Nucleophile-Initiated Thiol-Michael Reactions: Effect of Organocatalyst, Thiol, and Ene

Identifying Quinones in Complex Aqueous Environmental Media (Biochar Extracts) through Tagging with Cysteine and Cysteine-Contained Peptides and High Resolution Mass Spectrometry Analysis

Quinones are among the most important components in natural organic matter (NOM) for redox reactions; however, no quinones in complex environmental media have been identified. To aid the identification of quinone-containing molecules in ultracomplex environmental samples, we developed a chemical tagging method that makes use of a Michael addition reaction between quinones and thiols (-SH) in cysteine (Cys) and cysteine-contained peptides (CCP). After the tagging, candidates of quinones in representative aqueous environmental samples (water extractions of biochar) were identified through high-resolution mass spectrometry (HRMS) analysis. The MS and UV spectra analysis showed rapid reactions between Cys/CCP and model quinones with β-carbon from the same benzene ring available for Michael addition. The tagging efficiency was not influenced by other co-occurring nonquinone representative compounds, including caffeic acid, cinnamic acid, and coumaric acid. Cys and CCP were used to tag quinones in water extractions of biochars, and possible candidates of quinones (20 and 53 based on tagging with Cys and CCP, respectively) were identified based on the HRMS features for products of reactions with Cys/CCP. This study has successfully demonstrated that such a Michael addition reaction can be used to tag quinones in complex environmental media and potentially determine their identities. The method will enable an in-depth understanding of the redox chemistry of NOM and its critical chemical compositions and structures.

Click Chemistry: Reaction Rates and Their Suitability for Biomedical Applications

Click chemistry has become a commonly used synthetic method due to the simplicity, efficiency, and high selectivity of this class of chemical reactions. Since their initial discovery, further click chemistry methods have been identified and added to the toolbox of click chemistry reactions for biomedical applications. However, selecting the most suitable reaction for a specific application is often challenging, as multiple factors must be considered, including selectivity, reactivity, biocompatibility, and stability. Thus, this review provides an overview of the benefits and limitations of well-established click chemistry reactions with a particular focus on the importance of considering reaction rates, an often overlooked criterion with little available guidance. The importance of understanding each click chemistry reaction beyond simply the reaction speed is discussed comprehensively with reference to recent biomedical research which utilized click chemistry. This review aims to provide a practical resource for researchers to guide the selection of click chemistry classes for different biomedical applications.

Bioinspired Mechanochromic Liquid Crystal Materials: From Fundamentals to Functionalities and Applications

Inspired by intriguing color changeable ability of natural animals, the design and fabrication of artificial mechanochromic materials capable of changing colors upon stretching or pressing have attracted intense scientific interest. Liquid crystal (LC) is a self-organized soft matter with anisotropic molecular alignment. Due to the sensitivity to various external stimulations, LC has been considered as an emerging and appealing responsive building block to construct intelligent materials and advanced devices. Recently, mechanochromic LC materials have becoming a hot topic in multifields from flexible artificial skins to visualized sensors and smart biomimetic devices. In this review, the recent progress of mechanochromic LCs is comprehensively summarized. Firstly, the mechanism and functionalities of mechanochromic LC is introduced, followed by preparation of various functional materials based on mechanochromic LCs. Then the applications of mechanochromic LCs are provided. Finally, the conclusion and outlooks of this field is given. This overview is hoped to provide inspiration in fabrication of advanced functional soft materials for scientists and engineers from multidisciplines including materials science, elastomers, chemistry, and physical science.

Metal-Free Click-Chemistry: A Powerful Tool for Fabricating Hydrogels for Biomedical Applications

Increasing interest in the utilization of hydrogels in various areas of biomedical sciences ranging from biosensing and drug delivery to tissue engineering has necessitated the synthesis of these materials using efficient and benign chemical transformations. In this regard, the advent of "click" chemistry revolutionized the design of hydrogels and a range of efficient reactions was utilized to obtain hydrogels with increased control over their physicochemical properties. The ability to apply the "click" chemistry paradigm to both synthetic and natural polymers as hydrogel precursors further expanded the utility of this chemistry in network formation. In particular, the ability to integrate clickable handles at predetermined locations in polymeric components enables the formation of well-defined networks. Although, in the early years of "click" chemistry, the copper-catalyzed azide-alkyne cycloaddition was widely employed, recent years have focused on the use of metal-free "click" transformations, since residual metal impurities may interfere with or compromise the biological function of such materials. Furthermore, many of the non-metal-catalyzed "click" transformations enable the fabrication of injectable hydrogels, as well as the fabrication of microstructured gels using spatial and temporal control. This review article summarizes the recent advances in the fabrication of hydrogels using various metal-free "click" reactions and highlights the applications of thus obtained materials. One could envision that the use of these versatile metal-free "click" reactions would continue to revolutionize the design of functional hydrogels geared to address unmet needs in biomedical sciences.

A Study on Thiol-Michael Addition to Semi-Synthetic Elastin-Hyaluronan Material for Electrospun Scaffolds

Thiol-Michael addition is a chemical reaction extensively used for conjugating peptides to polysaccharides with applications as biomaterials. In the present study, for designing a bioactive element in electrospun scaffolds as wound dressing material, a chemical strategy for the semi-synthesis of a hyaluronan-elastin conjugate containing an amide linker (ELAHA) was developed in the presence of tris(2-carboxyethyl)phosphine hydrochloride (TCEP ⋅ HCl). The bioconjugate was electrospun with poly-D,L-lactide (PDLLA), obtaining scaffolds with appealing characteristics in terms of morphology and cell viability of dermal fibroblast cells. For comprehending the factors influencing the efficiency of the bioconjugation reaction, thiolated amino acids were also investigated as nucleophiles toward hyaluronan decorated with Michael acceptors in the presence of TCEP ⋅ HCl through the evaluation of byproducts formation.

Click Chemistry in Lithium-Metal Batteries

Lithium-metal batteries (LMBs) are considered the "holy grail" of the next-generation energy storage systems, and solid-state electrolytes (SSEs) are a kind of critical component assembled in LMBs. However, as one of the most important branches of SSEs, polymer-based electrolytes (PEs) possess several native drawbacks including insufficient ionic conductivity and so on. Click chemistry is a simple, efficient, regioselective, and stereoselective synthesis method, which can be used not only for preparing PEs with outstanding physical and chemical performances, but also for optimizing the stability of solid electrolyte interphase (SEI) layer and elevate the cycling properties of LMBs effectively. Here it is primarily focused on evaluating the merits of click chemistry, summarizing its existing challenges and outlining its increasing role for the designing and fabrication of advanced PEs. The fundamental requirements for reconstructing artificial SEI layer through click chemistry are also summarized, with the aim to offer a thorough comprehension and provide a strategic guidance for exploring the potentials of click chemistry in the field of LMBs.

Copolymerization Involving Sulfur-Containing Monomers

Incorporating sulfur (S) atoms into polymer main chains endows these materials with many attractive features, including a high refractive index, mechanical properties, electrochemical properties, and adhesive ability to heavy metal ions. The copolymerization involving S-containing monomers constitutes a facile method for effectively constructing S-containing polymers with diverse structures, readily tunable sequences, and topological structures. In this review, we describe the recent advances in the synthesis of S-containing polymers via copolymerization or multicomponent polymerization techniques concerning a variety of S-containing monomers, such as dithiols, carbon disulfide, carbonyl sulfide, cyclic thioanhydrides, episulfides and elemental sulfur (S8). Particularly, significant focus is paid to precise control of the main-chain sequence, stereochemistry, and topological structure for achieving high-value applications.

NHC-catalyzed enantioselective access to β-cyano carboxylic esters via in situ substrate alternation and release

A carbene-catalyzed asymmetric access to chiral β-cyano carboxylic esters is disclosed. The reaction proceeds between β,β-disubstituted enals and aromatic thiols involving enantioselective protonation of enal β-carbon. Two main factors contribute to the success of this reaction. One involves in situ ultrafast addition of the aromatic thiol substrates to the carbon-carbon double bond of the enal substrate. This reaction converts almost all enal substrate to a Thiol-click Intermediate, significantly reducing aromatic thiol substrates concentration and suppressing the homo-coupling reaction of enals. Another factor is an in situ release of enal substrate from the Thiol-click Intermediate for the desired reaction to proceed effectively. The optically enriched β-cyano carboxylic esters from our method can be readily transformed to medicines that include γ-aminobutyric acids derivatives such as Rolipram. In addition to synthetic utilities, our control of reaction outcomes via in situ substrate modulation and release can likely inspire future reaction development.

A library of new organofunctional silanes obtained by thiol-(meth)acrylate Michael addition reaction

A simple and efficient method for the synthesis of organofunctional silanes by the thiol-(meth)acrylate addition reaction is presented. At first, systematic studies were carried out to select an optimum initiator/catalyst of the addition reaction for the model reaction involving 3-mercaptopropyltrimethoxysilane (MPTMS) and hexyl acrylate. Photoinitiators (in the presence of UV light energy), thermal initiators (such as aza compound and peroxide) as well as catalysts (primary and tertiary amines, phosphines and Lewis acid) were studied. After selecting an effective catalytic system and optimizing the reaction conditions, reactions between the thiol group (i.e. 3-mercaptopropyltrimethoxysilane) and (meth)acrylates containing various functional groups were carried out. All derivatives obtained were characterized by 1H, 13C, 29Si NMR and FT-IR analysis. In reactions carried out at room temperature, in an air atmosphere and in the presence of dimethylphenylphosphine (DMPP) as a catalyst, quantitative conversions of both substrates were obtained within a few minutes. The library of organofunctional silanes was expanded by compounds (containing various functional groups, i.e. alkenyl, epoxy, amino, ether, alkyl, aralkyl, fluoroalkyl) which were obtained in the thiol-Michael addition of 3-mercaptopropyltrimethoxysilane to a group of organofunctional (meth)acrylic acid esters.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Mechanism-Guided Design of Chain-Growth Click Polymerization Based on a Thiol-Michael Reaction

The development of chain-growth click polymerization is challenging yet desirable in modern polymer chemistry. In this work, we reported a novel chain-growth click polymerization based on the thiol-Michael reaction. This polymerization could be performed efficiently under ambient conditions and spatiotemporally regulated by ultraviolet light, allowing the synthesis of sulfur-containing polymers in excellent yields and high molecular weights. Density functional theory calculations indicated that the thiolate addition to the Michael acceptor is the rate-determining step, and introducing the phenyl group could facilitate the chain-growth process. This polymerization is a new type of chain-growth click polymerization, which will provide a unique approach to creating functional polymers.

Direct in situ photolithography of perovskite quantum dots based on photocatalysis of lead bromide complexes

Photolithography has shown great potential in patterning solution-processed nanomaterials for integration into advanced optoelectronic devices. However, photolithography of perovskite quantum dots (PQDs) has so far been hindered by the incompatibility of perovskite with traditional optical lithography processes where lots of solvents and high-energy ultraviolet (UV) light exposure are required. Herein, we report a direct in situ photolithography technique to pattern PQDs based on the photopolymerization catalyzed by lead bromide complexes. By combining direct photolithography with in situ fabrication of PQDs, this method allows to directly photolithograph perovskite precursors, avoiding the complicated lift-off processes and the destruction of PQDs by solvents or high-energy UV light, as PQDs are produced after lithography exposure. We further demonstrate that the thiol-ene free-radical photopolymerization is catalyzed by lead bromide complexes in the perovskite precursor solution, while no external initiators or catalysts are needed. Using direct in situ photolithography, PQD patterns with high resolution up to 2450 pixels per inch (PPI), excellent fluorescence uniformity, and good stability, are successfully demonstrated. This work opens an avenue for non-destructive direct photolithography of high-efficiency light-emitting PQDs, and potentially expands their application in various integrated optoelectronic devices.

Perovskite nanomaterials may suffer degradation during conventional photolithography. Here, the authors report a non-destructive method for patterning perovskite quantum dots based on direct photopolymerization catalyzed by lead bromide complexes.

Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks

While the Michael addition has been employed for more than 130 years for the synthesis of a vast diversity of compounds, the reversibility of this reaction when heteronucleophiles are involved has been generally less considered. First applied to medicinal chemistry, the reversible character of the hetero-Michael reactions has recently been explored for the synthesis of Covalent Adaptable Networks (CANs), in particular the thia-Michael reaction and more recently the aza-Michael reaction. In these cross-linked networks, exchange reactions take place between two Michael adducts by successive dissociation and association steps. In order to understand and precisely control the exchange in these CANs, it is necessary to get an insight into the critical parameters influencing the Michael addition and the dissociation rates of Michael adducts by reconsidering previous studies on these matters. This review presents the progress in the understanding of the thia-Michael reaction over the years as well as the latest developments and plausible future directions to prepare CANs based on this reaction. The potential of aza-Michael reaction for CANs application is highlighted in a specific section with comparison with thia-Michael-based CANs.

Click Step-Growth Polymerization and E/Z Stereochemistry Using Nucleophilic Thiol-yne/-ene Reactions: Applying Old Concepts for Practical Sustainable (Bio)Materials

Polymer sustainability is synonymous with "bioderived polymers" and the zeitgeist of "using renewable feedstocks". However, this sentiment does not adequately encompass the requirements of sustainability in polymers. In addition to recycling considerations and mechanical performance, following green chemistry principles also needs to be maximized to improve the sustainability of polymer synthesis. The synthetic cost (i.e., maximizing atom economy, reducing chemical hazards, and lowering energy requirements) of producing polymers should be viewed as equally important to the monomer source (biomass vs petrol platform chemicals). Therefore, combining the use of renewable feedstocks with efficient syntheses and green chemistry principles is imperative to delivering truly sustainable polymers. The high efficiency, atom economy, and single reaction trajectories that define click chemistry reactions position them as ideal chemical approaches to synthesize polymers in a sustainable manner while simultaneously expanding the structural scope of accessible polymers from sustainably sourced chemicals.Click step-growth polymerization using the thiol-yne Michael addition, a reaction first reported over a century ago, has emerged as an extremely mild and atom-efficient pathway to yield high-performance polymers with controllable E/Z stereochemistry along the polymer backbone. Building on studies of aromatic thiol-yne polymers, around 10 years ago our group began investigating the thiol-yne reaction for the stereocontrolled synthesis of alkene-containing aliphatic polyesters. Our early studies established a convenient path to high-molecular-weight (>100 kDa) E-rich or Z-rich step-growth polymers by judiciously changing the catalyst and/or reaction solvent. This method has since been adapted to synthesize fast-degrading polyesters, high-performance polyamides, and resilient hydrogel biomaterials. Across several systems, we have observed dramatic differences in material properties among polymers with different alkene stereochemistry.We have also explored the analogous thiol-ene Michael reaction to create high-performance poly(ester-urethanes) with precise E/Z stereochemistry. In contrast to the stereoselective thiol-yne polymerization, here the use of monomers with predefined E/Z (geometric) isomerism (arising from either alkenes or the planar rigidity of ring units) affords polymers with total control over stereochemistry. This advancement has enabled the synthesis of tough, degradable materials that are derived from sustainable monomer feedstocks. Employing isomers of sugar-derived isohexides, bicyclic rigid-rings possessing geometric isomerism, led to degradable polymers with fundamentally opposing mechanical behavior (i.e., plastic vs elastic) simply by adjusting the stereochemistry of the isohexide.In this Account, we feature our investigation of thiol-yne/-ene click step-growth polymers and efforts to establish structure-property relationships toward degradable materials with practical mechanical performance in the context of sustainable polymers and/or biomaterials. We have paid attention to installing and controlling geometric isomerism by using these click reactions, an overarching objective of our work in this research area. The exquisite control of geometric isomerism that is possible within polymer backbones, as enabled by convenient click chemistry reactions, showcases a powerful approach to creating multipurpose degradable polymers.

Eliminating Transition State Calculations for Faster and More Accurate Reactivity Prediction in Sulfa-Michael Additions Relevant to Human Health and the Environment

Fast and accurate computational approaches to predicting reactivity in sulfa-Michael additions are required for high-throughput screening in toxicology (e.g., predicting excess aquatic toxicity and skin sensitization), chemical synthesis, covalent drug design (e.g., targeting cysteine), and data set generation for machine learning. The kinetic glutathione chemoassay is a time-consuming in chemico method used to extract kinetic data in the form of log(k GSH) for organic electrophiles. In this work, we use density functional theory to compare the use of transition states (TSs) and enolate intermediate structures following C-S bond formation in the prediction of log(k GSH) for a diverse group of 1,4 Michael acceptors. Despite the widespread use of transition state calculations in the literature to predict sulfa-Michael reactivity, we observe that intermediate structures show much better performance for the prediction of log(k GSH), are faster to calculate, and easier to obtain than TSs. Furthermore, we show how linear combinations of atomic charges from the isolated Michael acceptors can further improve predictions, even when using inexpensive semiempirical quantum chemistry methods. Our models can be used widely in the chemical sciences (e.g., in the prediction of toxicity relevant to the environment and human health, synthesis planning, and the design of cysteine-targeting covalent inhibitors), and represent a low-cost, sustainable approach to reactivity assessment.

Biocatalytic 3D Actuation in Liquid Crystal Elastomers via Enzyme Patterning

Liquid crystal elastomers (LCEs) are stimuli-responsive materials that undergo large shape transformations after undergoing an order-disorder transition. While shape reconfigurations in LCEs are predominantly triggered by heat, there is a considerable interest in developing highly specific triggers that work at room temperature. Herein, we report the fabrication of biocatalytic LCEs that respond to the presence of urea by covalently immobilizing urease within chemically responsive LCE networks. The hydrogen-bonded LCEs developed in this work exhibited contractile strains of up to 36% upon exposure to a base. Notably, the generation of ammonia by immobilized urease triggered a disruption in the supramolecular network and a large reduction of liquid crystalline order in the films when the LCEs were exposed to urea. This reduction in order was macroscopically translated into a strain response that could be modulated by changing the concentration of urea or exposure time to the substrate. Local control of the mechanical response of the LCE was realized by spatially patterning the enzyme on the surface of the films. Subsequent exposure of enzymatically patterned LCE to urea-triggered 3D shape transformations into a curl, arch, or accordion-like structure, depending on the motif patterned on the film surface. Furthermore, we showed that the presence of salt was critical to prevent bridging of the network by the presence of ammonium ions, thereby enabling such macroscopic 3D shape changes. The large actuation potential of LCEs and the ability to translate the biocatalytic activity of enzymes to macroscopic 3D shape transformations could enable use in applications ranging from cell culture, medicine, or antifouling.

Synthetic dynamic hydrogels promote degradation-independent in vitro organogenesis

Epithelial organoids are most efficiently grown from mouse-tumour-derived, reconstituted extracellular matrix hydrogels, whose poorly defined composition, batch-to-batch variability and immunogenicity limit clinical applications. Efforts to replace such ill-defined matrices for organoid culture have largely focused on non-adaptable hydrogels composed of covalently crosslinked hydrophilic macromolecules. However, the excessive forces caused by tissue expansion in such elastic gels severely restrict organoid growth and morphogenesis. Chemical or enzymatic degradation schemes can partially alleviate this problem, but due to their irreversibility, long-term applicability is limited. Here we report a family of synthetic hydrogels that promote extensive organoid morphogenesis through dynamic rearrangements mediated by reversible hydrogen bonding. These tunable matrices are stress relaxing and thus promote efficient crypt budding in intestinal stem-cell epithelia through increased symmetry breaking and Paneth cell formation dependent on yes-associated protein 1. As such, these well-defined gels provide promising versatile matrices for fostering elaborate in vitro morphogenesis.

Theranostic Copolymers Neutralize Reactive Oxygen Species and Lipid Peroxidation Products for the Combined Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) results in the generation of reactive oxygen species (ROS) and lipid peroxidation product (LPOx), including acrolein and 4-hydroxynonenal (4HNE). The presence of these biochemical derangements results in neurodegeneration during the secondary phase of the injury. The ability to rapidly neutralize multiple species could significantly improve outcomes for TBI patients. However, the difficulty in creating therapies that target multiple biochemical derangements simultaneously has greatly limited therapeutic efficacy. Therefore, our goal was to design a material that could rapidly bind and neutralize both ROS and LPOx following TBI. To do this, a series of thiol-functionalized biocompatible copolymers based on lipoic acid methacrylate and polyethylene glycol monomethyl ether methacrylate (FW ∼ 950 Da) (O950) were prepared. A polymerizable gadolinium-DOTA methacrylate monomer (Gd-MA) was also synthesized starting from cyclen to facilitate direct magnetic resonance imaging and in vivo tracking of accumulation. These neuroprotective copolymers (NPCs) were shown to rapidly and effectively neutralize both ROS and LPOx. Horseradish peroxidase absorbance assays showed that the NPCs efficiently neutralized H₂O₂, while R-phycoerythrin protection assays demonstrated their ability to protect the fluorescent protein from oxidative damage. ¹H NMR studies indicated that the thiol-functional NPCs rapidly form covalent bonds with acrolein, efficiently removing it from solution. In vitro cell studies with SH-SY5Y-differentiated neurons showed that NPCs provide unique protection against toxic concentrations of both H₂O₂ and acrolein. NPCs rapidly accumulate and are retained in the injured brain in controlled cortical impact mice and reduce post-traumatic oxidative stress. Therefore, these materials show promise for improved target engagement of multiple biochemical derangements in hopes of improving TBI therapeutic outcomes.

Albumin Biomolecular Drug Designs Stabilized through Improved Thiol Conjugation and a Modular Locked Nucleic Acid Functionalized Assembly

Albumin-nucleic acid biomolecular drug designs offer modular multifunctionalization and extended circulatory half-life. However, stability issues associated with conventional DNA nucleotides and maleimide bioconjugation chemistries limit the clinical potential. This work aims to improve the stability of this thiol conjugation and nucleic acid assembly by employing a fast-hydrolyzing monobromomaleimide (MBM) linker and nuclease-resistant nucleotide analogues, respectively. The biomolecular constructs were formed by site-selective conjugation of a 12-mer oligonucleotide to cysteine 34 (Cys34) of recombinant human albumin (rHA), followed by annealing of functionalized complementary strands bearing either a fluorophore or the cytotoxic drug monomethyl auristatin E (MMAE). Formation of conjugates and assemblies was confirmed by gel shift analysis and mass spectrometry, followed by investigation of serum stability, neonatal Fc receptor (FcRn)-mediated cellular recycling, and cancer cell killing. The MBM linker afforded rapid conjugation to rHA and remained stable during hydrolysis. The albumin-nucleic acid biomolecular assembly composed of stabilized oligonucleotides exhibited high serum stability and retained FcRn engagement mediating FcRn-mediated cellular recycling. The MMAE-containing assembly exhibited cytotoxicity in the human MIA PaCa-2 pancreatic cancer cell line with an IC50 of 342 nM, triggered by drug release from breakdown of an acid-labile linker. In summary, this work presents rHA-nucleic acid module-based assemblies with improved stability and retained module functionality that further promotes the drug delivery potential of this biomolecular platform.

Rapid Synthesis of Polyester Based Single-Chain Polymeric Nanoparticles via Intra-Molecular Aza Michael Addition Reaction

An alternative strategy was developed to prepare intramolecularly crosslinked soft nanoparticles using aza Michael addition reaction on a linear polyester (PCA) containing in-chain reactive triple bonds. The linear precursor was...

4D Printing of Liquid Crystals: What's Right for Me?

Recent years have seen major advances in the developments of both additive manufacturing concepts and responsive materials. When combined as 4D printing, the process can lead to functional materials and devices for use in health, energy generation, sensing, and soft robots. Among responsive materials, liquid crystals, which can deliver programmed, reversible, rapid responses in both air and underwater, are a prime contender for additive manufacturing, given their ease of use and adaptability to many different applications. In this paper, selected works are compared and analyzed to come to a didactical overview of the liquid crystal-additive manufacturing junction. Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work. The educational text is closed with proposals for future research in this crossover field.

Anisotropic Iridescence and Polarization Patterns in a Direct Ink Written Chiral Photonic Polymer

The iridescence of structural color and its polarization characteristics originate from the nanoscale organization of materials. A major challenge in materials science is generating the bright, lustrous hues seen in nature through nanoscale engineering, while simultaneously controlling interaction of the material with different light polarizations. In this work, a suitable chiral nematic liquid crystal elastomer ink is synthesized for direct ink writing, which self-assembles into a chiral photonic structure. Tuning the writing direction and speed leads to the programmed formation of a slanted photonic axis, which exhibits atypical iridescence and polarization selectivity. After crosslinking, a freely programmable, chiroptical photonic polymer material is obtained. The strongly perspective-dependent appearance of the material can function as specialized anticounterfeit markers, as optical elements in decorative iridescent coatings, or, as demonstrated here, in optically based signaling features.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

A Strong Dual-Component Bioadhesive Based on Solventless Thiol-isocyanate Click Chemistry

Isocyanate is an efficient tissue anchor for engineering of strong bioadhesives. However, isocyanate-containing adhesives were seldom manufactured due to their requirement of water-free administration and time-consuming moisture-induced solidification. To address this issue, here, a solventless dual-component bioadhesive based on thiol-isocyanate cross-linking chemistry is reported. This dual-component bioadhesive consists of a hyperbranched polymer with thiol groups (HBPTE) and an isocyanate-modified polyethylene glycol (PEGNCO). HBPTE and PEGNCO are low-viscosity fluids at room temperature and hence could be used directly as adhesive components, in the absence of a catalyst and a solvent. The thiol-isocyanate click chemistry of components provides the HBPTE-PEGNCO mixture with a gelation time of 1.8-3 min, which makes it acceptable for practical applications. The abundance of isocyanate groups in the adhesive molecule provides strong bonding strength through formation of chemical linkages with reactive groups on the tissue. Moreover, in vitro and in vivo evaluations showed excellent biocompatibility of the HBPTE-PEGNCO adhesive. This dual-component bioadhesive based on solventless thiol-isocyanate click chemistry displayed a fast gelation time and excellent bonding performance, providing a pioneering idea for engineering isocyanate-containing bioadhesives.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Click Nucleophilic Conjugate Additions to Activated Alkynes: Exploring Thiol-yne, Amino-yne, and Hydroxyl-yne Reactions from (Bio)Organic to Polymer Chemistry

The 1,4-conjugate addition reaction between activated alkynes or acetylenic Michael acceptors and nucleophiles (i.e., the nucleophilic Michael reaction) is a historically useful organic transformation. Despite its general utility, the efficiency and outcomes can vary widely and are often closely dependent upon specific reaction conditions. Nevertheless, with improvements in reaction design, including catalyst development and an expansion of the substrate scope to feature more electrophilic alkynes, many examples now present with features that are congruent with Click chemistry. Although several nucleophilic species can participate in these conjugate additions, ubiquitous nucleophiles such as thiols, amines, and alcohols are commonly employed and, consequently, among the most well developed. For many years, these conjugate additions were largely relegated to organic chemistry, but in the last few decades their use has expanded into other spheres such as bioorganic chemistry and polymer chemistry. Within these fields, they have been particularly useful for bioconjugation reactions and step-growth polymerizations, respectively, due to their excellent efficiency, orthogonality, and ambient reactivity. The reaction is expected to feature in increasingly divergent application settings as it continues to emerge as a Click reaction.

Expanding the Thiol-X Toolbox: Photoinitiation and Materials Application of the Acid-Catalyzed Thiol-ene (ACT) Reaction

The acid-catalyzed thiol-ene reaction (ACT) is a unique thiol-X conjugation strategy that produces S,X-acetal conjugates. Unlike the well-known radical-mediated thiol-ene and anion-mediated thiol-Michael reactions that produce static thioether bonds, acetals provide unique function for various fields such as drug delivery and protecting group chemistries; however, this reaction is relatively underutilized for creating new and unique materials owing to the unexplored reactivity over a broad set of substrates and potential side reactions. Solution-phase studies using a range of thiol and alkene substrates were conducted to evaluate the ACT reaction as a conjugation strategy. Substrates that efficiently undergo cationic polymerizations, such as those containing vinyl functional groups, were found to be highly reactive to thiols in the presence of catalytic amounts of acid. Additionally, sequential initiation of three separate thiol-X reactions (thiol-Michael, ACT, and thiol-ene) was achieved in a one-pot scheme simply by the addition of the appropriate catalyst demonstrating substrate selectivity. Furthermore, photoinitiation of the ACT reaction was achieved for the first time under 470 nm blue light using a novel photochromic photoacid. Finally, using multifunctional monomers, solid-state polymer networks were formed using the ACT reaction producing acetal crosslinks. The presence of S,X-acetal bonds results in an increased glass transition temperature of 20 °C as compared with the same polymeric film polymerized through the radical thiol-ene mechanism. This investigation demonstrates the broad impact of the ACT reaction and expands upon the diverse thiol-X library of conjugation strategies towards the development of novel materials systems.

Phosphonium Tetraphenylborate: A Photocatalyst for Visible-Light-Induced, Nucleophile-Initiated Thiol-Michael Addition Photopolymerization

A photoinitiation system that utilizes phosphonium tetraphenylborate as the key component was developed for the visible light-triggered nucleophile-catalyzed thiol-Michael addition reaction. This highly reactive catalyst was composed of a photocaged phosphine (methyldiphenylphosphonium tetraphenylborate, MDPP·HBPh₄), a photosensitizer (isopropylthioxanthone, ITX), and a radical scavenger (TEMPO). Unlike the prevailing photobase catalysts, this photoactivatable phosphine system triggers the thiol-Michael addition polymerization by a nucleophile-catalyzed mechanism and provides a controlled stoichiometric reaction between the thiol and the vinyl precursors. This approach enables the formation of homogeneous polymer networks upon low-energy visible light exposure and, thus, broadens its potential applications in bulk polymer materials synthesis and UV-sensitive bioscaffold formation.

Controlling the crystallinity and solubility of functional PCL with efficient post-polymerisation modification

Varying the size of an alkyl side-chain group, installed by thiol–ene addition of alkylthiols to poly(ε-allyl caprolactone), the semi-crystallinity and lipophilicity of functional PCLs could be modulated to achieve divergent physico-chemical properties.

Oxa-Michael polyaddition of vinylsulfonylethanol for aliphatic polyethersulfones

The reactive AB-type monomer vinylsulfonylethanol polymerizes in the presence of common organic and inorganic bases forming a novel aliphatic polyethersulfone. A detailed analysis revealed different mechanisms for the organic and inorganic catalysts.

Hyperbranched polymer with dynamic thiol-aldehyde crosslinking and its application as a self-healable bioadhesive

Bioadhesives crosslinked with dynamic bonds exhibit shear-thinning, self-healing, and on-demand detachment properties, but generally show a weak bonding performance due to their poor bulk strength. Obtaining a strong bioadhesive with reversible crosslinking remains a challenge. To address this issue, herein we engineered a dynamic thiol-aldehyde crosslinked solvent-free adhesive based on hyperbranched polymer. The adhesive was obtained by directly mixing a liquid hyperbranched polymer with thiol end groups (HBPTE) and benzaldehyde-terminated polyethylene glycol (PEGCHO) without any additional catalyst or solvent. The solvent-free strategy yielded a dense crosslinking structure with many aldehyde groups, so this HBPTE-PEGCHO adhesive can strongly bond to tissue and various non-biological substrates. In addition, the HBPTE-PEGCHO adhesive has self-healing and thermo-reversible bonding properties due to the dynamic thiol-aldehyde crosslinking matrix. In vivo wound healing experiments show that this HBPTE-PEGCHO adhesive is tissue-benign, suggesting it can be applied in clinical practice. Combining the hyperbranched polymer-based solvent-free strategy and dynamic thiol-aldehyde crosslinking chemistry provides a simple but effective way to engineer a multifunctional bioadhesive with the desired bonding performance.

TEMPO driven thiol–ene reaction for the preparation of polymer functionalized silicon wafers

TEMPO driven thiol–ene reaction was utilized to prepare silicon (Si) wafers modified with a variety of polymer brushes, such as poly(N-isopropyl acrylamide), polystyrene, poly(isobornyl acrylate), poly(acrylic acid), and functionalized cysteine.

Fabrication of a Self-Healing, 3D Printable, and Reprocessable Biobased Elastomer

A novel self-healable, fully reprocessable, and inkjet three-dimensional (3D) printable partially biobased elastomer is reported in this work. A long-chain unsaturated diacrylate monomer was first synthesized from canola oil and then cross-linked with a partially oxidized silicon-based copolymer containing free thiol groups and disulfide bonds. The elastomer is fabricated through inkjet 3D printing utilizing the photoinitiated thiol-ene click chemistry and reprocessed by compression molding exploiting the dynamic nature of disulfide bond. Self-healing is enabled by phosphine-catalyzed disulfide metathesis. The elastomer displayed a tensile strength of ∼52 kPa, a breaking strain of ∼24, and ∼86% healing efficiency at 80 °C temperature after 8 h. Moreover, the elastomer showed excellent thermal stability, and the highest thermal degradation temperature was recorded to be ∼524 °C. After reprocessing through compression molding, the elastomer fully recovered its mechanical and thermal properties. These properties of the elastomer yield an ecofriendly alternative of fossil fuel-based elastomers that can find broad applications in soft robotics, flexible wearable devices, strain sensors, health care, and next-generation energy-harvesting and -storage devices.

Byproducts formed During Thiol-Acrylate Reactions Promoted by Nucleophilic Aprotic Amines: Persistent or Reactive?

The nucleophile-initiated mechanism of thiol-Michael reactions naturally leads to the formation of undesired nucleophile byproducts. Three aza-Michael compounds representing nucleophile byproducts of thiol-acrylate reactions initiated by 4-dimethylaminopyridine (DMAP), 1-methylimidazole (MIM), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) have been synthesized and their reactivity in the presence of thiolate has been investigated. Spectroscopic analysis shows that each nucleophile byproduct reacts with thiolate to produce a desired thiol-acrylate product along with liberated aprotic amines DMAP, MIM, or DBU, thus demonstrating that these byproducts are reactive rather than persistent. Density functional theoretical computations support experimental observations and predict that a β-elimination mechanism is favored for converting each nucleophile byproduct into a desired thiol-acrylate product, though an S_N 2 process can be competitive (i. e. within <2.5 kcal/mol) in less polar solvents.

Poly(N-vinylpyrrolidone) Antimalaria Conjugates of Membrane-Disruptive Peptides

The concepts of polymer-peptide conjugation and self-assembly were applied to antimicrobial peptides (AMPs) in the development of a targeted antimalaria drug delivery construct. This study describes the synthesis of α-acetal, ω-xanthate heterotelechelic poly(N-vinylpyrrolidone) (PVP) via reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization, followed by postpolymerization deprotection to yield α-aldehyde, ω-thiol heterotelechelic PVP. A specific targeting peptide, GSRSKGT, for Plasmodium falciparum-infected erythrocytes was used to sparsely decorate the α-chain ends via reductive amination while cyclic decapeptides from the tyrocidine group were conjugated to the ω-chain end via thiol-ene Michael addition. The resultant constructs were self-assembled into micellar nanoaggregates whose sizes and morphologies were determined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The in vitro activity and selectivity of the conjugates were evaluated against intraerythrocytic P. falciparum parasites.

Modular design of a hybrid hydrogel for protease-triggered enhancement of drug delivery to regulate TNF-α production by pro-inflammatory macrophages

Systemic drug administration has conventionally been prescribed to alleviate persistent local inflammation which is prevalent in chronic diseases. However, this approach is associated with drug-induced toxicity, particularly when the dosage exceeds that necessitated by pathological conditions of diseased tissues. Herein, we developed a modular hybrid hydrogel which could be triggered to release an anti-inflammatory drug upon exposure to elevated protease activity associated with inflammatory diseases. Modular design of the hybrid hydrogel enabled independent optimization of its protease-cleavable and drug-loaded subdomains to facilitate hydrogel formation, cleavability by matrix-metalloprotease-9 (MMP-9), and tuning drug release rate. In vitro study demonstrated the protease-triggered enhancement of drug release from the hybrid hydrogel system for effective inhibition of TNF-α production by pro-inflammatory macrophages and suggested its potential to mitigate drug-induced cytotoxicity. Using non-invasive imaging to monitor the activity of reactive oxygen species in biomaterial-induced host response, we confirmed that the hybrid hydrogel and its constituent materials did not induce adverse immune response after 5 days following their subcutaneous injection in immuno-competent mice. We subsequently incorporated this hybrid hydrogel onto a commercial wound dressing which could release the drug upon exposure to MMP-9. Together, our findings suggested that this hybrid hydrogel might be a versatile platform for on-demand drug delivery via either injectable or topical application to modulate inflammation in chronic diseases.

Decomplexation as a rate limitation in the thiol-Michael addition of N-acrylamides

The thiol-Michael addition is a popular, selective, high-yield "click" reaction utilized for applications ranging from small-molecule synthesis to polymer or surface modification. Here, we combined experimental and quantum mechanical modeling approaches using density functional theory (DFT) to examine the thiol-Michael reaction of N-allyl-N-acrylamide monomers used to prepare sequence-defined oligothioetheramides (oligoTEAs). Experimentally, the reaction was evaluated with two fluorous tagged thiols and several monomers at room temperature (22 °C and 40 °C). Using the Eyring equation, the activation energies (enthalpies) were calculated, observing a wide range of energy barriers ranging from 28 kJ mol-1 to 108 kJ mol-1 within the same alkene class. Computationally, DFT coupled with the Nudged Elastic Band method was used to calculate the entire reaction coordinate of each monomer reaction using the B97-D3 functional and a hybrid implicit-explicit methanol solvation approach. The thiol-Michael reaction is traditionally rate-limited by the propagation or chain-transfer steps. However, our test case with N-acrylamides and fluorous thiols revealed experimental and computational data produced satisfactory agreement only when we considered a previously unconsidered step that we termed "product decomplexation", which occurs as the product physically dissociates from other co-reactants after chain transfer. Five monomers were investigated to support this finding, capturing a range of functional groups varying in alkyl chain length (methyl to hexyl) and aromaticity (benzyl and ethylenephenyl). Increased substrate alkyl chain length increased activation energy, explained by the inductive effect. Aromatic ring-stacking configurations significantly impacted the activation energy and contributed to improved molecular packing density. Hydrogen-bonding between reactants increased the activation energy emphasizing the rate-limitation of the product decomplexation. Our findings begin to describe a new structure-kinetic relationship for thiol-Michael acceptors to enable further design of reactive monomers for synthetic polymers and biomaterials.