMI shielding applications. The shielding capability was further demonstrated using a Tesla coil simulation experiment. When the Tesla coil was energized, the connected light emitting diode (LED) bulb lit up immediately. However, once the PHD-HA/CNT composite was placed between the coil and the bulb, the LED was promptly extinguished (Fig. 6h), clearly indicating effective electromagnetic shielding in a practical scenario. Importantly, the PHD-HA/CNT composite retained full solubility in DMF, allowing the material to be recast into various customized shapes (Supplementary Fig. 45), showcasing its reprocessability and design versatility. These findings demonstrate a promising strategy for recycling and functionally upcycling thermosetting polyurethanes into high-value, reprocessable composites with advanced EMI shielding capabilities, offering significant potential for promoting the sustainable development of polyurethane-based materials.

Methods
Materials
Polytetramethylene ether glycol with a number-average molecular weight (Mn) of 2000 g mol–1 (PTMEG2000), diethanolamine (DEA, purity ≥99%), 1,5-pentanediol (PDO, purity ≥97%), n-propanol (purity ≥97%), and dicyclohexylmethane 4,4’-diisocyanate (HMDI, purity ≥90%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Triethanolamine (TEA, purity ≥99%), n-hexylamine (HA, purity ≥99%), 1,6-hexanediamine (HDA, purity ≥99%), n-propylamine (purity ≥97%), cyclohexyl isocyanate (purity ≥98%), and diethylamine (purity ≥99%) were obtained from Aladdin Reagent Co. (Shanghai, China). Dibutyltin dilaurate (DBTDL, purity ~95%) was supplied by Sigma-Aldrich. N, N’-Dimethylacetamide (DMAc, purity ~99.8%, water ≤50 ppm) and N, N-dimethylformamide (DMF, purity ~99.8%, water ≤50 ppm) were purchased from Energy Chemical. Glass fiber (GF) was obtained from Xiongxian Yujing Building Materials Co., Ltd., and multi-walled carbon nanotubes (CNTs) (purity ~95%) were provided by XFNANO Materials Tech Co., Ltd. Trichloromethane (purity ≥99.0%), acetone (purity ≥99.5%), diethyl ether (purity ≥99.5%), and ethanol (purity ~99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further purification.

Synthesis of PHD networks
Four grams of PTMEG2000 was added to a 100 mL flask and heated to 120°C under stirring for 2 h to remove residual moisture. After cooling to 80°C under a nitrogen atmosphere, 4 mL of DMF, 0.02 g of DBTDL, and 1.2 g of HMDI were sequentially added. The mixture was stirred at 80°C for 1 h. The temperature was then reduced to 70°C, and 0.14 g of DEA (pre-dissolved in 10 mL of DMF) was added. The reaction continued at 70°C for another hour. During the reaction, additional DMF was added as needed to control viscosity and avoid gelation. The resulting solution was poured into a silicone mold, vacuum-degassed at room temperature, then cured at 60°C for 12 h and further dried at 80°C for 24 h to yield the PHD elastomer. For large-scale production, a 100 L double-layer glass reactor was used, and the raw material quantities were proportionally scaled up based on the laboratory formulation.

Synthesis of linear PHP polymers and PHT networks
The synthesis followed a similar procedure. Briefly, 4 g of PTMEG2000 was heated to 120°C under stirring for 2 h to remove moisture. After cooling to 80°C under nitrogen, 4 mL of DMF, 0.02 g of DBTDL, and 1.2 g of HMDI were added sequentially. The mixture was stirred at 80°C for 1 h. After lowering the temperature to 70°C, either 0.208 g of PDO or 0.199 g of TEA (each dissolved in 10 mL of DMF) was introduced. The reaction was continued at 70°C for 1 h, with DMF added as needed to maintain proper viscosity. The final solution was cast into silicone molds, vacuum-degassed at room temperature, and cured at 60°C for 12 h, followed by drying at 80°C for 24 h to obtain the PHP or PHT elastomers.

Healing, recovery, and decrosslinking of PHD networks
Elastomer healing process: the elastomer was scratched with a blade and subsequently healed by heating at 120°C for 2 h to obtain the healed elastomer.

Hot-pressing recovery process of elastomer: the elastomer was chopped into small pieces and hot-pressed at 120°C under a pressure of 10 MPa for 2 h to produce the recycled elastomer.

Solvent decrosslinking: approximately 0.3 g of the elastomer was cut into small pieces and added to 10 mL of a 0.1 M HA solution in DMF. The mixture was heated and stirred until complete dissolution, followed by solvent evaporation to obtain the decrosslinked PHD-HA elastomer.

Hot-pressing decrosslinking: about 0.5 g of the elastomer was immersed in a DCM solution containing 20 mg of HA and then dried at room temperature to remove the solvent. The dried product was subsequently hot-pressed at 120°C under 10 MPa for 2 h to yield the decrosslinked PHD-HA elastomer. The decrosslinking process for PHD-HDA elastomers was similar to that for PHD-HA elastomers.

Screw extrusion decrosslinking: approximately 0.5 g of the elastomer was immersed in a DCM solution containing 20 mg of HA, dried at room temperature, and then chopped into small granules. The granules were fed into an extruder with the feed inlet maintained at 60°C and the barrel at 150°C to facilitate chain exchange reactions, resulting in the decrosslinked PHD-HA elastomer.

Determination of solvent resistance of samples
To determine the gel fraction, the sample of initial weight (W1) was immersed in THF for 24 h at 25°C, followed by vacuum drying at 60°C until constant mass. The process was repeated three times, and the sample was weighed (W2). The gel fraction was calculated using the following equation:

η(%) = W2/W1 × 100% (2)

To determine the residue weight, samples were immersed in selected solvents (DMSO, DMF, THF, DCM, EtOH, AC, 1M HCl, 1M NaOH, and water) at 25°C for 12 h, followed by vacuum drying at 80°C for 24 h. The residue weight was calculated using the following equation:

Residual weight (%) = (minitial - mfinal)/minitial × 100% (3)

Fabrication of PHD-HA/GF composites
One gram of PHD-HA was dissolved in 15 mL of DMF by heating and stirring at 80°C for 2 h. The resulting solution was uniformly brushed onto 2.3 g of GF fabric. Prior to use, the GF fabric was refluxed in acetone and ethanol at 60°C for 24 h to remove surface impurities. After impregnation, the sample was vacuum-degassed at room temperature, followed by heating at 60°C for 12 h. Finally, the sample was dried in an oven at 80°C for 24 h to obtain the PHD-HA/GF composite.

Fabrication of PHD-HA/CNT x composites
One gram of PHD-HA was dissolved in 15 mL of DMF by heating and stirring at 80°C for 2 h. Subsequently, 0.05, 0.1, and 0.15 g of CNTs were added to the solution to prepare PHD-HA/CNT composites with CNTs contents of 5%, 10%, and 15%, respectively. The mixtures were sonicated for 6 h and then stirred at room temperature for an additional 12 h to ensure homogeneous dispersion. The resulting solutions were poured into silicone molds and vacuum-degassed at room temperature. The molds were then placed in an oven at 60°C for 12 h, followed by drying at 80°C for 24 h to yield the PHD-HA/CNT composites.

Synthesis of propyl cyclohexylcarbamate
Under a nitrogen atmosphere, 0.5 g of cyclohexyl isocyanate, 1 mg of DBTDL, 0.25 g of n-propanol, and 6 mL of trichloromethane were added to a round-bottom flask and stirred at 60°C for 6 h. Upon completion of the reaction, the solvent was removed by rotary evaporation. The crude product was then washed several times with diethyl ether and subjected to rotary evaporation again. Finally, the product was dried under vacuum to yield propyl cyclohexylcarbamate.

Synthesis of 3-cyclohexyl-1,1-diethylurea
Under a nitrogen atmosphere, 0.5 g of cyclohexyl isocyanate, 0.292 g of diethylamine, and 6 mL of trichloromethane were added to a round-bottom flask and stirred at room temperature for 12 h. After completion of the reaction, the solvent was removed by distillation under reduced pressure. The resulting product was washed three times with diethyl ether, each followed by distillation under reduced pressure. Finally, the product was dried under vacuum to obtain 3-cyclohexyl-1,1-diethylurea.

Characterizations
GPC (Agilent 1260) was used to determine the molecular weight of the samples, employing THF as the mobile phase at a flow rate of 1 mL min−1. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were collected using a Bruker Tensor II spectrometer (Germany) over the range of 600–4000 cm−1. Variable-temperature FTIR measurements were performed on a NICOLET 6700 spectrometer, with a heating rate of 5°C min−1 across a temperature range of 25–150°C. 2D correlation FTIR spectra were analyzed using 2D Shige software (version 1.3) to evaluate thermal response behavior within the same temperature range. 1H NMR spectra were recorded on a Bruker AVANCE III spectrometer using DMSO-d6 as the solvent. HR-MS analysis was carried out using an Agilent Q-TOF 6510 spectrometer (USA). Thermal stability was assessed via TGA on a TGA 5500 instrument with a heating rate of 10°C min−1. XRD patterns were obtained using a Rigaku SmartLab SE diffractometer equipped with Cu Kα radiation (λ=1.5418 Å). SAXS measurements were performed using a Xeuss 3.0 system (Xenocs, France), utilizing a Cu Kα X-ray source with a wavelength of 0.154 nm. The periodicity (L) was calculated by the Bragg’s law:

L = 2π/qmax (4)

where qmax represents the peak position of the SAXS curve.

Photographic images were captured using a smartphone (iQOO Neo8 Pro, Shenzhen, China). Differential scanning calorimetry (DSC) measurements were conducted using a TA DSC-250 instrument at a heating rate of 10°C min−1 over a temperature range of −80 to 120°C. Dynamic mechanical analysis (DMA) was performed on a DMA 850 analyzer in tensile mode from −100 to 120°C, with a heating rate of 5°C min−1 and a fixed frequency of 1 Hz. Rheological properties were measured using an Anton Paar MCR 302 rheometer, where the samples were heated from 20 to 190°C at a rate of 10°C min−1, also at a constant frequency of 1 Hz. Polymer morphology was characterized by SEM using a Hitachi SU8010 microscope equipped with an XFlash 6160 detector. Prior to imaging, samples were sputter-coated with platinum for 10 min and observed at an accelerating voltage of 10 kV. Electromagnetic parameters in the frequency range of 2–18 GHz were obtained using a vector network analyzer (VNA, Agilent PNA N5244A) via the coaxial transmission line method. The sample particles were extruded into filaments using an SJ15 single-screw extruder. The feed section was maintained at 60°C, while the barrel heating temperature was set to 150°C. A GR-100 variable-speed glass reactor (Shandong Borui Instrument Co., Ltd.) equipped with a polytetrafluoroethylene (PTFE) stirring paddle was used for the large-scale preparation of samples. The reaction temperature was precisely controlled using a ZTM-50-200-30 closed-loop refrigeration and heating system.

Stress–strain curves were recorded using an Instron 3344 universal testing machine. Rectangular specimens (50.0 mm × 5.0 mm × 0.5 mm) were prepared and subjected to uniaxial tensile testing at a crosshead speed of 100 mm min−1. For cyclic tensile testing, samples were stretched to a maximum strain of 500% at a loading rate of 50 mm min−1. Puncture resistance was measured using a universal testing machine equipped with a custom sample holder and a needle probe, which was advanced at a speed of 10 mm min−1. Fracture energy was determined using single-edge notched specimens with a width of 5.0 mm, stretched at a tensile rate of 50 mm min−1. The fracture energy was calculated using the following equation:

Fracture energy = W/c * sqrt(λc) (5)

where c represents the notch length (1 mm), λc is equal to (elongation at break + 1) of the notched sample, and W is the strain energy calculated by integrating the stress–strain curve of the unnotched specimen up to εc (εc = λc − 1).

Computational details
Density functional theory (DFT) calculations were performed using the Gaussian 16 program package. To gain deeper insights into the formation and dynamic behavior of urea bonds, the PBE0 hybrid functional combined with the D3BJ dispersion correction was employed for all calculations. Additionally, calculations were conducted at the B3LYP/6–31+G(d) level of theory with DFT-D3 dispersion corrections, using n-propylamine and 3-cyclohexyl-1,1-diethylurea as model reactants. Furthermore, the resonance energies of both hindered and conventional urea bonds were calculated to elucidate the differences in their dynamic behavior.

Statistical analysis
Statistical analysis was conducted using the Origin software. Data are presented as means ± standard deviation (SD, n=3) unless otherwise stated.

Data availability
The data supporting the findings of the study are available in the article and its Supplementary Information. The data generated in this study are provided in the Supplementary Information/Source Data/Supplementary Data. All data are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper is highly impressive, presenting an innovative strategy that tackles the significant environmental challenge of unrecyclable thermoset polyurethanes. It enables complete reprocessing and upcycling while impressively maintaining excellent mechanical performance and multifunctionality.\nI think this paper needs some revisions before publication to make it more valuable.\n1.\nSpecifically, while the 1st amine-induced decrosslinking is undeniably a pivotal breakthrough for enabling the upcycling of thermosets, the manuscript also showcases equally important features such as self-healing and thermal reprocessability achieved through dynamic urea exchange reactions. These characteristics are not merely byproducts but fundamental aspects of the dynamic polyurethane network, significantly extending the material's lifespan and utility. Emphasizing these attributes in the title would convey a more complete and compelling picture of the material's capabilities, potentially attracting a broader readership interested in smart, sustainable, and multi-functional polymers.\n2. Contextualizing Thermosets vs. Thermoplastics in the Introduction\nTo further underscore the significance of this work, the introduction could benefit from a more explicit discussion distinguishing the processing advantages of thermoplastic elastomers (TPEs) from the limitations of thermoset elastomers (TSEs). This would effectively highlight the unique contribution of your strategy in bridging this gap.\nConsider incorporating points such as:\nThermoplastic Elastomers (TPEs): When producing new product shapes, TPEs offer remarkable flexibility in manufacturing. They allow for a wide selection of molding techniques, diverse injection molding machines, and various mold designs. Crucially, any scrap generated during in-factory processing can be effectively recycled internally, contributing to material efficiency and reduced waste.\nThermoset Elastomers (TSEs): In stark contrast, TSEs typically require specific, fixed processing equipment, injection molding machines, and molds tailored for a particular curing process. Creating new product geometries often necessitates significant re-tooling and re-setting of the entire manufacturing line, leading to higher capital expenditure, longer lead times, and limited recycling options for process scrap.\nBy elucidating these conventional differences, the introduction can more powerfully set the stage for how your innovative decrosslinking and upcycling strategy transforms the paradigm for thermosetting polyurethanes, endowing them with thermoplastic-like processability and recyclability while retaining their robust thermoset performance.\n3. Emphasizing Solvent-Based Processing and Tunable Crosslinking\nThe discussion on polyurethane processing and recycling primarily focuses on thermal methods. However, it's crucial to acknowledge the significant role of solvent-based processing in the polyurethane industry. For instance, the production of spandex-type polyurethanes often involves solvent-intensive techniques like wet or dry spinning, where even slightly crosslinked (e.g., 5% crosslinked) polyurethanes are dissolved in highly toxic solvents like DMF or DMSO. These solvents pose considerable environmental and health concerns, making their widespread use a significant industry challenge.\nI propose that your invented crosslinked polyurethane could potentially be dissolved in relatively less toxic solvents at elevated temperatures, facilitating solvent-based recycling. This might require a slight adjustment to the degree of crosslinking to optimize solubility. My point is that polyurethane processing and recycling are not solely confined to thermal methods; solvent-based processes also play a major role. Furthermore, the distinction between thermosets and thermoplastics often lies in a gradient of crosslinking density. It may not always be necessary to achieve complete decrosslinking with primary amines; partial decrosslinking to enable solubility or melt flow could suffice for certain recycling or reprocessing applications.\n4. Supporting References for Recycling Mechanisms\nTo strengthen the discussion on both solvent and thermal recycling, I recommend referencing relevant literature:\nFor the discussion around solvent recycling and its implications, please refer to:\nNature Communications, volume 10, Article number: 2601 (2019)\nFor the aspects concerning thermal recycling at high temperatures, please consider referencing:\nNature Communications, volume 12, Article number: 621 (2021)", "reviewer_2": "This manuscript describes a strategy for the decrosslinking and upcycling of thermoset polyurethanes based on dynamic hindered urea bonds (HUBs). However, dynamic hindered urea bonds, as a typical class of dynamic covalent bonds, have already been extensively studied in the context of self-healing materials and recyclable materials for sustainable cycles. Previous research, such as that by Cheng's group (Adv. Mater. 2016, 28, 7646–7651; Nat. Commun. 2014, 5, 3218; J. Am. Chem. Soc. 2014, 136, 16974−16977; ACS Appl. Mater. Interfaces 2020, 12, 35403−35414), has reported a series of studies on the dynamic chemistry and self-healing polymers based on hindered urea bonds. Although this manuscript demonstrates the application potential of the designed dynamic hindered urea bond-based material (PHD) in decrosslinking and upcycling, the overall manuscript does not present innovative dynamic chemical mechanisms. Therefore, I am concerned that this work may not meet the high standards of Nature Communications.\nAdditionally, several questions are as follows:\n1. The thermoset polyurethanes designed in this manuscript emphasize that their dynamic network contains hindered urea bonds. In the temperature-dependent FTIR spectra, the dynamics observed primarily correspond to the rupture of hydrogen bonds. However, it remains unclear whether the dynamic rupture of these hydrogen bonds originates from the hindered urea bonds or the hydrogen bonds between urethane groups. In addition, a deeper analysis of the contributions of the hindered urea bonds and dynamic hydrogen bonds to the dynamics in thermoset polyurethanes is necessary.\n2. In Figure 3d, the PHD sample exhibits good self-healing properties after 2 hours at 120°C. How about the healing propperty of PHD samples at under different temperatures? It is recommended to include additional comparative experiments at different healing temperatures.\n3. The PHD sample, when assisted by small molecule amine HA, undergoes decrosslinking via thermal pressing to yield PHD-HA. Thermal analysis revealed an increase in the glass transition temperature (Tg) from −56.4 °C to −50.2 °C. Why does the Tg of the PHD-HA sample, obtained after decrosslinking with a small molecule amine, appear higher than that of the original PHD? Additionally, does a similar phenomenon occur when using the diamine molecule 1,6-hexanediamine (HAD) for decrosslinking to obtain the PHD-HDA sample?\n4. In the stress relaxation curves (Figure 3b), it is observed that the stress relaxation time of the PHD sample significantly decreases at 120°C, indicating that the dynamics play a role in network rearrangement. However, in the rheological curves, the flow temperature (Tf) of the PHD sample is 162.7 °C. Please explain why there is such a noticeable discrepancy between these two observations.\n5. In this manuscript, the dynamic hindered urea bonds in thermoset polyurethanes are crosslinked using diethanolamine (DEA). Structurally, the steric hindrance formed by the overall structure of DEA is not as advantageous as the amine crosslinking agents with tert-butyl groups, which have been previously reported. There is a lack of necessary explanation regarding the design and selection of crosslinking agents in the manuscript. A more in-depth discussion on the molecular structure design would be appreciated.\n6. In the discussion of the PHD sample’s properties, there is a lack of comparative studies with other self-healing and recyclable materials prepared with hindered urea bonds. It is suggested to include relevant discussions to clearly highlight the advantages of the thermoset polyurethanes designed in this manuscript.", "reviewer_3": "This study proposes an innovative strategy for the upcycling of thermoset polymers through the design of polyurethane networks incorporating dynamic hindered urea bonds (HUBs). The work demonstrates strong systematic rigor, encompassing material synthesis, characterization of dynamic behavior, investigation of decrosslinking mechanisms, and practical applications (composite reinforcement and electromagnetic interference shielding). It holds significant value for advancing the circular materials economy.\nHowever, there are some issues should be supplemented to enhance the manuscript's completeness prior to acceptance.\n1、Figure 4 demonstrates HUB exchange using a small-molecule model， it is suggested to supplement this with direct evidence proving the same reaction pathway in the polymer network (e.g., monitoring the reaction between PHD and HA via in situ FTIR).\n2、The article mentions that the Mn of PHD-HDA is approximately twice that of PHD-HA, likely due to chain extension initiated by the diamine. It would be valuable to verify whether new crosslinks form during the recycling process.\n3. While the article mentions excellent recyclability, it only presents data from a single recycling cycle (Figures 3f, 5e). It would be valuable to supplement this with data on the changes in mechanical properties after multiple cycles (e.g., strength/toughness after 3-5 cycles of decrosslinking-reprocessing).\n4. Why does Figure 2c show a tensile strain >1500% for PHD, whereas the cyclic test (Figure 2d) only reaches 500%? It would be helpful to clarify this discrepancy.\n5. Thermal stability of CNT composites (Supplementary Figure 38): PHD-HA/CNT15% exhibits a lower decomposition temperature than PHD-HA/CNT5%. It is recommended to explain this anomalous phenomenon.\n6. For the FTIR spectra in the article, peak shifts should be labeled with specific wavenumber changes.\n7. It is recommended to specify the practical application context for the EMI shielding effectiveness of 28.1 dB (Figure 6f) (e.g., does it meet typical requirements for electronic devices, which often demand >30 dB?)."}, "author_response": "Response to reviewers’ comments\nComments and Response\nReviewer #1\nThis paper is highly impressive, presenting an innovative strategy that tackles the\nsignificant environmental challenge of unrecyclable thermoset polyurethanes. It\nenables complete reprocessing and upcycling while impressively maintaining excellent\nmechanical performance and multifunctionality. I think this paper needs some revisions\nbefore publication to make it more valuable.\nR: Thank you for your recognition of our work. Please find our responses to each of\nyour points below.\n1.Specifically, while the 1st amine-induced decrosslinking is undeniably a pivotal\nbreakthrough for enabling the upcycling of thermosets, the manuscript also showcases\nequally important features such as self-healing and thermal reprocessability achieved\nthrough dynamic urea exchange reactions. These characteristics are not merely\nbyproducts but fundamental aspects of the dynamic polyurethane network, significantly\nextending the material's lifespan and utility. Emphasizing these attributes in the title\nwould convey a more complete and compelling picture of the material's capabilities,\npotentially attracting a broader readership interested in smart, sustainable, and multi-\nfunctional polymers.\nR: Based on your suggestion, the title has been modified in the revised manuscript:\n“Dynamic decrosslinking enables self-healing, reprocessability, and upcycling in\npolyurethane networks”\n2. Contextualizing Thermosets vs. Thermoplastics in the Introduction\nTo further underscore the significance of this work, the introduction could benefit from\na more explicit discussion distinguishing the processing advantages of thermoplastic\nelastomers (TPEs) from the limitations of thermoset elastomers (TSEs). This would\neffectively highlight the unique contribution of your strategy in bridging this gap.\nConsider incorporating points such as:\nThermoplastic Elastomers (TPEs): When producing new product shapes, TPEs offer\nremarkable flexibility in manufacturing. They allow for a wide selection of molding\ntechniques, diverse injection molding machines, and various mold designs. Crucially,\nany scrap generated during in-factory processing can be effectively recycled internally,\ncontributing to material efficiency and reduced waste.\nThermoset Elastomers (TSEs): In stark contrast, TSEs typically require specific, fixed\nprocessing equipment, injection molding machines, and molds tailored for a particular\ncuring process. Creating new product geometries often necessitates significant re-\ntooling and re-setting of the entire manufacturing line, leading to higher capital\nexpenditure, longer lead times, and limited recycling options for process scrap.\nBy elucidating these conventional differences, the introduction can more powerfully set\nthe stage for how your innovative decrosslinking and upcycling strategy transforms the\nparadigm for thermosetting polyurethanes, endowing them with thermoplastic-like\nprocessability and recyclability while retaining their robust thermoset performance.\nR: Thanks to your suggestion, we supplemented the background analysis of thermosets\nand thermoplastics in the introduction (Page 3 of the revised manuscript):\n“Thermoplastics have linear or lightly branched molecular chain structures that allow\nthem to be extremely flexible in the manufacture of new product shapes, supporting a\nvariety of molding techniques, diverse injection molding machines, and various mold\ndesigns, which makes them widely used in packaging, textiles, and consumer goods6-9.\nCrucially, any scrap generated during in-factory processing can be effectively recycled\ninternally, contributing to material efficiency and reduced waste10.”\n“However, processing thermosetting polymers typically requires specialized equipment,\nincluding injection molding machines and molds dedicated to a specific curing\nprocess15. Creating new product geometries often necessitates extensive retooling and\nreconfiguration of the manufacturing line, leading to significant capital expenditure,\nprotracted development cycles, and limited options for recycling process scrap16.”\n3. Emphasizing Solvent-Based Processing and Tunable Crosslinking\nThe discussion on polyurethane processing and recycling primarily focuses on thermal\nmethods. However, it's crucial to acknowledge the significant role of solvent-based\nprocessing in the polyurethane industry. For instance, the production of spandex-type\npolyurethanes often involves solvent-intensive techniques like wet or dry spinning,\nwhere even slightly crosslinked (e.g., 5% crosslinked) polyurethanes are dissolved in\nhighly toxic solvents like DMF or DMSO. These solvents pose considerable\nenvironmental and health concerns, making their widespread use a significant industry\nchallenge.\nR: Thank you for highlighting this issue, we agree with your viewpoint that solvent-\nbased processing plays an important role in the polyurethane industry. We also realized\nthat solvents such as DMF and DMSO pose significant environmental and health\nchallenges, so we tried to choose N,N’-dimethylacetamide (DMAc) (Supplementary\nTable 2 in the revised Supplementary Information file), which is commonly\nrecommended for the industrial production of polyurethanes, as an alternative solvent.\nSimilar to DMF and DMSO, DMAc is a polar aprotic solvent with a high boiling point\nand excellent solvating ability. In particular, DMAc exhibits lower toxicity and reduced\nhepatotoxicity, and remains relatively stable at high temperatures without significant\ndecomposition, thereby helping to maintain the stability of both the production process\nand product quality. The relevant content description has been added to Page 8 of the\nrevised manuscript:\n“Considering environmental and health factors, N,N’-dimethylacetamide (DMAc) with\nlow toxicity and high stability can also be used instead of DMF as the solvent for\npolymer synthesis to improve safety during the synthesis process (Supplementary Table\n2).”\nSupplementary Table 2. Safety studies of DMF, DMSO, and DMAc.\nDMF DMSO DMAc\nOral: LD50 - rat\nOral: LD50 - rat\n(male/female) - 28300\nOral: LD50 - rat (male/female) - (male/female) - 5830\nmg/kg bw.\n3010 mg/kg bw. mg/kg bw.\nGHS toxicity Dermal: lethal dose - rat Dermal: LD50 - rat Dermal: approximate\nclassification (male/female)- > 3160 mg/kg bw. mg/kg bw. lethal dose - rabbit\n(female) - 5 000 mg/kg bw.\nInhalation: LC50 - rat\n(male/female) -> 5.85 mg/L air.\nInhalation: LC0 - rat\nInhalation: LC50 - rat\n(a m ir. a le/female) -> 5.33 mg/L\n(female) - 8.8 mg/L air.\nDMF is clearly hepatotoxic. DMAc exhibits\nDMSO has significant\nLong-term exposure could result significantly lower\ndamage to the liver at high\nHepatotoxicity in liver cell death (hepatocyte hepatotoxicity, and the\nconcentrations, long-term\nn fi e b c r r o o s s is is 1 ) . , leading to liver\nor large intake.\nr g e e s n u e l r t a in ll g y l l e iv ss e r s e d v a e m re a 2.\ng e is\nEuropean Union (EU):\nDMSO is not approved for\nthe treatment of any disease\nEuropean Union (EU): The\nother than interstitial\nEuropean Union’s REACH\ncystitis or as a dietary\n(Registration, Evaluation,\nsupplement or health\nAuthorization and Restriction of\nproduct.\nChemicals) regulation imposes\nstringent restrictions on the use United States: DMSO is\nof DMF. not approved for the\nNo country has\ntreatment of any disease\nUnited States: The EPA implemented regulations\nother than interstitial\nApplication (Environmental Protection for DMAc that involve a\ncystitis or as a dietary\nAgency) also regulates the use complete prohibition or\nrestrictions and emissions of DMF to protect supplement or health strict authorization of use,\nthe environment. like the regulations for\nDMF\nCanada: DMSO is not\nCanada: Health Canada\napproved for the treatment\nregulates the use of DMF and\nof other diseases or health\nissues relevant guidelines.\nproducts other than\nJapan: Japan’s Industrial Safety interstitial cystitis.\nand Health Act restricts\nChina: China's Cosmetics\noccupational exposure to DMF.\nSafety Technical\nSpecification restricts the\nuse of DMSO.\nThe boiling point of DMSO The boiling point of DMAc\nThe boiling point of DMF is is approximately 189 °C. is approximately 165 °C,\nThermal approximately 153 °C. When When heated to this slightly higher than that of\nvolatility heated to this temperature, DMF temperature for a long time, DMF. Therefore, DMAc\nbegins to evaporate rapidly. DMSO will begin to slowly has slightly lower volatility\ndecompose. when heated.\nI propose that your invented crosslinked polyurethane could potentially be dissolved in\nrelatively less toxic solvents at elevated temperatures, facilitating solvent-based\nrecycling. This might require a slight adjustment to the degree of crosslinking to\noptimize solubility. My point is that polyurethane processing and recycling are not\nsolely confined to thermal methods; solvent-based processes also play a major role.\nFurthermore, the distinction between thermosets and thermoplastics often lies in a\ngradient of crosslinking density. It may not always be necessary to achieve complete\ndecrosslinking with primary amines; partial decrosslinking to enable solubility or melt\nflow could suffice for certain recycling or reprocessing applications.\nR: Thanks to your suggestion, we selected DMAc—a solvent with lower toxicity—as\na substitute for DMF. We carried out synthesis experiments on dynamic polyurethane\nnetworks and found that replacing DMF with DMAc had no adverse effect on the\nmaterial properties (Supplementary Fig. 3). These results demonstrate that DMAc is a\nviable alternative to DMF for the production and recycling of polyurethane elastomers.\nMoreover, during depolymerization, introducing a limited amount of primary amine for\npartial decrosslinking enables effective recycling by either increasing the dissolution\ntemperature or extending the depolymerization time.\nA related discussion was added to Page 10 of the revised manuscript:\n“Furthermore, replacing DMF with DMAc as the solvent has virtually no effect on the\nmechanical properties of the synthesized material (Supplementary Fig. 3).”\nSupplementary Fig. 3 | Stress-strain curves of PHD samples synthesized by using\nDMF or DMAc as the solvent.\n4. Supporting References for Recycling Mechanisms\nTo strengthen the discussion on both solvent and thermal recycling, I recommend\nreferencing relevant literature: For the discussion around solvent recycling and its\nimplications, please refer to: Nature Communications, volume 10, Article number:\n2601 (2019). For the aspects concerning thermal recycling at high temperatures, please\nconsider referencing: Nature Communications, volume 12, Article number: 621 (2021).\nR: Based on your suggestion, we have added these references of the revised manuscript.\nReviewer #2\nThis manuscript describes a strategy for the decrosslinking and upcycling of thermoset\npolyurethanes based on dynamic hindered urea bonds (HUBs). However, dynamic\nhindered urea bonds, as a typical class of dynamic covalent bonds, have already been\nextensively studied in the context of self-healing materials and recyclable materials for\nsustainable cycles. Previous research, such as that by Cheng's group (Adv. Mater. 2016,\n28, 7646–7651; Nat. Commun. 2014, 5, 3218; J. Am. Chem. Soc. 2014, 136,\n16974−16977; ACS Appl. Mater. Interfaces 2020, 12, 35403−35414), has reported a\nseries of studies on the dynamic chemistry and self-healing polymers based on hindered\nurea bonds. Although this manuscript demonstrates the application potential of the\ndesigned dynamic hindered urea bond-based material (PHD) in decrosslinking and\nupcycling, the overall manuscript does not present innovative dynamic chemical\nmechanisms. Therefore, I am concerned that this work may not meet the high standards\nof Nature Communications.\nR: We sincerely thank the reviewer for the thoughtful and constructive comments. We\nfully acknowledge that dynamically hindered urea bonds (HUBs), as a class of dynamic\ncovalent bonds, have indeed been widely investigated in the development of self-\nhealing and recyclable materials. In particular, the pioneering and systematic work by\nCheng and co-workers has provided valuable mechanistic insights and has greatly\ninspired our own research. We have carefully cited and discussed these representative\nstudies in the revised manuscript (Refs. 46, 49-51).\nBuilding upon these foundational studies, our work moves beyond the conventional\nscope of HUB-based systems by addressing a critical unsolved challenge—the circular\nregeneration of polyurethane elastomers at the end of their service life. Specifically, we\nintroduce a dynamic competitive reaction strategy that enables the decrosslinking of\ndynamic polyurethane networks, converting permanently crosslinked elastomers into\nreprocessable thermoplastics. Through a combination of small-molecule model\nexperiments and computational chemistry, we elucidated the underlying reaction\nmechanism, providing both theoretical and experimental validation for this\ntransformation.\nImportantly, after successful decrosslinking, we further demonstrated the upcycling of\nthe regenerated materials into high-value composites, achieving enhanced mechanical\nand functional properties. This approach therefore integrates self-healing,\nreprocessability, and upcycling within a single polymer platform—an achievement not\nreported in previous HUB-based systems.\nWe respectfully disagree with the reviewer’s concern regarding novelty. The conceptual\nadvancement lies in the transition from self-healing/recyclable systems to truly circular\npolyurethane materials capable of end-of-life regeneration and functional upgrading.\nWe believe that this framework provides a significant step forward toward sustainable\npolymer design and meets Nature Communications’ criteria for both novelty and impact.\nTo further highlight the novelty of our work, related discussions were added to Page 6\nof the revised manuscript:\n“The exceptional dynamic properties of HUB-based PUNs have garnered widespread\ninterest. Notably, systematic studies by Cheng and co-workers on HUBs have paved\nthe way for the development of dynamic networks with exceptional reprocessability,\nrecyclability, and self-healing performance49-51.”\n“This work not only retains the advantageous properties of dynamic HUB-based\npolymer networks, but also introduces an innovative dynamic competition reaction\nstrategy. The strategy efficiently converts chemically crosslinked elastomers into\nreprocessable thermoplastics, which can then be upgraded into high-performance\ncomposites. This approach therefore integrates self-healing, reprocessability, and\nupcycling within a single polymer platform—an achievement not reported in previous\nHUB-based systems.”\nAdditionally, several questions are as follows:\n1. The thermoset polyurethanes designed in this manuscript emphasize that their\ndynamic network contains hindered urea bonds. In the temperature-dependent FTIR\nspectra, the dynamics observed primarily correspond to the rupture of hydrogen bonds.\nHowever, it remains unclear whether the dynamic rupture of these hydrogen bonds\noriginates from the hindered urea bonds or the hydrogen bonds between urethane\ngroups. In addition, a deeper analysis of the contributions of the hindered urea bonds\nand dynamic hydrogen bonds to the dynamics in thermoset polyurethanes is necessary.\nR: Thank you for highlighting this issue. As you have commented, hydrogen bond\nfracture can be observed in the temperature-dependent FTIR spectra. By further\nanalyzing the assignment of hydrogen bonds in two-dimensional infrared spectra, the\norder of functional groups changing with temperature can be revealed. It can be seen\nfrom Supplementary Table 3 that the change of the urethane preferentially changes with\nthe increase of temperature, indicating that thermal perturbation may lead to the\npreferred dynamic fracture of the hydrogen bond between the urethane. Considering\nthat non-covalent hydrogen bonds are much weaker than covalent bonds, when the\ntemperature is raised, the weaker non-covalent interactions in the elastomer, i.e.,\nhydrogen bonds between urethane groups or hydrogen bonds between urea bonds, will\nbe broken earlier by thermal disturbances. The dynamics of HUBs are essentially\nreversible chemical reactions involving the formation/breaking of covalent bonds, so\nhigher activation energies are required.\nSubsequently, the dynamics of the materials were characterized by stress relaxation\nexperiments, and the magnitude of its activation energy was calculated by the Arrhenius\nequation, which was significantly higher than the energy range required for hydrogen\nbond (25-40 kJ/mol) fracture (Gai, et al. ChemPlusChem 2019, 84, 432–440). The\nsubsequent molecular model experiments and DFT theoretical calculations further\nconfirmed that the HUB can undergo an exchange reaction with small molecule amines,\nindicating the dynamic reversibility of the HUB. However, under the same conditions,\nthe carbamate bond did not exchange. From the control experiment in Fig. 3h, it can be\nseen that although the hydrogen bond dissociation occurs when the PHT sample is\nheated, the material recovery cannot be realized due to the lack of dynamic covalent\nbond exchange reaction. This result highlights the critical role of HUBs in building\ndynamic networks.\nIn summary, while both HUBs and dynamic hydrogen bonds contribute to the dynamic\nbehavior of the elastomer, the results of the above characterizations, small-molecule\nmodel experiments, and calculations indicate that the dynamic behavior observed in\nthis study is mainly governed by HUBs. The relevant content description has been\nadded to Page 13 of the revised manuscript:\n“The hydrogen bonds between elastomer chains, being much weaker than covalent\nbonds, break first under thermal agitation. As the temperature rises further, the\ndissociation and recombination rates of dynamic covalent bonds in polymers accelerate,\nthereby enhancing the activity of chain segments and the ability of network\nreconstruction, and significantly improving the dynamic performance of the material.”\nA related discussion was added to Page 14 of the revised manuscript:\n“Control experiments reveal that the PHT sample, despite undergoing hydrogen bond\ndissociation upon heating, cannot be recycled because it lacks dynamic covalent bond\nexchange. This result underscores that HUBs in the elastomer are essential for granting\nthe polymer its dynamic properties.”\n2. In Figure 3d, the PHD sample exhibits good self-healing properties after 2 hours at\n120°C. How about the healing property of PHD samples at under different\ntemperatures? It is recommended to include additional comparative experiments at\ndifferent healing temperatures.\nR: Based on your suggestion, we supplemented the stress-strain curves of the PHD\nsamples at different healing temperatures, as shown in Supplementary Fig. 7. The\nrelated discussions were added to Page 13 of the revised manuscript:\n“With increasing temperature, the self-healing performance of the damaged samples\nsignificantly improves (Supplementary Fig. 7).”\nSupplementary Fig. 7 | Stress-strain curves of the damaged samples healed for 2 h at\ndifferent temperatures (100, 110, 120, and 130 ℃).\n3. The PHD sample, when assisted by small molecule amine HA, undergoes\ndecrosslinking via thermal pressing to yield PHD-HA. Thermal analysis revealed an\nincrease in the glass transition temperature (T ) from −56.4 °C to −50.2 °C. Why does\ng\nthe T of the PHD-HA sample, obtained after decrosslinking with a small molecule\ng\namine, appear higher than that of the original PHD?\nR: Thank you for highlighting this issue. During the degradation process, the\ntransformation of HUBs promotes changes in the dynamic network structure, leading\nto an increase in the glass transition temperature (T ) of the elastomer (Lyu, et al. Angew.\ng\nChem. Int. Ed. 2024, 63, e202411397). The increase in T of the PHD sample after\ng\ndepolymerization is attributed to two possible factors:\n1. Conversion of hindered urea linkages to ordinary urea linkages: With the\nassistance of HA, the original HUBs in the dynamic network of the PHD sample\nare converted into new ordinary urea bonds. The hydrogen bond density between\nthe urea bonds increases to form a stronger intermolecular force, thereby inhibiting\nthe movement of the chain segment and consequently raising T .\ng\n2. Disruption of dynamic crosslinking and increased chain regularity: The\nexchange reaction between HA and the HUBs in the PHD sample disrupts the\ndynamically crosslinked structure. However, the increase in chain regularity leads\nto an increase in non-covalent interactions between chains and reduces the free\nvolume, causing T to rise.\ng\nA related discussion was added to Page 20 of the revised manuscript:\n“Under the action of HA, the original HUBs in the dynamic network of PHD samples\nare transformed into ordinary urea bonds, and the intermolecular forces are enhanced,\ninhibiting the movement of chain segments. Meanwhile, the disruption of dynamic\ncrosslinking enhances the regularity of the chain, increases non-covalent interactions\nand reduces the free volume, thereby raising the T of the elastomer.”\ng\nAdditionally, does a similar phenomenon occur when using the diamine molecule 1,6-\nhexanediamine (HAD) for decrosslinking to obtain the PHD-HDA sample?\nR: When the diamine molecule HAD was used to decrosslink PHD, an increase in T\ng\nwas also observed. The related discussions were added to Page 20 of the revised\nmanuscript:\n“As shown in Supplementary Fig. 38, after the PHD sample was depolymerized into\nPHD-HDA sample, the T increased from –56.4 °C to –47.1 °C.”\ng\n4. In the stress relaxation curves (Figure 3b), it is observed that the stress relaxation\ntime of the PHD sample significantly decreases at 120°C, indicating that the dynamics\nplay a role in network rearrangement. However, in the rheological curves, the flow\ntemperature (T) of the PHD sample is 162.7 °C. Please explain why there is such a\nf\nnoticeable discrepancy between these two observations.\nR: In response to your question, we provide the following explanation for the difference\nbetween the two temperatures.\nStress relaxation experiments are used to study the dynamic behavior of polymer\nnetworks. The significant shortening of the stress relaxation time at 120 °C indicates\nthat local dynamic exchange is rapid enough at this temperature to enable\nreorganization of the network topology. This process primarily involves local bond\nexchange, while the overall network remains intact, and the sample stays in a solid state\nwithout macroscopic flow.\nThe viscous flow temperature (T) refers to the critical temperature of the polymer from\nf\na high elastic state to a viscous flow state, which usually corresponds to the temperature\nwhen the storage modulus (G′) and loss modulus (G″) intersect (G′ = G″). At this\ntemperature, the thermal motion of molecular chains becomes sufficient to overcome\nentanglements and interactive forces, allowing entire chain segments to move and\nresulting in macroscopic flow. This process requires significantly higher energy than\nthat needed for local rearrangement.\nIn summary, the stress relaxation experiment reflects the temperature required for local\nbond exchange, while the viscous flow temperature corresponds to the temperature at\nwhich the entire network breaks down and macroscopic flow occurs, thus generally\nrequiring a higher temperature (Zhang, et al. Macromolecules 2017, 50, 5051–5060;\nZhang, et al. Chem. Eng. J. 2022, 429, 131848).\nA related discussion was added to Page 13 of the revised manuscript:\n“The stress relaxation temperature, which reflects the onset of local segmental motion\nwithout causing macroscopic flow, is generally lower than the temperature required for\nmacroscopic viscous flow.”\n5. In this manuscript, the dynamic hindered urea bonds in thermoset polyurethanes are\ncrosslinked using diethanolamine (DEA). Structurally, the steric hindrance formed by\nthe overall structure of DEA is not as advantageous as the amine crosslinking agents\nwith tert-butyl groups, which have been previously reported. There is a lack of\nnecessary explanation regarding the design and selection of crosslinking agents in the\nmanuscript. A more in-depth discussion on the molecular structure design would be\nappreciated.\nR: In the manuscript, the diethanolamine (DEA) molecule used to construct the\ndynamic polyurethane network exhibits less pronounced steric hindrance compared to\nthe tert-butylamine crosslinker previously reported by Cheng et al. However, the focus\nof this study is not on the self-healing and reprocessing properties of the material, but\nrather on the decrosslinking and upcycling of the dynamic network.\nAlthough the bulky tert-butyl group provides stronger steric hindrance, which enhances\nthe dynamics and reversibility of the polymer network, its application may be limited\nby temperature and environmental constraints, resulting in a relatively narrow scope of\napplicability. In contrast, the dynamic bonds with relatively small steric hindrance used\nin this study have higher bond energy, exhibit better stability, and are less prone to\nfracture. The moderate dynamics of the dynamic polymer network can maintain a high\ndegree of stability and almost no dissociation occurs, thus giving the material good\nmechanical strength and stability. At a specific temperature, the polymer network can\nachieve effective bond fracture and recombination, thereby enabling the rearrangement\nof molecules and damage repair, while avoiding excessive dynamics that could lead to\nmaterial softening or instability. Furthermore, the dynamic network decrosslinking and\nupcycling strategies proposed in this study not only demonstrate applicability and\nguiding significance for highly sterically hindered dynamic networks, but also provide\na theoretical foundation and methodological reference for the sustainable development\nof other dynamic polymer systems.\nWe have added a discussion on the design and selection of crosslinking agents to Page\n6 of the revised manuscript:\n“Considering that the bulky tert-butyl group provides stronger steric hindrance, which\nenhances the dynamics and reversibility of the polymer network, its application may be\nlimited by temperature and environmental constraints. In this work, we designed a\ndynamic network utilizing HUBs with lower steric hindrance, which provides enhanced\nstability and sustainable performance.”\n6. In the discussion of the PHD sample’s properties, there is a lack of comparative\nstudies with other self-healing and recyclable materials prepared with hindered urea\nbonds. It is suggested to include relevant discussions to clearly highlight the advantages\nof the thermoset polyurethanes designed in this manuscript.\nR: Thank you for your thoughtful comments and valuable suggestions. We added a\ncomparison figure with other materials containing dynamic HUBs to Supplementary\nFig. 19, and the related discussions were added to Page 14 of the revised manuscript:\n“In addition, compared with other materials containing dynamic HUBs reported in\nprevious studies56-61, the PHD sample shows reasonably good recyclability and healing\nperformance (Supplementary Fig. 19). The reversible nature of HUBs enables the\nnetwork to rearrange under suitable conditions, facilitating efficient repair and\nrecycling without compromising mechanical integrity, thereby achieving a balance\nbetween high performance and sustainability.”\nSupplementary Fig. 19 | Comparison of the healing efficiency and recyclability\nbetween PHD sample and other samples with HUBs.\nReviewer #3\nThis study proposes an innovative strategy for the upcycling of thermoset polymers\nthrough the design of polyurethane networks incorporating dynamic hindered urea\nbonds (HUBs). The work demonstrates strong systematic rigor, encompassing material\nsynthesis, characterization of dynamic behavior, investigation of decrosslinking\nmechanisms, and practical applications (composite reinforcement and electromagnetic\ninterference shielding). It holds significant value for advancing the circular materials\neconomy. However, there are some issues should be supplemented to enhance the\nmanuscript's completeness prior to acceptance.\nR: Thank you for your recognition of our work. Our responses to each point are\nprovided below.\n1. Figure 4 demonstrates HUB exchange using a small-molecule model, it is suggested\nto supplement this with direct evidence proving the same reaction pathway in the\npolymer network (e.g., monitoring the reaction between PHD and HA via in situ FTIR).\nR: Based on your suggestion, we monitored the reaction process between PHD and HA\nusing in situ FTIR spectroscopy to demonstrate the depolymerization of the polymer\nnetwork. The result was shown in Supplementary Fig. 27. We also added a description\nof the relevant content to Page 19 of the revised manuscript:\n“The successful synthesis of PHD-HA was confirmed by 1H NMR spectroscopy and in\nsitu FTIR spectroscopy, and GPC analysis showed a M of 34.8 kg mol−1 and a PDI of\nn\n3.3 (Supplementary Figs. 26–28).”\nSupplementary Fig. 27 | In situ FTIR spectra showing the time-dependent reaction\nbetween PHD samples and HA at 120 ℃.\n2. The article mentions that the M of PHD-HDA is approximately twice that of PHD-\nn\nHA, likely due to chain extension initiated by the diamine. It would be valuable to verify\nwhether new crosslinks form during the recycling process.\nR: Thank you for your careful review and valuable suggestions. Regarding your\nsuggestion that new chemical crosslinks may occur during the recycling process, we\nhave conducted the following analysis and explanation:\nIn this manuscript, the dynamic polyurethane network is constructed by reacting a\ndifunctional polyurethane prepolymer with trifunctional diethanolamine (DEA). When\nHDA molecules are added to the PHD samples, the crosslinking points formed by\nHUBs in the reversible network are broken, and the polyurethane is depolymerized into\na soluble linear polymer. During the depolymerization process, the effective\nfunctionality of diethanolamine decreases, while the newly introduced bifunctional\nHDA molecules act as chain extenders, increasing the molecular weight of the polymer.\nOverall, the depolymerization of the PHD sample effectively undergoes two chain\nextension reactions, thereby preventing the formation of new crosslinked structures.\nIn addition, we investigated the solubility of the recycled material to demonstrate the\nelimination of crosslinking. The recycled PHD sample could be readily dissolved in\nTHF solvent, confirming that the polymer underwent decrosslinking and did not form\nnew crosslinked structures. If a new crosslinked network had formed, it would typically\nexhibit insolubility with an inability to swell or only partial swelling in the solvent. The\nfact that our recycled material dissolved easily fully demonstrates that its three-\ndimensional crosslinked structure was effectively broken down, with no remaining\ncrosslinking points.\nIn summary, the addition of HDA to the PHD sample functions both as a\ndepolymerizing agent and a chain extender, thereby preventing the formation of new\ncrosslinked networks. To enable the formation of new crosslinked structures, the\nintroduction of multifunctional amine molecules with three or more reactive sites is\nrequired. A related discussion was added to Pages 20 and 21 of the revised manuscript:\n“During depolymerization, the difunctional HDA converts the HUBs in the reversible\nnetwork into normal urea bonds, but it struggles to form new crosslinking structures\nwith the difunctional isocyanate, serving only to effectively extend the polymer\nmolecular chains.”\n3. While the article mentions excellent recyclability, it only presents data from a single\nrecycling cycle (Figures 3f, 5e). It would be valuable to supplement this with data on\nthe changes in mechanical properties after multiple cycles (e.g., strength/toughness\nafter 3-5 cycles of decrosslinking-reprocessing).\nR: Based on your suggestion, we conducted multiple recycling experiments on PHD\nand PHD-HA samples, and measured the mechanical properties of the recycled samples.\nThe results were shown in Fig. 3f and Supplementary Fig. 34, and the related\ndiscussions were added to Pages 14 and 20 of revised manuscript:\n“Mechanical testing before and after recycling (Fig. 3f) showed that the reprocessed\nsample retained comparable mechanical performance, confirming its outstanding\nreprocessability.”\n“The PHD-HA samples were capable of solvent recycling, and their mechanical\nproperties showed negligible changes after multiple recycling cycles (Supplementary\nFig. 34), confirming their excellent recyclability.”\nFig. 3 | f Stress–strain curves of the original and repeatedly regenerated PHD samples.\nSupplementary Fig. 34 | Stress–strain curves of the original and repeatedly\nregenerated PHD-HA samples.\n4. Why does Figure 2c show a tensile strain >1500% for PHD, whereas the cyclic test\n(Figure 2d) only reaches 500%? It would be helpful to clarify this discrepancy.\nR: Thank you for your suggestions. We would like to clarify that Fig. 2c and 2d reflect\ndifferent mechanical characteristics of the elastomer. The stress–strain test in Fig. 2c\nrepresents a one-time loading of the original sample to its mechanical limit, primarily\nreflecting the material’s intrinsic ductility and toughness. In contrast, the cyclic\nloading–unloading test in Fig. 2d involves repeated tensile cycles within the material’s\nsafe deformation range, aiming to evaluate its response to cyclic stress, fatigue\nresistance, and mechanical stability under repeated loading. Cycling the material near\nits ultimate failure strain would cause rapid and irreversible damage within only a few\ncycles, thus preventing meaningful assessment of the fatigue behavior we intended to\ninvestigate.\nMoreover, in most practical applications, materials are often subjected to repeated\ndeformations at magnitudes significantly lower than their fracture strain to ensure long-\nterm durability. Therefore, a strain level of 500%, consistent with values reported in\nrelevant literature (Chen, et al. Adv. Sci., 2024, 11, 2405941; Zhang, et al. Chem. Eng.\nJ., 2025, 520, 165986; Wu, et al. Polymer, 2025, 334, 128745; Yang, et al. Small, 2025,\n21, e06269), was selected for the cyclic tensile tests.\n5. Thermal stability of CNT composites (Supplementary Figure 38): PHD-HA/CNT\n15%\nexhibits a lower decomposition temperature than PHD-HA/CNT .\n5%\nIt is recommended to explain this anomalous phenomenon.\nR: Thank you for your careful review. The decomposition temperature of PHD-\nHA/CNT sample is lower than that of PHD-HA/CNT sample. There may be the\n15% 5%\nfollowing reasons:\n1. The introduction of CNTs can disrupt the ordered arrangement of polyurethane\nchains or their rich hydrogen bonding network. Simultaneously, strong non-covalent\ninteractions exist between CNT and polyurethane segments. These interactions\ncompete with the intrinsic hydrogen bonding between polymer chains, thereby\nweakening the strong interactions between polymer segments. A higher CNT content\nintensifies this disruptive effect, leading to a looser overall polymer structure and\nreduced intermolecular forces. This is macroscopically manifested as a decrease in\nthermal stability, meaning an earlier thermal decomposition temperature.\n2. CNTs possess extremely high thermal conductivity, which can easily lead to local\noverheating, making the entire network more susceptible to dissociation and\ndecomposition under thermal stress. A large number of areas with weak thermal\nstability will be generated at the interface between the polymer and CNTs, and the\nthermal resistance of the interface will cause heat to accumulate locally and the\ntemperature will increase rapidly. With increasing CNT content, the interfacial defects\nand localized overheating regions become more pronounced, serving as initiation sites\nfor thermal decomposition. This process progressively accelerates the dissociation and\nbreakdown of the entire polymer network, resulting in a lower overall decomposition\ntemperature.\nA related discussion was added to Page 24 of the revised manuscript:\n“Samples with a high CNT content may exhibit a lower decomposition temperature,\nlikely because the highly thermally conductive CNTs disrupt the ordered structure of\nthe polyurethane chains and induce localized overheating, thereby accelerating the\ndissociation of the polymer network.”\n6. For the FTIR spectra in the article, peak shifts should be labeled with specific\nwavenumber changes.\nR: Thanks to your suggestion, we supplemented the wavenumber changes of peak shifts\nin the FTIR spectra of the revised manuscript (Page 24):\n“The observed shift in the carbonyl absorption peak from 1713 cm−1 to 1716 cm−1,\nalong with the increased intensity at 1698 cm−1 in the FTIR spectra, suggests strong\nsupramolecular interactions between the CNTs and the PHD-HA matrix\n(Supplementary Fig. 43).”\n7. It is recommended to specify the practical application context for the EMI shielding\neffectiveness of 28.1 dB (Figure 6f) (e.g., does it meet typical requirements for\nelectronic devices, which often demand >30 dB?).\nR: According to relevant research reports (Ryu, et al. J. Mater. Chem. A 2022, 10,\n4446-4455; Shen, et al. ACS Sustainable Chem. Eng. 2019, 7, 6259-6266; Zhao, et al.\nCompos. Sci. Technol. 2024, 253, 110635; Liang, et al. Sci. Bull., 2020, 65, 616-622;\nMei, et al. Carbon, 2016, 109, 149-153), while materials with a shielding effectiveness\nranging from 20 to 30 dB may not meet the stringent requirements of high-demand\napplications such as military, aerospace, or medical equipment, they are still widely\nused in various commercial scenarios, including consumer electronics, textiles and\napparel, office, and industrial equipment. The EMI shielding effectiveness of our\ncomposite material (28.1 dB) exceeds 20 dB, enabling it to provide sufficient EMI\nattenuation to ensure the normal and stable operation of electronic devices in most\ncommon working environments. Additionally, the EMI shielding performance can be\nfurther enhanced by increasing the carbon nanotube content to meet more advanced\nshielding requirements.\nThe related discussion was added to Page 24 of the revised manuscript:\n“Notably, the composite with 15 wt% CNTs achieved an average shielding\neffectiveness (SE) of 28.1 dB, effectively attenuating approximately 99.8% of incident\nelectromagnetic radiation, which is higher than the 20 dB EMI shielding requirement\nfor standard commercial applications65 (consumer electronics, office, and industrial\nequipment, etc.).”"}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed most of the concerns raised by my and other reviewers. The manuscript can be accepted without further revision."}, "author_response": ""}]

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