Minimal N-hydroxyphthalimide-urethane bonds enable superior thermomechanical stability for covalent adaptable networks

Domain: physics
Published: 2026-03-04
Editorial outcome: minor_revision

Abstract
Compared to classic thermosets, the reprocessability of covalent adaptable networks (CANs) endowed by dynamic covalent bonds (DCBs) often comes at the expense of mechanical performance and thermomechanical stability. Herein, we report a “High Activity & Low Content” strategy for CANs to achieve superior thermomechanical stability, which is enabled by dynamic N-hydroxyphthalimide-urethane bonds (NUBs). The catalyst-free addition reaction between N-hydroxyphthalimides and isocyanates proceeds to an near-quantitative conversion within 2 h at room temperature in dimethyl sulfoxide, while the formed bonds dissociate even up to ~28% at 120 °C. The dual high activity, characterized by a high degree of dissociation and fast association kinetics, allows for an effective reduction in DCB content within CANs while preserving their dynamic characteristics. We incorporate only 5 mol% of dynamic units to develop poly(N-hydroxyphthalimide-urethanes) (PNU) networks. The “High Activity & Low Content” design endows PNUs with superior mechanical properties, exceptional crack tolerance, and remarkable mechanical stability at high temperatures. Furthermore, even with minimal DCB participation, the PNUs exhibit excellent reprocessability and mild degradability in neutral aqueous conditions. This study proposes a compelling strategy that enables CANs to achieve excellent reprocessability while retaining their mechanical strength and thermomechanical robustness—overcoming the traditional trade-off between these properties.

Introduction
Thermosets with permanently crosslinked structures exhibit mechanical strength, solvent resistance, and durability, but also raise significant sustainability concerns1. Recent studies have reported innovative strategies to convert thermosets into reprocessable covalent adaptable networks (CANs) by incorporating dynamic covalent bonds (DCBs) such as transalkylation2,3, aza-Michael addition4, radical-mediated reaction5, disulfide/diselenide6, C=C/C=N metathesis7,8, nucleophilic aromatic substitution9. However, the developed CAN systems often face a fundamental trade-off: insufficient network dynamics reduce reprocessability, whereas excessive bond exchange degrades mechanical properties, especially the thermomechanical stability10–14. Consequently, it is a significant scientific challenge and practical imperative to pursue a delicate balance between mechanical stability and dynamic adaptability for the design of high-performance CANs.

The macroscopic behaviors of CANs are derived from the thermodynamic and kinetic parameters of DCBs governing the reconfiguration process and the mean lifetime of network crosslinks, as well as the macromolecular topology15–17. To avoid excessive flow of CANs at high temperatures, chemical approaches typically involve tailoring the kinetics of DCBs through trapping reactive counterparts18, developing DCBs with high activation energy barriers14, or introducing multiple types of dynamic bonds with disparate lifetimes, etc16,19–21. In parallel, topological design strategies include tuning cross-linking density22,23, and manipulating phase separation or nanoscale domains24,25 to optimize the balance between mechanical robustness and creep. Despite these significant advancements, most existing studies primarily focus on the service performance of CANs at low temperatures, where dynamic exchange reactions are sluggish or kinetically suppressed by limited polymer chain motion (<Tg or Tm)13,14,26. However, for demanding applications subject to high temperatures, such as thermal insulation and sealing, mechanical stability at elevated temperatures is important. We propose that a more general and straightforward strategy is to minimize the incorporation of dynamic bonds to mitigate the detrimental effects on material properties while enabling the reprocessing of as-formed CANs. For this purpose, DCBs must be carefully designed to satisfy two critical criteria: (i) efficient thermal dissociation (large dissociation equilibrium constant, Kd) at elevated temperatures to facilitate reprocessing, and (ii) rapid association reaction kinetics (large association rate constant, ka) at low temperatures to ensure complete network reformation at low DCB content (Fig. 1).

Reversible isocyanate chemistry, which focuses on the inherent reversibility of urethane/urea bonds, has garnered particular attention due to the diverse substrate structures available for modulating their dynamics1,27–31. This feature provides a promising candidate to meet the abovementioned stringent requirements. Isocyanates are highly reactive and usually react with active hydrogen compounds rapidly (large ka), which is typically accompanied by a substantial enthalpy change (large |ΔH|). Nevertheless, this usually results in a low degree of dissociation (low Kd, Fig. 1) at elevated temperatures27,32,33, as enthalpy changes typically dominate the overall Gibbs free energy change of a reaction. Therefore, developing a reversible chemistry that simultaneously exhibits both large Kd and ka remains challenging15,34.

Herein, we report a type of reversible isocyanate chemistry, N-hydroxyphthalimide-urethane bonds (NUBs), which enable concurrent high dissociability and fast reformation kinetics and demonstrate their efficacy in minimizing DCB content to enhance the mechanical properties and thermomechanical stability of the corresponding CANs. The dynamic chemistry of NUBs is controlled by the electronic effects of the phthalimide moiety in urethane. Specifically, the strong electron-withdrawing capability of the N-hydroxyphthalimide (NHPI) structure renders it a good leaving group5, thus promoting NUB dissociation with a conversion far higher than those of previously reported reversible isocyanate chemistry27,32,33. The high reactivity with isocyanates originates from the unique dipolar tautomer of NHPI, contributing to lowering the transition state activation energy. As a proof-of-concept, we have developed a series of poly(N-hydroxyphthalimide-urethanes) (PNUs) with a minimal proportion of NUBs (5 mol% of the total diisocyanate) as the dynamic building blocks. The resulting PNUs exhibit extraordinary tensile strength, toughness, and resistance to crack propagation, attributable to the reduction of dynamic weak bonds. This “High Activity & Low Content” design ensures thermal reprocessability and mild degradation while effectively maintaining thermomechanical stability of PNUs, as evidenced by excellent dimensional persistence, stress relaxation resistance, and network integrity at elevated temperatures. Consequently, the PNUs demonstrate significant potential as sustainable alternatives to conventional PUs, and this work provides a robust framework for manufacturing high-performance CANs by dynamic chemical design to overcome the traditional trade-off in dynamic networks.

Results
Molecular dynamic chemistry
We first synthesized a series of model compounds (Supplementary Figs. S1–7, S37–48). The kinetics of the addition reaction between methoxy-NHPI(1) and hexyl isocyanate were investigated using in situ Fourier transform infrared spectroscopy (FTIR). The sharp drop of the characteristic peak at 2276 cm–1, corresponding to the asymmetric stretching of the NCO group, implies the rapid formation of N-hydroxyphthalimide-urethane bonds in the absence of a catalyst (Fig. 2a, b). The formation reaction of NUBs exhibits first-order kinetics for both N-hydroxyl compounds and isocyanates (rate = ka[NOH][NCO]) (Supplementary Fig. S8), achieving ~ 97% conversion after approximately 2 h at room temperature with a large association rate constant of 1.3 × 10–2 M–1·s–1 in dimethyl sulfoxide (DMSO). The association rate constant ka is comparable to previously reported oxime-urethane and pyrazole-urea bonds32,33. Interestingly, the reaction rate in N,N-dimethylformamide (DMF) dramatically decreased, but it is still much faster than that of conventional uncatalyzed urethane and thiourethane bonds (Fig. 2c and Supplementary Fig. S9)32,35. This phenomenon implies a distinct reaction mechanism for NUBs, since previously reported reversible isocyanate chemistries exhibit comparable reaction kinetics in these two solvents due to their similar hydrogen bonding abilities30,32,36.

Density functional theory (DFT) calculations were conducted to elucidate the mechanism of the addition reaction. The results revealed that NHPI can undergo an intramolecular 1,4-proton transfer (TS1) to form the 1,3-dipolar nitrone isomer (INT1), with an activation energy barrier of 20.5 kcal mol–1 (endergonic by 19.9 kcal mol–1). The nitrone subsequently may react with isocyanate via either a concerted or a stepwise mechanism. The concerted pathway begins with the formation of an isocyanate-nitrone complex (endergonic by 5.2 kcal mol–1) and is featured by the simultaneous hydrogen transfer and nucleophilic attack via a seven-membered ring transition state (TS2, ΔG≠=10.2 kcal mol–1), ultimately yielding the final product (endergonic by 24.1 kcal mol–1). The stepwise pathway in which nucleophilic attack precedes proton transfer is energetically unfavorable because the activation energy is approximately 12.2 kcal mol–1 higher than that of the concerted process. Additionally, theoretical calculations were performed for the direct addition between N-hydroxy and isocyanate via a concerted four-membered ring transition state, analogous to the typical reaction mechanism of isocyanates with alcohols. This pathway is ruled out for the disfavored activation energy barrier as high as 32.8 kcal mol–1 (Fig. 2d and Supplementary Figs. S10, S11). It’s worth noting that the favored reaction mechanism involves a thermodynamically unstable key intermediate, the 1,3-dipolar nitrone tautomer. Given that dipolar isomers (e.g., nitrones) typically exhibit enhanced stability in highly polar solvents (those with large dielectric constants (ε), like DMSO) due to the effective solvation and stabilization of the separated charges via dipole-dipole or ion-dipole interactions, this finding accounts for the remarkably accelerated reaction kinetics we observed in the order of DMSO > DMF > 1, 4-dioxane (εDMSO = 45 > εDMF = 36 > ε1,4-dioxane = 2)37,38.

Thermodynamic studies using in situ variable-temperature 1H NMR revealed that the NUB dissociated to NHPIs and isocyanates at elevated temperatures (Fig. 2e, f). The dissociation reaction reached equilibrium very rapidly in less than 20 min at 80 °C (Supplementary Figs. S12, S13). At 120 °C, the thermal dissociation ratio of NUB was up to 28%, with a dissociation equilibrium constant Kd of 2.72 × 10–2 M (Fig. 2f and Supplementary Table S1). Compared to oxime-urethane (Kd = 3.2 × 10–3 M), pyrazole-urea (Kd = 2.6 × 10–3 M), and imidazole-urea (Kd = 6.2 × 10–3 M, estimated by the Van’t Hoff equation)27,32,33, the dissociation ratio of the NUB endowed by the stronger electron-withdrawing NHPI group was much higher at the same temperature. Thermodynamically, the large dissociation equilibrium constant Kd is attributed to the small enthalpy change (ΔHd = 10.8 ± 0.7 kcal mol–1 from the Van’t Hoff plot) of the dissociation reaction (Fig. 2g, h). Subsequently, the reorganization was proved by the reversible recombination of the dissociated counterparts. When two model compounds, AB and CD, were mixed in anhydrous acetonitrile and heated at 80 °C, as expected, the formed exchange products CB and AD were successfully identified at about 10 min and 12 min by high-performance liquid chromatography (HPLC) (Fig. 2i and Supplementary Fig. S14). Overall, these results demonstrate that the dynamic chemistry of NUBs is different from previously reported dynamic chemistry27,32,33,39–41, and is characterized by a high degree of dissociation and a rapid association rate (large Kd and ka, Supplementary Table S2).

Superior mechanical properties
Mechanical robustness is a prerequisite for CANs to meet the requirements of actual applications. Therefore, the mechanical deterioration associated with the incorporation of DCBs should be minimized as much as possible. With this in mind, we employed HDI, BDO as hard segments, PTMEG, Tri-NH2 as soft segments, and only a small amount (5 mol% relative to HDI) of bifunctional dynamic building blocks (Bi-NOH) to prepare a series of PNUs, denoted as PNU-A, PNU-B and PNU-C, with sequentially decreasing hard/soft segment ratios of 13/6.5, 12/7.5, 11/8.5 (Table 1, Figs. 1, 3a and Supplementary Figs. S15, S19, Table S3). Notably, the introduction of Bi-NOH did not significantly compromise the techno-economic feasibility (Supplementary Table S8). Swelling experiments conducted in different solvents verified that the gel fractions of PNUs remain above 95%, suggesting the well-defined crosslinked networks (Supplementary Fig. S20). Remarkably, PNU-A demonstrated a tensile strength of 43.2 ± 4.3 MPa, attaining an extraordinary elongation-at-break of 1891 ± 72% and a toughness of 404.4 MJ m-3, surpassing other covalently crosslinked dynamic PUs reported to date (Fig. 3b, c and Supplementary Fig. S21, Table S9)7–9,22,32,33,42–49. Their characteristic ‘J-shaped’ stress-strain curves indicated the advent of strain-induced crystallization (SIC) during stretching, a pivotal mechanism underpinning elastomer reinforcement. Consequently, the PNUs exhibited a pretty high true stress at break of up to 1.0 GPa, enabling a 5 kg weight to be lifted by 0.1 g PNU-A elastomer (Supplementary Fig. S22). In sequential cyclic tensile tests, the hysteresis loops increased significantly as the applied strain increased, indicating a progressive energy dissipation mechanism (Fig. 3d and Supplementary Fig. S23).

In addition to minimal incorporation of relatively weak NUBs, the aggregated structure of polymers also plays a crucial role in determining PNU performance. The carbonyl (C=O) stretching vibration regions (1800~1600 cm–1) in the FTIR spectra were deconvolved into multiple subpeaks, suggesting the presence of hierarchical hydrogen bonding. Quantitative analysis revealed that the content of hydrogen-bonded C=O in PNU-A was 73%, which is higher than that in PNU-B (69%) and PNU-C (39%) (Fig. 3e and Supplementary Fig. S24, Table S10). As the temperature increased from 30 to 160 °C, the spectral shifts such as ν(C=O), δ(N-H), ν(C-N) and ν(C-O-C) suggested the progressive dissociation of hydrogen bonds, which allows for effective energy dissipation under deformation (Fig. 3f). In situ stretching wide-angle X-ray scattering (WAXS) was performed to shed more light on the polymer morphology evolution during deformation. When unstretched, PNU-A showed concentric circles in the 2D WAXS spectra, typical of amorphous elastomers. As tensile strain increased, the pattern evolved from concentric circles to distinct spots (Fig. 3g and Supplementary Fig. S25). Concurrently, the broad hump in the 1D WAXS spectra became increasingly sharper, signifying enhanced chain alignment and increasingly ordered molecular packing (Fig. 3h). The underlying molecular mechanism involves the controlled release of soft segments composed of polyether chain segments during the initial stages of deformation. At large strains, the hard segments comprising the symmetric structure of HDI and the aromatic planarity of Bi-NOH withstand more stress and gradually align parallel to the tensile direction. This would facilitate crystallization and further reinforce PNUs (Fig. 3i). These evidences provide a mechanistic explanation for the pronounced nonlinear behavior observed in the stress-strain curves.

Exceptional crack tolerance
In addition to their remarkable ductility and toughness, the orientation of the abundant symmetric structure (from HDI) and aromatic planarity (from Bi-NOH) under deformation also empowers PNUs with exceptional crack tolerance. The PNU-A exhibited a fracture energy of 92.1 kJ m–2 (Fig. 4a), and a tear energy up to 460.1 kJ m–2 (Fig. 4b). In puncture resistance experiments, a 0.5 mm-radius needle tip penetrated a 0.2 mm-thick PNU-A film, yielding a puncture force of 13.2 N and a displacement of 11.2 mm (Fig. 4c). In situ stretching SEM was employed to elucidate the microscale crack growth behavior. A controlled displacement (Δd) was incrementally applied to a pre-cracked PNU-A specimen using an in situ stretching stage (Fig. 4d and Supplementary Fig. S26). During the initial deformation, SEM observations revealed rapid crack-tip blunting. As Δd increased, the crack underwent substantially transverse opening rather than propagating through the specimen. Notably, when Δd increased from 4.5 mm to 5.4 mm, the horizontal width at the narrowest section of the specimen decreased by only 0.077 mm, demonstrating strong resistance to crack advancement. At the final deformation stage, the cross-sectional area further decreased, and the specimen could no longer sustain the increasing applied load, triggering rapid flaw initiation and horizontal propagation, and ultimately resulting in catastrophic failure. Analysis of fracture morphology confirmed a significant deviation of the crack trajectory from its initial path (Fig. 4e, f). Digital image correlation (DIC) strain nephograms indicated that crack tip blunting efficiently alleviated stress concentration by redistributing localized stress to the surrounding region. This enabled the polymer chains near the crack tip to experience substantially greater vertical deformation, which activated SIC and led to significant reinforcement along the vertical axis, thereby inducing anisotropy within the material (Fig. 4g–i). Consequently, the crack tip no longer functioned as a stress concentration region in the horizontal direction, as vertical reinforcement effectively suppressed horizontal crack propagation.

Remarkable thermomechanical stability
To systematically investigate the influence of DCB content on the thermomechanical behavior of CANs, we synthesized two dynamic PNUs (PNU-15 mol% and PNU-30 mol%) alongside NUB-free reference thermoset (CPU) and thermoplastic (TPU) controls, all with the same hard/soft segment ratio as PNU-A (Table 1, Fig. 5a and Supplementary Figs. S15, S16, Table S4, S5). Tensile testing showed that the incorporation of NUBs with rigid ring structures endowed PNU-A with superior comprehensive mechanical properties compared to TPU and CPU. However, as the NUB content increased further, the mechanical response of the PNUs markedly changed from strong and tough to soft and weak, indicating that an overabundance of weak NUBs reduced the capacity of PNUs to withstand high loads and resistance to deformation (Fig. 5b).

Then, the mechanical behavior and dynamic properties at elevated temperatures were systematically evaluated. Dynamic mechanical analysis (DMA) revealed a similar two-stage thermal transition in the storage modulus (E’), attributable to the distinct thermal transitions of the hard and soft segments. Notably, PNU-A retained a constant modulus plateau in the high-temperature region up to 160 °C, a characteristic also observed in CPU. This behavior suggested that the incorporation of a small fraction (5 mol%) of dynamic units minimally perturbed the integrity of the overall crosslinked network at high temperatures. However, at higher DCB contents, this plateau progressively evolved into a viscous-flow transition similar to TPU, featuring a continuously declining modulus (Fig. 5c, Supplementary Figs. S27, S28 and Table S11, S12). The temperature-strain curves quantitatively corroborated this trend. At lower temperatures, all PNUs deformed slowly with the same temperature dependency. Above 50 °C, however, strain increased suddenly, because of the softening of hard segments and the accelerated dissociation/exchange of DCBs. At 80 °C, the strain of PNU-A reached only ~9.0%, second to CPU (~4.1%), and much lower than PNU-15 mol% (~13.1%) and PNU-30 mol% (~20.9%) (Fig. 5d). Optical images further illustrated the dimensional stability at high temperatures: the triangular geometry of PNU-A was preserved upon heating, whereas PNU-30 mol% underwent pronounced viscous flow (Fig. 5e). Considering their similar transition temperatures from DMA, the poor dimensional stability is mainly due to the significant increase in network plasticity caused by dissociation of excessive DCBs.

The dynamic properties of networks were further studied by temperature-dependent stress relaxation experiments (Fig. 5f, g and Supplementary Fig. S29). As the temperature increased from 120 to 150 °C, the characteristic relaxation time (τ*) of PNU-A decreased from 4049 s to 339 s. In contrast, PNU-15 mol% and PNU-30 mol% exhibited notably shorter τ* even at 80 °C, reflecting accelerated network reorganization with increasing DCB content. In high-temperature time sweeps, PNU-A showed no discernible changes in storage (G’) and loss (G”) moduli over at least 60 min, indicating excellent thermomechanical stability of PNUs upon heating (Fig. 5h). Frequency sweeps revealed that PNU-A maintained a stable crosslinked network, similar to CPU, with its G’ consistently exceeding G” and both moduli remaining temperature-insensitive at 100~140 °C. In contrast, the network of PNU-15 mol% was much more susceptible to heat, resulting in a pronounced reduction in its modulus. Strikingly, PNU-30 mol% displayed a G’/G” crossover at a lower temperature of 100 °C, indicative of a transition to a viscous-flow dominant state (Fig. 5i–k and Supplementary Fig. S30). Additionally, PNUs with a minimal NUBs exhibit comparable hygrothermal aging resistance to the CPU (Supplementary Fig. S31 and Table S13). Taken together, these results fully reflect that the “High Activity & Low Content” design preserves an integrated network with a high-temperature mechanical stability, which is significant for high-temperature applications of CANs, while still enabling dynamic adaptability.

Reprocessability and mild degradability
Arrhenius analysis revealed that the PNUs with varying hard/soft segment ratios exhibited similar stress relaxation activation energies of 23.9~26.9 kcal mol–1, suggesting that the plasticity of these networks is governed by the rearrangement of NUBs. Due to the high dissociation and the rapid reformation of the NUBs, PNUs containing only 5 mol% DCB could be efficiently hot pressed at 140 °C under 10 MPa for 30 min (Fig. 6a, b). Tensile tests revealed that the PNUs containing 5 mol% NUBs retained nearly unchanged tensile strength and elongation at break after repeated reprocessing, whereas both poly(oxime-urethane) (POU) containing 5 mol% oxime-urethane bonds with a relatively low degree of dissociation (12% at 120 °C vs. 28% at 120 °C for NUBs) and CPU (without NUBs) exhibit poor reprocessability and pronounced deterioration due to insufficient topological rearrangement (Table 1, Fig. 6c–e and Supplementary Table S6, Figs. S17, S32, S33). These results demonstrate that our “High activity & Low content” strategy strikes a balance between efficient network rearrangement and mechanical retention.

In addition to thermal reprocessability, we achieved degradation of PNUs under mild natural aqueous conditions and direct reuse of the degradation products. Urethane bonds are intrinsically stable, so the conventional chemical recycling of PU usually requires harsh conditions such as high temperature, acids/bases, or catalysts1,50,51. In contrast, our strategy exploited the high thermal dissociation tendency of NUBs and captured the generated isocyanates with water, thereby irreversibly shifting the equilibrium toward dissociation under mild conditions (Fig. 6f). Model experiments showed that the NUBs dissociated into NHPIs and isocyanates, with the latter subsequently reacting with water to form urea. This process was evidenced by a urea N-H proton appearing at δ 5.73 ppm in the 1H NMR spectrum, alongside an exchangeable proton at δ 10.80 ppm attributable to the N-hydroxy compound. At 100 °C, nearly complete degradation was attained after 8 h (Fig. 6g and Supplementary Fig. S34). Guided by this insight, the linear PNU-D bearing NUBs (Supplementary Fig. S18, Table S7) were prepared and then degraded in DMF containing 10% water at 100 °C. After 3 days, the weight-average molecular weight (Mw) of the PNU-D decreased from 50.0 kDa to 22.2 kDa (Fig. 6h). The resulting oligomers, enriched with polar groups (urea and urethane), were directly repurposed as adhesives. They demonstrated robust adhesion to diverse substrates via hydrogen bonding or coordination interactions (Fig. 6i, j and Supplementary Fig. S35, S36). These investigations provided valuable insights for designing reprocessable and degradable polymers by skillfully employing dual-active NUBs.

Methods
Synthesis of the crosslinked PNUs
Typical procedure: polyols (e.g., BDO, PTMEG) were vacuum dehydrated for 12 h before use, and the flasks were dried in a vacuum oven at 120 °C. HDI (10 equiv., 30 mmol, 5.046 g) and Bi-NOH (0.5 equiv., 1.5 mmol, 0.510 g) were dissolved in 30 mL anhydrous DMF, and the mixture was stirred overnight at room temperature under N2 atmosphere. Then PTMEG (5.5 equiv., 16.5 mmol, 16.500 g), BDO (2.5 equiv., 7.5 mmol, 0.676 g), and DBTDL (0.030 g) were added, and the solution was mechanically stirred at 80 °C for 3 h under N2 atmosphere. The reaction system was then diluted with 100 mL anhydrous 1,4-dioxane and cooled to room temperature. Tri-NH2 (1 equiv., 3 mmol, 1.320 g) dissolved in 10 mL anhydrous 1,4-dioxane was slowly added to the reaction solution under vigorous stirring, followed by a rapid increase in viscosity and gelation. The prepared PNU gels were placed in a fume hood to evaporate the solvent at room temperature for 24 h and then placed in a vacuum oven at 60 °C for 12 h to remove the residual solvent.

Synthesis of the reference TPU and CPU
TPU procedure: polyols (e.g., BDO, PTMEG) were vacuum dehydrated for 12 h before use, and the flasks were dried in a vacuum oven at 120 °C. HDI (10 equiv., 30 mmol, 5.046 g), PTMEG (7 equiv., 21 mmol, 21.000 g), BDO (3 equiv., 9 mmol, 0.811 g), and DBTDL (0.030 g) were dissolved in 30 mL anhydrous DMF, and the mixture was mechanically stirred at 80 °C for 3 h under N2 atmosphere. The prepared TPU was placed in a fume hood to evaporate the solvent at room temperature for 24 h and then placed in a vacuum oven at 60 °C for 12 h to remove the residual solvent.

CPU procedure: polyols (e.g., BDO, PTMEG) were vacuum dehydrated for 12 h before use, and the flasks were dried in a vacuum oven at 120 °C. HDI (10 equiv., 30 mmol, 5.046 g), PTMEG (5.5 equiv., 16.5 mmol, 16.500 g), BDO (3 equiv., 9 mmol, 0.811 g,) and DBTDL (0.030 g) were dissolved in 30 mL anhydrous DMF, and the mixture was mechanically stirred at 80 °C for 3 h under N2 atmosphere. The reaction system was then diluted with 100 mL anhydrous 1,4-dioxane and cooled to room temperature. Tri-NH2 (1 equiv., 3 mmol, 1.320 g) dissolved in 10 mL anhydrous 1,4-dioxane was slowly added to the reaction solution under vigorous stirring, followed by a rapid increase in viscosity and gelation. The prepared CPU gel was placed in a fume hood to evaporate the solvent at room temperature for 24 h and then placed in a vacuum oven at 60 °C for 12 h to remove the residual solvent.

Synthesis of the reference POU
The synthesis procedure for POU was analogous to that for crosslinked PNUs. In the formulation, Bi-NOH (0.5 equiv., 1.5 mmol, 0.510 g) was replaced by dimethylglyoxime (0.5 equiv., 1.5 mmol, 0.174). All other steps remained identical.

Synthesis of the linear PNU (PNU-D)
PEG was vacuum dehydrated for 12 h before use, and the flasks were dried in a vacuum oven at 120 °C. HDI (10 equiv., 30 mmol, 5.046 g) and Bi-NOH (1 equiv., 3 mmol, 1.021 g) were dissolved in 30 mL anhydrous DMF, and the sealed mixture was stirred overnight at room temperature. After adding PEG (9 equiv., 27 mmol, 27.000 g) and DBTDL (0.030 g), the solution was mechanically stirred at 80 °C for 3 h under a N2 atmosphere.

Reprocessing of the crosslinked PUs
Reprocessing experiments were carried out by hot pressing. The specimens were sectioned into small pieces and subsequently hot pressed within a rectangular mold at 120 °C or 140 °C under 10 MPa for 30 min. After cooling to room temperature, the reprocessed samples (recycle 1) were demolded and then cut into dumbbell-shaped specimens for uniaxial tensile measurements, as well as rectangular specimens for DMA. The recycle 1 specimens were reprocessed repeatedly following the abovementioned procedure to obtain recycle 2, recycle 3, and recycle 4 specimens, respectively.

Mild degradation of PNU-D
The chemical degradation mechanism of NUBs was investigated by 1H NMR using the model compound (CD). CD (0.384 g, 1.2 mmol) and 324 µL of H2O (18 mmol) were added into DMSO-d6 to prepare a 0.2 M solution. The mixture was then stirred at different temperatures, and samples were taken out at different periods for 1H NMR (Supplementary Fig. S21). Upon heating, CD dissociated into 1 and 2, and the latter reacted irreversibly with water to form the transient 3, which was consumed by the next dissociated 2 to form 4 with a urea structure, contributing to a continuous shift of the chemical equilibrium of CD in the direction of dissociation. The percentage of degradation was determined by the integral ratio of b (belonging to the starting compound CD) and b’ (belonging to the degradation product 4).

We then investigated the degradation of PNU-D bearing the NUBs (Supplementary Table S6). A DMF solution (containing 10% water) of PNU-D was stirred at 100 °C, and samples were taken out at selected times for GPC monitoring of molecular weight change. The resulting oligomers were cast into a film for the next adhesion experiments.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Zhao et al. reported a “High Activity & Low Content” strategy for CANs to achieve superior thermomechanical stability, which is enabled by a novel type of dynamic N-hydroxyphthalimide-urethane bonds (NUBs). The preparation, dynamic properties, mechanical properties and crack tolerance and recyclability were systematically investigated. Besides, the data were organized very well and the manuscript is well written. Thus, I think this work can be published in Nature communications after minor revisions Some suggestions are as follows.\n1. In introduction section: Currently, it gives a broad overview of the challenges in transforming thermoset PUs into CANs, but a more precise statement of the problem, such as the exact limitations of existing strategies in balancing mechanical and re - processing properties, would strengthen the introduction.\n2. Although authors used swelling test to prove the network feature of CANs, gel content should be measured to further clearly demonstrate the network integrity of the CANs.\n3. Authors used in - situ Fourier transform infrared spectroscopy (FTIR) experiment for studying the addition reaction between methoxy - NHPI and hexyl isocyanate. It’s better to use more experimental techniques to further validate the kinetic of this reaction. For example, differential scanning calorimetry (DSC) could be used to study the thermal behavior during the reaction.\n4. To prove the advance of this work, it’s better to include a table or a figure comparing the properties of PNUs with other relevant CANs reported in the literature.\n5. Long - term stability tests such as hygrothermal aging resistance should be conducted to assess the durability of PNUs in practical applications.\n6. An analysis of the scalability of the synthesis process and an estimate of the cost - effectiveness of using NUBs in CANs should be provided.", "reviewer_2": "This manuscript proposes an approach coined \"high activity & low content\" to prepare CANs with high performance. The idea is to introduce only small percentage of fastly exchangeable bonds. the manuscript describes the behaviour of polyurethanes materials containing small fractions of active N-hydroxyphthalimide-urethanes bonds.\nThis manuscript does not bring much to the field of vitrimer. indeed the so called \"high activity & low content\" approach is not new. It is well known that decreasing the amount of exchangeable bonds will slow down the topological rearrangement and decrease creep and thus help maintain mechanical properties.\nIn addition, the authors do not present relevant reference materials to compare their materials to. for example the reference materials devoid of any N-hydroxyphthalimide-urethanes bonds are presented, although the reference thermoset and the reference thermoplastic version of their material would be very useful and reinforce the case the authors are trying to make.\nSimilarly, a systematic study of the effect of the crosslinking density woudl have been useful. indeed the crosslinking density varies from one material to teh other, but this is not clearly discussed( and the denomination of the aterials is misleading the percentage values annonced are actually not percentages).\nThere are also a few unsustantiated claims. for example the authors say: \" Overall, these results demonstrate that the dynamic chemistry of NUBs is different from previously reported reversible isocyanate chemistry,32 and is characterized by a high degree of dissociation and a rapid association rate (large Kd and ka), which makes it possible to reprocess the corresponding CANs with a lower DCBs content.\" The authors do not prove at that stage that lower kinetics would not be sufficient to reprocess CAns acontaining low amonts of exchangeable bonds. and in effect , at that point in teh manuscriptn the authors have not proven yet that the CANs they propose are indeed CANs , and can be reprocessed.\nThe manusript is also not easy to follow since the actual formulation (and hence structural differences) as well as their main properties (Tg, Tdeg, Moduules etc...) are not tabulated in teh main text.\nIn the current state, I cannot recommend it for publication in Nature communications as the degree of novelty is questionnable, and the work lacks convincing benchmarking.", "reviewer_3": "The manuscript “Minimal N-Hydroxyphthalimide-Urethane Bonds Enable Superior Thermomechanical Stability for Covalent Adaptable Networks” describes a well-design system of reversible isocyanate-based PUR with comparably high thermomechanical stability while being reprocessable. The manuscript is well structured and written, contributing to the field.\n1. The design criteria to enable the comparably high thermomechanical stability for the PNUs are clearly communicated within the manuscript and is defined as “High activity & Low Content”. Have the authors considered the applicability of these criteria to other systems, for example by evaluating literature on CANs with low incorporation of dynamic bonds and compared their activity of association to this system and could the authors suggest other systems where these design criteria could or do apply? Such comparisons would contribute to the generality of the concept discussed. The authors have introduced similar structures in an ester-based CAN system previously (JACS, 2023, 145, 38, 20927) and putting this system into a bigger context would contribute by elucidating how general the concept is or if it is specific to specific structures.\n2. For clarity, the authors are suggested to include all relevant parameters when comparing or describing their system. This includes (but is not limited to): 1. The authors refer to mild degradation. As this is not a very clear description of a degradation procedure or what this actually means, the authors are asked to clarify this. 2. Line 116. Please include the temperature. 3. Line 147. At what conversion? 4. Line 150. Please include the values for comparison. 5. Line 156. Please include solvent used. 6. Line 180. For ease of reading, please include the ratios between hard and soft domains. 7. Line 158. Please include values. 8. Line 181. Please include solvent for the swelling experiments. 9. Line 316. Please include how the materials were reprocessed. 10. Line 322. Please include temperature.\n3. Although well written, the introduction of the manuscript is rather long. Please consider shortening it. It is also not completely clear how the authors have chosen their references. For example, reference 2-3 do not focus on the applications of PUR, ref 4-5 do not concentrate of the accumulation of PUR waste and do ref 6-13 also apply to the statement “The incorporation of DCBs with relatively low bond energies into classical thermosets, through crucial for achieving reprocesability, inevitably sacrifice their mechanical properties, especially thermomechanical stability”. Line 52. Here three references are used to describe topological alterations, that are not later cited when the different topologies are described. Do these references suggest other macromolecular topologies? Could another topological approach be to combine permanent covalent bonds with DCB? Would this lead to a similar result as what is presented here?\n4. Line 154. It is not clear how a small enthalpy could drive the reaction?\n5. Line 181. How can swelling experiments in one solvent verify that the PNUs possess well-defined crosslinked networks? Please also include swelling and gel-content values for more relevant solvents.\n6. Line 184. The mechanical properties of the materials will depend on the structure. The authors here compare the properties of their materials to other DCB containing PU-materials. This only becomes relevant if similar chemical structures are compared if the idea is to highlight the superiority of the chosen amount and structure of the DCB. In the introduction the authors also highlight other PU-containing CANS, but these are not incorporated in the comparison for the mechanical properties. It could of course be that mechanical properties were not evaluated in these systems or that their structure deviates from the comparison made here, but please clarify how the materials used for comparison were chosen. Please also include some clarification to the difference in properties between PNU-A, PNU-B and PNU-C. Similarly, it is not clear on line 192 what the authors are comparing to. A suggestion to consider is to include a reference not containing the DCB to compare with. Line 285-299. That increasing the DCB amount would decrease thermal stability and reduce relaxation time is quite intuitive, hence the “low content” part in the design criteria is proven. However, it is not clear how the importance of “high activity” is shown.\n7. For PNU-A (and for PNU-B and PNU-C) it would be interesting to evaluate the degree of crosslinking. Also, for all systems the activation energy would be interesting to include.\n8. Line 332-338. It is not clear why linear structures containing PEG were introduced and why they were evaluated as an adhesive. Also, what is robust adhesion and how does it compare to other systems?\n9. Supplementary Information. Please include all relevant parameters in the figure captions to facilitate understanding. Materials: please double check the naming of the chemicals used and also include company they were attained from. Instrumentation: Please include calibration range. NMR spectra: Please include assignments."}, "author_response": "ResponsetoReviewers’Comments Manuscript#NCOMMS-25-78293\nReviewer#1(Remarks totheAuthor):\nZhao et al. reported a “High Activity & Low Content” strategy for CANs to achieve superior\nthermomechanical stability, which is enabled by a novel type of dynamic\nN-hydroxyphthalimide-urethane bonds (NUBs). The preparation, dynamic properties,\nmechanical properties and crack tolerance and recyclability were systematically investigated.\nBesides, the data were organized very well and the manuscript is well written. Thus, I think\nthis work can be published in Nature communications after minor revisions Some suggestions\nare as follows.\nReply: Thank you very much for your careful review and positive evaluation of our work.\nYourconstructive comments help improvethequality of ourmanuscript.\n1. In introduction section: Currently, it gives a broad overview of the challenges in\ntransforming thermoset PUs into CANs, but a more precise statement of the problem, such as\nthe exact limitations of existing strategies in balancing mechanical and re-processing\nproperties, would strengthen theintroduction.\nReply: We appreciate the reviewer’s suggestion. In the revised version of the introduction\n(Page 3), we have clarified that existing strategies often suffer from either insufficient network\ndynamics, leading to poor reprocessability, or excessive bond exchange, compromising\nmechanical strength and thermal stability.This clarification allows us to present the novelty of\nourstudy inaddressing this long-standing trade-offmore clearly.\n2. Although authors used swelling test to prove the network feature of CANs, gel content\nshould bemeasured to further clearly demonstrate thenetwork integrity of theCANs.\nReply:We are grateful for your insightful review.The gel fraction and swelling ratio of PNUs\nwere determined through equilibrium swelling experiments, and the detailed results have been\nprovided in Supplementary Fig. S8. Specifically, the swelling ratios of PNU-A, PNU-B, and\nPNU-C in THF are 697%, 818%, and 903%, while their gel fractions are 98.3%, 96.8%, and\n94.9%, respectively. Following Reviewer #3’s advice, we have also conducted equilibrium\nswelling experiments in DMF, acetone, and 1,4-dioxane. The resulting gel fractions of all\nsamples remain above 95%, further confirming the well-developed crosslinking networks\nwithinthePNUs.\nSupplementary Fig. S8 a Swelling ratio and b gel fraction of PNUs obtained by swelling\nexperimentsin different solvents.\n3. Authors used in-situ Fourier transform infrared spectroscopy (FTIR) experiment for\nstudying the addition reaction between methoxy-NHPI and hexyl isocyanate. It’s better to use\nmore experimental techniques to further validate the kinetic of this reaction. For example,\ndifferential scanning calorimetry (DSC) could beused tostudy thethermal behaviorduring the\nreaction.\nReply: We thank the reviewer for the valuable suggestion. In response, we attempted to\ninvestigate the addition reaction between methoxy-NHPI and hexyl isocyanate using DSC.\nMethoxy-NHPI and hexyl isocyanate were prepared as 0.1 M solutions in DMSO, and\nisothermal DSC investigation was carried out at 25°C.Aclear exothermic signal was observed\nin the heat-flow curve, confirming the occurrence of the addition reaction. However, as the\nreaction inevitably begins during sample loading and mixing, the initial stage of the reaction\ncould not be reliably captured, making quantitative kinetic analysis from the DSC data\nunreliable (Fig. R1). Furthermore, since the addition reaction must be conducted in a suitable\nsolvent, DSC is not the most optimal technique for accurately studying its kinetics. Therefore,\nwe believe that in-situ FTIR, which allows real-time quantitative monitoring of functional\ngroup evolution, remains a more reliable method for investigating the kinetics of this addition\nreaction in ourstudy.\nFig. R1 DSC kinetic profile of 0.1 M methoxy-NHPI and 0.1 M hexyl isocyanate in DMSO at\n25°C.\nIn addition, DSC is commonly employed for determining the dissociation temperatures of\nreversible adducts (Maes, S.et al., 2023; Chen, X. X. et al., 2002).Accordingly,we conducted\nDSC and TGA analyses on our model compounds to investigate their thermal dissociation\nbehavior (Fig. R2). However, no pronounced endothermic peaks indicative of dissociation\nwere observed in the lower temperature region. The absence of detectable dissociation could\nbe attributed to the relatively low enthalpy change of the reaction (ΔH = 10.8 kcal mol-1),\nd\nwhich is hardly observed via DSC. Unlike the typical Diels-Alder system, the dissociation\nexhibits distinct heat flow signals because of the large reaction enthalpy (ΔH = 37 kcal mol-1,\nd\ndetermined by 1H NMR) (Buonerba, A. et al., 2017). Besides, the highly crystalline nature of\nthe model compounds may suppress thermal dissociation. The peaks observed in the\nhigh-temperature region are attributable to the melting and subsequent irreversible\ndecomposition of the model compounds. TGA revealed a 5% weight loss starting around\n200°C and completedecomposition at 250°C for all compounds.\nFig. R2 aChemical structure, b DSCandcTGAcurves ofthemodel compounds.\nSource:\nMaes, S. et al. N-Sulfonyl Urethanes to Design Polyurethane Networks with\nTemperature-Controlled Dynamicity.Macromolecules 2023,56,1934-1944.\nChen, X. X. et al.AThermally Re-mendable Cross-Linked Polymeric Material. Science 2002,\n295,1698-1702.\nBuonerba,A. et al.AComprehensive Depiction of the Furan-Maleimide Coupling via Kinetic\nand Thermodynamic Investigations of the Diels-Alder Reaction of\nPoly(styrene-co-2-vinylfuran) withMaleimides. ChemistrySelect 2017,2,1605-1612.\n4. To prove the advance of this work, it’s better to include a table or a figure comparing the\nproperties ofPNUs with otherrelevant CANs reported intheliterature.\nReply: Many thanks for this advice. The PNUs exhibit outstanding mechanical properties. As\nshown in Fig. 3c, PNU-A demonstrated an extraordinary elongation-at-break of 1891 ± 72%\nand a toughness of 404.4 MJ m-3, surpassing other covalently crosslinked dynamic PUs\nreported to date. The detailed comparison data has been provided in Supplementary Table S8\noftherevised Supplementary Information.\nFig. 3c Comparison between PNUs and other representative covalently crosslinked dynamic\nPUs interms oftoughness and elongation-at-break.\nSupplementaryTableS8. Comparison ofmechanical properties between PNUs and other\nrepresentative covalently crosslinked dynamicPUs.\nElongation-at-break Toughness\nLiterature Sample\n(%) (MJ m-3)\nAngew.Chem.Int. C=C/C=NPU 7 2.8\nEd. 63,e202406708\nC=NPU 6 2.1\n(2024) [7]\nJ.Am. Chem. Soc.\n146,21612-21622 V1 Network 111 24.7\n(2024) [8]\nCNPU-H4 127 49.8\nAdv.Mater.36,\nCNPU-H5 124 62.0\n2407854(2024)[9]\nCNPU-H7 164 84.0\nDAG-PU-1 1040 51.4\nSci.Adv.11,eadt0825\nDAG-PU-2 717 94.7\n(2025)[22]\nDAG-PU-3 567 105.4\nJ.Am. Chem. Soc. 6a 672 88.3\n139,8678-8684 6b 677 123.5\n(2017)[32] 6c 491 92.7\n7a 505 109.1\nNat. Commun. 10,\n7b 345 72.9\n4753 (2019)[33]\n7c 284 37.4\nAngew.Chem.Int. CPUU-FA 624 81.5\nEd. 64,e202505848\nCPUU-BA 574 55.6\n(2025)[39]\nTNB0.75 291 65.0\nAngew.Chem.Int.\nTNB0.75-10IL 386 64.8\nEd. 63,e202410335\nTNB0.75-30IL 550 32.3\n(2024)[40]\nTNB0.75-50IL 497 5.0\nNature631,783-788\n3Dprinted elastomer 919 310.4\n(2024)[41]\nPDMS1000disulfide-D 290 21.4\nAngew.Chem.Int.\nPDMS3000disulfide-D 532 19.4\nEd. 62,e202310989\nPDMS5000disulfide-D 884 9.3\n(2023)[42]\nPDMS3000disulfide-R 150 1.5\nAdv.Mater.35,\nCPUDTC4 782 73.4\n2211009(2023)[43]\nPU0.9 13 2.4\nJ.Am. Chem. Soc.\nPU1.0 13 2.0\n142,21852-21860\nPU1.1 14 3.1\n(2020)[44]\nPU1.2 14 2.7\nAngew.Chem.Int.\nEd. 56,2016-2021 BTNPU 432 0.3\n(2017)[45]\nvirgin 1510 189.3\nDrawing ratio=4 1190 272.9\nAdv.Funct. Mater.28,\nDrawing ratio=10 856 265.8\n1706050 (2018)[46] Drawing ratio=16 547 193.5\nDrawing ratio=18 419 183.1\nDrawing ratio=19 336 154.5\nPNU-A 1891 404.4\nThis work PNU-B 2127 349.1\nPNU-C 2367 124.8\n5. Long-term stability tests such as hygrothermal aging resistance should be conducted to\nassess thedurability ofPNUs in practical applications.\nReply: We thank the reviewer for raising this important point. The long-term stability of\nCANs remains a key challenge for theirpractical application.\nTo address this concern, we conducted hygrothermal aging tests on the PNUs according to\nISO 4611-1987. Dumbbell-shaped specimens for uniaxial tensile measurements and\nrectangular specimens for DMA investigation were placed in a constant temperature and\nhumidity chamber (ICTHI-150, STIK Instrument) maintained at 40 °C and 100% relative\nhumidity. After 7 d exposure, the aged specimens were removed and allowed to fully\nequilibrate under ambient conditions. They were subsequently subjected to tensile testing,\nDMAtemperature sweeps, and FTIR spectroscopy to evaluate the structure and performance\nchanges ofthePNUs induced byhygrothermal aging (Supplementary Information, Page 7).\nFor comparison, a conventional polyurethane thermoset was synthesized without NUBs\n(denoted as CPU, Supplementary Table S4). Following hygrothermal exposure, the PNUs\nexhibited a noticeable decrease in Young’s modulus, tensile strength and elongation-at-break,\nwhereas the DMAcurves and FTIR spectra showed nosignificant changes.Asimilar degree of\nmechanical degradation was also observed in CPU after the same hygrothermal aging.\nAlthough PNUs contain NUBs that are considered to be relatively unstable, their content is\nextremely limited. This indicates that the reduction in mechanical properties after combined\nheat and moisture exposure more likely arises from water-induced plasticization or alterations\nin the microphase-separated structure, rather than the potential molecular chain scission\ncaused by the minimal NUBs in PNUs. We have added the discussion at lines 302-303 in the\nrevised Manuscript, and the corresponding experimental results have been provided in the\nrevised Supplementary Information (Supplementary Fig. S18andTableS12).\nSupplementary Fig. S18 The influence of hygrothermal aging on PNUs and CPU\nperformance. Stress-strain curves, DMAtemperature sweeps and FTIR spectra of a-c PNU-A,\nd-fPNU-B, g-i PNU-C and j-l CPUbefore and after hygrothermal exposure.\nSupplementaryTableS12. Comparisonof themechanical properties ofPNUs and CPU\nbefore and after hygrothermal aging.\nYoung’s modulus Tensilestrength Elongation-at-break\n(MPa) (MPa) (%)\nPNU-A 3.9±0.5 43.2±4.3 1891± 72\nHygrothermal aged\n3.0±0.1 30.5±1.4 1558± 37\nPNU-A\nPNU-B 3.7±0.5 40.3±3.0 2127± 68\nHygrothermal aged\n2.3±0.3 24.5± 3.1 1811± 51\nPNU-B\nPNU-C 1.8±0.1 15.2±0.7 2367± 83\nHygrothermal aged\n1.2±0.1 10.0±0.9 2091± 77\nPNU-C\nCPU 2.6±0.1 25.2±0.2 1646± 12\nHygrothermal aged\n2.1±0.5 19.1±2.5 1336± 104\nCPU\n6. An analysis of the scalability of the synthesis process and an estimate of the\ncost-effectiveness ofusing NUBs in CANs should beprovided.\nReply: We greatly appreciate the reviewer’s insightful suggestion to discuss the\ntechno-economic feasibility of incorporating NUBs into CANs. The approximate market\nquotations for themain raw materials used in this study were provided in SupplementaryTable\nS7.\nSupplementaryTableS7.The estimated cost ofprimary raw materialsfor PNU.\nApprox. Market\nRawMaterial Chemical Structure Price Function\n(USD·kg-1)\n4,4’-Oxydiphthalicanhydride Monomerfor\n5-30\n(ODPA) Bi-NOH\nMonomerfor\nHydroxylaminehydrochloride NH OH·HCl 5-12\nSolventfor\nPyridine 1.5-3.5\nBi-NOH\nHexamethylenediisocyanate Diisocyanatefor\n4-8\n(HDI) PNU\nPolytetramethyleneether\nglycol 5-9 PolyolforPNU\n(PTMG,M ≈1000)\nn\nPolyethyleneglycol\n2-4 PolyolforPNU\n(PEG,M ≈1000)\nn\nChainextender\n1,4-Butanediol(BDO) 1.8-3.5\nforPNU\nJeffamine®T-403 Crosslinker\n7-11\n(polyetheramine,M ≈440)\nn\nforPNU\nNote:Thelisted pricesareapproximateindustrial marketquotationsobtainedfrompubliclyavailabledatabases\nandplatformsofchemicalsuppliers.Thesevaluesarebasedonthespecificmonomerusedinthisstudyandserve\nas representative references to illustrate the economic feasibility. Actual market prices may vary depending on\npurity,purchasequantity,andregionalfactors.\nIn our study, the key NUB-forming monomer (denoted as Bi-NOH in the Manuscript) is\nsynthesized from 4,4’-oxydiphthalic anhydride (ODPA) and hydroxylamine hydrochloride,\nwhich are both widely available industrial chemicals. ODPA is a widely used dianhydride in\nthe large-scale production of high-performance polyimides and epoxy resins, while\nhydroxylamine hydrochloride is a low-cost reagent commonly employed in organic synthesis.\nThe synthetic route to Bi-NOH is straightforward, and the crude product can be readily\npurified by simple filtration and water washing, without the need for chromatographic\nseparation or other complex purification procedures. Moreover, the high yield (>80%) further\ndemonstrates its practicality and scalability. Based on the aforementioned raw material prices\nand monomer synthesis yields, and neglecting additional factors such as labor, energy\nconsumption, and equipment depreciation, the estimated synthesis cost of Bi-NOH is\napproximately 9.5-43.2 USD·kg-1. Notably, this cost is comparable to that of ODPA,\nindicating that the Bi-NOH synthesis does not introduce a substantial economic burden at the\nmonomerlevel.\nIn the subsequent preparation of PNU, only a small amount of Bi-NOH is incorporated into\nthe conventional polyurethane synthesis process. All other components, including\ndiisocyanates, polyols, and chain extenders, as well as the synthetic conditions, remain\nunchanged. For example, the overall material cost of PNU-A is estimated to be 4.9-9.5\nUSD·kg-1, of which Bi-NOH accounts for approximately 2.1-18.7%, depending on the\nassumed price scenario. This is comparable to the market price of commercial thermosetting\npolyurethanes (3-8 USD·kg-1)(Supplementary Information, Page 21-22).\nConsidering the low-cost raw materials, the high monomer yield (>80%), and the\nstraightforward synthesis, Bi-NOH represents a manageable proportion of the total cost.\nConsequently, incorporating NUBs does not significantly compromise techno-economic\nfeasibility. These results suggest that NUB-incorporated CANs show promise for scalable,\ncost-effective, and industry-compatibleapplications.\nReviewer#2(Remarks totheAuthor):\nThis manuscript proposes an approach coined \"high activity & low content\" to prepare CANs\nwith high performance. The idea is to introduce only small percentage of fastly exchangeable\nbonds. the manuscript describes the behaviour of polyurethanes materials containing small\nfractions ofactive N-hydroxyphthalimide-urethanes bonds.\nReply: We sincerely thank the reviewer for the insightful comments, which greatly\ncontributed to improving our manuscript. We hope that the detailed explanations and\ncorresponding revisions belowhave addressed thereviewer’s concerns.\nThis manuscript does not bring much to the field of vitrimer. indeed the so called \"high\nactivity & low content\" approach is not new. It is well known that decreasing the amount of\nexchangeable bonds will slow down the topological rearrangement and decrease creep and\nthus help maintain mechanical properties.\nReply: Thanks for your critical comments. We agree that decreasing the amount of\nexchangeable bonds will slow down the topological rearrangement, thereby reducing creep\nand preserving mechanical properties. However, this often comes at the loss of significantly\nreduced reprocessability. Therefore, the developed CANs (not only vitrimers) often face a\nfundamental trade-off: insufficient network dynamics reduces reprocessability and excessive\nbond exchange degrades mechanical properties, especially the thermomechanical stability.\nWe proposed that a more general and straightforward strategy is to minimize the\nincorporation of dynamic bonds to mitigate the detrimental effects on material properties\nwhile enabling the reprocessing of as-formed CANs. To this end, these DCBs should satisfy\ntwo key requirements: (i) pronounced thermal cleavage at elevated temperatures (large K ) to\nd\nenable efficient reprocessing, and (ii) rapid re-bonding kinetics (high k ) at lower\na\ntemperatures to ensure full network restoration. Achieving this is challenging and has been\nlargelyoverlooked inprevious studiesas shown inTableR3.\nTableR3.Comparison ofk and K between NUBs and other dissociativeDCBs.\na d\nk (M-1·s-1)at 25°C K (M)at 110°C\na d\nOxime-urethane 8.5×10-3 1.4×10-3 High k ,lowK\na d\nPyrazole-urea 9.7×10-2 1.3×10-3 High k ,lowK\na d\nImidazole-urea a - 3.9×10-3 lowK\nd\nDonor-acceptor\n1.9×10-3 1.25 Lowk ,High K\nStenhouseb a d\nFuran-maleimideb 3.6×10-6 2.3×10-2 Lowk ,High K\na d\nThis work 1.3×10-2 1.7×10-2 Large k & K\na d\naTheK isestimatedbytheVan’tHoffequation;bThek isestimatedbytheArrheniusequation.\nd a\nTo enable a clearer and more rigorous comparison, we synthesized two reference samples\n(Table R4): (i) dynamic poly(oxime-urethane) (POU) networks containing 5 mol%\noxime-urethane bonds and (ii) conventional thermosetting PU (CPU, without NUBs). The\ndynamic oxime-urethane bond shows a similar k to NUB but a relatively low degree of\na\ndissociation(12% at 120°C vs. 28%at 120°C for NUBs).\nTableR4.Composition ofthe referencePOUand CPU.\nHDI Bi-NOH BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPNU-A\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.510g 0.676g 16.500g 1.320g\nHDI Dimethylglyoxime BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPOU\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.174g 0.676g 16.500g 1.320g\nHDI - BDO PTMEG Tri-NH\n2\n10equiv. - 3equiv. 5.5equiv. 1equiv.\nCPU\n30mmol, 9mmol, 16.5mmol, 3mmol,\n-\n5.046g 0.811g 16.500g 1.320g\nReprocessing experiments showed that the PNU-A could be efficiently reprocessed into a\nuniform, smooth sheet under 120°C, 10 MPa for 30 min. In contrast, both POU and CPU\nfailed to achieve a defect-free film under identical conditions (Supplementary Fig. S20). This\nindicates 5 mol% oxime-urethane bond is insufficient to endow POU reprocessability. When\nthe processing temperature increased to 140 °C, all three samples could be molded into\nvisually uniform sheets. However, tensile tests revealed that both POU and CPU exhibit\npronounced deterioration. This deterioration is more likely due to insufficient topological\nrearrangement, resulting in weak interfacial bonding. In contrast, the PNU-A retained nearly\nunchangedtensilestrength and elongation at break after repeated reprocessing (Fig. 6c-e).\nThese results demonstrate that simply reducing the content of DCBs, although beneficial\nfor creep resistance, is insufficient to ensure effective reprocessability. In contrast, our “High\nactivity & Low content” strategy strikes a balance between efficient network rearrangement\nand mechanical retention, offering a practical and distinguishable advantage for recyclable\nCANs.\nFig. R4 The effect of DCB dissociation degree on the reprocessability of CANs. a\nPhotographs of PNU-A, POU and CPU reprocessed under different conditions\n(Supplementary Fig. S20). The mechanical properties of the b PNU-A(Fig. 6c), c POU (Fig.\n6d)and d CPU(Fig. 6e) after multiplereprocessing cycles (140°C, 10MPa,30min).\nSource:\nLiu, W.-X. et al. Oxime-Based and Catalyst-Free Dynamic Covalent Polyurethanes. J. Am.\nChem. Soc. 2017,139,8678-8684.\nLiu, W.-X. et al. Dynamic multiphase semi-crystalline polymers based on thermally\nreversiblepyrazole-urea bonds.Nat Commun2019,10,4753.\nZhang, D.et al. Self-Catalysis Highly Reversible Imidazole-Based Dynamic Covalent\nPolyurethane-Ureas. Macromolecules 2024,57,2965-2973.\nGarcia Villatoro, E. A. Donor-Acceptor Stenhouse Adducts as Intrinsically Photoswitchable\nDynamicCovalent Bonds. J.Am. Chem. Soc. 2025,147,42,38452-38464.\nBuonerba,A. et al.AComprehensive Depiction of the Furan-Maleimide Coupling via Kinetic\nand Thermodynamic Investigations of the Diels-Alder Reaction of\nPoly(styrene-co-2-vinylfuran) withMaleimides. ChemistrySelect 2017,2,1605-1612.\nGandini,A. et al. Reversible click chemistry at the service of macromolecular materials. Part\n1: Kinetics of the Diels-Alder reaction applied to furan-maleimide model compounds and\nlinearpolymerizations.European Polymer Journal 2008,44,4029-4036.\nIn addition, the authors do not present relevant reference materials to compare their materials\nto. for example the reference materials devoid of any N-hydroxyphthalimide-urethanes bonds\nare presented, although the reference thermoset and the reference thermoplastic version of\ntheirmaterial would bevery useful and reinforcethe case theauthors are trying to make.\nReply: Thanks for your constructive suggestions.We synthesized the reference thermoplastic\n(TPU) and thermoset (CPU) devoid of NUBs (Supplementary Table S4). The relevant\nexperimental results and discussion have been added in the revised Supplementary\nInformationand Manuscript (Pages 18-21).\nSupplementaryTableS4. Composition ofthereferenceTPUand CPU.\nHDI Bi-NOH BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPNU-A\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.510g 0.676g 16.500g 1.320g\n10equiv. - 3equiv. 7equiv. -\nTPU\n30mmol, 9mmol, 21mmol,\n- -\n5.046g 0.811g 21.000g\n10equiv. - 3equiv. 5.5equiv. 1equiv.\nCPU\n30mmol, 9mmol, 16.5mmol, 3mmol,\n-\n5.046g 0.811g 16.500g 1.320g\nMechanical characterizations of TPU and CPU illustrated that the introduction of a small\nnumber of rigid structural NUBs imparted PNU-A with superior comprehensive mechanical\nperformance relative to both TPU and CPU. In addition, PNU-A exhibited excellent\nthermomechanical stability comparable to that of CPU, as evidenced by the presence of a\nmodulus plateau in the high-temperature region of the DMA curve (Fig. 5c), excellent\ndimensional stability (Fig. 5d), and prolonged stress relaxation (Fig. 5g). As expected, the\nviscous flow transition was observed in the high-temperature region for TPU. These results\nemphasize that the “High Activity & Low Content” strategy strikes a balance between\nefficient network rearrangement and mechanical retention.\nFig. 5 | The influence of DCB content on mechanical performances and\nthermomechanical behaviors. a Schematic illustration of PNUs with different DCB\ncontents and their dynamic behaviors at elevated temperatures. b Comparison of the\nstress-strain curves of CPU, TPU and PNUs. c Storage modulus and tan δ of CPU, TPU and\nPNUs as a function of temperature. d Dilatometry measurements of CPU and PNUs when\nheated from 0 to 160 °C at a temperature ramp rate of 3 °C/min, with an applied stress of 10\nkPa. e Optical photographs showing the dimensional stability of PNUs upon heating. f\nNormalized stress-relaxation analysis of PNU-A. g Comparison of stress-relaxation behavior\nof CPU and PNUs at 80 °C. h Time sweeps of PNU-A at elevated temperatures, with an\noscillation strain of 0.1% and frequency of 1 Hz for 60 min. Frequency sweeps of i PNU-A, j\nPNU-15 mol% and k PNU-30 mol% with a fixed strain of 0.1%, in the range of 0.01 to 100\nrad s−1at different temperatures.\nSupplementary Fig. S17 Frequency sweeps of CPU with a fixed strain of 0.1%, in the range\nof0.01to 100rad s−1at different temperatures.\nSimilarly, a systematic study of the effect of the crosslinking density woudl have been useful.\nindeed the crosslinking density varies from one material to teh other, but this is not clearly\ndiscussed (and the denomination of the aterials is misleading the percentage values annonced\nare actually not percentages).\nReply: Thanks for your professional suggestion. We conducted DMAtemperature sweeps on\nall PNUs. The cross-linking density (v) and the molecular weight between cross-links (M )\nwere evaluated bythefollowing equation:\nc\n3RT\nE’3vRT \nM\nc\nWhere E’is the tensile storage modulus measured at 120 °C by DMA, ρ is the density of\nthePNUs, Ris thegas constant and Trefers totheabsolutetemperature (393 K).\nSupplementary Fig. S14 DMAtemperature sweeps of PNUs with varying hard/soft segment\nratios.\nUpon heating, the hard domains progressively softened and the highly active NUBs\ndissociated, allowing reorganization of the primary network. In the high-temperature rubbery\nplateau, E’ depended on the concentration of elastically active network strands, which is\nmainly controlled by the fundamental network topology (such as elastically active covalent\ncross-links, dangling chains, and cyclic loops) and the dissociation/association behavior of\ntheNUBs.\nBased on the E’measured byDMAat 120 °C , PNU-A, PNU-B and PNU-C exhibit similar\nand stable cross-linking densities (147 ~ 133 mol·m-3) in the rubber plateau region. Since the\ncontent of NUBs is minimal, their E’values are primarily governed by the similar topological\nstructure of the network, yielding comparable E’ values across the three samples. These\nresults demonstrate that the introduction of minimal NUBs does not compromise the integrity\nofthenetwork at high temperatures.\nSupplementary Table S10. Cross-linking density analysis of PNUs with varying hard/soft\nsegment ratios.\nρ E’ v M c\n(g·mL-1) (MPa) (mol·m-3) (kg·mol-1)\nPNU-A 1.069 1.44 147 7.3\nPNU-B 1.070 1.37 140 7.7\nPNU-C 1.076 1.30 133 8.1\nWith increasing NUB content, the impact of the dynamics of reversible NUBs on the\ncross-linking density becomes more pronounced, with bond dissociation reducing\ncross-linking density. DMA measurements showed that the calculated crosslinking density\nsignificantly decreased from 147 to 3 mol·m-3 as the NUB content increased from 5 mol% to\n30 mol%. These findings indicated that high NUB loadings markedly reduced the effective\nnetwork connectivity at elevated temperatures, compromising thethermomechanical stability.\nSupplementary Table S11. Cross-linking density analysis of PNUs with different NUB\ncontents.\nρ E’ v M c\n(g·mL-1) (MPa) (mol·m-3) (kg·mol-1)\nPNU-A 1.069 1.44 147 7.3\nPNU-15 mol% 1.109 0.43 44 25.3\nPNU-30 mol% 1.131 0.03 3 369.5\nWe apologize for any misunderstanding caused by the sample names. To avoid ambiguity,\nthe samples “PNU-15%” and “PNU-30%” have been written as “PNU-15 mol%” and\n“PNU-30mol%” in therevised Manuscript and Supplementary Information.\nThere are also a few unsustantiated claims. for example the authors say: \" Overall, these\nresults demonstrate that thedynamic chemistry of NUBs is different from previously reported\nreversible isocyanate chemistry,32 and is characterized by a high degree of dissociation and a\nrapid association rate (large Kd and ka), which makes it possible to reprocess the\ncorresponding CANs with a lower DCBs content.\" The authors do not prove at that stage that\nlower kinetics would not be sufficient to reprocess CAns acontaining low amonts of\nexchangeable bonds. and in effect, at that point in teh manuscriptn the authors have not\nproven yet that theCANs they proposeare indeed CANs ,and can be reprocessed.\nReply: Thanks for the reviewer’s thorough review. The original statement has been removed\nand the sentence has been changed to “Overall, these results demonstrate that the dynamic\nchemistry of NUBs is different from previously reported reversible isocyanate chemistry,32\nand ischaracterized bya high degree of dissociation and a rapid association rate (large K and\nd\nk ).” Such phrasing is more objective and accurate at this stage. Additionally, the\na\naforementioned control experiments (POU and CPU) better illustrated that the novel design\nof “High Activity & Low Content” in pursuing thermomechanical stability while ensuring\nsufficienttopological rearrangement during CANreprocessing.\nThe manusript is also not easy to follow since the actual formulation (and hence structural\ndifferences) as well as their main properties (Tg, Tdeg, Moduules etc...) are not tabulated in\nteh main text.\nReply: Many thanks for the careful review. We have included detailed formulations\n(including equivalents, molar amounts, and masses) in the revised experimental section and\nsupplementary tables, and the formulation and principal properties (T , T , Young’s\ng d5%\nmodulus, etc.) of each sample discussed in the main text have been summarized in Table 1 in\ntherevised manuscript.\nTable1|The formulation andprincipal properties of thecrosslinked PUs.\nYoung’s Tensile Elongation-a\nT (°C)b\ng T c E’d\nd5%\nMolarratio a modulus strength t-break\n(°C) (MPa)\nT T\ngs gh (MPa) (MPa) (%)\nPNU-A 10/0.5/2.5/5.5/1 -36 114 307 1.44 3.9± 0.5 43.2± 4.3 1891± 72\nPNU-B 10/0.5/1.5/6.5/1 -38 93 311 1.37 3.7± 0.5 40.3± 3.0 2127± 68\nPNU-C 10/0.5/0.5/7.5/1 -40 71 315 1.30 1.8± 0.1 15.2± 0.7 2367± 83\nPNU-15 mol% 10/1.5/1.5/5.5/1 -33 96 296 0.43 1.4± 0.1 20.7± 1.3 2490± 31\nPNU-30 mol% 10/3/ -/5.5/1 -33/11 - 292 0.03 0.7± 0.1 2.9± 0.1 1998± 39\nTPU 10/-/3/7/ - -48 - 275 2.71 1.3±0.0 26.2± 0.4 2352± 15\nCPU 10/ -/3/5.5/1 -41 122 301 1.10 2.6± 0.1 25.2± 0.2 1646± 12\nPOU 10/0.5/2.5/5.5/1 -37 67 297 3.11 3.1± 0.2 27.1± 0.9 1657± 37\na The molar ratio of HDI: Bi-NOH: BDO: PTMEG: Tri-NH , in POU, Bi-OH was replaced with\n2\ndimethylglyoxime; b T value for soft segment and T value for hard segment obtained from DMA curves; c\ngs gh\nOnsettemperaturesafter5%weightlossobservedviaTGA;dE’at120°CacquiredfromDMAcurves.\nSupplementary Fig. S7cTGAcurves of thereferences. dDMAcurves ofthe POU.\nIn the current state, I cannot recommend it for publication in Nature communications as the\ndegreeof novelty isquestionnable, and thework lacks convincing benchmarking.\nReply: We appreciate the reviewer’s careful evaluation. We believe that the additional\nexperiments and corresponding discussions have substantially strengthened the novelty ofour\nwork: mechanically robust and efficiently reprocessable CANs without compromising their\nthermomechanical performance.Thank you again foryour valuableinput.\nReviewer#3(Remarks totheAuthor):\nThe manuscript “Minimal N-Hydroxyphthalimide-Urethane Bonds Enable Superior\nThermomechanical Stability for Covalent Adaptable Networks” describes a well-design\nsystem of reversible isocyanate-based PUR with comparably high thermomechanical stability\nwhile being reprocessable. The manuscript is well structured and written, contributing to the\nfield.\nReply: We sincerely appreciate the reviewer’s encouraging comments and valuable\nsuggestions. These comments have been very instrumental in guiding the revision and\nimprovingthe clarity ofour manuscript.\n1.The design criteria to enable the comparably high thermomechanical stability for the PNUs\nare clearly communicated within the manuscript and is defined as “High activity & Low\nContent”. Have the authors considered the applicability of these criteria to other systems,\nfor example by evaluating literature on CANs with low incorporation of dynamic bonds and\ncompared their activity of association to this system and could the authors suggest other\nsystems where these design criteria could or do apply? Such comparisons would contribute to\nthe generality of the concept discussed. The authors have introduced similar structures in an\nester-based CAN system previously (JACS, 2023, 145, 38, 20927) and putting this system\ninto a bigger context would contribute by elucidating how general the concept is or if it is\nspecific tospecific structures.\nReply: We thank the reviewer for the valuable comment regarding the generality of the\n“High activity & Low content” design principle. The core of this concept is that these\n“low-content” DCBs must exhibit “high activity”, satisfying two key requirements: (i)\npronounced thermal cleavage at elevated temperatures (large K ) to enable efficient\nd\nreprocessing, and (ii) rapid re-bonding kinetics (high k ) at lower temperatures to ensure full\na\nnetwork restoration. When the two requirements are met, in principle, this concept should be\napplicable to other CAN systems, independent of the specific chemistry. However, as shown\nin Table R3, developing DCBs that exhibit both large k at low temperatures and high K at\na d\nelevated temperatures is challenging.\nTableR3.Comparison ofk and K between NUBs and other dissociativeDCBs.\na d\nk (M-1·s-1)at 25°C K (M)at 110°C\na d\nOxime-urethane 8.5×10-3 1.4×10-3 High k ,lowK\na d\nPyrazole-urea 9.7×10-2 1.3×10-3 High k ,lowK\na d\nImidazole-urea a - 3.9×10-3 lowK\nd\nDonor-acceptor\n1.9×10-3 1.25 Lowk ,High K\nStenhouseb a d\nFuran-maleimideb 3.6×10-6 2.3×10-2 Lowk ,High K\na d\nThis work 1.3×10-2 1.7×10-2 Large k & K\na d\naTheK isestimatedbytheVan’tHoffequation;bThek isestimatedbytheArrheniusequation.\nd a\nOur previously reported ester metathesis mechanism of N-acyloxyphthalimide is distinct\nfrom the typical dissociative NUBs. In that system, a very small fraction of reversible\nhomolysis of the ester C-O bond generates transient radical intermediates that participate in\nsubsequent fast associative exchange reactions. The molecular exchange kinetics and the\nrheological behavior oftheresulting CANs are similartothose oftheassociative type.\nIn summary, previous studies have often overlooked the intrinsic interplay between the\nnature of dynamic covalent chemistry and the optimal content of DCBs.Therefore, our study\nseeks to highlight a broadly applicable strategy for balancing dynamicity and stability by\nintroducing the“High activity & Lowcontent” design principle.\nSource:\nLiu, W.-X. et al. Oxime-Based and Catalyst-Free Dynamic Covalent Polyurethanes. J. Am.\nChem. Soc. 2017,139,8678-8684.\nLiu, W.-X. et al. Dynamic multiphase semi-crystalline polymers based on thermally\nreversiblepyrazole-urea bonds.Nat Commun2019,10,4753.\nZhang, D.et al. Self-Catalysis Highly Reversible Imidazole-Based Dynamic Covalent\nPolyurethane-Ureas. Macromolecules 2024,57,2965-2973.\nGarcia Villatoro, E. A. Donor-Acceptor Stenhouse Adducts as Intrinsically Photoswitchable\nDynamicCovalent Bonds. J.Am. Chem. Soc. 2025,147,42,38452-38464.\nBuonerba,A. et al.AComprehensive Depiction of the Furan-Maleimide Coupling via Kinetic\nand Thermodynamic Investigations of the Diels-Alder Reaction of\nPoly(styrene-co-2-vinylfuran) withMaleimides. ChemistrySelect 2017,2,1605-1612.\nGandini,A. et al. Reversible click chemistry at the service of macromolecular materials. Part\n1: Kinetics of the Diels-Alder reaction applied to furan-maleimide model compounds and\nlinearpolymerizations.European Polymer Journal 2008,44,4029-4036.\n2. For clarity, the authors are suggested to include all relevant parameters when comparing or\ndescribing their system. This includes (but is not limited to): 1. The authors refer to mild\ndegradation. As this is not a very clear description of a degradation procedure or what this\nactually means, the authors are asked to clarify this. 2. Line 116. Please include the\ntemperature. 3. Line 147. At what conversion? 4. Line 150. Please include the values for\ncomparison. 5. Line 156.Please include solvent used. 6. Line 180. For ease of reading, please\ninclude the ratios between hard and soft domains. 7. Line 158. Please include values. 8. Line\n181. Please include solvent for the swelling experiments. 9. Line 316. Please include how the\nmaterials were reprocessed. 10.Line 322. Pleaseincludetemperature.\nReply: Thank you for these careful comments. 1. By “mild degradation”, we mean that,\nunlike the harsh conditions (such as high temperature, acids/bases, or catalysts) usually\nrequired for the conventional chemical recycling of PU, the degradation of PNU-D can be\nachieved solely by driving the dissociation equilibrium irreversibly using water in solution\nunder much gentler conditions. For details of the specific degradation procedure, please refer\nto the “Mild Degradation of PNU-D” in the Methods section. 2. “at room temperature” has\nbeen added at line 112 in the revised submission. 3. At 80 °C, the thermal dissociation\nconversion of NUB was 14%, with a dissociation equilibrium constant K of 5.7×10-3 M (Fig.\nd\n2f and Supplementary Table S1). 4. The K = 3.2×10-3 M at 120 °C of oxime-urethane, K =\nd d\n2.6×10-3 M at 120 °C of pyrazole-urea and K = 6.2×10-3 M (estimated by Van’t Hoff\nd\nequation) at 120 °C of imidazole-urea have been added at lines 146-147 in the revised\nsubmission. 5. The solvent used, anhydrous acetonitrile, has been added at line 153 in the\nrevised submission. 6.The hard/soft segment ratios of 13/6.5, 12/7.5, 11/8.5 have been added\nat lines 180-181 in the revised submission. 7. The retention times of CB and AD have been\nadded at line 154 in the revised submission. 8. Swelling experiments were conducted in\ndifferent solvents including THF, DMF, acetone and 1,4-dioxane. 9. The original\n“reprocessed” has been replaced with “hot pressed” at line 324 in the revised submission.10.\nTheexperimental temperature has been provided at line342intherevised submission.\n3. Although well written, the introduction of the manuscript is rather long. Please consider\nshortening it. It is also not completely clear how the authors have chosen their references.\nFor example, reference 2-3 do not focus on the applications of PUR, ref 4-5 do not concentrate of\nthe accumulation of PUR waste and do ref 6-13 also apply to the statement “The\nincorporation of DCBs with relatively low bond energies into classical thermosets, through\ncrucial for achieving reprocesability, inevitably sacrifice their mechanical properties,\nespecially thermomechanical stability”. Line 52. Here three references are used to describe\ntopological alterations, that are not later cited when the different topologies are described.\nDo these references suggest other macromolecular topologies? Could another topological\napproach be to combine permanent covalent bonds with DCB? Would this lead to a similar\nresult as what ispresented here?\nReply: Thanks for your thorough review. In response to Reviewer #1’s comment that\nbeginning the introduction with CANs as a solution to thermoset PU recycling was overly\nbroad, we removed the discussion and associated citations specific to PU recycling. Instead,\nwe emphasized a concise statement of the long-standing trade-off in current CAN systems:\ninsufficient network dynamics result in poor reprocessability, whereas excessive bond\nexchange leads to deteriorated mechanical properties, especially thermomechanical stability.\nThisenables a morefocused and concise introduction,as you suggested.\nFor clarification, the citations listed as refs 6-13 (refs 2-9 in the revised submission) were\nintended to survey recent advances in CANs. Refs 14-17 (refs 10-14 in the revised\nsubmission) were chosen specifically to illustrate the aforementioned trade-off. Refs 18-20,\nline 52 (refs 15-17 in the revised submission) were three related review articles that provided\na broader context on the factors governing the macroscopic behaviour of CANs. Subsequent,\nmore specific citations directed readers to original research elaborating on the particular\napproaches discussed.\nThe topological strategy of combining DCBs with permanent (non-dynamic) covalent\nbonds could achieve similar effects. For example, vitrimers with excellent thermomechanical\nstabilityand reprocessability can beobtained byprecisely regulating thedensity ofpermanent\ncross-links (Li, L. et al.,2018).\nSource:\nLi, L. et al. Vitrimers Designed Both To Strongly Suppress Creep and To Recover Original\nCross-Link Density after Reprocessing: Quantitative Theory and Experiments.\nMacromolecules 2018,51,5537−5546.\n4. Line 154.It is not clear howa smallenthalpy could drive thereaction?\nReply: Many thanks for your question. From G H TS RT lnK ,\nwe obtained\nd d d\nH S\n d d\nK eG/RT e RT R .\nd\nAs shown, K is determined by both the enthalpy and entropy changes; in most cases, the\nd\nenthalpy change plays a predominant role compared to the entropic contribution (TΔS) in the\noverall change in Gibbs free energy. To improve clarity, we have removed that confusing\nstatement; the text now reads “Thermodynamically, the large dissociation equilibrium\nconstant K is attributed to the small enthalpy change (ΔH = 10.8 ± 0.7 kcal mol-1 from the\nd d\nVan’t Hoffplot)of thedissociation reaction.”\n5. Line 181. How can swelling experiments in one solvent verify that the PNUs possess\nwell-defined crosslinked networks? Please also include swelling and gel-content values for\nmorerelevant solvents.\nReply: Thanks for your kind advice. We have also conducted equilibrium swelling\nexperiments in DMF, acetone, and 1,4-dioxane. The swelling behavior of PNUs depended\nstrongly on the solvent: for example, PNU-A exhibited swelling ratios of 697% in THF,\n549% in DMF, 208% in acetone, and 672% in 1,4-dioxane. The gel fractions of all samples\nexceeded 95%, further confirming the well-developed crosslinking networks (Supplementary\nFig.S8).\nSupplementary Fig. S8 a Swelling ratio and b gel fraction of PNUs obtained by swelling\nexperimentsin different solvents.\n6. Line 184. The mechanical properties of the materials will depend on the structure. The\nauthors here compare the properties of their materials to other DCB containing PU-materials.\nThis only becomes relevant if similar chemical structures are compared if the idea is to\nhighlight the superiority of the chosen amount and structure of the DCB. In the introduction\nthe authors also highlight other PU-containing CANS, but these are not incorporated in the\ncomparison for the mechanical properties. It could of course be that mechanical properties\nwere not evaluated in these systems or that their structure deviates from the comparison made\nhere, but please clarify how the materials used for comparison were chosen. Please also\ninclude some clarification to the difference in properties between PNU-A, PNU-B and\nPNU-C. Similarly, it is not clear on line 192 what the authors are comparing to.Asuggestion\nto consider is to include a reference not containing the DCB to compare with. Line 285-299.\nThat increasing the DCB amount would decrease thermal stability and reduce relaxation time\nis quite intuitive, hence the “low content” part in the design criteria is proven. However, it is\nnotclear howtheimportance of“high activity” isshown.\nReply: Many thanks! We agree that comparisons of macroscopic mechanical properties are\nmeaningful when the chemical structures are similar.Accordingly, the examples we chose for\ncomparison are all covalently crosslinked dynamic PUs, excluding non-covalently\ncrosslinked PUs and TPUs. As you suspected, refs 6 and 10 cited in the introduction did not\nreport mechanical tests, and refs 7-9 and 11-13 (refs 44-46 and 7-9 in the revised submission) \ndid not provide explicit numerical values. We estimated their elongation-at-break and\ntoughness approximately from published stress-strain curves, and these estimated values have\nbeen included in therevised Fig.3c and SupplementaryTableS8.\nFig. 3c Comparison between PNUs and other representative covalently crosslinked dynamic\nPUs interms oftoughness and elongation-at-break.\nSupplementaryTableS8. Comparison ofmechanical properties between PNUs and other\nrepresentative covalently crosslinked dynamicPUs.\nElongation-at-break Toughness\nLiterature Sample\n(%) (MJ m-3)\nAngew.Chem.Int. C=C/C=NPU 7 2.8\nEd. 63,e202406708\nC=NPU 6 2.1\n(2024) [7]\nJ.Am. Chem. Soc.\n146,21612-21622 V1 Network 111 24.7\n(2024) [8]\nCNPU-H4 127 49.8\nAdv.Mater.36,\nCNPU-H5 124 62.0\n2407854(2024)[9]\nCNPU-H7 164 84.0\nDAG-PU-1 1040 51.4\nSci.Adv.11,eadt0825\nDAG-PU-2 717 94.7\n(2025)[22]\nDAG-PU-3 567 105.4\nJ.Am. Chem. Soc. 6a 672 88.3\n139,8678-8684 6b 677 123.5\n(2017)[32] 6c 491 92.7\n7a 505 109.1\nNat. Commun. 10,\n7b 345 72.9\n4753 (2019)[33]\n7c 284 37.4\nAngew.Chem.Int. CPUU-FA 624 81.5\nEd. 64,e202505848\nCPUU-BA 574 55.6\n(2025)[39]\nTNB0.75 291 65.0\nAngew.Chem.Int.\nTNB0.75-10IL 386 64.8\nEd. 63,e202410335\nTNB0.75-30IL 550 32.3\n(2024)[40]\nTNB0.75-50IL 497 5.0\nNature631,783-788\n3Dprinted elastomer 919 310.4\n(2024)[41]\nPDMS1000disulfide-D 290 21.4\nAngew.Chem.Int.\nPDMS3000disulfide-D 532 19.4\nEd. 62,e202310989\nPDMS5000disulfide-D 884 9.3\n(2023)[42]\nPDMS3000disulfide-R 150 1.5\nAdv.Mater.35,\nCPUDTC4 782 73.4\n2211009(2023)[43]\nPU0.9 13 2.4\nJ.Am. Chem. Soc.\nPU1.0 13 2.0\n142,21852-21860\nPU1.1 14 3.1\n(2020)[44]\nPU1.2 14 2.7\nAngew.Chem.Int.\nEd. 56,2016-2021 BTNPU 432 0.3\n(2017)[45]\nvirgin 1510 189.3\nDrawing ratio=4 1190 272.9\nAdv.Funct. Mater.28,\nDrawing ratio=10 856 265.8\n1706050 (2018)[46] Drawing ratio=16 547 193.5\nDrawing ratio=18 419 183.1\nDrawing ratio=19 336 154.5\nPNU-A 1891 404.4\nThis work PNU-B 2127 349.1\nPNU-C 2367 124.8\nThe hard/soft segment ratios in the formulations of PNU-A, PNU-B, and PNU-C\nprogressively decreased from 13/6.5 12/7.5 to 11/8.5. PNU-A with the highest hard-segment\ncontent displayed the highest Young’s modulus and tensile strength, while PNU-C with the\nhighest soft-segment content showed the greatest elongation-at-break. The specific\nmechanical property parameters were summarizedinTable1ofthemain text.\nTable1|The formulation andprincipal properties of thecrosslinked PUs.\nYoung’s Tensile Elongation-a\nT (°C)b\ng T c E’d\nd5%\nMolarratio a modulus strength t-break\n(°C) (MPa)\nT T\ngs gh (MPa) (MPa) (%)\nPNU-A 10/0.5/2.5/5.5/1 -36 114 307 1.44 3.9± 0.5 43.2± 4.3 1891± 72\nPNU-B 10/0.5/1.5/6.5/1 -38 93 311 1.37 3.7± 0.5 40.3± 3.0 2127± 68\nPNU-C 10/0.5/0.5/7.5/1 -40 71 315 1.30 1.8± 0.1 15.2± 0.7 2367± 83\nPNU-15 mol% 10/1.5/1.5/5.5/1 -33 96 296 0.43 1.4± 0.1 20.7± 1.3 2490± 31\nPNU-30 mol% 10/3/ -/5.5/1 -33/11 - 292 0.03 0.7± 0.1 2.9± 0.1 1998± 39\nTPU 10/-/3/7/ - -48 - 275 2.71 1.3±0.0 26.2± 0.4 2352± 15\nCPU 10/ -/3/5.5/1 -41 122 301 1.10 2.6± 0.1 25.2± 0.2 1646± 12\nPOU 10/0.5/2.5/5.5/1 -37 67 297 3.11 3.1± 0.2 27.1± 0.9 1657± 37\na The molar ratio of HDI: Bi-NOH: BDO: PTMEG: Tri-NH , in POU, Bi-OH was replaced with\n2\ndimethylglyoxime; b T value for soft segment and T value for hard segment obtained from DMA curves; c\ngs gh\nOnsettemperaturesafter5%weightlossobservedviaTGA;dE’at120°CacquiredfromDMAcurves.\nWe deleted the ambiguous text on line 192 and synthesized the reference thermoplastic\n(TPU) and thermoset (CPU) devoid of NUBs (Supplementary Table S4). The relevant\nexperimental results and discussion have been added to the revised Supplementary\nInformationand Manuscript (Pages 18-21).\nSupplementaryTableS4. Composition ofthereferenceTPUand CPU.\nHDI Bi-NOH BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPNU-A\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.510g 0.676g 16.500g 1.320g\n10equiv. - 3equiv. 7equiv. -\nTPU\n30mmol, 9mmol, 21mmol,\n- -\n5.046g 0.811g 21.000g\n10equiv. - 3equiv. 5.5equiv. 1equiv.\nCPU\n30mmol, 9mmol, 16.5mmol, 3mmol,\n-\n5.046g 0.811g 16.500g 1.320g\nMechanical characterizations of TPU and CPU illustrated that the introduction of a small\nnumber of rigid structural NUBs imparted PNU-A with superior comprehensive mechanical\nperformance relative to both TPU and CPU. In addition, PNU-A exhibited excellent\nthermomechanical stability comparable to that of CPU, as evidenced by the presence of a\nmodulus plateau in the high-temperature region of the DMA curve (Fig. 5c), excellent\ndimensional stability (Fig. 5d), and prolonged stress relaxation (Fig. 5g). As expected, the\nviscous flow transition was observed in the high-temperature region for TPU. These results\nemphasize that the “High Activity & Low Content” strategy strikes a balance between\nefficient network rearrangement and mechanical retention.\nFig. 5 | The influence of DCB content on mechanical performances and\nthermomechanical behaviors. a Schematic illustration of PNUs with different DCB\ncontents and their dynamic behaviors at elevated temperatures. b Comparison of the\nstress-strain curves of CPU, TPU and PNUs. c Storage modulus and tan δ of CPU, TPU and\nPNUs as a function of temperature. d Dilatometry measurements of CPU and PNUs when\nheated from 0 to 160 °C at a temperature ramp rate of 3 °C/min, with an applied stress of 10\nkPa. e Optical photographs showing the dimensional stability of PNUs upon heating. f\nNormalized stress-relaxation analysis of PNU-A. g Comparison of stress-relaxation behavior\nof CPU and PNUs at 80 °C. h Time sweeps of PNU-A at elevated temperatures, with an\noscillation strain of 0.1% and frequency of 1 Hz for 60 min. Frequency sweeps of i PNU-A, j\nPNU-15 mol% and k PNU-30 mol% with a fixed strain of 0.1%, in the range of 0.01 to 100\nrad s−1at different temperatures.\nSupplementary Fig. S17 Frequency sweeps of CPU with a fixed strain of 0.1%, in the range\nof0.01to 100rad s−1at different temperatures.\nTo prove the importance of “high activity”, we synthesized two reference samples (Table\nR4): (i) dynamic poly(oxime-urethane) (POU) networks containing 5 mol% oxime-urethane\nbonds and (ii) conventional thermosetting PU (CPU, without NUBs). The dynamic\noxime-urethane bond shows similar a k to NUB but a relatively low degree of dissociation\na\n(12%at 120°Cvs. 28%at 120°Cfor NUBs).\nTableR4.Composition ofthe referencePOUand CPU.\nHDI Bi-NOH BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPNU-A\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.510g 0.676g 16.500g 1.320g\nHDI Dimethylglyoxime BDO PTMEG Tri-NH\n2\n10equiv. 0.5equiv. 2.5equiv. 5.5equiv. 1equiv.\nPOU\n30mmol, 1.5mmol, 7.5mmol, 16.5mmol, 3mmol,\n5.046g 0.174g 0.676g 16.500g 1.320g\nHDI - BDO PTMEG Tri-NH\n2\n10equiv. - 3equiv. 5.5equiv. 1equiv.\nCPU\n30mmol, 9mmol, 16.5mmol, 3mmol,\n-\n5.046g 0.811g 16.500g 1.320g\nReprocessing experiments showed that the PNU-A could be efficiently reprocessed into a\nuniform, smooth sheet under 120°C, 10 MPa for 30 min. In contrast, both POU and CPU\nfailed to achieve a defect-free film under identical conditions (Supplementary Fig. S20). This\nindicates 5 mol% oxime-urethane bond is insufficient to endow POU reprocessability. When\nthe processing temperature increased to 140 °C, all three samples could be molded into\nvisually uniform sheets. However, tensile tests revealed that both POU and CPU exhibit\npronounced deterioration. This deterioration is more likely due to insufficient topological\nrearrangement, resulting in weak interfacial bonding. In contrast, the PNU-A retained nearly\nunchangedtensilestrength and elongation at break after repeated reprocessing (Fig. 6c-e).\nThese results demonstrate that simply reducing the content of DCBs, although beneficial\nfor creep resistance, is insufficient to ensure effective reprocessability. In contrast, our “High\nactivity & Low content” strategy strikes a balance between efficient network rearrangement\nand mechanical retention, offering a practical and distinguishable advantage for recyclable\nCANs.\nFig. R4 The effect of DCB dissociation degree on the reprocessability of CANs. a\nPhotographs of PNU-A, POU and CPU reprocessed under different conditions\n(Supplementary Fig. S20). The mechanical properties of the b PNU-A(Fig. 6c), c POU (Fig.\n6d)and d CPU(Fig. 6e) after multiplereprocessing cycles (140°C, 10MPa,30min).\n7. For PNU-A(and for PNU-B and PNU-C) it would be interesting to evaluate the degree of\ncrosslinking.Also, for all systems theactivation energy would beinteresting to include.\nReply: Thanks for your constructive suggestion. We conducted DMAtemperature sweeps on\nPNU-A, PNU-B and PNU-C.The cross-linking density (v) and the molecular weight between\ncross-links (M ) were evaluated bythefollowing equation:\nc\n3RT\nE’3vRT \nM\nc\nWhere E’is the tensile storage modulus measured at 120 °C by DMA, ρ is the density of\nthePNUs, Ris thegas constant and Trefers totheabsolutetemperature (393 K).\nSupplementary Fig. S14 DMAtemperature sweeps of PNUs with varying hard/soft segment\nratios.\nUpon heating, the hard domains progressively softened and the highly active NUBs\ndissociated, allowing reorganization of the primary network. In the high-temperature rubbery\nplateau, E’ depended on the concentration of elastically active network strands, which is\nmainly controlled by the fundamental network topology (such as elastically active covalent\ncross-links, dangling chains, and cyclic loops) and the dissociation/association behavior of\ntheNUBs.\nBased on the E’measured byDMAat 120 °C , PNU-A, PNU-B and PNU-C exhibit similar\nand stable cross-linking densities (147 ~ 133 mol·m-3) in the rubber plateau region. Since the\ncontent of NUBs is minimal, their E’values are primarily governed by the similar topological\nstructure of the network, yielding comparable E’ values across the three samples. These\nresults demonstrate that the introduction of minimal NUBs does not compromise the integrity\nofthenetwork at high temperatures.\nSupplementaryTableS10.Cross-linking analysis ofPNU-A, PNU-B and PNU-C.\nρ E’ v M c\n(g·mL-1) (MPa) (mol·m-3) (kg·mol-1)\nPNU-A 1.069 1.44 147 7.3\nPNU-B 1.070 1.37 140 7.7\nPNU-C 1.076 1.30 133 8.1\nWe conducted stress relaxation experiments on PNU-A, PNU-B and PNU-C at different\ntemperatures. Arrhenius analysis revealed that the three samples exhibited similar relaxation\nactivation energies of 23.9 ~ 26.9 kcal mol-1, suggesting that the plasticity of the PNUs is\ngoverned bytherearrangement of NUBs.\nFig. R4Thenormalized stress relaxation ofaPNU-A(Fig. 5f), bPNU-B (Supplementary Fig.\nS16 a) and c PNU-C (Supplementary Fig. S16 b). d Determination of relaxation activation\nenergy (E ) fromArrhenius analysis of the characteristic relaxation time τ* versus 1000/T for\na\nPNUs (Fig. 6b).\n8. Line 332-338. It is not clear why linear structures containing PEG were introduced and\nwhy they were evaluated as an adhesive. Also, what is robust adhesion and how does it\ncompare to othersystems?\nReply: We appreciate your insightful review. Although 1H NMR readily detects the\nhydrolysis of NUB at elevated temperatures for the small molecule, this hydrolysis process is\ndifficult to observe within PNU networks in our initial attempts. This is caused by the low\nNUB content and the hydrophobicity of the PTMEG. Therefore, we employed a linear,\nPEG-containing model polymer (PNU-D) to explore the hydrolytic behavior at the\nmacromolecular level and the molecular weight changes could be easily monitored by GPC\novertime.This approach provides adirect measure ofhydrolysis-driven degradation ofPNU.\nPU is a class of versatile polymer material with diverse product forms. Besides foams and\nelastomers, another significant application is as an adhesive. It is promising to convert\ndiscarded PUs into adhesives via depolymerization, offering an alternative approach to waste\nmaterial management.\nTo demonstrate the robust adhesive performance, we compared the binding ability of the\nadhesive derived from degraded oligomers with three typical commercial PU adhesives on\ndiverse substrates (Supplementary Fig. S23). Overall, the performance of this adhesive was\nstronger than 3M-2665, comparable to Loctite-3542, but inferior to Lubrizol-5713 (Sun, P. et\nal., 2021). Therefore, we used the term “robust adhesion” to describe its universal\napplicability to multiple substrates and excellent adhesion comparable to commercial\nadhesives.\nSupplementary Fig. S23 Comparison ofadhesion properties between thedegraded oligomer\nand other representativecommercial PUadhesives.\nSource:\nSun,P.et al. SuperStrong and Multi-ReusableSupramolecular EpoxyHot MeltAdhesives.\nACSMaterials Lett. 2021,3,1003-1009.\n9. Supplementary Information. Please include all relevant parameters in the figure captions\nto facilitate understanding. Materials: please double check the naming of the chemicals used\nand also include company they were attained from. Instrumentation: Please include\ncalibrationrange. NMRspectra: Please includeassignments.\nReply: Thank you for your helpful reminder. In the revised version, the figure captions now\ninclude the relevant parameters for clarity. The chemical names have been verified, and the\nsupplier information has been added. The instrument calibration ranges are specified, and the\nNMRspectra are fully assigned.Thank you again for your valuable input."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have carefully revised the manuscript according to the comments, thus, I think it can be published in Nature Communications.", "reviewer_2": "The authors have adequately answered to the issues I had raised. The revised manuscript is now acceptable for publication.", "reviewer_3": "Upon revisions, the manuscript by Yin et.al. has been improved and many of the reviewer’s comments have been addressed and resolved. The answers and additional experiments however raise a new question, and there is one comment that has not been completely elucidated.\n1. The authors are emphasizing their CAN design to be “high activity and low content”, meaning that a combination of an efficient thermal decomposition, and rapid association chemistry for dynamic covalent bonds in a low content, improves, for example, the mechanical properties. As this is suggested to be a novel design criterion, and the focus of the manuscript, it must be set in a larger context, where other dynamic chemistries and systems are included and compared (not only for PURs). Similar design criteria have been incorporated in other dynamic chemistries, although maybe not explicitly stated. This was highlighted by two of the reviewers in the first round of comments. The authors must compare the design criteria to other dynamic chemistries, showcasing the novelty of them in the larger context of all CANs. A suggestion is to focus on CANs where the “low content” concept is already satisfied. This would showcase the novelty of the concept, while simultaneously enabling the scientific community to broaden the concept.\n2. The authors have now included hygrothermal aging experiments, which contributes to the understanding of the PNUs properties under use. The authors interpretation of the reduction in Youngs modulus, tensile strength and elongation-at-break is that it is a consequence of plasticization or alterations in microphase separation. Still, plasticization is commonly associated with a decrease in Tg and increase in elongation. As hygrothermal aging will be a key aspect for use, an elucidation of what is the prevailing mechanism would be of high interest. It also raises the question if there is an effect on properties at use conditions without elevated temperature. Performing the same experiment at room temperature with 100% humidity and also simply measuring if there are changes in properties as a function of time in room temperature with uncontrolled humidity would strengthen the study and possibly elucidate if plasticization or microstructure rearrangements dominate.\n3. In table S8 and figure 3c, the authors compare elongation-at-break with toughness. As elongation contributes to toughness, would a clearer message be sent if elongation-at-break is compared to strength or strain-at-break for the materials?"}, "author_response": "Reviewer#1(Remarks totheAuthor):\nAuthors have carefully revised the manuscript according to the comments, thus, I think it can\nbepublished in Nature Communications.\nReply:Thank you very muchfor recommending publication ofourwork.\nReviewer#2(Remarks totheAuthor):\nThe authors have adequately answered to the issues I had raised. The revised manuscript is\nnowacceptable forpublication.\nReply:Weare grateful forthe recommendationto publish ourwork.\nReviewer#3(Remarks totheAuthor):\nUpon revisions, the manuscript by Yin et.al. has been improved and many of the reviewer’s\ncomments have been addressed and resolved. The answers and additional experiments\nhowever raise a new question, and there is one comment that has not been completely\nelucidated.\nReply:Thank you forthecareful evaluation of ourwork and theconstructive comments.\n1. The authors are emphasizing their CAN design to be “high activity and low content”,\nmeaning that a combination of an efficient thermal decomposition, and rapid association\nchemistry for dynamic covalent bonds in a low content, improves, for example, the\nmechanical properties.As this is suggested to be a novel design criterion, and the focus of the\nmanuscript, it must be set in a larger context, where other dynamic chemistries and systems\nare included and compared (not only for PURs). Similar design criteria have been\nincorporated in other dynamic chemistries, although maybe not explicitly stated. This was\nhighlighted by two of the reviewers in the first round of comments. The authors must\ncompare the design criteria to other dynamic chemistries, showcasing the novelty of them in\nthe larger context of all CANs. A suggestion is to focus on CANs where the “low content”\nconcept is already satisfied. This would showcase the novelty of the concept, while\nsimultaneouslyenabling thescientificcommunity to broaden theconcept.\nReply: We sincerely appreciate the insightful comments regarding the broader context of our\n“high activity & low content” design criterion. We agree that situating our work within the\nlarger landscape of CANs significantly enhances its scientific value. While this strategy is\nmore straightforward and simpler, it has been significantly overlooked in prior studies.\nPrevious studies (Cash, JJ. et al., 2018; Li, L. et al., 2018) have primarily focused on\nenhancing creep resistance by reducing dynamic bond density (i.e., “low content”). However,\nthe intrinsic relationship between reversible bond kinetics/thermodynamics (i.e., the “high\nactivity” aspect) and recyclability remains largely absent. To further underscore the novelty\nand generality of our concept, we have benchmarked the NUB against various other dynamic\nchemistries (Supplementary Table S2), illustrating that simultaneously achieving large k and\na\nhigh K is challenging. When these two criteria are satisfied, this concept should theoretically\nd\nbe applicable to other CAN systems, regardless of the specific chemistry involved. These\ndiscussionshave been updated inthe Manuscript and Supplementary Information.\nSource:\nCash, JJ. et al. Maximizing the symbiosis of static and dynamic bonds in self-healing boronic\nester networks. Polymer Chemistry 2018,9,2011-2020.\nLi, L. et al. Vitrimers Designed Both To Strongly Suppress Creep and To Recover Original\nCross-Link Density after Reprocessing: Quantitative Theory and Experiments.\nMacromolecules 2018,51,5537−5546.\n2. The authors have now included hygrothermal aging experiments, which contributes to the\nunderstanding of the PNUs properties under use. The authors interpretation of the reduction\nin Youngs modulus, tensile strength and elongation-at-break is that it is a consequence of\nplasticization or alterations in microphase separation. Still, plasticization is commonly\nassociated with a decrease in T and increase in elongation. As hygrothermal aging will be a\ng\nkey aspect for use, an elucidation of what is the prevailing mechanism would be of high\ninterest. It also raises the question if there is an effect on properties at use conditions without\nelevated temperature. Performing the same experiment at room temperature with 100%\nhumidity and also simply measuring if there are changes in properties as a function of time in\nroom temperature with uncontrolled humidity would strengthen the study and possibly\nelucidate ifplasticization ormicrostructure rearrangements dominate.\nReply: Thank you for the careful review. We apologize for the inaccurate phrasing and agree\nwith the reviewer that plasticization is typically accompanied by a decrease in T and an\ng\nincrease in elongation-at-break. Since the hygrothermal aging experiment was conducted at a\nrelatively low temperature (40 °C), the dissociation of NUBs is negligible, which was proven\nby the IR experiments (Supplementary Fig. S31). As illustrated in the DMA results\n(Supplementary Fig. S31), the PNU did not exhibit a decrease in T after hygrothermal aging.\ng\nTherefore, the deterioration in mechanical properties is likely attributed to alterations in the\naggregation structure (e.g., changes in microphase separation or hydrogen bonding networks)\nrather than chemical degradation orplasticization.\n3. In table S8 and figure 3c, the authors compare elongation-at-break with toughness. As\nelongation contributes to toughness, would a clearer message be sent if elongation-at-break is\ncompared to strength orstrain-at-break for thematerials?\nReply: Thanks for your suggestion. We have provided an additional comparison of tensile\nstrength and elongation at break in theSupplementary Information.\nSupplementary Fig. S21 Comparison between PNUs and other representative covalently\ncrosslinked dynamic PUsin terms of tensilestrength and elongation-at-break.\n3"}]

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