Amine Induced Retardation of the Radical-Mediated Thiol-Ene Reaction via the Formation of Metastable Disulfide Radical Anions

Interrupting marine fouling with active buffered coatings

Biofouling on marine surfaces causes immense material and financial harm for maritime vessels and related marine industries. Previous reports have shown the effectiveness of amphiphilic coating systems based on poly(dimethylsiloxane) (PDMS) against such marine foulers. Recent studies on biofouling mechanisms have also demonstrated acidic microenvironments in biofilms and stronger adhesion at low-pH conditions. This report presents the design and utilization of amphiphilic polymer coatings with buffer functionalities as an active disruptor against four different marine foulers. Specifically, this study explores both neutral and zwitterionic buffer systems for marine coatings, offering insights into coating design. Overall, these buffer systems were found to improve foulant removal, and unexpectedly were the most effective against the diatom Navicula incerta.

UV-curable thiol-ene system for broadband infrared transparent objects

Conventional infrared transparent materials, including inorganic ceramic, glass, and sulfur-rich organic materials, are usually processed through thermal or mechanical progress. Here, we report a photo-curable liquid material based on a specially designed thiol-ene strategy, where the multithiols and divinyl oligomers were designed to contain only C, H, and S atoms. This approach ensures transparency in a wide range spectrum from visible light to mid-wave infrared (MWIR), and to long-wave infrared (LWIR). The refractive index, thermal properties, and mechanical properties of samples prepared by this thiol-ene resin were characterized. Objects transparent to LWIR and MWIR were fabricated by molding and two-photon 3D printing techniques. We demonstrated the potential of our material in a range of applications, including the fabrication of IR optics with high imaging resolution and the construction of micro-reactors for temperature monitoring. This UV-curable thiol-ene system provides a fast and convenient alternative for the fabrication of thin IR transparent objects.

Traceless Thioacid-Mediated Radical Cyclization of 1,6-Dienes

Five-membered ring systems are ubiquitous throughout natural products and synthetic therapeutics, and thus, efficient methods to access this essential scaffold are required. Herein, we report the thioacid-mediated, 5-exo-trig cyclization of various 1,6-dienes, with high yields of up to 98%. The labile thioester functionality can be exploited to generate a free thiol residue which can be used as a functional handle or removed entirely to provide the traceless cyclized product.

Cysteine-Selective Installation of Functionally Diverse Boronic Acid Probes on Peptides

The current methods for direct late-stage and residue-selective installation of a versatile boronic acid (BA) repertoire on peptides are inadequate for a wide range of applications. Here, we show the suitability and efficiency of thiol-ene radical click chemistry to install functionally versatile BA derivatives on numerous bioactive, native peptides. Our work highlights that the methodology is operationally simple and adaptable for applications with BA-modified peptides, such as cyclization, conjugation, and functional group alteration.

Self-healing mixed matrix membranes containing metal-organic frameworks

Mixed-matrix membranes (MMMs) provide a means to formulate metal-organic frameworks (MOFs) into processable films that can help to advance their use in various applications. Conventional MMMs are inherently susceptible to craze or tear upon exposure to impact, cutting, bending, or stretching, which can limit their intended service life and usage. Herein, a simple, efficient, and scalable in situ fabrication approach was used to prepare self-healing MMMs containing Zr(iv)-based MOFs. The ability of these MMMs to self-heal at room temperature is based on the reversible hydrolysis of boronic-ester conjugates. Thiol-ene 'photo-click' polymerization yielded robust MMMs with ∼30 wt% MOF loading and mechanical strength that varied based on the size of MOF particles. The MMMs could undergo repeated self-healing with good retention of mechanical strength. In addition, the MMMs were catalytically active toward the degradation of the chemical warfare agent (CWA) simulant dimethyl-4-nitrophenyl phosphate (DMNP) with no change in activity after two damage-healing cycles.

Investigation of N-Substituted Morpholine Structures in an Amphiphilic PDMS-Based Antifouling and Fouling-Release Coating

Biofouling is a major disruptive process affecting the fuel efficiency and durability of maritime vessel coatings. Previous research has shown that amphiphilic coatings consisting of a siloxane backbone functionalized with hydrophilic moieties are effective marine antifouling and fouling-release materials. Poly(ethylene glycol) (PEG) has been the primary hydrophilic component used in such systems. Recently, the morpholine group has emerged as a promising compact alternative in antifouling membranes but is yet to be studied against marine foulants. In this work, the use of morpholine moieties to generate amphiphilicity in a poly(dimethylsiloxane) (PDMS)-based antifouling and fouling-release coating was explored. Two separate coating sets were investigated. The first set examined the incorporation of an N-substituted morpholine amine, and while these coatings showed promising fouling-release properties for Ulva linza, they had unusually high settlement of spores compared to controls. Based on those results, a second set of materials was synthesized using an N-substituted morpholine amide to probe the source of the high settlement and was found to significantly improve antifouling performance. Both coating sets included PEG controls with varying lengths to compare the viability of the morpholine structures as alternative hydrophilic groups. Surfaces were evaluated through a combination of bubble contact angle goniometry, profilometry, X-ray photoelectron spectroscopy (XPS), and marine bioassays against two soft fouling species, U. linza and Navicula incerta, known to have different adhesion characteristics.

A disulfide-based linker for thiol-norbornene conjugation: formation and cleavage of hydrogels by the use of light

Photolabile groups are the key components of photo-responsive polymers, dynamically tunable materials with multiple applications in materials and life sciences. They usually consist of a chromophore and a labile bond and are inherently light sensitive. An exception are disulfides, simple reversible linkages, which become photocleavable upon addition of a photoinitiator. Despite their practical features, disulfides are rarely utilized due to their impractical formation. Here, we report a disulfide-based linker series bearing norbornene terminals for facile crosslinking of thiol-functionalized macromers via light-triggered thiol-ene conjugation (TEC). Besides finding a highly reactive lead compound, we also identify an unexpected TEC-retardation, strongly dependent on the molecular linker structure and affecting hydrogel stability. Finally, we present a useful method for localized disulfide cleavage by two-photon irradiation permitting micropatterning of disulfide-crosslinked networks.

Photoimageable Organic Coating Bearing Cyclic Dithiocarbonate for a Multifunctional Surface

We report the fabrication of photocross-linkable and surface-functionalizable polymeric thin films using reactive cyclic dithiocarbonate (DTC)-containing copolymers. The chemical functionalities of these material surfaces were precisely defined with light illumination. The DTC copolymers, namely, poly(dithiocarbonate methylene methacrylate-random-alkyl methacrylate)s, were synthesized via reversible addition-fragmentation chain transfer polymerization, and the reaction kinetics was thoroughly analyzed. The copolymers were cross-linked into a coating using a bifunctional urethane cross-linker that contains a photolabile o-nitrobenzyl group and releases aniline upon exposure to light. The nucleophilic attack of the aromatic amine opens the DTC group, forming a carbamothioate bond and generating a reactive thiol group in the process. The surface concentrations of the unreacted DTC and thiol were effectively controlled by varying the amounts of the copolymer and the cross-linker. The use of methacrylate comonomers led to additional reactive surface functionality such as carboxylic acid via acid hydrolysis. The successful transformations of the resulting DTC, thiol, and carboxylic acid groups to different functionalities via sequential nucleophilic ring opening, thiol-ene, and carbodiimide coupling reactions under ambient conditions were confirmed quantitatively using X-ray photoelectron spectroscopy. The presented chemistries were readily adapted to the immobilization of complex molecules such as a fluorophore and a protein in lithographically defined regions, highlighting their potential in creating organic coatings that can have multiple functional groups under ambient conditions.

One-Pot Construction of Sulfur-Rich Thermoplastic Elastomers Enabled by Metal-Free Self-Switchable Catalysis and Air-Assisted Coupling

Construction of well-defined sulfur-rich macromolecules in a facile manner is an interesting but challenging topic. Herein, we disclose how to readily construct well-defined triblock sulfur-rich thermoplastic elastomers via a self-switchable isothiocyanate/episulfide copolymerization and air-assisted oxidative coupling strategy. During self-switchable polymerization, alternating copolymerization of isothiocyanate and episulfide occurs initially due to the lower energy barrier for isothiocyanate insertion with respect to successive episulfide ring-opening. After exhaustion of isothiocyanate, ring-opening polymerization of episulfide begins, providing diblock polymers. Subsequent exposure of the reaction to air leads to a transformation of diblock copolymers into triblock thermoplastic elastomers. This protocol can be extended to diverse isothiocyanates and episulfides, allowing fine-tuning of the performance of the produced sulfur-rich thermoplastic elastomers.

Effect of cell culture media on photopolymerizations

Radical polymerization is one of the most widely used methods for the synthesis of polymeric materials for biomedical applications, such as drug delivery, 3D cell culture, and regenerative medicine. Among radical polymerization reactions, thiol-ene click chemistry has shown excellent orthogonality in diverse reaction conditions. However, our preliminary investigations revealed that it fails in cell culture environment. Herein, we investigate the mechanisms by which cell culture media interfere with radical photoreactions. Three different models including free radical linear photopolymerization (N,N-dimethylacrylamide photopolymerization), free radical photohydrogelation (poly(ethylene glycol) diacrylate photohydrogelation), and thiol-ene photohydrogelation (4-arm poly(ethylene glycol)-norbornene thiol-ene photohydrogelation) were investigated. We showed that common cell culture media ingredients can interfere with radical polymerization by two different pathways; namely, radical chain transfer and radical scavenging effects. Thiol-ene photoclick hydrogelation was seriously affected by cell culture media especially under the alkaline conditions of many of them, due to the impact of deprotonation of the thiol reactant. We intend these findings to serve as a reference guide to researchers employing free radical-based molecular synthesis in cell culture settings. The nonbenign impact of media components, pH, and concentration should provide a cue for future studies that aim to prepare well-defined polymeric materials in the presence of cell culture media.

Visible-Light Photoinitiation of (Meth)acrylate Polymerization with Autonomous Post-conversion

Conversion plateaus rapidly in radical photopolymerizations (RPPs) following discontinuation of irradiation due to rapid termination of reactive radicals, which restricts the wider use of RPPs in applications that involve nonuniform light access including those with attenuated light transmission or irregular surfaces. Based on our recent report of a radical dark-curing photoinitiator (DCPI) that continues polymerization beyond the cessation of irradiation by enabling latent redox initiation with photo-released amine in the presence of a suitable oxidant, we developed a new DCPI with an absorption spectrum that extends well into the visible range. Our design process involved a series of computational investigations of candidate molecules, including a systematic study of substituents and their position-dependent effects on absorption characteristics, electronic transitions, and the photochemical mechanism and its associated energetics. Our quantum chemical computations identified the target compound 5,7-dimethoxy-6-bromo-3-aroylcoumarin-DMPT/BPh4 and predicted that it would facilitate the dark-curing mechanism by concurrent photo-radical generation and photo-induced release of an efficient redox reductant under visible irradiation. This reductant-tethered chromophore was then synthesized and optically characterized with UV-vis spectroscopy that revealed its strong visible-light absorption with a molar absorptivity of 5710 M-1 cm-1 at 405 nm and 50 M-1 cm-1 at 455 nm. We then demonstrated extensive dark-curing of >35% additional conversion over 25 min following brief activation of the shelf-stable one-part system by irradiation with a 455 nm LED that was ceased at 20% conversion. In contrast, shuttering irradiation of the control formulation at that same point resulted in immediate cessation of conversion, which plateaued at 20%. We determined a remarkable initiator efficiency of 2.82 that results from the additional redox-generated radicals with a 77% photo-reductant generation quantum yield. The combination of superior photo- and dark-curing efficiencies of this new visible DCPI is expected to open new application opportunities in RPP, especially those involving resins that are highly light attenuating, surfaces that possess irregular features that produce uneven irradiance, and production lines where continued dark-curing downstream of the light activation step enhances line efficiencies.

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.

Photochemical Methods for Peptide Macrocyclisation

Photochemical reactions have been the subject of renewed interest over the last two decades, leading to the development of many new, diverse and powerful chemical transformations. More recently, these developments have been expanded to enable the photochemical macrocyclisation of peptides and small proteins. These constructs benefit from increased stability, structural rigidity and biological potency over their linear counterparts, providing opportunities for improved therapeutic agents. In this review, an overview of both the established and emerging methods for photochemical peptide macrocyclisation is presented, highlighting both the limitations and opportunities for further innovation in the field.

The Effect of Thiol Structure on Allyl Sulfide Photodegradable Hydrogels and their Application as a Degradable Scaffold for Organoid Passaging

Intestinal organoids are useful in vitro models for basic and translational studies aimed at understanding and treating disease. However, their routine culture relies on animal-derived matrices that limit translation to clinical applications. In fact, there are few fully defined, synthetic hydrogel systems that allow for the expansion of intestinal organoids. Here, an allyl sulfide photodegradable hydrogel is presented, achieving rapid degradation through radical addition-fragmentation chain transfer (AFCT) reactions, to support routine passaging of intestinal organoids. Shear rheology to first characterize the effect of thiol and allyl sulfide crosslink structures on degradation kinetics is used. Irradiation with 365 nm light (5 mW cm-2 ) in the presence of a soluble thiol (glutathione at 15 × 10-3 m), and a photoinitiator (lithium phenyl-2,4,6-trimethylbenzoylphosphinate at 1 × 10-3 m), leads to complete hydrogel degradation in less than 15 s. Allyl sulfide hydrogels are used to support the formation of epithelial colonies from single intestinal stem cells, and rapid photodegradation is used to achieve repetitive passaging of stem cell colonies without loss in morphology or organoid formation potential. This platform could support long-term culture of intestinal organoids, potentially replacing the need for animal-derived matrices, while also allowing systematic variations to the hydrogel properties tailored for the organoid of interest.

Reaction Environment Effect on the Kinetics of Radical Thiol-Ene Polymerizations in the Presence of Amines and Thiolate Anions

Because of facile implementation, quantitative conversions, and an insensitivity to oxygen, water, and most organic functional groups, radical-mediated thiol-ene coupling (TEC) reactions have emerged as a valuable tool in macromolecule synthesis. It was recently demonstrated that the kinetics and conversions of thiyl radical-mediated reactions are adversely affected in the presence of basic amines by the formation of retardive thiolate anions. Herein, the performance of TEC polymerizations is evaluated under a variety of reaction environments with the intention to aid in the optimal formulation design of TEC reactions in the presence of amines. Results from both bulk and aqueous-phase network photopolymerizations established that sensitivity to amine basicity and pH is dependent on the thiol acidity, although norbornene-type alkenes exhibit a unique ability to achieve high conversions, where allyl ethers, vinyl ether, and vinyl siloxanes are highly inhibited. Additionally, the protic solvents such as alcohols and acetic acid are established as ideal solvents or additives to suppress or eliminate amine-induced retardation.

Rational Design of Efficient Amine Reductant Initiators for Amine-Peroxide Redox Polymerization

Amine-peroxide redox polymerization (APRP) has been highly prevalent in industrial and medical applications since the 1950s, yet the initiation mechanism of this radical polymerization process is poorly understood so that innovations in the field are largely empirically driven and incremental. Through a combination of computational prediction and experimental analysis, we elucidate the mechanism of this important redox reaction between amines and benzoyl peroxide for the ambient production of initiating radicals. Our calculations show that APRP proceeds through S_N2 attack by the amine on the peroxide but that homolysis of the resulting intermediate is the rate-determining step. We demonstrate a correlation between the computationally predicted initiating rate and the experimentally measured polymerization rate with an R² = 0.80. The new mechanistic understanding was then applied to computationally predict amine reductant initiators with faster initiating kinetics. This led to our discovery of N-(4-methoxyphenyl)pyrrolidine (MPP) as amine reductant, which we confirmed significantly outperforms current state-of-the-art tertiary aromatic amines by ∼20-fold, making it the most efficient amine-peroxide redox initiator to date. The application of amines with superior kinetics such as MPP in APRP could greatly accelerate existing industrial processes, facilitate new industrial manufacturing methods, and improve biocompatibility in biomedical applications conducted with reduced initiator concentrations yet higher overall efficiency.

Kinetic Model under Light-Limited Condition for Photoinitiated Thiol-Ene Coupling Reactions

Thiol-ene click chemistry has become a powerful paradigm in synthesis, materials science, and surface modification in the past decade. In the photoinitiated thiol-ene reaction, an induction period is often observed before the major change in its kinetic curve, for which a possible mechanism is proposed in this report. Briefly, light soaking generates radicals following the zeroth-order reaction kinetics. The radical is the reactant that initializes the chain reaction of thiol-ene coupling, which is a first-order reaction. Combining both and under the light-limited conditions, a surprising kinetics represented by a Gaussian-like model evolves that is different from the exponential model used to describe the first-order reaction of the final product. The experimental data are fitted well with the new model, and the reaction kinetic constants can be pulled out from the fitting.

Photopolymerized dynamic hydrogels with tunable viscoelastic properties through thioester exchange

The extracellular matrix (ECM) constitutes a viscoelastic environment for cells. A growing body of evidence suggests that the behavior of cells cultured in naturally-derived or synthetic ECM mimics is influenced by the viscoelastic properties of these substrates. Adaptable crosslinking strategies provide a means to capture the viscoelasticity found in native soft tissues. In this work, we present a covalent adaptable hydrogel based on thioester exchange as a biomaterial for the in vitro culture of human mesenchymal stem cells. Through control of pH, gel stoichiometry, and crosslinker structure, viscoelastic properties in these crosslinked networks can be modulated across several orders of magnitude. We also propose a strategy to alter these properties in existing networks by the photo-uncaging of the catalyst 4-mercaptophenylacetic acid. Mesenchymal stem cells encapsulated in thioester hydrogels are able to elongate in 3D and display increased proliferation relative to those in static networks.

A user's guide to the thiol-thioester exchange in organic media: scope, limitations, and applications in material science

The dynamic exchange of thiols and thioesters in organic media was explored, leading to room temperature plasticity in crosslinked polymers.