Dual-Factor-Controlled Dynamic Precursors Enable On-Demand Thermoset Degradation and Recycling.

Thermosets are well known for their advantages such as high stability and chemical resistance. However, developing sustainable thermosets with degradability and recyclability faces several principal challenges, including reconciling the desired characteristics during service with the recycling and reprocessing properties required at the end of life, establishing efficient methods for large-scale synthesis, and aligning with current manufacturing process. Here a general strategy is presented for the on-demand degradation and recycling of thermosets under mild conditions utilizing dynamic precursors with dual-factor-controlled reversibility. Specifically, dynamic triazine crosslinkers are introduced through dynamic nucleophilic aromatic substitution (SNAr) into the precursor polyols used in polyurethane (PU) synthesis. Upon removal of the catalyst and alcohol, the reversibility of SNAr is deactivated, allowing for the use of standard PU polymerization techniques such as injection molding, casting, and foaming. The resulting cyanurate-crosslinked PUs maintain high stability and diverse mechanical properties of traditional crosslinked PUs, yet offer the advantage of easy on-demand depolymerization for recycling by activating the reversibility of SNAr under specific but mild conditions-a combination of base, alcohol, and mild heat. It is envisioned that this approach, involving the pre-installation of dual-factor-controlled dynamic crosslinkers, can be broadly applied to current thermosetting plastic manufacturing processes, introducing enhanced sustainability.

Oxindolyl-based Radicals with Tunable Mechanochromic and Thermochromic Behavior.

Organic radicals based dynamic covalent chemistry is promising in preparing stimuli-responsive chromic materials, due to their simplicity of dissociation/association, accompanied with distinct color changes during the process. However, suitable organic radicals for dynamic covalent chemistry have not been widely explored yet. Herein, a series of oxindolyl-based mono-radicals (OxRs) with different substituents were successfully synthesized and studied systematically as potential building blocks for stimuli-responsive chromic materials. These OxRs would dimerize spontaneously to form their corresponding dimers. The structures of dimers were unambiguously confirmed through low-temperature 1H-NMR and single-crystal X-ray diffraction analyses. Dynamic interconversion between monomers and dimers was achieved by reversible cleavage and recovery of the σ-bond upon soft external stimuli (temperature, pressure, and solvent polarity), accompanied by significant color changes. It is interesting that the stability of the mono-radical could be tuned through changing different substituents, and consequently altering the bond dissociation energy of the dynamic covalent bond between monomers. These new OxRs characterized by appreciable properties are entitled to more opportunities in developing mechanochromic and thermochromic materials, where their responsiveness to stimuli can be readily controlled by the substituents adhered.

Pnictogen-Assisted Self-Assembly As A Means To Study Mediated Thiol/Disulfide Exchange in Supramolecular Dynamic Covalent Assemblies.

Thiol-disulfide interchange has been an active field of study for biochemists and physical organic chemists alike due to its prevalence within biological systems and fundamentally interesting dynamic nature. More recently, efforts have been made to harness the power of this reversible reaction to make self-assembling systems of macrocyclic and cage-like molecules. However, less effort has focused on the fundamental study of isolating these assemblies and analyzing the factors that control the assembly and sorting of these emerging cyclic systems. We have shown previously that pnictogen-assisted self-assembly enables formation of discrete disulfide macrocycles and cages without competition from polymer formation for a wide variety of alkyl thiols. Herein we report the expansion of these methods to form disulfide macrocycles from aryl thiol containing ligands, allowing access to previously unreported molecules. More importantly, the development of this new self-assembly chemistry allows for a comparison of aryl vs alkyl disulfide exchange and self-assembly. These studies complement classical physical organic and chemical biology studies on the kinetics and thermodynamics of aryl thiol oxidation to disulfides, and we show that this self-assembly method revises some prevailing wisdom from these key classical studies by providing new product distributions and new isolable products in cyclic disulfide formation.

Rewritable Surface-Grafted Polymer Brushes with Dynamic Covalent Linkages.

Surface grafting of polymer brushes drastically modifies surface properties, including wettability, compatibility, responsiveness, etc. A broad variety of functionalities can be introduced to the surface via different types of polymers. Bringing together properties of two or more types of polymer brushes to one surface opens up even more possibilities in brush-modified materials. However, while it is generally feasible to introduce several chemical compositions along the brushes via copolymerization, it is challenging to vary the types of polymer brushes along a surface. Although previous studies have demonstrated binary brushes via orthogonal polymerization techniques or partial deactivation/regrafting, they commonly limit the number of polymer types to two. Here, we propose a strategy to introduce dynamic covalent diketoenamine linkages at the root of polymer brushes. The grafting density could be precisely tuned during surface functionalization. The surface-anchored polymer brushes were cleaved by the addition of small molecule amines. New polymer brushes can be regrafted from the surface following refunctionalization of exposed sites. The maneuverability allows tuning of the types and densities of the polymer brushes, pointing the way to the preparation of a new generation of well-defined brush-modified materials with mixed grafts, with potential applications in the design of smart materials and surfaces.

Dynamic Gas-Bridged Bond: An Opportunity of Fabricating Dynamic Assembled Materials with Gas.

Gas molecules, as a family of unique polyatomic building blocks, have long been considered hard to involve in molecular assembly or construct assembled materials due to their structural simplicity yet paucity of defined interacting sites. To solve this non-trivial challenge, a core idea is to break the limit of current ways of bonding gas molecules, endowing them with new modes of interactions that match the basic requirements of molecular assembly. In recent years, a new concept, named the dynamic gas-bridged bond (DGB), has emerged, which allows for gas molecules to constitute a dynamic bridging structure between other building blocks with the aid of frustrated Lewis pairs. This makes it possible to harness gas in a supramolecular or dynamic manner. Herein, this perspective discusses distinct dynamic natures of DGBs and manifests their particular functions in various fields, including the control of molecular/polymeric self-assembly nanostructures, creation of multidimensional assembled materials, and recyclable catalysts. The future research direction and challenges of dynamic gas-bridged chemistry toward gas-programmed self-assembly and gas-constructed adaptive materials are highlighted.

Ionic Covalent Organic Frameworks Consisting of Tetraborate Nodes and Flexible Linkers.

Covalent organic frameworks (COFs) have emerged as versatile materials with many applications, such as carbon capture, molecular separation, catalysis, and energy storage. Traditionally, flexible building blocks have been avoided due to their potential to disrupt ordered structures. Recent studies have demonstrated the intriguing properties and enhanced structural diversity achievable with flexible components by judicious selection of building blocks. This study presents a novel series of ionic COFs (ICOFs) consisting of tetraborate nodes and flexible linkers. These ICOFs use borohydrides to irreversibly deprotonate the alcohol monomers to achieve a high degree of polymerization. Structural analysis confirms the dia topologies. Reticulation is explored using various monomers and metal counterions. Also, these frameworks exhibit excellent stability in alcohols and coordinating solvents. The materials have been tested as single-ion conductive solid-state electrolytes. ICOF-203-Li displays one of the lowest activation energies reported for ion conduction. This tetraborate chemistry is anticipated to facilitate further structural diversity and functionality in crystalline polymers.

Rapidly Self-Healable and Melt-Extrudable Polyethylene Reprocessable Network Enabled with Dialkylamino Disulfide Dynamic Chemistry.

Catalyst-free, radical-based reactive processing is used to transform low-density polyethylene (LDPE) into polyethylene covalent adaptable networks (PE CANs) using a dialkylamino disulfide crosslinker, BiTEMPS methacrylate (BTMA). Two versions of BTMA are used, BTMA-S2, with nearly exclusively disulfide bridges, and BTMA-Sn, with a mixture of oligosulfide bridges, to produce S2 PE CAN and Sn PE CAN, respectively. The two PE CANs exhibit identical crosslink densities, but the S2 PE CAN manifests faster stress relaxation, with average relaxation times ∼4.5 times shorter than those of Sn PE CAN over a 130 to 160 °C temperature range. The more rapid dynamics of the S2 PE CAN translate into a shorter compression-molding reprocessing time at 160 °C of only 5 min (vs 30 min for the Sn PE CAN) to achieve full recovery of crosslink density. Both PE CANs are melt-extrudable and exhibit full recovery within experimental uncertainty of crosslink density after extrusion. Both PE CANs are self-healable, with a crack fully repaired and the original tensile properties restored after 30 min for the S2 PE CAN or 60 min for the Sn PE CAN at a temperature slightly above the LDPE melting point and without the assistance of external forces.

Progressive endergonic synthesis of Diels-Alder adducts driven by chemical energy.

The overwhelming majority of artificial chemical reaction networks respond to stimuli by relaxing towards an equilibrium state. The opposite response - moving away from equilibrium - can afford the endergonic synthesis of molecules, of which only rare examples have been reported. Here, we report six examples of Diels-Alder adducts accumulated in an endergonic process and use this strategy to realize their stepwise accumulation. Indeed, systems respond to repeated occurrences of the same stimulus by increasing the amount of adduct formed, with the final network distribution depending on the number of stimuli received. Our findings indicate how endergonic processes can contribute to the transition from responsive to adaptive systems.

Hyaluronic Acid/Chondroitin Sulfate-Based Dynamic Thiol-Aldehyde Addition Hydrogel: An Injectable, Self-Healing, On-Demand Dissolution Wound Dressing.

Frequent removal and reapplication of wound dressings can cause mechanical disruption to the healing process and significant physical discomfort for patients. In response to this challenge, a dynamic covalent hydrogel has been developed to advance wound care strategies. This system comprises aldehyde functionalized chondroitin sulfate (CS-CHO) and thiolated hyaluronic acid (HA-SH), with the distinct ability to form in situ via thiol-aldehyde addition and dissolve on-demand via the thiol-hemithioacetal exchange reaction. Although rarely reported, the dynamic covalent reaction of thiol-aldehyde addition holds great promise for the preparation of dynamic hydrogels due to its rapid reaction kinetics and easy reversible dissociation. The thiol-aldehyde addition chemistry provides the hydrogel system with highly desirable characteristics of rapid gelation (within seconds), self-healing, and on-demand dissolution (within 30 min). The mechanical and dissolution properties of the hydrogel can be easily tuned by utilizing CS-CHO materials of different aldehyde functional group contents. The chemical structure, rheology, self-healing, swelling profile, degradation rate, and cell biocompatibility of the hydrogels are characterized. The hydrogel possesses excellent biocompatibility and proves to be significant in promoting cell proliferation in vitro when compared to a commercial hydrogel (HyStem® Cell Culture Scaffold Kit). This study introduces the simple fabrication of a new dynamic hydrogel system that can serve as an ideal platform for biomedical applications, particularly in wound care treatments as an on-demand dissolvable wound dressing.

Polymeric Prodrugs using Dynamic Covalent Chemistry for Prolonged Local Anesthesia.

Depot-type drug delivery systems are designed to deliver drugs at an effective rate over an extended period. Minimizing initial "burst" can also be important, especially with drugs causing systemic toxicity. Both goals are challenging with small hydrophilic molecules. The delivery of molecules such as the ultrapotent local anesthetic tetrodotoxin (TTX) exemplifies both challenges. Toxicity can be mitigated by conjugating TTX to polymers with ester bonds, but the slow ester hydrolysis can result in subtherapeutic TTX release. Here, we developed a prodrug strategy, based on dynamic covalent chemistry utilizing a reversible reaction between the diol TTX and phenylboronic acids. These polymeric prodrugs exhibited TTX encapsulation efficiencies exceeding 90 % and the resulting polymeric nanoparticles showed a range of TTX release rates. In vivo injection of the TTX polymeric prodrugs at the sciatic nerve reduced TTX systemic toxicity and produced nerve block lasting 9.7±2.0 h, in comparison to 1.6±0.6 h from free TTX. This approach could also be used to co-deliver the diol dexamethasone, which prolonged nerve block to 21.8±5.1 h. This work emphasized the usefulness of dynamic covalent chemistry for depot-type drug delivery systems with slow and effective drug release kinetics.

Dynamic Covalent Chemistry of Enamine-Ones: Exploring Tunable Reactivity in Vitrimeric Polymers and Covalent Organic Frameworks.

Dynamic covalent chemistry (DCC) has revolutionized the field of polymer science by offering new opportunities for the synthesis, processability, and recyclability of polymers as well as in the development of new materials with interesting properties such as vitrimers and covalent organic frameworks (COFs). Many DCC linkages have been explored for this purpose, but recently, enamine-ones have proven to be promising dynamic linkages because of their facile reversible transamination reactions under thermodynamic control. Their high stability, stimuli-responsive properties, and tunable kinetics make them promising dynamic cross-linkers in network polymers. Given the rapid developments in the field in recent years, this review provides a critical and up-to-date overview of recent developments in enamine-one chemistry, including factors that control their dynamics. The focus of the review will be on the utility of enamine-ones in designing a variety of processable and self-healable polymers with important applications in vitrimers and recyclable closed-loop polymers. The use of enamine-one linkages in crystalline polymers, known as COFs and their applications are also summarized. Finally, we provide an outlook for future developments in this field.

Ultra-Fast Selenol-Yne Click (SYC) Reaction Enables Poly(selenoacetal) Covalent Adaptable Network Formation.

The emergence of covalent adaptable networks (CANs) based on dynamic covalent bonds (DCBs) presents a promising avenue for achieving resource recovery and utilization. In this study, we discovered a dynamic covalent bond called selenacetal, which is obtained through a double click reaction between selenol and activated alkynes. Density functional theory (DFT) calculations demonstrated that the ΔG for the formation of selenoacetals ranges from 12 to 18 kJ mol-1, suggesting its potential for dynamic reversibility. Dynamic exchange experiments involving small molecules and polymers provide substantial evidence supporting the dynamic exchange properties of selenoacetals. By utilizing this highly efficient click reaction, we successfully synthesized dynamic materials based on selenoacetal with remarkable reprocessing capabilities without any catalysts. These materials exhibit chemical recycling under alkaline conditions, wherein selenoacetal (SA) can decompose into active enone selenide (ES) and diselenides. Reintroducing selenol initiates a renewed reaction with the enone selenide, facilitating material recycling and yielding a newly developed dynamic material exhibiting both photo- and thermal responsiveness. The results underscore the potential of selenoacetal polymers in terms of recyclability and selective degradation, making them a valuable addition to conventional covalent adaptable networks.

Functionalization of Supramolecular Polymers by Dynamic Covalent Boroxine Chemistry.

Molecular scaffolds that enable the combinatorial synthesis of new supramolecular building blocks are promising targets for the construction of functional molecular systems. Here, we report a supramolecular scaffold based on boroxine that enables the formation of chiral and ordered 1D supramolecular polymers, which can be easily functionalized for circularly polarized luminescence. The boroxine monomers are quantitatively synthesized in situ, both in bulk and in solution, from boronic acid precursors and cooperatively polymerize into 1D helical aggregates stabilized by threefold hydrogen-bonding and π-π stacking. We then demonstrate amplification of asymmetry in the co-assembly of chiral/achiral monomers and the co-condensation of chiral/achiral precursors in classical and in situ sergeant-and-soldiers experiments, respectively, showing fast boronic acid exchange reactions occurring in the system. Remarkably, co-condensation of pyrene boronic acid with a hydrogen-bonding chiral boronic acid results in chiral pyrene aggregation with circularly polarized excimer emission and g-values in the order of 10-3. Yet, the electron deficiency of boron in boroxine makes them chemically addressable by nucleophiles, but also sensitive to hydrolysis. With this sensitivity in mind, we provide first insights into the prospects offered by boroxine-based supramolecular polymers to make chemically addressable, functional, and adaptive systems.

Biodynamer Nano-Complexes and -Emulsions for Peptide and Protein Drug Delivery.

Background: Therapeutic proteins and peptides offer great advantages compared to traditional synthetic molecular drugs. However, stable protein loading and precise control of protein release pose significant challenges due to the extensive range of physicochemical properties inherent to proteins. The development of a comprehensive protein delivery strategy becomes imperative accounting for the diverse nature of therapeutic proteins. Methods: Biodynamers are amphiphilic proteoid dynamic polymers consisting of amino acid derivatives connected through pH-responsive dynamic covalent chemistry. Taking advantage of the amphiphilic nature of the biodynamers, PNCs and DEs were possible to be prepared and investigated to compare the delivery efficiency in drug loading, stability, and cell uptake. Results: As a result, the optimized PNCs showed 3-fold encapsulation (<90%) and 5-fold loading capacity (30%) compared to DE-NPs. PNCs enhanced the delivery efficiency into the cells but aggregated easily on the cell membrane due to the limited stability. Although DE-NPs were limited in loading capacity compared to PNCs, they exhibit superior adaptability in stability and capacity for delivering a wider range of proteins compared to PNCs. Conclusion: Our study highlights the potential of formulating both PNCs and DE-NPs using the same biodynamers, providing a comparative view on protein delivery efficacy using formulation methods.

Ultrafast Self-Healing Elastomer with Closed-Loop Recyclability.

The loss of function after prolonged periods of use is inevitable for all materials including plastics. Hence, self-healing capabilities are a key development to prolong the service lifetime of materials. One of such self-healing capabilities can be achieved by integrating dynamic bonds such as boronic ester linkages into polymeric materials, however the rate of self-healing in these materials is insufficient and current methods to accelerate it are limited. In this study, we report the rational design, synthesis and characterization of a fluorinated elastomer (FBE15) that utilizes enhanced interaction between polymer chains afforded by strong dipole-dipole interactions from -CF3, which showed a significant increase in binding energy to -7.71 Kcal/mol from -5.51 Kcal/mol, resulting in increased interaction between the boronic ester linkages and improving self-healing capabilities of boronic ester materials, drastically reducing the time required for stress relaxation by 900 %. The bulk elastomer is capable of ultrafast self-healing in a one-click fashion that can happen in mere seconds, which can then be stretched to 150 % of its original length. By utilising the dynamic cross-linking, FBE15 is also capable of both mechanical reprocessing into the same materials and chemical recycling into its starting materials, respectively, further allowing reconstruction of the elastomers that have comparable properties to the original ones at the end of its service lifespan.

Dynamic covalent surfactants and their uses in the development of smart materials.

Dynamic covalent chemistry, which leverages the dynamic nature of reversible covalent bonds controlled by the conditions of reaction equilibrium, has demonstrated great potential in diverse applications related to both the stability of covalent bonds and the possibility of exchanging building blocks, imparting to the systems the possibility of "error checking" and "proof-reading". By incorporating dynamic covalent bonds into surfactant molecular architectures, combinatorial libraries of surfactants with bespoke functionalities can be readily fabricated through a facile strategy, with minimum effort in organic synthesis. Consequently, a multidisciplinary field of research involving the creation and application of dynamic covalent surfactants has recently emerged, which has aroused great attention in surfactant and colloid science, supramolecular chemistry, self-assembly, smart materials, drug delivery, and nanotechnology. This review reports results in this field published over recent years, discusses the possibilities presented by dynamic covalent surfactants and their applications in developing smart self-assembled materials, and outlines some future perspectives.

Isothermal Phase Transitions in Liquid Crystals Driven by Dynamic Covalent Chemistry.

The dynamic nature of calamitic liquid crystals is exploited to perform isothermal phase transitions driven by dynamic covalent chemistry. For this purpose, nematic (N) arrays based on aldehyde 1 were treated with different amines (A-E) in an on-surface process, which resulted in different isothermal phase transitions. These phase transformations were caused by in situ imination reactions and are dependent on the nature of the added amine. Transitions from the N to crystal (1A, 1E), isotropic (1B), and smectic (Sm) (1C, 1D) phases were achieved, while the resulting materials feature thermotropic liquid crystal behavior. A sequential transformation from the N 1 to the Sm 1C and then to the N 1B was achieved by coupling an imination to a transimination processes and adjusting the temperature. All of these processes were well characterized by microscopic, spectroscopic, and X-ray techniques, unlocking not only the constitutional but also the structural aspects of the phase transitions. This work provides new insights into designing constitutionally and structurally adaptable liquid crystal systems, paving the way toward the conception of programable evolutive pathways and adaptive materials.

Kinetic Control of Complexity in Multiple Dynamic Libraries.

Multiple dynamic libraries of compounds are generated when more than one reversible reaction comes into play. Commonly, two or more orthogonal reversible reactions are used, leading to non-communicating dynamic libraries which share no building blocks. Only a few examples of communicating libraries have been reported, and in all those cases, building blocks are reversibly exchanged from one library to the other, constituting an antiparallel dynamic covalent system. Herein we report that communication between two different dynamic libraries through an irreversible process is also possible. Indeed, alkyl amines cancel the dynamic regime on the nucleophilic substitution of tetrazines, generating kinetically inert compounds. Interestingly, such amine can be part of another dynamic library, an imine-amine exchange. Thus, both libraries are interconnected with each other by an irreversible process which leads to kinetically inert structures that contain parts from both libraries, causing a collapse of the complexity. Additionally, a latent irreversible intercommunication could be developed. In such a way, a stable molecular system with specific host-guest and fluorescence properties, could be irreversibly transformed when the right stimulus was applied, triggering the cancellation of the original supramolecular and luminescent properties and the emergence of new ones.

Kinetic Trapping of an Out-of-Equilibrium Dynamic Library of Imines by Changing Solvent.

A well-behaved dynamic library composed of two imines and corresponding amines was subjected to the action of an activated carboxylic acid (ACA), whose decarboxylation is known to be base promoted, in different solvents, namely CD2Cl2, CD3CN, and mixtures of them. Two non-equilibrium systems are consequently obtained: i) a dissipative (CD2Cl2) and ii) an out-of-equilibrium (CD3CN) dynamic library whose composition goes back to equilibrium after a given time. In the former case, the library is fully coupled with the decarboxylation of the ACA, while in the latter, an energy ratchet operates. In the mixed solvents, the library exhibits a mediated behavior. Interestingly, in the presence of an excess of added ACA, the different behavior of the imine library in the two solvents is expected to manifest only when the excess acid is consumed.

Aromatic-Carbonyl Interactions as an Emerging Type of Non-Covalent Interactions.

Aromatic-carbonyl (Ar···C═O) interactions, attractive interactions between the arene plane and the carbon atom of carbonyl, are in the infancy as one type of new supramolecular bonding forces. Here the study and functionalization of aromatic-carbonyl interactions in solution is reported. A combination of aromatic-carbonyl interactions and dynamic covalent chemistry provided a versatile avenue. The stabilizing role and mechanism of arene-aldehyde/imine interactions are elucidated through crystal structures, NMR studies, and computational evidence. The movement of imine exchange equilibria further allowed the quantification of the interplay between arene-aldehyde/imine interactions and dynamic imine chemistry, with solvent effects offering another handle and matching the electrostatic feature of the interactions. Moreover, arene-aldehyde/imine interactions enabled the reversal of kinetic and thermodynamic selectivity and sorting of dynamic covalent libraries. To show the functional utility diverse modulation of fluorescence signals is realized with arene-aldehyde/imine interactions. The results should find applications in many aspects, including molecular recognition, assemblies, catalysis, and intelligent materials.