Acid-catalyzed Disulfide-mediated Reversible Polymerization for Recyclable Dynamic Covalent Materials

Supramolecular/Dynamic Covalent Design of High-Performance Pressure-Sensitive Adhesive from Natural Low-Molecular-Weight Compounds

Adhesive materials have played an essential role in the history of humanity. Natural adhesives composed of low-molecular-weight monomers have been overshadowed by modern petroleum-based glues. With the development of green economy, the demand for eco-friendly materials has increased. Herein, two natural biocompatible compounds, namely thioctic acid (TA) and malic acid (MA), are selected to prepare a high-performance pressure-sensitive adhesive poly[TA-MA]. This adhesive can be quantitatively obtained via a simple mixing and heating process. Poly[TA-MA] shows interesting and useful properties, including reversible flexibility, high elongation, and good self-healing, owing to its dynamic polymerization pattern and reversible cross-linking behavior. Poly[TA-MA] exhibits excellent adhesion performance under various extreme conditions, such as at low temperatures and in hot water. High values of shear strength (3.86 MPa), peel strength (7.90 N cm-1), loop tack (10.60 N cm-1), tensile strength (1.02 MPa), and shear resistance (1628 h) demonstrate the strong adhesive effect of poly[TA-MA]. Additionally, TA can be regenerated in the monomer forms from poly[TA-MA] with high recovery rate (>90%). Meanwhile, strong anti-bacterial behavior of poly[TA-MA] is recorded. This study not only reported a new pressure-sensitive adhesive but also fully displayed the feasibility of using natural small molecules to achieve robust surface adhesion.

Biological applications of lipoic acid-based polymers: an old material with new promise

Lipoic acid (LA) is a versatile antioxidant that has been used in the treatment of various oxidation-reduction diseases over the past 70 years. Owing to its large five-membered ring tension, the dynamic disulfide bond of LA is highly active, enabling the formation of poly(lipoic acid) (PLA) via ring-opening polymerization (ROP). Herein, we first summarize disulfide-mediated ROP polymerization strategies, providing basic routes for designing and preparing PLA-based materials. PLA, as a biologically derived, low toxic, and easily modified material, possesses dynamic disulfide bonds and universal non-covalent carboxyl groups. We also shed light on the biomedical applications of PLA-based materials based on their biological and structural features and further divide recent works into six categories: antibacterial, anti-inflammation, anticancer, adhesive, flexible electronics, and 3D-printed tissue scaffolds. Finally, the challenges and future prospects associated with the biomedical applications of PLA are discussed.

A Rational Design of Bio-Derived Disulfide CANs for Wearable Capacitive Pressure Sensor

Classic approaches to integrate flexible capacitive sensor performance are to on-demand microstructuring dielectric layers and to adjust dielectric material compositions via the introduction of insoluble carbon additives (to increase sensitivity) or dynamic interactions (to achieve self-healing). However, the sensor's enhanced performances often come with increased material complexity, discouraging its circular economy. Herein, a new intrinsic self-healable, closed-loop recyclable dielectric layer material, a fully nature-derived dynamic covalent poly(disulfide) decorated with rich H bonding and metal-catechol complexations is introduced. The polymer network possesses a mechanically ductile character with an Arrhenius-type temperature-dependent viscoelasticity. The assembled capacitive pressure sensor is able to achieve a sensitivity of up to 9.26 kPa-1, fast response/recovery time of 32/24 ms, and can deliver consistent signals of continuous consecutive cycles even after being self-healed or closed-loop recycled for real-time detection of human motions. This is expected to be of high interest for current capacitive sensing research to move toward a life-like, high performance, and circular economy direction.

Polymeric Nanoparticles for Drug Delivery

The recent emergence of nanomedicine has revolutionized the therapeutic landscape and necessitated the creation of more sophisticated drug delivery systems. Polymeric nanoparticles sit at the forefront of numerous promising drug delivery designs, due to their unmatched control over physiochemical properties such as size, shape, architecture, charge, and surface functionality. Furthermore, polymeric nanoparticles have the ability to navigate various biological barriers to precisely target specific sites within the body, encapsulate a diverse range of therapeutic cargo and efficiently release this cargo in response to internal and external stimuli. However, despite these remarkable advantages, the presence of polymeric nanoparticles in wider clinical application is minimal. This review will provide a comprehensive understanding of polymeric nanoparticles as drug delivery vehicles. The biological barriers affecting drug delivery will be outlined first, followed by a comprehensive description of the various nanoparticle designs and preparation methods, beginning with the polymers on which they are based. The review will meticulously explore the current performance of polymeric nanoparticles against a myriad of diseases including cancer, viral and bacterial infections, before finally evaluating the advantages and crucial challenges that will determine their wider clinical potential in the decades to come.

Reversibly Cross-Linked Isoporous Membranes Fabricated by the Recyclable Block Copolymer with Pendent Dithiolane Groups

The reversible formation and cleavage of disulfide bonds under physical/chemical stimuli make it a valuable motif in constructing dynamically cross-linked materials. In the present work, the block copolymer bearing pendent dithiolanes was synthesized and fabricated into isoporous membranes by the combination of self-assembly and nonsolvent-induced phase separation strategy. The cross-linking within the membrane was realized by the thiol-initiated ring-opening cascades of cyclic disulfides. Successful formation of disulfide bond networks within the isoporous membranes was proved by the Raman spectra, UV-vis diffuse reflectance spectroscopy, differential scanning calorimetry, and rheological analysis. The cross-linking in membranes was further demonstrated by the notably improved toughness and obviously enhanced swelling resistance to acid/alkaline solution as well as organic solvents. Importantly, the cross-linked isoporous membranes were fully dissolvable in solution containing dithiothreitol, which enabled the complete cleavage of disulfide bonds and successful recovery of the block copolymer that was able to be repeatedly fabricated into isoporous membranes with pore sizes identical to membranes prepared from the freshly synthesized copolymer. Our results indicate that dynamically cross-linked isoporous membranes with improved durability and good recyclability can be custom-made by simply incorporating active dithiolane moieties into self-assembling block copolymers.

Dynamic Chemistry Toolbox for Advanced Sustainable Materials

Developing dynamic chemistry for polymeric materials offers chemical solutions to solve key problems associated with current plastics. Mechanical performance and dynamic function are equally important in material design because the former determines the application scope and the latter enables chemical recycling and hence sustainability. However, it is a long-term challenge to balance the subtle trade-off between mechanical robustness and dynamic properties in a single material. The rise of dynamic chemistry, including supramolecular and dynamic covalent chemistry, provides many opportunities and versatile molecular tools for designing constitutionally dynamic materials that can adapt, repair, and recycle. Facing the growing social need for developing advanced sustainable materials without compromising properties, recent progress showing how the toolbox of dynamic chemistry can be explored to enable high-performance sustainable materials by molecular engineering strategies is discussed here. The state of the art and recent milestones are summarized and discussed, followed by an outlook toward future opportunities and challenges present in this field.

The Dynamic Chemistry of Orthoesters and Trialkoxysilanes: Making Supramolecular Hosts Adaptive, Fluxional, and Degradable

ConspectusThe encapsulation of ions into macro(bi)cyclic hosts lies at the core of supramolecular chemistry. While chemically inert hosts such as crown ethers (synthesis) and cyclodextrins (Febreze) have enabled real-world applications, there is a wider and accelerating trend toward functional molecules and materials that are stimuli-responsive, degradable, or recyclable. To endow supramolecular hosts with these properties, a deviation from ether C-O bonds is required, and functional groups that engage in equilibrium reactions under relatively mild conditions are needed.In this Account, we describe our group's work on supramolecular hosts that comprise orthoester and trialkoxysilane bridgeheads. In their simplest structural realization, these compounds resemble both Cram's crown ethers (macrocycles with oxygen donor atoms) and Lehn's cryptands (macrobicycles with 3-fold symmetry). It is therefore not surprising that these new hosts were found to have a natural propensity to bind cations relatively strongly. In recent work, we were also able to create anion-binding hosts by placing disubstituted urea motifs at the center of the tripodal architecture. Structural modifications of either the terminal substituents (e.g., H vs CH3 on the bridgehead), the diol (e.g., chiral), or the bridgehead atom itself (Si vs C) were found to have profound implications on the guest-binding properties.What makes orthoester/trialkoxysilane hosts truly unique is their dynamic covalent chemistry. The ability to conduct exchange reactions with alcohols at the bridgehead carbon or silicon atom is first and foremost an opportunity to develop highly efficient syntheses. Indeed, all hosts presented in this Account were prepared via templated self-assembly in yields of up to 90%. This efficiency is remarkable because the macrobicyclic architecture is established in one single step from at least five components. A second opportunity presented by dynamic bridgeheads is that suitable mixtures of orthoester hosts or their subcomponents can be adaptive, i.e. they respond to the presence of guests such that the addition of a certain guest can dictate the formation of a preferred host. In an extreme example of dynamic adaptivity, we found that ammonium ions can fulfill the dual role of catalyst for orthoester exchange and cationic template for efficient host formation, representing an unprecedented example of a fluxional supramolecular complex. The third implication of dynamic bridgeheads is due to the reaction of orthoesters and trialkoxysilanes with water instead of alcohols. We describe in detail how the hydrolysis rate differs strongly between O,O,O-orthoesters, S,S,S-trithioorthoesters, and trialkoxysilanes and how it is tunable by the choice of substituents and pH.We expect that the fundamental insights into exchange and degradation kinetics described in this Account will be useful far beyond supramolecular chemistry.

A Self-Assemble Supramolecular Film with Humidity Visualization Enabled by Clusteroluminescence

Clusteroluminescence (CL) has recently gained significant attention due to its unique through-space interactions associated with a high dependence on the aggregation of subgroups. These distinct features could easily transform the stimuli into visual fluorescence and monitor the fluctuation of the environment but have not received sufficient attention before. In this work, supramolecular films are designed based on the neutralization reaction of anhydride groups and the self-assembly of dynamic covalent disulfide bonds in NaOH aqueous solution. The self-assembly of hydrophilic carboxylate chromophores and hydrophobic disulfide-containing five-membered rings could be observed by the variation of the aggregation state of carboxylate in CL. Furthermore, the dynamic cross-linking films obtained with water-sensitive carboxylate chromophores could alter the aggregation distance stimulated by surrounding water vapor, causing the emission wavelength to change from 534 to 508 nm by varying the relative humidity. This work not only provides an approach to monitor the self-assembly of clusteroluminogens but also offers new strategies for designing stimuli-responsive materials that utilize the intrinsic features of CL.

Tunable Band Engineering Management on Perovskite MAPbBr3 /COFs Nano-Heterostructures for Efficient S-S Coupling Reactions

Efficient artificial photosynthesis of disulfide bonds holds promises to facilitate reverse decoding of genetic codes and deciphering the secrets of protein multilevel folding, as well as the development of life science and advanced functional materials. However, the incumbent synthesis strategies encounter separation challenges arising from leaving groups in the ─S─S─ coupling reaction. In this study, according to the reaction mechanism of free-radical-triggered ─S─S─ coupling, light-driven heterojunction functional photocatalysts are tailored and constructed, enabling them to efficiently generate free radicals and trigger the coupling reaction. Specifically, perovskites and covalent organic frameworks (COFs) are screened out as target materials due to their superior light-harvesting and photoelectronic properties, as well as flexible and tunable band structure. The in situ assembled Z-scheme heterojunction MAPB-M-COF (MAPbBr3 = MAPB, MA+ = CH3 NH2 + ) demonstrates a perfect trade-off between quantum efficiency and redox chemical potential via band engineering management. The MAPB-M-COF achieves a 100% ─S─S─ coupling yield with a record photoquantum efficiency of 11.50% and outstanding cycling stability, rivaling all the incumbent similar reaction systems. It highlights the effectiveness and superiority of application-oriented band engineering management in designing efficient multifunctional photocatalysts. This study demonstrates a concept-to-proof research methodology for the development of various integrated heterojunction semiconductors for light-driven chemical reaction and energy conversion.

Controlled and Regioselective Ring-Opening Polymerization for Poly(disulfide)s by Anion-Binding Catalysis

Poly(disulfide)s are an emerging class of sulfur-containing polymers with applications in medicine, energy, and functional materials. However, the constituent dynamic covalent S-S bond is highly reactive in the presence of the sulfide (RS-) anion, imposing a persistent challenge to control the polymerization. Here, we report an anion-binding approach to arrest the high reactivity of the RS- chain end to control the synthesis of linear poly(disulfide)s, realizing a rapid, living ring-opening polymerization of 1,2-dithiolanes with narrow dispersity and high regioselectivity (Mw/Mn ∼ 1.1, Ps ∼ 0.85). Mechanistic studies support the formation of a thiourea-base-sulfide ternary complex as the catalytically active species during the chain propagation. Theoretical analyses reveal a synergistic catalytic model where the catalyst preorganizes the protonated base and anionic chain end to establish spatial confinement over the bound monomer, effecting the observed regioselectivity. The catalytic system is amenable to monomers with various functional groups, and semicrystalline polymers are also obtained from lipoic acid derivatives by enhancing the regioselectivity.

Reversible Amidation Chemistry Enables Closed-Loop Chemical Recycling of Carbon Fiber Reinforced Polymer Composites to Monomers and Fibers

Highly efficient recycling of carbon fiber reinforced polymer composites into monomers and fibers is a formidable challenge. Herein, we present a closed-loop recycling approach for carbon fiber reinforced polymer composites using reversible amidation chemistry, which enables the complete recovery of intact carbon fibers and pure monomers. The polymer network, synthesized by amidation between a macromonomer linear polyethyleneimine and a bifunctional maleic anhydride cross-linker, serves as a matrix for the construction of composites with exceptional mechanical properties, thermal stability and solvent resistance. The matrices can be fully depolymerized under the acidic condition at ambient temperature, allowing the effective separation and recovery of both carbon fibers and the two monomers. The reclaimed carbon fibers retain nearly identical mechanical properties to pristine ones, while pure monomers are recycled with high separation yields (>93 %). They can be reused in for multiple cycles for the manufacture of new composites, whose mechanical properties recover over 95 % of their original properties. This line of research presents a promising approach for the design of high-performance and sustainable thermoset composites, offering significant environmental and economic benefits.

Controlled-Radical Polymerization of α-Lipoic Acid: A General Route to Degradable Vinyl Copolymers

Here, we present the synthesis and characterization of statistical and block copolymers containing α-lipoic acid (LA) using reversible addition-fragmentation chain-transfer (RAFT) polymerization. LA, a readily available nutritional supplement, undergoes efficient radical ring-opening copolymerization with vinyl monomers in a controlled manner with predictable molecular weights and low molar-mass dispersities. Because lipoic acid diads present in the resulting copolymers include disulfide bonds, these materials efficiently and rapidly degrade when exposed to mild reducing agents such as tris(2-carboxyethyl)phosphine (Mn = 56 → 3.6 kg mol-1). This scalable and versatile polymerization method affords a facile way to synthesize degradable polymers with controlled architectures, molecular weights, and molar-mass dispersities from α-lipoic acid, a commercially available and renewable monomer.

Emerging Trends in the Chemistry of End-to-End Depolymerization

Over the past couple of decades, polymers that depolymerize end-to-end upon cleavage of their backbone or activation of a terminal functional group, sometimes referred to as "self-immolative" polymers, have been attracting increasing attention. They are of growing interest in the context of enhancing polymer degradability but also in polymer recycling as they allow monomers to be regenerated in a controlled manner under mild conditions. Furthermore, they are highly promising for applications as smart materials due to their ability to provide an amplified response to a specific signal, as a single sensing event is translated into the generation of many small molecules through a cascade of reactions. From a chemistry perspective, end-to-end depolymerization relies on the principles of self-immolative linkers and polymer ceiling temperature (Tc). In this article, we will introduce the key chemical concepts and foundations of the field and then provide our perspective on recent exciting developments. For example, over the past few years, new depolymerizable backbones, including polyacetals, polydisulfides, polyesters, polythioesters, and polyalkenamers, have been developed, while modern approaches to depolymerize conventional backbones such as polymethacrylates have also been introduced. Progress has also been made on the topological evolution of depolymerizable systems, including the introduction of fully depolymerizable block copolymers, hyperbranched polymers, and polymer networks. Furthermore, precision sequence-defined oligomers have been synthesized and studied for data storage and encryption. Finally, our perspectives on future opportunities and challenges in the field will be discussed.

Highly Elastic, Self-Healing, Recyclable Interlocking Double-Network Liquid-Free Ionic Conductive Elastomers via Facile Fabrication for Wearable Strain Sensors

Liquid-free ionic conductive elastomers (ICEs) are ideal materials for wearable strain sensors in increasingly flexible electronic devices. However, developing recyclable ICEs with high elasticity, self-healability, and recyclability is still a great challenge. In this study, we fabricated a series of novel ICEs by in situ polymerization of lipoic acid (LA) in poly(acrylic acid) (PAA) solution and cross-linking by coordination bonding and hydrogen bonding. One of the obtained dynamically cross-linked interlocking double-network ICEs, PLA-PAA_4-1% ICE, showed excellent mechanical properties, with high elasticity (90%) and stretchability (610%), as well as rapid self-healability (mechanical self-healing within 2 h and electrical recovery within 0.3 s). The PLA-PAA_4-1% ICE was used as a strain sensor and possessed excellent linear sensitivity and highly cyclic stability, effectively monitoring diverse human motions with both stretched and compressed deformations. Notably, the PLA-PAA_4-1% ICE can be fully recycled and reused as a new strain sensor without any structure change or degradation in performance. This work provided a viable path to fabricate conductive materials by solving the two contradictions of high mechanical property and self-healability, and structure stability and recyclability. We believe that the superior overall performance and feasible fabrication make the developed PLA-PAA_4-1% ICE hold great promise as a multifunctional strain sensor for practical applications in flexible wearable electronic devices and humanoid robotics.

Pressure-Sensitive Supramolecular Adhesives Based on Lipoic Acid and Biofriendly Dynamic Cyclodextrin and Polyrotaxane Cross-Linkers

Slide-ring materials are polymer networks with mobile cross-links that exhibit impressive stress dissipation and fracture resistance owing to the pulley effect. On account of their remarkable ability to dissipate the energy of deformation, these materials have found their way into advanced materials such as abrasion-resistant coatings and elastic battery electrode binders. In this work, we explore the role of mobile cross-links on the properties of a biofriendly pressure-sensitive adhesive made using composites of cyclodextrin-based macromolecules and poly(lipoic acid). We modify cyclodextrin-based hosts and polyrotaxanes with pendant groups of lipoic acid (a commonly ingested antioxidant) to incorporate them as cross-links in poly(lipoic acid) networks obtained by simple heating in open air. By systematically varying the adhesive formulations while probing their mechanical and adhesive properties, we uncover trends in structure-property relationships that enable one to tune network properties and access biofriendly, high-tack adhesives.