The furan/maleimide Diels–Alder reaction: A versatile click–unclick tool in macromolecular synthesis

Highly flexible yet strain-insensitive conjugated polymer

Imparting excellent electrical properties, mechanical robustness, suppleness, conduction stability during deformation, and self-healing to intrinsic conducting polymers is a challenging endeavor. The reversibly interlocked macromolecular networks (RILNs) approach is utilized to tackle this problem. Specifically, poly(3,4-ethylenedioxythiophene) (PEDOT) is mixed with flexible polysulfonic acid networks crosslinked by reversible Diels-Alder bonds, while rigid polyaniline networks crosslinked by reversible Schiff base bonds act as molecular staples. Owing to the joint actions of the doping effect of polyaniline on PEDOT, the specific interlocking architecture and synergy between the component materials, the electrical conductivity (59.3-980.5 S cm-1), tensile strength (8.4-81.6 MPa) and elongation at break (44.5-411.0%) of the resultant PEDOT/RILNs films is significantly tunable according to different usage scenarios by adjusting the PEDOT content from 1.48 to 22.24 wt%. More importantly, the electrical resistance of PEDOT/RILNs remains constant during not only a single large extension and deflection but also repeated stretching (up to 1500 cycles) and bending (up to 106 cycles). The built-in reversible covalent bonds enable the PEDOT/RILNs to autonomously restore damaged mechanical and electrical performance. These record-breaking results and the demonstration of self-powered sensor made of PEDOT/RILNs suggest that the proposed approach successfully satisfies various conflicting requirements of flexible electronics regarding the properties of conducting polymers.

Selective uranyl ion-imprinting with clickable amidoxime-functionalized pullulan

Manufacturing a highly effective sorbent for removing UO22+ ions from aqueous effluents is vital for safeguarding the environment and recovering valuable resources. This research presents an innovative strategy employing adsorbents derived from pullulan, specifically tailored with furfuryl-amidoxime (FAO), to improve their affinity for UO22+ ions. The formation of a UO22+ ion-imprinted sorbent (U-II-P) was achieved by crosslinking the UO22+/FAO-modified pullulan (FAO-P) complex with bis(maleimido)ethane (BME) via click Diels-Alder (DA) cyclization, enhancing its attraction and specificity for UO22+ ions. Detailed characterization of the synthesis was performed using NMR and FTIR spectroscopy, and the sorbent's external textures were analyzed using scanning electron microscopy (SEM). The U-II-P sorbent showcased outstanding preference for UO22+ over other metallic ions, with the most efficient adsorption occurring at pH 5. It exhibited a significant adsorption capacity of 262 mg/g, closely aligning with the predictions of the Langmuir adsorption model and obeying pseudo-second-order kinetic behavior. This investigation underlines the effectiveness of FAO-P as a specialized solution for UO22+ ion extraction from wastewater, positioning it as a viable option for the remediation of heavy metals.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Synthesis of furan-modified cationic cellulose for stereo-specific imprinting and separation of S-indacrinone via Diels-Alder reaction

This study introduces a novel approach for the separation of indacrinone (IC) enantiomers, crucial in treating edema, hypertension, and hyperuricemia. A cationic biopolymer from furan-2-ylmethylhydrazine-cellulose (FUH-CE), derived from cyanoethyl cellulose (CEC), serving as a substrate in molecular imprinting. A key innovation is the use of the Diels-Alder reaction for efficient cross-linking with bis(maleimido)ethane (BME). This chemical strategy resulted in molecularly imprinted microparticles with high selectivity for the S-IC enantiomer, which can be eluted by adjusting the solution's pH. Extensive characterization confirmed the chemical modifications and selective binding efficacy of these biopolymers. Utilizing separation columns, our method achieved an impressive chiral resolution of (±)-IC, with an enantiomeric excess (ee) of 95 % for R-IC during the loading phase and 97 % for S-IC during elution. Under optimized conditions, the biopolymer demonstrated a maximum binding capacity of 131 mg/g at pH 6. This advanced approach represents a significant advancement in chiral separation technology, offering a robust and efficient technique for the selective isolation of enantiomers. This method not only enhances potential targeted therapeutic applications but also provides a scalable solution for industrial chiral separations.

IPN based hydrogels for in-vivo wound dressings; catalytic wound healing dynamics and isothermal adsorption models

Interpenetrating network (IPN) methacrylated chitosan or methacrylated flaxseed gum based hydrogels have been utilized to make outstanding in-vivo wound dressings. The photopolymerization process was accomplished in presence of Eosin-Y photoinitiator with average exposure time of 13-14 s for gelation. Spectroscopic structural investigations of 1H NMR. ATR-FTIR, TGA, and AFM techniques were used. In-vitro hemolysis test provided evidence of no cytotoxicity in both hydrogels observed. The in-vivo wound dressings were monitored for five mice coated with each hydrogel and another uncoated five mice for control (self-healing). All measurements were performed in quintuplicate (n = 5) and expressed as mean ± SD values. In wound healing dynamics, our data confirmed that wound healing pass through two stages; hemostasis and inflammation for stage 1, and proliferation and remodeling for stage 2. It also provided evidence of 1st order kinetics with descending rate of healing. Consequently, catalytic role of hydrogels in wound healing was checked via half-life (δ) and negative change of activation energy values (ΔEa). Various isothermal adsorption models demonstrated spontaneous and high binding affinities of hydrogels. It also confirmed the two-stage healing process in presence of hydrogels. Conclusively, the outstanding properties of the two hydrogels suggest their potential applications in treating venous ulcers and diabetic wound healing dressings.

Chemical and linguistic considerations for encoding Chinese characters: an embodiment using chain-end degradable sequence-defined oligourethanes created by consecutive solid phase click chemistry

Sequence-defined polymers (SDPs) are currently being investigated for use as information storage media. As the number of monomers in the SDPs increases, with a corresponding increase in mathematical base, the use of tandem-MS for de novo sequencing becomes more challenging. In contrast, chain-end degradation routines are truly de novo, potentially allowing very large mathematical bases for encoding. While alphabetic scripts have a few dozen symbols, logographic scripts, such as Chinese, can have several thousand symbols. Using a new in situ consecutive click reaction approach on an oligourethane backbone for writing, and a previously reported chain-end degradation routine for reading, we encoded/decoded a confucius proverb written in Chinese characters using two encoding schemes: Unicode and Zhèng Mă. Unicode is an internationally standardized arbitrary string of hexadecimal (base-16) symbols which efficiently encodes uniquely identifiable symbols but requires complete fidelity of transmission, or context-based inferential strategies to be interpreted. The Zhèng Mă approach encodes with a base-26 system using the visual characteristics and internal composition of Chinese characters themselves, which leads to greater ambiguity of encoded strings, but more robust retrievability of information from partial or corrupted encodings. The application of information-encoded oligourethanes to two different encoding systems allowed us to establish their flexibility and versatility for data storage. We found the oligourethanes immensely adaptable to both encoding schemes for Chinese characters, and we highlight the expected tradeoff between the efficiency and uniqueness of Unicode encoding on the one hand, and the fidelity to a scripts' particular visual characteristics on the other.

Diels-Alder clickable furan-thiosemicarbazide cellulose for selective ruthenium (III) imprinting

Developing an efficient adsorbent for Ru3+ ions in wastewater is crucial for both environmental protection and resource recovery. This study introduces a novel approach using cellulose-based adsorbents, specifically modified with furan-thiosemicarbazide (FTC), to enhance their selectivity for Ru3+ ions. By cross-linking the Ru3+/FTC-modified cellulose (FTC-CE) complex with a bis(maleimido)ethane (BME) cross-linker, we created a Ru3+ ion-imprinted sorbent (Ru-II-CE) that exhibits a strong affinity and selectivity for Ru3+ ions. The synthesis process was thoroughly characterized using NMR and FTIR spectroscopy, while the surface morphology of the sorbent particles was examined with scanning electron microscopy. The Ru-II-CE sorbent demonstrated exceptional selectivity for Ru3+ among competing metal cations, achieving optimal adsorption at a pH of 5. Its adsorption capacity was notably high at 215 mg/g, fitting well with the Langmuir isotherm model, and it followed pseudo-second-order kinetics. This study highlights the potential of FTC-CE for targeted Ru3+ removal from wastewater, offering a promising solution for heavy metal decontamination.

Access to Advanced Functional Materials through Postmodification of Biomimetic Assemblies via Click Chemistry

The design, synthesis, and fabrication of functional nanomaterials with specific properties remain a long-standing goal for many scientific fields. The self-assembly of sequence-defined biomimetic synthetic polymers presents a fundamental strategy to explore the chemical space beyond biological systems to create advanced nanomaterials. Moreover, subsequent chemical modification of existing nanostructures is a unique approach for accessing increasingly complex nanostructures and introducing functionalities. Of these modifications, covalent conjugation chemistries, such as the click reactions, have been the cornerstone for chemists and materials scientists. Herein, we highlight some recent advances that have successfully employed click chemistries for the postmodification of assembled one-dimensional (1D) and two-dimensional (2D) nanostructures to achieve applications in molecular recognition, mineralization, and optoelectronics. Specifically, biomimetic nanomaterials assembled from sequence-defined macromolecules such as peptides and peptoids are described.

Reversible Postsynthetic Modification in a Metal-Organic Framework

Postsynthetic modification (PSM) of metal-organic frameworks (MOFs) provides access to functional materials and advanced porous solid engineering. Herein, we report the reversible PSM of a multivariate isoreticular MOF by applying dynamic furan-maleimide Diels-Alder (DA) chemistry. The key step involves incorporating a furan group into the MOF via "click" PSM, which can then undergo repeated cycles of modification and de-modification with maleimides. The structural integrity, crystallinity, and porosity of the furan-appended MOF remained intact even after three consecutive PSM/de-modification cycles using three different functionalized maleimides.

Kinetic Study of the Diels-Alder Reaction between Maleimide and Furan-Containing Polystyrene Using Infrared Spectroscopy

The Diels-Alder (D-A) reaction between furan and maleimide is a thermally reversible reaction that has become a vital chemical technique for designing polymer structures and functions. The kinetics of this reaction, particularly in polymer bulk states, have significant practical implications. In this study, we investigated the feasibility of utilizing infrared spectroscopy to measure the D-A reaction kinetics in bulk-state polymer. Specifically, we synthesized furan-functionalized polystyrene and added a maleimide small-molecule compound to form a D-A adduct. The intensity of the characteristic absorption peak of the D-A adduct was quantitatively measured by infrared spectroscopy, and the dependence of conversion of the D-A reaction on time was obtained at different temperatures. Subsequently, the D-A reaction apparent kinetic coefficient kapp and the Arrhenius activation energy Ea,D-A were calculated. These results were compared with those determined from 1H-NMR in the polymer solution states.

Synthesis and characterization of gellan gum-based hydrogels for drug delivery applications

In this study, gellan gum (Gel) derivatives were allowed to interact via aqueous Diels-Alder chemistry without the need for initiators, producing a crosslinked hydrogel network that exhibited good potential as a drug carrier using tramadol as a drug model. Hydrogel conjugation was achieved by the synthesis of a maleimide and furan-functionalized Gel, and the pre- and post-gelation chemical structure of the resulting hydrogel precursors was fully investigated. Potential uses of the developed hydrogel in the pharmaceutical industry were also evaluated by looking at its gelation duration, temperature, morphologies, swelling, biodegradation, and mechanical characteristics. The Gel-FM hydrogels were safe, showed good antimicrobial activity, and had a low storage modulus, which meant that they could be used in many biochemical fields. The encapsulation and release of tramadol from the hydrogel system in phosphate-buffered saline (PBS) at 37 °C were investigated under acidic and slightly alkaline conditions, replicating the stomach and intestinal tracts, respectively. The in-vitro release profile showed promising results for drug encapsulation, revealing that the drug could safely be well-encapsulated in acidic stomach environments and released more quickly in slightly alkaline intestinal environments. This implies that the hydrogels produced could work well as polymers for specifically delivering medication to the colon.

Smart carbon-based sensors for the detection of non-coding RNAs associated with exposure to micro(nano)plastics: an artificial intelligence perspective

Micro(nano)plastics (MNPs) are pervasive environmental pollutants that individuals eventually consume. Despite this, little is known about MNP's impact on public health. In this article, we assess the evidence for potentially harmful consequences of MNPs in the human body, concentrating on molecular toxicity and exposure routes. Since MNPs are present in various consumer products, foodstuffs, and the air we breathe, exposure can occur through ingestion, inhalation, and skin contact. MNPs exposure can cause mitochondrial oxidative stress, inflammatory lesions, and epigenetic modifications, releasing specific non-coding RNAs in circulation, which can be detected to diagnose non-communicable diseases. This article examines the most fascinating smart carbon-based nanobiosensors for detecting circulating non-coding RNAs (lncRNAs and microRNAs). Carbon-based smart nanomaterials offer many advantages over traditional methods, such as ease of use, sensitivity, specificity, and efficiency, for capturing non-coding RNAs. In particular, the synthetic methods, conjugation chemistries, doping, and in silico approach for the characterization of synthesized carbon nanodots and their adaptability to identify and measure non-coding RNAs associated with MNPs exposure is discussed. Furthermore, the article provides insights into the use of artificial intelligence tools for designing smart carbon nanomaterials.

Polyurethane Adhesives with Chemically Debondable Properties via Diels-Alder Bonds

Covalent adaptable networks (CANs) represent a pioneering advance in polymer science, offering unprecedented versatility in materials design. Unlike conventional adhesives with irreversible bonds, CAN-based polyurethane adhesives have the unique ability to undergo chemical restructuring through reversible bonds. One of the strategies for incorporating these types of reactions in polyurethanes is by functionalisation with Diels-Alder (DA) adducts. By taking advantage of the reversible nature of the DA chemistry, the adhesive undergoes controlled crosslinking and decrosslinking processes, allowing for precise modulation of bond strength. This adaptability is critical in applications requiring reworkability or recyclability, as it allows for easy disassembly and reassembly of bonded components without compromising the integrity of the material. This study focuses on the sustainable synthesis and characterisation of a solvent-based polyurethane adhesive, obtained by functionalising a polyurethane prepolymer with DA diene and dienophiles. The characterisation of the adhesives was carried out using different experimental techniques: nuclear magnetic resonance spectroscopy (NMR), Brookfield viscosity, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and T-peel strength testing of leather/adhesive/rubber joints to determine the adhesive properties, both before and after the application of external stimuli. The conversion of both the DA and retro-Diels-Alder (r-DA) reactions was confirmed by 1H-NMR. The adhesive properties were not altered by the functionalisation of the adhesive prepolymer, showing similar thermal resistance and good rheological and adhesive properties, even exceeding the most demanding technical requirements for upper-to-sole joints in footwear. After the application of an external thermal stimuli, the bonded materials separated without difficulty and without damage, thus facilitating their separation, recovery and recycling.

Investigation of the enhanced antitumour potency of STING agonist after conjugation to polymer nanoparticles

Intravenously administered cyclic dinucleotides and other STING agonists are hampered by low cellular uptake and poor circulatory half-life. Here we report the covalent conjugation of cyclic dinucleotides to poly(β-amino ester) nanoparticles through a cathepsin-sensitive linker. This is shown to increase stability and loading, thereby expanding the therapeutic window in multiple syngeneic tumour models, enabling the study of how the long-term fate of the nanoparticles affects the immune response. In a melanoma mouse model, primary tumour clearance depends on the STING signalling by host cells-rather than cancer cells-and immune memory depends on the spleen. The cancer cells act as a depot for the nanoparticles, releasing them over time to activate nearby immune cells to control tumour growth. Collectively, this work highlights the importance of nanoparticle structure and nano-biointeractions in controlling immunotherapy efficacy.

Evidence through Thermal Analysis of Retro Diels-Alder Reaction in Model Networks Based on Anthracene Modified Polyester Resins

The present work is focused on polyester resins obtained from the diglycidyl ether of bisphenol A and anthracene modified 5-maleimidoisophthalic acid. Because the maleimide-anthracene Diels-Alder (DA) adduct is stable at high temperatures, it is considered a good option for high performance polymers. However, the information related to the retroDA reaction for this type of adduct is sometimes incoherent. A detailed thermal study (conventional TGA, HiRes TGA, MTGA, DSC, MDSC) was performed in order to establish whether the rDA reaction can be revealed for this type of anthracene modified polyester resins. The TGA method confirmed the cleavage of the anthracene-maleimide DA adduct, while the DSC demonstrated the presence of anthracene in the system. At high temperatures, unprotected maleimide homopolymerizes and/or reacts with allyl groups according to the -ene reaction. Therefore, the thermal DA reaction is not displayed anymore upon the subsequent cooling, and the glass transition region is registered at a higher temperature range during the second heating. The use of sample-controlled thermal analysis (HiRes TGA) and MTGA improved the TGA result; however, it was not possible to separate the very complex degradation processes that are interconnected.

Dynamic Properties of Di(cyclopentadienecarboxylic Acid) Dimethyl Esters

Di(cyclopentadienecarboxylic acid) dimethyl ester (DCPDME) is a potential dynamic covalent system. When such molecules are used as dynamic crosslinkers in polymers, understanding the reversibility of cyclopentadiene dimerization is crucial to determine optimal melt processing conditions. To this end, we synthesized DCPDME, which consists of three regioisomers with different physicochemical properties, which were investigated by isolating them and further characterizing them using 1H NMR, FTIR and DSC. There have been many attempts to improve the synthesis process to increase the reaction yield and purity of isomer 3, and this goal remains a challenge today. In this work, we show that pure isomers 1 and 2 irreversibly convert to the more stable DCPDME isomer 3 at temperatures between 120 and 140 °C in N2. This shows that isolation of the pure isomer 3 from the DCPDME isomer mixture is not necessary. The DCPDME isomer 3 is reversibly cleaved to the monomeric cyclopentadienecarboxylic acid methyl ester (CPME), as confirmed with GC-MS and the resulting mass spectrum. The conversion of DCPDME isomers 1 and 2 to isomer 3 was confirmed by heating the synthesized mixture of DCPDME isomers at 135 °C for 5 min in N2, producing an almost pure isomer 3 which increased its synthesis yield by 35%.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

Neighboring Molecular Engineering in Diels-Alder Chemistry Enabling Easily Recyclable Carbon Fiber Reinforced Composites

Although a variety of dynamic covalent bonds have been successfully used in the development of diverse sustainable thermosetting polymers and their composites, solving the trade-off between recovery efficiency and comprehensive properties is still a major challenge. Herein, a "one-stone-two-birds" strategy of lower rotational energy barrier (Er ) phosphate-derived Diels-Alder (DA) cycloadditions was proposed for easily recyclable carbon fiber (CF)-reinforced epoxy resins (EPs) composites. In such a strategy, the phosphate spacer with lower Er accelerated the segmental mobility and dynamic DA exchange reaction for network rearrangement to achieve high-efficiency repairing, reprocessing of the EPs matrix and its composites and rapid nondestructive recycling of CF; meanwhile, incorporating phosphorus-based units especially reduced their fire hazards. The resulting materials simultaneously showed excellent thermal/mechanical properties, superb fire safety and facile recyclability, realizing the concept of recycling for high-performance thermosetting polymers and composites. This strategy is of great significance for understanding and enriching the molecular connotation of DA chemistry, making it potentially applicable to the design and development of a wide range of dynamic covalent adaptable materials toward practical cutting-edge-tech applications.

Solvent-Free Environmentally Friendly Method to Prepare Thermo-Reversible Fully Bio-Based Elastomers

Most rubber products come from petrochemical resources, which are increasingly in short supply. Rubber products that formed irreversible chemical bonds in the vulcanization process are difficult to recycle, resulting in a serious waste of resources. Therefore, it is important to prepare a kind of reprocessable biobased elastomers. Using furfuryl methacrylate (FMA) as the modified monomer, poly(dibutyl itaconate-myrcene-furfuryl methacrylate) (PDBIMFA) was synthesized by high-temperature emulsion polymerization successfully. The structure and compositions of PDBIMFA were characterized by Fourier transform infrared and 1H NMR, and the effects of different FMA contents on the structures and properties of PDBIMFA were systematically studied. Based on the Diels-Alder reaction, bismaleimide (BMI) and carbon black (CB) were introduced into PDBIMFA as cross-linking agents and reinforcing fillers, respectively, by the melt blending method, and PDBIMFA-BMI elastomer materials and CB/PDBIMFA-BMI elastomer composites with thermo-reversible cross-linking characteristics were prepared. The effects of the ratio of FMA and BMI on the mechanical properties of PDBIMFA-BMI were studied. PDBIMFA-BMI and CB/PDBIMFA-BMI were reprocessed twice, and the recovery rate of tensile strength was both more than 90%. The addition of CB was found to play a reinforcing role in the elastomer and with the introduction of the amount of CB, the reprocessability of composite remained at a good level. It is hoped that this research will provide a new strategy for the sustainable development of bio-based elastomer materials.

Electroactive Thermo-Pneumatic Soft Actuator with Self-Healing Features: A Critical Evaluation

Soft actuators that operate with overpressure have been successfully implemented as soft robotic grippers. Naturally, as these pneumatic devices are prone to cuts, self-healing properties are attractive. Here, we prepared a gripper that operates based on the liquid-gas phase transition of ethanol within its hollow structure. The gripping surface of the device is coated with a self-healing polymer that heals with heat. This gripper also includes a stainless steel wire along the device that heats the entire structure through resistive heating. This design results in a soft robotic gripper that actuates and heals in parallel driven by the same practical stimulus, that is, electricity. Compared to other self-healing soft grippers, this approach has the advantage of being simple and having autonomous self-healing. However, there remain fundamental drawbacks that limit its implementation. The current work critically assesses this overpressure approach and concludes with a broad perspective regarding self-healing soft robotic grippers.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Inside Block Copolymer Micelles-Tracing Interfacial Influences on Crosslinking Efficiency in Nanoscale Confined Spaces

Recently, several studies have demonstrated the excellent capabilities of tip-enhanced Raman spectroscopyfor in-depth investigations of structural properties of matter with unprecedented resolution and chemical specificity. These capabilities are utilized here to study the internal structure of core-crosslinked micelles, which are formed by self-assembly of the diblock terpolymer poly(ethylene oxide)-block-poly(furfuryl glycidylether-co-tert-butylglycidyl ether). Supplementing force-volume atomic force microscopy experiments address additionally the nanomechanical properties. Particularly, TERS enables investigating the underlying principles influencing the homogeneity and efficiency of the Diels-Alder core-crosslinking process in the confined hydrophobic core. While the central core region is homogenously crosslinked, a breakdown of the crosslinking reaction is observed in the core-corona interfacial region. The results corroborate that a strong crosslinking efficiency is directly correlated to the formation of a mixed zone of the glycidyl ether and PEO corona blocks reaching ≈5 nm into the core region. Concomitantly a strong exclusion of the encapsulated bismaleimide crosslinker from the interfacial region is observed. It is conceivable that a changed structure, chemical composition and altered nanomechanical properties of this interfacial region may also influence the crosslinking efficiency across the entire core region by a modification of the solubility of the crosslinker in the interfacial core-corona region.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Magnetic-responsive Covalent Adaptable Networks

Covalent adaptable networks (CANs) are reprocessable polymers whose structural arrangement is based on the recombination of dynamic covalent bonds. Composite materials prepared by incorporating magnetic particles into CANs attract much attention due to their remote and precise control, fast response speed, high biological safety and strong penetration of magnetic stimuli. These properties often involve magnetothermal effect and direct magnetic-field guidance. Besides, some of them can also respond to light, electricity or pH values. Thus, they are favorable for soft actuators since various functions are achieved such as magnetic-assisted self-healing (heating or at ambient temperature), welding (on land or under water), shape-morphing, and so on. Although magnetic CANs just start to be studied in recent two years, their advances are promised to expand the practical applications in both cutting-edge academic and engineering fields. This review aims to summarize recent progress in magnetic-responsive CANs, including their design, synthesis and application.

Understanding the Effect of Side Reactions on the Recyclability of Furan-Maleimide Resins Based on Thermoreversible Diels-Alder Network

We studied the effect of side reactions on the reversibility of epoxy with thermoreversible Diels-Alder (DA) cycloadducts based on furan and maleimide chemistry. The most common side reaction is the maleimide homopolymerization which introduces irreversible crosslinking in the network adversely affecting the recyclability. The main challenge is that the temperatures at which maleimide homopolymerization can occur are approximately the same as the temperatures at which retro-DA (rDA) reactions depolymerize the networks. Here we conducted detailed studies on three different strategies to minimize the effect of the side reaction. First, we controlled the ratio of maleimide to furan to reduce the concentration of maleimide groups which diminishes the effects of the side reaction. Second, we applied a radical-reaction inhibitor. Inclusion of hydroquinone, a known free radical scavenger, is found to retard the onset of the side reaction both in the temperature sweep and isothermal measurements. Finally, we employed a new trismaleimide precursor that has a lower maleimide concentration and reduces the rate of the side reaction. Our results provide insights into how to minimize formation of irreversible crosslinking by side reactions in reversible DA materials using maleimides, which is important for their application as novel self-healing, recyclable, and 3D-printable materials.

3D printed tissue models: From hydrogels to biomedical applications

The development of new advanced constructs resembling structural and functional properties of human organs and tissues requires a deep knowledge of the morphological and biochemical properties of the extracellular matrices (ECM), and the capacity to reproduce them. Manufacturing technologies like 3D printing and bioprinting represent valuable tools for this purpose. This review will describe how morphological and biochemical properties of ECM change in different tissues, organs, healthy and pathological states, and how ECM mimics with the required properties can be generated by 3D printing and bioprinting. The review describes and classifies the polymeric materials of natural and synthetic origin exploited to generate the hydrogels acting as "inks" in the 3D printing process, with particular emphasis on their functionalization allowing crosslinking and conjugation with signaling molecules to develop bio-responsive and bio-instructive ECM mimics.

Bond breaking of furan-maleimide adducts via a diradical sequential mechanism under an external mechanical force

Substituted furan-maleimide Diels-Alder adducts are bound by dynamic covalent bonds that make them particularly attractive mechanophores. Thermally activated [4 + 2] retro-Diels-Alder (DA) reactions predominantly proceed via a concerted mechanism in the ground electronic state. We show that an asymmetric mechanical force along the anchoring bonds in both the endo and exo isomers of proximal dimethyl furan-maleimide adducts favors a sequential pathway. The switching from a concerted to a sequential mechanism occurs at external forces of ≈1 nN. The first bond rupture occurs for a projection of the pulling force on the scissile bond at ≈4.3 nN for the exo adduct and ≈3.8 nN for the endo one. The reaction is inhibited for external forces up to ≈3.4 nN for the endo adduct and 3.6 nN for the exo one after which it is activated. In the activated region, at 4 nN, the rupture rate of the first bond for the endo adduct is computed to be ≈3 orders of magnitude larger than for the exo one in qualitative agreement with recent sonication experiments [Z. Wang and S. L. Craig, Chem. Commun., 2019, 55, 12263-12266]. In the intermediate region of the path between the rupture of the first and the second bond, the lowest singlet state exhibits a diradical character for both adducts and is close in energy to a diradical triplet state. The computed values of spin-orbit coupling along the path are too small for inducing intersystem crossings. These findings open the way for the rational design of DA mechanophores for polymer science and photochemistry.

Design of Surface-Aligned Main-Chain Liquid-Crystal Networks Prepared under Ambient, Light-Free Conditions Using the Diels-Alder Cycloaddition

Surface-aligned liquid-crystal networks (LCNs) offer a solution for developing functional materials capable of performing a range of tasks, including actuation, shape memory, and surfaces patterning. Here we show that Diels-Alder cycloaddition can be used to prepare the backbone of planar aligned LCNs under mild ambient conditions without the addition of additives or UV irradiation. The mechanical properties of the networks have robust viscoelastic modulus and stiffness with a reversible local free volume change upon physical aging. This study shows new opportunities to design surface-aligned LCNs based on additive free step-growth Diels-Alder polymerization and enables the potential to incorporate a wider range of photochromic materials into LCNs.

Self- and Cross-Fusing of Furan-Based Polyurea Gels Dynamically Cross-Linked with Maleimides

Bio-based polyureas (PUs) with main-chain furan rings were synthesized by the polyaddition of 2,5-bis(aminomethyl)furan with various diisocyanates, such as methylene diphenyl diisocyanate. Several PU's were soluble in polar organic solvents, and were cast to form thermomechanically stable films with softening temperatures of over 100 °C. The furan rings of the PU main chains underwent a dynamic Diels-Alder (DA) reaction with bismaleimide (BMI) cross-linkers. While the mixed solution of PU and BMI did not show any apparent signs of reaction at room temperature, the DA reaction proceeded to form gels upon heating to 60 °C, which became a solution again by further heating to 80 °C (retro-DA reaction). The solution phase was maintained by rapid quenching from 80 °C to room temperature, while the gel was reformed upon slow cooling. The recovered gels exhibited self-healing properties. A scratch made by a hot knife at temperatures above 80 °C disappeared spontaneously. When two different gels were cut using a knife at room temperature, placed in contact with each other, and heated to 60 °C, they fused. The ability to control the DA/retro-DA reaction allowed gels of varying composition to heal.

HYDRHA: Hydrogels of hyaluronic acid. New biomedical approaches in cancer, neurodegenerative diseases, and tissue engineering

In the last decade, hyaluronic acid (HA) has attracted an ever-growing interest in the biomedical engineering field as a biocompatible, biodegradable, and chemically versatile molecule. In fact, HA is a major component of the extracellular matrix (ECM) and is essential for the maintenance of cellular homeostasis and crosstalk. Innovative experimental strategies in vitro and in vivo using three-dimensional (3D) HA systems have been increasingly reported in studies of diseases, replacement of tissue and organ damage, repairing wounds, and encapsulating stem cells for tissue regeneration. The present work aims to give an overview and comparison of recent work carried out on HA systems showing advantages, limitations, and their complementarity, for a comprehensive characterization of their use. A special attention is paid to the use of HA in three important areas: cancer, diseases of the central nervous system (CNS), and tissue regeneration, discussing the most innovative experimental strategies. Finally, perspectives within and beyond these research fields are discussed.

Nanocellulose-based hydrogels as versatile drug delivery vehicles: A review

Hydrogels designed with nanocellulose (i.e. cellulose nanocrystals (CNC), cellulose nanofibrils (CNF), and bacterial cellulose (BC)) have significant advantages as drug carriers due to their environmentally-benign features and excellent properties. Nanocellulose hydrogels have been demonstrated to sustainably deliver various kinds of drugs via different routes of administration, in which nanocellulose significantly improves the hydrogel properties and tunes the drug releasing profile. This article comprehensively summarizes the recent research progress on nanocellulose hydrogels in drug delivery. We carefully assessed the gelation methods for nanocellulose hydrogel design and highlighted the influence of nanocellulose on hydrogel properties and drug release behaviors. In particular, it is the first time to summarize the research on nanocellulose hydrogel-based drug carriers regarding specific routes of administration. This work provides a critical review of nanocellulose-based hydrogels as drug delivery vehicles, and also underlines the outlook in this field, with the objective to inspire/prompt future work, especially the practical applications of nanocellulose hydrogels in designing controlled drug delivery systems.

The Effect of Molecular Weight on the (Re)-Processability and Material Properties of Bio-Based, Thermoreversibly Cross-Linked Polyesters

A (partially) bio-based short-chain polyester is prepared through interfacial polycondensation of furan-functionalized diphenolic acid with terephthalic chloride. The furan groups along the backbone of the obtained polyester are able to form a covalent network (PE-fur/Bism) with various ratios of 1,1′-(methylenedi-4,1-phenylene)bismaleimide via the thermoreversible Diels–Alder (DA) reaction. Several techniques have been employed to characterize the polyester network, including 1H-NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). The polyester base polymer displays a glass transition temperature of 115 °C, whereas the temperatures at which the retro-Diels–Alder (rDA) reaction takes place lie above 130 °C for the various polyester/bismaleimide networks. Excellent thermoreversibility and recyclability of the polyester resin have been shown through DSC and DMTA measurements.

Engineering Hydrogels for Modulation of Material-Cell Interactions

Hydrogels are a recurrent platform for Tissue Engineering (TE) strategies. Their versatility and the variety of available methods for tuning their properties highly contribute to hydrogels' success. As a result, the design of advanced hydrogels has been thoroughly studied, in the quest for better solutions not only for drugs- and cell-based therapies but also for more fundamental studies. The wide variety of sources, crosslinking strategies, and functionalization methods, and mostly the resemblance of hydrogels to the natural extracellular matrix, make this 3D hydrated structures an excellent tool for TE approaches. The state-of-the-art information regarding hydrogel design, processing methods, and the influence of different hydrogel formulations on the final cell-biomaterial interactions are overviewed herein. This article is protected by copyright. All rights reserved.

Thermoreversible Cross-Linked Rubber Prepared via Melt Blending and Its Nanocomposites

A covalent adaptable network based on the thermoreversible cross-linking of an ethylene-propylene rubber through Diels-Alder (DA) reaction was prepared for the first time through melt blending as an environmental-friendly alternative to traditional synthesis in organic solvents. Functionalization of the rubber with furan groups was performed in a melt blender and subsequently mixed with different amounts of bismaleimide in a microextruder. Cross-linking was confirmed by FT-IR spectroscopy and insolubility at room temperature, while its thermoreversible character was confirmed by a solubility test at 110 °C and by remolding via hot-pressing. Mechanical and thermomechanical properties of the obtained rubbers showed potential to compete with conventionally cross-linked elastomers, with stiffness in the range 1-1.7 MPa and strain at break in the range 200-500%, while allowing recycling via a simple melt processing step. Nanocomposites based on the thermoreversible rubber were prepared with reduced graphene oxide (rGO), showing significantly increasing stiffness up to ca. 8 MPa, ∼2-fold increased strength, and thermal conductivity up to ∼0.5 W/(m K). Results in this paper may open for industrially viable and sustainable applications of thermoreversible elastomers.

Rational mechanochemical design of Diels-Alder crosslinked biocompatible hydrogels with enhanced properties

An important but often overlooked feature of Diels-Alder (DA) cycloadditions is the ability for DA adducts to undergo mechanically induced cycloreversion when placed under force. Herein, we demonstrate that the commonly employed DA cycloaddition between furan and maleimide to crosslink hydrogels results in slow gelation kinetics and "mechanolabile" crosslinks that relate to reduced material strength. Through rational computational design, "mechanoresistant" DA adducts were identified by constrained geometries simulate external force models and employed to enhance failure strength of crosslinked hydrogels. Additionally, utilization of a cyclopentadiene derivative, spiro[2.4]hepta-4,6-diene, provided mechanoresistant DA adducts and rapid gelation in minutes at room temperature. This study illustrates that strategic molecular-level design of DA crosslinks can provide biocompatible materials with improved processing, mechanical durability, lifetime, and utility.