Reversible Imine Research Articles

Scalable Fabrication of Crystalline COF Membranes from Amorphous Polymeric Membranes

AbstractCovalent organic framework (COF) membranes hold potential for widespread applicability, but scalable fabrication is challenging. Here, we demonstrate the disorder‐to‐order transformation from amorphous polymeric membrane to crystalline COF membrane via monomer exchange. Solution processing is used to prepare amorphous membrane and the replacing monomer is selected based on the chemical and thermodynamical stability of the final framework. Reversible imine bonds allow the extraneous monomers to replace the pristine monomers within amorphous membrane, driving the transformation from disordered network to ordered framework. Incorporation of intramolecular hydrogen bonds enables the crystalline COF to imprint the amorphous membrane morphology. The COF membranes harvest proton conductivity up to 0.53 S cm−1 at 80 °C. Our strategy bridges amorphous polymeric and crystalline COF membranes for large‐scale fabrication of COF membranes and affords guidance on materials processing.

A high ion-conducting, self-healing and nonflammable polymer electrolyte with dynamic imine bonds for dendrite-free lithium metal batteries

High-performance polymer electrolytes with the capability to self-heal mechanical damage can effectively enhance reliability, safety and service life of the batteries. Herein, a novel polymer electrolyte (PBPE) with excellent self-healing capacity and high ionic conductivity was developed. PBPE with polymer networks cross-linked by highly reversible imine bonds was fabricated by the Schiff-base reaction between poly(ethylene glycol) diamine and benzene-1,3,5-tricarbaldehyde. PBPE can quickly repair the cut damage spontaneously within 1 h at room temperature and recover its mechanical strength and electrochemical properties. The healed PBPE exhibits almost the same ionic conductivities and mechanical properties as the original PBPE shows. PBPE possesses the highest ionic conductivity (4.79 × 10−3 S cm−1 at 30 °C) among the self-healing polymer electrolytes. PBPE can promote the generation of LiF-rich SEI and effectively suppress dendrite growth on Li metal anodes. LiFePO4 (LFP) cells assembled with PBPE exhibit excellent rate capability (discharge capacity of 118.2 mAh g−1 at 5 C rate) and good cycling performance (capacity retention of 97.8% over 125 cycles). More importantly, the healed PBPE can completely recover its properties from damage in LFP cells and enable the cells to possess cycle performance identical to original PBPE.

Biobased dynamic hydrogels by reversible imine bonding for controlled release of thymopentin

Thymopentin (TP5) is widely used in the treatment of autoimmune diseases, but the short in vivo half-life of TP5 strongly restricts its clinical applications. A series of blank and TP5 loaded hydrogels were synthesized via reversible dual imine bonding by mixing water soluble O-carboxymethyl chitosan (CMCS) with a dynamer (Dy) prepared from Jeffamine and benzene-1,3,5-tricarbaldehyde. TP5 release from hydrogels was studied at 37 °C under in vitro conditions. The molar mass of CMCS, drug loading conditions and drug content were varied to elucidate their effects on hydrogel properties and drug release behaviors. Density functional theory was applied to theoretically confirm the chemical connections between TP5 or CMCS with Dy. All hydrogels exhibited interpenetrating porous architecture with average pore size from 59 to 83 μm, and pH-sensitive swelling up to 10,000% at pH 8. TP5 encapsulation affected the rheological properties of hydrogels as TP5 was partially attached to the network via imine bonding. Higher TP5 loading led to higher release rates. Faster release was observed at pH 5.5 than at pH 7.4 due to lower stability of imine bonds in acidic media. Fitting of release data using Higuchi model showed that initial TP5 release was essentially diffusion controlled. All these findings proved that the dynamic hydrogels are promising carriers for controlled delivery of hydrophilic drugs, and shed new light on the design of drug release systems by both physical mixing and reversible covalent bonding.

Improved processability and high fire safety of wood plastic composites via assembling reversible imine crosslinking network

Wood plastic composites (WPCs) are considered as the next-generation construction and decorative materials because of their mechanical stability and eco-friendliness. The manufacture of high-performance multifunctional WPCs with improved processability is, however, significantly challenging. In this study, we designed a temperature-responsive flame-retardant WPC consisting of a reversible imine crosslinking network. The reversibility of the imine crosslinking network allowed the formation of a loosely crosslinked network during the melt‐processing of the material (170 °C, 100 Hz), resulting in improved processability, with a low melt viscosity of 1.7 kPa‧s and a short stress relaxation time of 1.16 min. Meanwhile, upon cooling, the imine crosslinking network is reconstructed, imparting the flame-retardant WPC with outstanding stress/strain transfer and mechanical properties. More importantly, the unique imine-based crosslinking network accelerates the catalytic charring effect of the WPC, leading to remarkable improvements in terms of the restriction of heat transfer and inhibition of flammable gases, easy achievement of the UL-94 V-0 rating, and effective inhibition of the peak heat release rate (57.3%) and total smoke production (28.5%). Considering the simultaneous improvements in the processability, mechanical properties, and fire resistance of the WPC, this strategy of reversible crosslinking network may open up new opportunities in the development of innovative fibre-reinforced composites.

Formation of Organogel In Situ Based on a Dynamic Imine Bond

A simple approach for creating organogel in situ through formation of a reversible imine bond known as a dynamic covalent bond is described. As the condensations of the glutamate-based amine compou...

Ion-conductive self-healing polymer network based on reversible imine bonding for Si electrodes

Si electrodes have attracted considerable attention as materials for next-generation lithium-ion batteries owing to their high theoretical capacity and natural abundance. However, many limitations must be overcome before Si electrodes can be commercialized, particularly the steady degradation in battery performance caused by large volume changes in the active material. Here, an ion-conductive self-healing polymer binder is developed for high-performance silicon electrodes. Glycol chitosan with dialdehyde-terminated polyethylene glycol as a macro-crosslinker is used in a simple process to form the crosslinked polymer network based on imine double bonds, which further improves ion conductivity. Strong and reversible imine double bonds in the crosslinked polymer provide self-healing ability. Si electrodes using the developed polymer network results in an initial Coulombic efficiency of 82.2%, a discharge capacity of 2141 mAh g−1 after 150 cycles, and a reversible capacity of 2700 mAh g−1 at a current density of 3C. These outstanding electrochemical performances demonstrate that the self-healable network and ion-conductive functionality of the developed polymeric binder significantly improves the operation of Si electrodes.

Double-Network Heparin Dynamic Hydrogels: Dynagels as Anti-bacterial 3D Cell Culture Scaffolds.

Double cross-linked dynamic hydrogels, dynagels, have been prepared through reversible imine bonds and supramolecular interactions, which showed good pH responsiveness, injectability, self-healing property and biocompatibility. With the further encapsulation of heparin, the obtained hydrogels exhibited good anti-bacterial activity and promotion effects for 3D cell culture.

A Theoretical Model for Release Dynamics of an Antifungal Agent Covalently Bonded to the Chitosan

The aim of the study was to create a mathematical model useful for monitoring the release of bioactive aldehydes covalently bonded to the chitosan by reversible imine linkage, considered as a polymer–drug system. For this purpose, two hydrogels were prepared by the acid condensation reaction of chitosan with the antifungal 2-formyl-phenyl-boronic acid and their particularities; influencing the release of the antifungal aldehyde by shifting the imination equilibrium to the reagents was considered, i.e., the supramolecular nature of the hydrogels was highlighted by polarized light microscopy, while scanning electron microscopy showed their microporous morphology. Furthermore, the in vitro fungicidal activity was investigated on two fungal strains and the in vitro release curves of the antifungal aldehyde triggered by the pH stimulus were drawn. The theoretical model was developed starting from the hypothesis that the imine-chitosan system, both structurally and functionally, can be assimilated, from a mathematical point of view, with a multifractal object, and its dynamics were analyzed in the framework of the Scale Relativity Theory. Thus, through Riccati-type gauges, two synchronous dynamics, one in the scale space, associated with the fungicidal activity, and the other in the usual space, associated with the antifungal aldehyde release, become operational. Their synchronicity, reducible to the isomorphism of two SL(2R)-type groups, implies, by means of its joint invariant functions, bioactive aldehyde compound release dynamics in the form of “kink–antikink pairs” dynamics of a multifractal type. Finally, the theoretical model was validated through the experimental data.

A Hexapodal Capsule for the Recognition of Anions.

We herein describe the preparation, characterization, and recognition characteristics of novel hexapodal capsule 1 composed of two benzenes joined by six hydrogen bonding (HB) groups to encircle space. This barrel-shaped host was obtained by reversible imine condensation of hexakis-aldehyde 2 and hexakis-amine 3 in the presence of oxyanions or halides acting as templates. Fascinatingly, capsule 1 includes 18 HB donating (Csp2-H and N-H) and 12 HB accepting groups (C═O and C═N) surrounding a binding pocket (78 Å3). In this regard, the complexation of fluoride, chloride, carbonate, sulfate, and hydrogen phosphate was probed by NMR spectroscopy (DMSO) and X-ray diffraction analysis to disclose the adaptive nature of 1 undergoing an adjustment of its conformation to complement each anionic guest. Furthermore, the rate by which encapsulated chloride was substituted by sulfate or hydrogen phosphate was slow (>7 days) while the stability of [SO4⊂1]2- was greatest in the series with Ka > 107 M-1 in highly competitive DMSO. With facile access to 1, the stage is set to probe this modular, polyvalent, and novel host to further improve the extraction of tetrahedral oxyanions from waste and the environment or control their chemistry in living systems.

Imine as a linchpin approach for meta-C\u2013H functionalization

Despite the widespread applications of C–H functionalization, controlling site selectivity remains a significant challenge. Covalently attached directing groups (DGs) served as ancillary ligands to ensure ortho-, meta- and para-C–H functionalization over the last two decades. These covalently linked DGs necessitate two extra steps for a single C–H functionalization: introduction of DG prior to C–H activation and removal of DG post-functionalization. Here we report a temporary directing group (TDG) for meta-C–H functionalization via reversible imine formation. By overruling facile ortho-C–H bond activation by imine-N atom, a suitably designed pyrimidine-based TDG successfully delivered selective meta-C–C bond formation. Application of this temporary directing group strategy for streamlining the synthesis of complex organic molecules without any necessary pre-functionalization at the meta position has been explored.

Self‐Healable and Transparent Elastomers Based on Dual Reversible Networks

AbstractThe fabrication of silicon elastomer with good mechanical properties and self‐healing ability is still a challenge. Herein, a facile method to fabricate a transparent healable double‐network (DN) elastomer by introducing polyborosiloxane (PBS) into self‐healing polydimethylsiloxane (HPDMS) networks is demonstrated. Tensile testing shows that the mechanical properties of PBS and HPDMS are both improved. The DN elastomer shows high transparency across the visible spectrum with an average transmittance of 75%. Because of the breakage and reconstruction of reversible imine bonds and BO dative bonds, the elastomer exhibits excellent self‐healing properties with a maximum healing efficiency of 98% at room temperature. Meanwhile, the elastomer can be repeatedly reprocessed with a recovery of 85% of virgin mechanical strengths. This work provides a new insight to design transparent, self‐healing silicone elastomers with a simple and eco‐friendly method.

Ion-Conductive Self-Healing Polymer Network Based on Reversible Imine Bonding for Si Electrodes

Lithium-ion battery (LIB), one of the most promising energy storage devices due to higher energy density than other secondary batteries, have continuously developed, and their market has been growing steadily. Currently, the efforts have been produced for using LIB in electric vehicles (EVs) and energy storage systems (ESS) storage beyond small portable devices. Higher energy density and faster charge/discharge rate are required for the practical application of LIB to these large-scale energy devices, and new materials to meet these requirements are hence necessary. Silicon has been reported as an anode material for the next-generation LIB due to its outstanding theoretical capacity and natural abundance. However, there are many limitations in practical application due to the continuous deterioration of the battery performances by enormous volume change of this active material. These problems are mainly originated from the structural deterioration of the electrode causing loss of conductive pathway and the active area for the electrode. Hence, many approaches to securing the structural stability of the Si electrode through various methods, including polymeric binders with good adhesion with Si were developed.In this study, self-healable polymeric binder was designed through a crosslinking polymer network using a reversible imine chemistry. Additionally, the ion conductive polymer was introduced to improve ion conductivities of this self-healable binder. The synthesis, characterization and the electrical properties of the proposed polymeric binder for high-performance silicon electrodes will be discussed in detail.

Dynameric Collagen Self-Healing Membranes with High Mechanical Strength for Effective Cell Growth Applications.

The fabrication of biocompatible adaptive materials with high stiffness and self-healing properties for medical applications is a challenging endeavor. Collagen is a major extracellular matrix component acting as a substrate for cell adhesion and migration. Dynamers are constitutional polymers whose monomeric components are linked through reversible bonds, able to modify their constitution through reversible exchange of their components. In the current work, we demonstrate that the rational combination of collagen and dynameric networks connected with reversible covalent imine bonds is a very important and previously unreported strategy to provide biocompatible membranes with self-healing ability and excellent mechanical strength. The key challenge in the construction of such membranes is the required adaptive interaction between collagen chains and the dynamic cross-linkers, preventing the formation of defects. For example, by varying structure and molecular lengths of the dynamers, the tensile strength of the dynameric membranes reach over 80 MPa, more than 400 % higher than that observed for the reference collagen membrane, and the highest value for break strain found, was 19 %. The self-healing properties were observed when reconnecting two membrane pieces or even from crushed status of the membranes. Moreover, both MTT assay and confocal laser scanning microscopy method demonstrated the good biocompatibility of the collagen membranes, leaving more than 90 % viability for NIH 3T3 cells after 24 h co-culture.

A Green Strategy for Developing a Self-Healing Gelatin Resistive Memory Device

Self-healing gelatin resistive random access memory (RRAM), namely, SHG RRAM, is utilized by incorporating the reversible imine bond as self-healing points into the gelatin. With the reformation of...

Diastereodivergent chiral aldehyde catalysis for asymmetric 1,6-conjugated addition and Mannich reactions

Chiral aldehyde catalysis is a burgeoning strategy for the catalytic asymmetric α-functionalization of aminomethyl compounds. However, the reaction types are limited and to date include no examples of stereodivergent catalysis. In this work, we disclose two chiral aldehyde-catalysed diastereodivergent reactions: a 1,6-conjugate addition of amino acids to para-quinone methides and a bio-inspired Mannich reaction of pyridinylmethanamines and imines. Both the syn- and anti-products of these two reactions can be obtained in moderate to high yields, diastereo- and enantioselectivities. Four potential reaction models produced by DFT calculations are proposed to explain the observed stereoselective control. Our work shows that chiral aldehyde catalysis based on a reversible imine formation principle is applicable for the α-functionalization of both amino acids and aryl methylamines, and holds potential to promote a range of asymmetric transformations diastereoselectively.

Poly-Epsilon-Lysine Hydrogels with Dynamic Crosslinking Facilitates Cell Proliferation.

The extracellular matrix (ECM) is a three-dimensional network within which fundamental cell processes such as cell attachment, proliferation, and differentiation occur driven by its inherent biological and structural cues. Hydrogels have been used as biomaterials as they possess many of the ECM characteristics that control cellular processes. However, the permanent crosslinking often found in hydrogels fails to recapitulate the dynamic nature of the natural ECM. This not only hinders natural cellular migration but must also limit cellular expansion and growth. Moreover, there is an increased interest in the use of new biopolymers to create biomimetic materials that can be used for biomedical applications. Here we report on the natural polymer poly-ε-lysine in forming dynamic hydrogels via reversible imine bond formation, with cell attachment promoted by arginine-glycine-aspartic acid (RGD) incorporation. Together, the mechanical properties and cell behavior of the dynamic hydrogels with low poly-ε-lysine quantities indicated good cell viability and high metabolic activity.

Skin-Inspired Electret Nanogenerator with Self-Healing Abilities

Summary Flexible electronics have been rapidly developed in recent years. However, there are still some concerns about the degradation and short life caused by mechanical damage during their operation. Here, we present a self-healing electret nanogenerator (SENG) that can restore its structure and function after mechanical damage, benefiting from a healable polymer with reversible imine bond and the instantaneous healing ability of liquid metal. After destructive testing comprising four cutting-healing cycles the output current of the SENG autonomously recovers to the original values before damage. Finally, as a practical device, the SENG is demonstrated to work in both energy harvesting and human physiology-monitoring applications. This research offers a feasible route for improving reliability and lifetime of electret generators or other flexible electronics that suffer from mechanical damage.

Dynamic Covalent Self-Sorting and Kinetic Switching Processes in Two Cyclic Orders: Macrocycles and Macrobicyclic Cages.

Dynamic covalent component self-sorting processes have been investigated for constituents of different cyclic orders, macrocycles and macrobicyclic cages based on multiple reversible imine formation. The progressive assembly of the final structures from dialdehyde and polyamine components involved the generation of kinetic products and mixtures of intermediates which underwent component selection and self-correction to generate the final thermodynamic constituents. Importantly, constitutional dynamic networks (CDNs) of macrocycles and macrobicyclic cages were set up either from separately prepared constituents or by in situ assembly from their components. Over time, these CDNs underwent conversion from a kinetically trapped out-of-equilibrium distribution of constituents to the thermodynamically self-sorted one through component exchange in different dimensional orders.

Unveiling the Local Structure of Palladium Loaded into Imine-Linked Layered Covalent Organic Frameworks for Cross-Coupling Catalysis.

Layered covalent organic frameworks (2D-COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen-enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine-linked 2D-COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine-linked 2D-COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki-Miyaura cross-coupling reactions.

Unveiling the Local Structure of Palladium Loaded into Imine‐Linked Layered Covalent Organic Frameworks for Cross‐Coupling Catalysis

AbstractLayered covalent organic frameworks (2D‐COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen‐enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine‐linked 2D‐COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine‐linked 2D‐COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki–Miyaura cross‐coupling reactions.