Bond Exchange Reactions Research Articles

Light-Driven, Reversible Spatiotemporal Control of Dynamic Covalent Polymers.

Dynamic covalent polymer networks exhibit a cross-linked structure like conventional thermosets and elastomers, although their topology can be reorganized through externally triggered bond exchange reactions. This characteristic enables a unique combination of repairability, recyclability and dimensional stability, crucial for a sustainable industrial economy. Herein the application of a photoswitchable nitrogen superbase is reported for the spatially resolved and reversible control over dynamic bond exchange within a thiol-ene photopolymer. By the exposure to UV or visible light, the associative exchange between thioester links and thiol groups is successfully gained control over, and thereby the macroscopic mechanical material properties, in a locally controlled manner. Consequently, the resulting reorganization of the global network topology enables to utilize this material for previously unrealizable advanced applications such as spatially resolved, reversible reshaping as well as micro-imprinting over multiple steps. Finally, the presented concept contributes fundamentally to the evolution of dynamic polymers and provides universal applicability in covalent adaptable networks relying on a base-catalyzed exchange mechanism.

Synthesis and Characterization of Rebondable Polyurethane Adhesives Relying on Thermo-Activated Transcarbamoylation.

Dynamic polymer networks combine the noteworthy (thermo)mechanical features of thermosets with the processability of thermoplastics. They rely on externally triggered bond exchange reactions, which induce topological rearrangements and, at a sufficiently high rate, a macroscopic reflow of the polymer network. Due to this controlled change in viscosity, dynamic polymers are repairable, malleable, and reprocessable. Herein, several dynamic polyurethane networks were synthetized as model compounds, which were able to undergo thermo-activated transcarbamoylation for the use in rebondable adhesives. Ethylenediamine-N,N,N',N'-tetra-2-propanol (EDTP) was applied as a transcarbamoylation catalyst, which participates in the curing reaction across its four -OH groups and thus, is covalently attached within the polyurethane network. Both bond exchange rate and (thermo)mechanical properties of the dynamic networks were readily adjusted by the crosslink density and availability of -OH groups. In a last step, the most promising model compound was optimized to prepare an adhesive formulation more suitable for a real case application. Single-lap shear tests were carried out to evaluate the bond strength of this final formulation in adhesively bonded carbon fiber reinforced polymers (CFRP). Exploiting the dynamic nature of the adhesive layer, the debonded CFRP test specimens were rebonded at elevated temperature. The results clearly show that thermally triggered rebonding was feasible by recovering up to 79% of the original bond strength.

A self-healing composite solid electrolyte with dynamic three-dimensional inorganic/organic hybrid network for flexible all-solid-state lithium metal batteries

Composite solid electrolytes (CSEs), which combine the advantages of solid polymer electrolytes and inorganic solid electrolytes, are considered to be promising electrolytes for all-solid-state lithium metal batteries. However, the current CSEs suffer from defects such as poor inorganic/organic interface compatibility, lithium dendrite growth, and easy damage of electrolyte membrane, which hinder the practical application of CSEs. Herein, a CSE (PBHL@LLZTO@DDB) with polyurethane (PBHL) as the polymer matrix and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) modified by silane coupling agent (DDB) as inorganic fillers (LLZTO@DDB) has been prepared. Disulfide bond exchange reactions between PBHL and LLZTO@DDB enable PBHL@LLZTO@DDB to form a dynamic three-dimensional (3D) inorganic/organic hybrid network, which promotes the uniform dispersion of LLZTO in PBHL@LLZTO@DDB, improves the Li+ conductivity (1.24 ± 0.08 × 10−4 S cm−1 at 30 ℃), and broadens the electrochemical stability window (5.16 V vs. Li+/Li). Moreover, a combination of hydrogen bonds and disulfide bonds endows PBHL@LLZTO@DDB with excellent self-healing properties. As such, both all-solid-state symmetric and full cells exhibit excellent cycle performance at ambient temperature. More importantly, the healed PBHL@LLZTO@DDB can almost completely restore its original electrochemical properties, indicating its application potential in flexible electronic products.

Creep and recovery of vitrimers under thermo-chemo-mechanical coupling effects

Reprocessable, repairable, and recyclable (“3R”) vitrimers have experienced rapid development over the past decade and demonstrate significant potential for diverse applications. The creep performance of vitrimers is crucial for their dimensional stability under load-bearing conditions at relatively low temperatures and serves as a key metric for evaluating their reprocessability at high temperatures. However, the impact of dynamic covalent polymer networks (DCPNs) on vitrimer mechanics, particularly creep properties, remains debated. Systematic experimental studies on the creep of vitrimers across a wide temperature range are lacking. We use the classic epoxy vitrimer as a model system to investigate the impact of DCPNs on vitrimer mechanics, especially focusing on the creep and recovery behavior across a wide temperature range, spanning from room temperature to the glass transition temperature (Tg) and further to the topology freezing transition temperature (Tv). We systematically examined the effects of temperature, stress, and catalysts on vitrimer creep, revealing the influence of DCPNs. Our findings demonstrate significant thermo-chemo-mechanical coupling effects in the creep mechanics of vitrimers, a facet not comprehensively acknowledged in existing studies. In addition, at temperatures below Tg, vitrimers exhibit superior creep resistance compared to pure epoxy resin due to metal coordination, ensuring excellent dimensional stability under load-bearing conditions. Conversely, at high temperatures, active bond exchange reactions in vitrimers accelerate creep and result in greater residual deformation, highlighting exceptional reprocessability. This study provides new insights into the materials and mechanics of vitrimers.

Functional Upcycling of Polyurethane Thermosets into Value-Added Thermoplastics via Small-Molecule Carbamate-Assisted Decross-Linking Extrusion.

The cross-linked structures of most commodity polyurethanes (PUs) hinder their recycling by common mechanical/chemical approaches. Catalyzed dynamic carbamate exchange emerges as a promising PU recycling strategy, which converts traditional static PU thermosets into reprocessable covalent adaptable networks (CANs). However, this approach has been limited to thermoset-to-thermoset reprocessing of PU CANs, accompanied by their well-preserved network structures and extremely high viscosities, which pose challenges to processing and certain applications. This study reports a catalytic decross-linking extrusion process aided by small-molecule carbamates, which can upcycle PU thermosets into easily processable and functional PU thermoplastics in a solvent-free and high-throughput manner. Key to this process is the employment of small-molecule carbamates as decross-linkers to simultaneously deconstruct cross-linked PUs and functionalize the decross-linked PU chains, through catalyzed carbamate exchange reactions in a twin-screw extruder. This strategy applies to both aromatic and aliphatic cross-linked PU films and foams, and the amount of small-molecule carbamates required to decross-link PU networks is determined through thermal, chemical, and structural analyses of the resulting extrudates. This approach is generalizable to small-molecule carbamates with various steric/electronic structures and chemical functionalities including methacrylate, anthracene, and stilbene groups. The chain-end functionalization is confirmed by analyzing the purified decross-linked extrudates after dialysis. This thermoset-to-thermoplastic extrusion process represents a powerful approach for upcycling postconsumer PU thermosets into a library of thermoplastic PUs with controlled molecular weights and chain-end functionalities for diverse applications, including adhesives, photoresins, and stimuli-responsive materials, as demonstrated herein. In the future, this strategy could be extended to upcycle many other step-growth networks capable of undergoing catalytic bond exchange reactions, such as cross-linked polyureas and polyesters, contributing to plastic waste management in general.

Influence of monomer structure and catalyst concentration on topological transition and dynamic properties of dicarboxylic acid‐epoxy vitrimers

AbstractThis study delineates the dependence of thermophysical behavior of acid‐epoxy vitrimers on their formulation. The stress relaxation due to the bond exchange reaction and the glass transition temperature of acid epoxy vitrimers are investigated, with respect to the influence of catalyst content and acid chain length. This is carried out for a range of dicarboxylic acids and catalyst concentrations formulated and characterized using calorimetry and dynamic mechanical analysis. The influence of acid chain length on the bond exchange rate, topological transition, and glass transition temperatures of the vitrimers is found to be significant. The activation energy of the exchange reaction varies over a wide range from 73 to 104 kJ/mol and the topology freezing temperature from 66 to 136°C with the behavior governed by the interplay between crosslinking density, network flexibility and density and distance of functional groups, with an increase of catalyst concentration leading to lower topological transition temperature and the dependence on chain length showing non‐monotonic behavior. The glass transition decreases by about 30°C as the carbon chain length increases from 6 to 14 carbons due to enhanced monomer flexibility and is not affected by the concentration of catalyst.

Temperature- and Rate-Dependent Fracture in Disulfide Vitrimers.

The fracture behaviors of disulfide vitrimers are highly rate-dependent. Our investigation revealed that the temperature-dependent fracture behaviors of disulfide vitrimers cannot be entirely explained by a simple time-temperature superposition model. This Letter explores the impact of the dynamic nature of molecular defects on the temperature- and rate-dependent fracture behaviors of disulfide vitrimers. Considering that the high cross-linking density remains constant during the associated bond exchange reaction, we identify loop defects in the network as the primary dynamic defects. By employing small amplitude oscillatory shear, we measured the loop defect fraction in EPS25 disulfide vitrimers at varied temperatures, revealing an increased presence of loop defects at elevated temperatures. Furthermore, our findings indicate that the temperature-dependent fracture behaviors are attributed to the temperature-dependent number of loop defects in disulfide vitrimers.

Topological Programmability of Isomerizable Polymers.

Topology isomerizable networks (TINs) can be programmed into numerous polymers exhibiting unique and spatially defined (thermo-) mechanical properties. However, capturing the dynamics in topological transformations and revealing the intrinsic mechanisms of mechanical property modulation at the microscopic level is a significant challenge. Here, we use a combination of coarse-grained molecular dynamics simulations and reaction kinetic theory to reveal the impact of dynamic bond exchange reactions on the topology of branched chains. We find that, the grafted units follow a geometric distribution with a converged uniformity, which depends solely on the average grafted units of branched chains. Furthermore, we demonstrate that the topological structure can lead to spontaneous modulation of mechanical properties. The theoretical framework provides a research paradigm for studying the topology and mechanical properties of TINs.

Effect of molecular structure on the dynamics and viscoelasticity of vitrimers

Vitrimers are a class of dynamic polymer networks that perform bond-exchange reactions (BERs) under environmental stimuli while preserving network connectivity. The viscoelastic properties of vitrimers are not only dependent on the kinetics of BERs, but also the architectures of polymer matrix. Here, we investigate the influence of polymer architectures on the dynamics and linear viscoelasticity of vitrimers through varying the sticker (reactive bead) distribution in the precursor chain and the BERs energy barrier (Ub) by performing hybrid molecular dynamics-Monte Carlo simulations. Increasing Ub, vitrimers with block and gradient distributions exhibit considerably faster relaxations of chain ends, Rouse mode, and stress and lower zero-shear viscosity than the analogs with random and uniform distributions, in qualitative agreement with the predictions of the sticky Rouse model. In contrast, the distribution of stickers has a small influence on the relaxation of dynamic bonds regardless of the Ub value. By comparing the molecular structures of vitrimers with different distributions of stickers, we find that the molecular defects, especially the relatively longer dangling end in block and gradient distributions, impair the effect of dynamic bonds on the dynamics and viscoelasticity of vitrimers. This suggests that the dangling defects can be leveraged for further development of recyclable and healable materials with fast stress relaxation and improved flowability.

Programming Chain Distribution of Branched Polymers via Bond Exchange Reactions

Programming Chain Distribution of Branched Polymers via Bond Exchange Reactions

Reshapable bio-based thiol-ene vitrimers for nanoimprint lithography: Advanced covalent adaptability for tunable surface properties

Taking inspiration from natural surfaces known for their ability to adaptively remodel their shape, we fabricated stimuli-responsive microstructures by using UV-induced nanoimprint lithography. For this, a fully bio-based dynamic thiol-ene photopolymer was synthetized by the radical mediated addition of a trifunctional eugenol-based thiol (SH3E) crosslinker across an allylated linseed oil (ALELO). To catalyze the bond exchange reactions between the hydroxyl and ester groups within the network, a bio-based eugenol phosphate ester was introduced as a transesterification catalyst. Pure eugenol was further added as a reactive diluent to increase the number of hydroxyl groups and thus, accelerate the thermo-activated bond exchange reactions. Once cured by UV exposure, the dynamic photopolymer is thermally stable up to 250 °C and is able to relax 63% of the original stress within 62 min at 160 °C. Subsequently, films with micropillars, having an aspect ratio of 1:2.5 were prepared by using nanoimprint lithography. The macroscopic reflow capability of the dynamic network enabled a reorientation of the imprinted structures during a thermal reshaping step. The imprints were characterized by 2D/3D optical microscopy, μCT imaging and static water contact angle measurements. Based on the orientation of the micropillars, the water contact angle was varied between 118° and 95°, giving rise to a possible application in microfluidic devices.

Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review.

Covalent adaptable networks and vitrimers are novel polymers with dynamic reversible bond exchange reactions for crosslinks, enabling them to modulate their properties between those of thermoplastics and thermosets. They have been gathering interest as materials for their recycling and self-healing properties. In this review, we discuss different molecular simulation efforts that have been used over the last decade to investigate and understand the nanoscale and molecular behaviors of covalent adaptable networks and vitrimers. In particular, molecular dynamics, Monte Carlo, and a hybrid of molecular dynamics and Monte Carlo approaches have been used to model the dynamic bond exchange reaction, which is the main mechanism of interest since it controls both the mechanical and rheological behaviors. The molecular simulation techniques presented yield sufficient results to investigate the structure and dynamics as well as the mechanical and rheological responses of such dynamic networks. The benefits of each method have been highlighted. The use of other tools such as theoretical models and machine learning has been included. We noticed, amongst the most prominent results, that stress relaxes as the bond exchange reaction happens, and that at temperatures higher than the glass transition temperature, the self-healing properties are better since more bond BERs are observed. The lifetime of dynamic covalent crosslinks follows, at moderate to high temperatures, an Arrhenius-like temperature dependence. We note the modeling of certain properties like the melt viscosity with glass transition temperature and the topology freezing transition temperature according to a behavior ruled by either the Williams-Landel-Ferry equation or the Arrhenius equation. Discrepancies between the behavior in dissociative and associative covalent adaptable networks are discussed. We conclude by stating which material parameters and atomistic factors, at the nanoscale, have not yet been taken into account and are lacking in the current literature.

Matrix curing and stress field development around fiber during the solvent evaporation-induced repolymerization of recycling composites

When recycling fiber-reinforced composites using depolymerized polymer solutions, oligomers reconnect through covalent bond exchange reactions (BERs), which ultimately forms a crosslinked network with mechanical properties comparable to those of virgin materials. This process offers exciting opportunities for the primary recycling of engineering composites, but it is intricately influenced by a few material and process parameters, such as heating temperature, duration, and fiber volume fractions. These parameters collectively determine the matrix's curing degree and the evolution of stress fields around the fiber, which are significant factors determining the quality and mechanical performance of recycled composites. This study introduces a diffusion-reaction-mechanics finite-element computational model to examine the solvent-induced repolymerization process in recycling composites. The model predicts the matrix curing degree, residual stress development around the fiber, and the composite's mechanical properties, aligning with experimental results. Parametric computational studies are then conducted to assess the influences of temperature, fiber content, solvent diffusivity, and the reactivity of BERs. The objective is to identify material and processing conditions that ensure efficient composite recycling while minimizing the development of residual stress. Overall, this study enhances our understanding of the mechanisms behind composite recycling using organic solvents and provides valuable insights for industrial stakeholders looking to optimize processing conditions and advance the commercialization and widespread adoption of this innovative recycling technique.

Dynamic Anion Enables Self‐Healing Single‐Ion Conductor Polymer Electrolyte for Lithium‐Metal Batteries

AbstractLithium‐metal batteries (LMBs) are regarded as a promising option for high‐energy‐density storage systems. However, lithium dendrite growth in LMBs can damage electrolytes and cause short circuits, which severely limits the practical application of LMBs. It is highly desired to develop an advanced polymer electrolyte with self‐healing capability and the ability to inhibit the growth of lithium dendrites for enhancing safety and prolonging the cycle life of the LMBs. Herein, sp3 boron‐based single‐ion conductor polymer electrolytes (B‐SIPEs) with excellent self‐healing capability and interfacial stability are constructed. The rapid bond exchange reactions of dynamic borate anions enable B‐SIPE to fast self‐heal in mild conditions, with a high self‐healing efficiency of 87% within 30 min at an ambient temperature. In particular, B‐SIPE displays a high Li+ transference number (0.88) and excellent lithium dendrite inhibition ability. The Li|B‐SIPE|Li cells exhibit a stable overpotential over 2000 h without short‐circuit. Additionally, the LiFePO4||Li cells with B‐SIPE exhibit superior long‐term cycling performance, with 93.5% capacity retention for 500 cycles. Significantly, B‐SIPE is capable of self‐healing the fracture in LiFePO4||Li cells and recovering the battery performance. These results demonstrate that the advanced self‐healing single‐ion conductor polymer electrolytes effectively promote the reliability and safety of LMBs.

Renewable Methacrylate Resins for 3D Printing Containing Dynamic Hydroxyester Linkages for Reprocessability

AbstractTo facilitate the ongoing transition toward a circular economy, renewable 3D print materials that are both sustainable and competitive must be accessible. However, the growing demand for bio‐based thermosetting resins, which are used as ink for vat photopolymerization, gives rise to environmental concerns in terms of plastic waste management. Therefore, photocurable materials that are renewable and recyclable at the same time are needed. In this work, a mechanically robust and reprocessable 3D printed photopolymer is developed from renewable feedstock. Reaction of malic acid with glycidyl methacrylate introduces both methacrylate moieties that can undergo photopolymerization in the 3D printer, and β‐hydroxyester linkages that can act as dynamic crosslinks via bond exchange reactions. By combining modified malic acid with reactive diluents, a photoinitiator, and phosphate catalyst, three distinct resins are formulated, resulting in bio‐based contents ranging from 43% to 49%. The formulations demonstrate good layer fusion and accurate print quality, while the 3D printed specimens are robust and thermally stable. Notably, the printed object with shortest relaxation time displayed Arrhenius flow behavior with an activation energy of 36.0 kJ mol−1, and its mechanical performance is maintained after being recycled three times. This contributes to the end‐of‐life perspective of photocurable resins in additive manufacturing.

Manipulating the Properties of Polymer Vitrimer Nanocomposites by Designing Dual Dynamic Covalent Bonds.

Polymer vitrimer is a novel material that contains dynamic covalent bonds (DCBs) allowing it to combine the desirable characteristics of both thermoplastics and thermosets. Similar to the traditional polymer nanocomposites, introducing nanoparticles into polymer vitrimer is also an effective strategy to further enhance its properties. However, a comprehensive understanding of matrix and interfacial bond exchange reactions (BERs) to tailor the properties of polymer vitrimer nanocomposites (PVNs) is still lacking. Herein, we utilized coarse-grained molecular dynamics simulations to investigate model PVNs in which there are two different kinds of DCBs in the vitrimer matrix and at the interface. Our results show that the normalized bond autocorrelation function (Csw) confirms the independence of BERs in the vitrimer matrix and in the interface. By varying the bond swap energy barrier () in the matrix or in the interface , or in both , a maximum mechanical property is observed at the moderate value of , , or. Meanwhile, the effect of on the stress relaxation and the bond orientation as a function of the time under a fixed strain is well probed, which both decay more slowly at greater . We simulated the tension-recovery curve to examine the effect of on the hysteresis loss and permanent deformation of PVNs, finding an optimal value to achieve its minimum energy dissipation and maximum recovery ratio. Lastly, we investigated the efficiency of self-healing by building and removing walls from the system. Interestingly, a maximum self-healing efficiency of the stress-strain behavior is observed at moderate . Overall, this study provides valuable insights into the relationship between the structure and properties of PVNs, offering implications for the manipulation of their mechanical properties and enhancement of their self-healing capabilities.

Tertiary amine-based autocatalytic transesterification epoxy vitrimer: Effect of tertiary amine structure and covalent incorporation on the properties of vitrimers

Since its first report in 2011, researches on vitrimer transesterification catalyst systems had been gradually focused on three aspects: low toxicity and low corrosion, good compatibility with resin matrix, and high catalytic performance for bond exchange reactions. However, it is difficult to obtain a catalytic system that can simultaneously meet the above three expectations. This research conducted the preparation of autocatalytic transesterification epoxy vitrimers by directly mixing small tertiary amine molecules with epoxy-anhydride systems and covalent incorporating tertiary amine structures into curing systems. And the influence of tertiary amine structures and covalent incorporation on the various properties of vitrimers was also discussed. A tertiary amine promoter composed of isofunctional group ratio reaction from di-n-hexamine and epoxy resin provided by this research overcame the problem of poor compatibility between tertiary amine and epoxy resin, introduced a highly active catalytic structure into vitrimer curing system. And the result indicated that the vitrimer prepared according to our new strategy had excellent comprehensive performance. In conclusion, a new idea of covalent incorporation was proposed to provide ideas and inspiration for the development of new tertiary amine-based autocatalytic transesterification epoxy vitrimers.

Hybrid Chemomechanical Yielding of Covalent Adaptable Networks for Toughening Ionic Semi-Crystalline Epoxy

Covalent adaptable network (CAN) in thermoset epoxy shows good recyclability due to its bond exchange reactions (BERs), which result in low mechanical strength and toughness. Many experimental studies on using the semi-crystalline structures and ions with the CAN have shown to improve the mechanical strength and toughness of these thermoset epoxies. However, their toughening mechanisms and coupling effects have not been well understood. Herein, we propose an extended Maxwell model to explore toughening mechanisms and coupling effects of the semi-crystallization process and formation of ion phases on CANs, which enables the thermoset epoxy with hybrid chemomechanical yielding. A constitutive stress–strain relationship is formulated to characterize the mechanical toughening and hybrid chemomechanical yielding behavior, where the first stage of yielding is originated from the effect of ions on amorphous phases, and the second stage of yielding is due to the coupling effect of semi-crystallization and formation of ions. The proposed models have been verified using the experimental results, and provide a fundamental approach to understanding the toughening mechanisms and coupling effects of semi-crystallization and ions in thermoset epoxy.

A phase-transition model of reprocessible thermadapt shape memory polymer

A new type of thermadapt shape memory polymer (SMP) has not only the processability of thermoplastic SMP, but also the excellent shape fixation of thermosetting SMP. To enhance the application of this thermadapt SMP within industrial sectors, a comprehensive constitutive model based on phase transition is being proposed as an indicative descriptor of the semi-crystalline thermadapt SMP’s salient features, predominantly related to its two-way shape memory effect (SME) and thermal reprocessability. The concept of cooling elongation is also introduced in this model for modeling the two-way SME during the crystallization process. The molecular mechanism of chain-packing has been studied and used to establish phenomenological formulas. In addition, to systematically assess the temperature-time dependence of the crystallization process, the Avrami equation is improved by incorporating the distribution of polymer chain segments. This strategy provides a detailed investigation into the evolving pattern of the crystallization process in response to various temperature and time conditions. Compared with the experimental results, it is found that our model can well capture mechanical behavior in multiple shape memory cycles, including the two-way SME and reshaping process caused by bond exchange reaction. Furthermore, the potential application of SMP in smart mandrels is explored because the cooling elongation feature is able to endow it with self-adaptive expansion ability.

Front Cover

The cover image by Simon van Hurne depicts the formation of a dynamic covalent boronate‐TetraAzaADamantane bond, which could be integrated in a covalent adaptable network. As a result of the on‐going dynamic bond exchange reactions within this network, the material could undergo stress relaxation. At the same time, the more robust triple bonding motif allowed for stronger materials compared to their conventional boronate ester‐based counterparts. DOI: 10.1002/pol.20230446 image