Thiol-acrylate Reaction Research Articles

Programming structural-colour modulation range of elastomer enables information storage

ABSTRACT Cholesteric liquid crystal elastomers (CLCEs) has garnered significant attention in the field of information display and storage. Obtaining multiple structural colours in a single elastomer film is highly desirable but presents a considerable challenge. Here, we have successfully achieved manipulable range of structural colours in a CLCE film by ingeniously introducing the photopolymerisation during thiol-acrylate reaction process. This particular strategy involves triggering spatial shrinkage of the polymer network in a pre-designed region following solvent evaporation, resulting in a blue shift of the structural colour compared to the background. Consequently, we can construct multi-colour and multi-pattern elastomer films by precisely controlling the localised polymerisation. Notably, these patterns can be concealed in a reversible manner as the film undergoes a specific degree of elongation and can be restored when external force is removed. This work demonstrates a straightforward method of programming structural-colour modulation range, eliminating the need for material changes or intricate preparation processes, broadening the applications of stretchable elastomers in information storage, encryption, anti-counterfeiting, and dynamic photonics.

Simultaneous photo-induced polymerization and surface modification by microfluidic spinning to produce functionalized polymer microfibers: the effect of their surface modification on cell adhesion

A capillary-based microfluidic device is used to prepare functionalized polymer microfibers in one-step involving monomer photopolymerization in the core phase and surface modification of the fibers by thiol-acrylate reactions at the interface.

Phenothiazine-based oxime ester as a photobase generator for thiol-acrylate reactions

In this work, a previously designed and synthesized phenothiazine-based oxime ester was investigated as a photobase generator for the formation of thiol-acrylate networks through the polymerization of thiol-acrylate reactions and more appealingly the base-catalyzed thiol-acrylate Michael addition reactions. This system accomplished a complete disappearance and a full conversion of the characteristic acrylate peak within 10 min of irradiation with a light-emitting diode (λ = 405 nm) or within 1 or 2 min of the same irradiation source and 24 h of heat exposure (T = 60 °C). Photobase generation was evidenced using phenol red as a pH-indicator, enabling to confirm the production of methyl amine (CH3NH2) as initiating species. Photoinitiation mechanism through which base reactive species are produced and the reactions by which the thiol-acrylate polymerization proceeds were investigated according to the base detection experiments, the obtained real-time Fourier transform infrared spectroscopy results, and according to the general photochemical base-generation reactions of oxime-esters. Finally, differential scanning calorimetry scans and Shore A and Shore D hardness experiments have been performed on the polymers obtained to examine their thermal properties and surface hardness features.

Thiol-Michael Addition Microparticles: Their Synthesis, Characterization, and Uptake by Macrophages.

Polymeric microparticles are promising biomaterial platforms for targeting macrophages in the treatment of disease. This study investigates microparticles formed by a thiol-Michael addition step-growth polymerization reaction with tunable physiochemical properties and their uptake by macrophages. The hexafunctional thiol monomer dipentaerythritol hexa-3-mercaptopropionate (DPHMP) and tetrafunctional acrylate monomer di(trimethylolpropane) tetraacrylate (DTPTA) were reacted in a stepwise dispersion polymerization, achieving tunable monodisperse particles over a size range (1-10 μm) relevant for targeting macrophages. An off-stoichiometry thiol-acrylate reaction afforded facile secondary chemical functionalization to create particles with different chemical moieties. Uptake of the microparticles by RAW 264.7 macrophages was highly dependent on treatment time, particle size, and particle chemistry with amide, carboxyl, and thiol terminal chemistries. The amide-terminated particles were non-inflammatory, while the carboxyl- and thiol-terminated particles induced pro-inflammatory cytokine production in conjunction with particle phagocytosis. Finally, a lung-specific application was explored through time-dependent uptake of amide-terminated particles by human alveolar macrophages in vitro and mouse lungs in vivo without inducing inflammation. The findings demonstrate a promising microparticulate delivery vehicle that is cyto-compatible, is non-inflammatory, and exhibits high rates of uptake by macrophages.

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 SN 2 process can be competitive (i. e. within <2.5 kcal/mol) in less polar solvents.

Evaluating Nucleophile Byproduct Formation during Phosphine- and Amine-Promoted Thiol-Methyl Acrylate Reactions.

The commonly accepted mechanism of nucleophile-initiated thiol-acrylate reactions requires the formation of undesired nucleophile byproducts. A systematic evaluation of the formation of such nucleophile byproducts has been carried out to understand the relationships between byproduct formation and nucleophile structure, stoichiometry, solvent, and reaction type. Three common nucleophiles for thiol-Michael reactions were investigated: dimethylphenylphosphine (DMPP), diethylamine (DEA), and hexylamine (HA). The formation of phosphonium ester and aza-Michael byproducts upon initiating a representative thiol-acrylate reaction between 1-hexanethiol and methyl acrylate at a range of initiator loading (0.01-10.0 equiv) and in different solvents (neat, DMSO, THF, and CHCl3) was determined by 1H NMR spectroscopy. The influence of reaction type was investigated by expanding from small molecule reactions to end group thiol-acrylate functionalization of PEG-diacrylate polymers and through investigations of polymer-polymer coupling reactions. Results indicate that the propensity of forming nucleophile byproducts varies with nucleophile type, solvent, and reaction type. Interestingly, for all but polymer-polymer ligation reactions, nucleophile byproduct formation is largely unobserved for nitrogen-centered nucleophiles DEA and HA and essentially nonexistent for the phorphorous-centered nucleophile DMPP. A rationale for the differences in nucleophile byproduct formation for DMPP, DEA, and HA is proposed and supported by experimental and computational analysis.

Effects of crosslinking agent/diluents/thiol on morphology of the polymer matrix and electro-optical properties of polymer-dispersed liquid crystal

ABSTRACTTo study effects of the crosslinking agent/diluents/thiol on morphology of the polymer matrix and the electro-optical properties of polymer-dispersed liquid crystal (PDLC) films, samples were prepared by ultraviolet (UV)-initiated polymerisation. Due to the interaction between thiol–acrylate reaction and acrylate monomers polymerisation, the sample compositions were the foremost determinant to the microstructures which in turn played an essential role on the electro-optical properties of the PDLC films. With the increasing content of the crosslinking agent, the LC droplet size decreased, while the thiol had a contrary effect on the LC droplet size. It was demonstrated that the superior properties of the low-driven voltage (37.2 V), the high contrast ratio (148.2), the short response time (14.9 ms) and the high saturation transmittance (86.6%) could benefit from a novel microstructure which had a dense surface and meshes with microspheres attached. It was of great significance for the optimisation and the potential applications of the PDLC films.

A Study on the Electro-Optical Properties of Thiol-Ene Polymer Dispersed Cholesteric Liquid Crystal (PDChLC) Films.

In this study, a polymer dispersed cholesteric liquid crystal (PDChLC) film obtained via a one-step fabrication technique based on photopolymerization of a thiol-acrylate reaction system was prepared and characterized for the first time. The effects of the chiral dopant, the influence of thiol monomer functionality and content on the morphology and subsequent performance of the PDChLC films were systematically investigated. It was demonstrated that the addition of a small amount of chiral dopant slightly increased the driving voltage, but decreased the off-state transmittance significantly. Furthermore, scanning electron micrographs (SEM) shown that the liquid crystal (LC) droplet size decreased at first and then increased with the increasing amount of thiol monomer functionality, while increasing the thiol content increased the LC droplet size. Correspondingly, the electro-optical switching behavior was directly dependent on LC droplet size. By tuning the raw material composition, PDChLC film with optimized electro-optical performance was prepared.

Effect of a Polymercaptan Material on the Electro-Optical Properties of Polymer-Dispersed Liquid Crystal Films.

Polymer-dispersed liquid crystal (PDLC) films were prepared by the ultraviolet-light-induced polymerization of photopolymerizable monomers in nematic liquid crystal/chiral dopant/thiol-acrylate reaction monomer composites. The effects of the chiral dopant and crosslinking agents on the electro-optical properties of the PDLC films were systematically investigate. While added the chiral dopant S811 into the PDLC films, the initial off-state transmittance of the films was decreased. It was found that the weight ratio among acrylate monomers, thiol monomer PETMP and the polymercaptan Capcure 3-800 showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol-acrylate reaction and acrylate monomer polymerization reaction. While adding polymercaptans curing agent Capcure 3-800 with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers.

Effects of functionality of thiol monomer on electro-optical properties of polymer-dispersed liquid crystal films

ABSTRACTIn this paper, polymer-dispersed liquid crystal (PDLC) films which based on the acrylate and the thiol monomers were first prepared by ultraviolet-initiated polymerisation. The electro-optical properties and morphologies of the PDLC films were systematically investigated. The functionality of thiol monomers and their feed ratio showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol–acrylate reaction and acrylate monomer polymerisation reaction. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers. When added four-functional thiol monomer PETMP with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved.

Copper-Coated Liquid-Crystalline Elastomer via Bioinspired Polydopamine Adhesion and Electroless Deposition.

This study explores the functionalization of main-chain nematic elastomers with a conductive metallic surface layer using a polydopamine binder. Using a two-stage thiol-acrylate reaction, a programmed monodomain is achieved for thermoreversible actuation. A copper layer (≈155 nm) is deposited onto polymer samples using electroless deposition while the samples are in their elongated nematic state. Samples undergo 42% contraction when heated above the isotropic transition temperature. During the thermal cycle, buckling of the copper layer is seen in the direction perpendicular to contraction; however, transverse cracking occurs due to the large Poisson effect experienced during actuation. As a result, the electrical conductivity of the layer reduced quickly as a function of thermal cycling. However, samples do not show signs of delamination after 25 thermal cycles. These results demonstrate the ability to explore multifunctional liquid-crystalline composites using relatively facile synthesis, adhesion, and deposition techniques.

Hybrid thiol-acrylate-epoxy polymer networks: Comparison of one-pot synthesis with sequential reactions and shape memory properties

Hybrid polymer networks are synthesized using thiol-, acrylate- and epoxide-functionalized reactants. The thiols react with both acrylate and epoxide groups, but acrylate and epoxide groups are unreactive with each other. Past research efforts on this system have employed a combination of sequential photo-initiated and thermally initiated reactions of thiols with (meth)acrylate and epoxide functional groups and have led to single-phase materials. Analysis of reaction kinetics shows that thiol-acrylate reactions are much faster than thiol-epoxy reactions; consequently, we have also developed a one-pot synthesis of thiol-acrylate-epoxy hybrid networks using room temperature reactions and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalyst. For the DBU-catalyzed reaction conditions tested, such one-pot reactions yield materials with properties identical within error to those of materials made by sequential reactions involving thiol-acrylate first and thiol-epoxide second. With low molecular weight reactants, homogeneous, single-phase materials are produced which exhibit the same thermal and mechanical properties regardless of whether reactions are sequential or simultaneous. With some higher molecular weight reactants, e.g., 2000 g/mol diacrylate, novel phase-separated thiol-acrylate-epoxide polymer networks are produced with properties that are highly tunable by sequential reaction order. Beyond good mechanical properties, some of the resulting phase-separated networks are very good shape memory polymers, with shape-fixity values above 95% and shape-recovery values above 99% after multiple cycles.

Transdermal thiol-acrylate polyethylene glycol hydrogel synthesis using near infrared light.

Light-induced polymerization has been widely applied for hydrogel synthesis, which conventionally involves the use of ultraviolet or visible light to activate a photoinitiator for polymerization. However, with these light sources, transdermal gelation is not efficient and feasible due to their substantial interactions with biological systems, and thus a high power is required. In this study, we used biocompatible and tissue-penetrating near infrared (NIR) light to remotely trigger a thiol-acrylate reaction for efficient in vivo gelation with good controllability. Our gelation system includes gold nanorods as a photothermal agent, a thermal initiator, diacrylate polyethylene glycol (PEG), and thiolated PEG. Irradiation with a low-power NIR laser (0.3 W cm(-2)) could induce gelation via a mixed-mode reaction with a small increase in temperature (∼5 °C) under the optimized conditions. We also achieved successful transdermal gelation via the NIR-assisted photothermal thiol-acryl reactions. This new type of NIR-assisted thiol-acrylate polymerization provides new opportunities for in situ hydrogel formation for injectable hydrogels and delivery of drugs/cells for various biomedical applications.

Tailorable and programmable liquid-crystalline elastomers using a two-stage thiol–acrylate reaction

A methodology is introduced to synthesize main-chain liquid-crystalline elastomers (LCEs) using a thiol–acrylate-based reaction. This method can program an aligned LCE monodomain and offer spatio-temporal control over liquid-crystalline behavior.

Photo-CuAAC Induced Wrinkle Formation in a Thiol–Acrylate Elastomer via Sequential Click Reactions

Two sequential click reactions are implemented in a facile methodology used to generate well-defined spatiotemporally controlled and persistent wrinkles on the surface of an elastomer. The click thiol-Michael addition reaction was utilized to form a cross-linked polymer with residual, reactive alkyne sites that remained tethered throughout the network. The latent, unreacted alkyne sites are subsequently reacted with diazide monomers via a photoinduced Cu(I)-catalyzed alkyne–azide cycloaddition (CuAAC) reaction to increase the cross-link density. Increased cross-linking raised the modulus and glass transition temperature from 1.6 MPa and 2 °C after the thiol–acrylate reaction to 4.4 MPa and 22 °C after the CuAAC reaction. However, the second-stage photopolymerization of the CuAAC reaction is significantly spatially restricted via limited Cu(II) ion diffusion into the thiol–acrylate elastomer, thereby creating the desired cross-linking gradient throughout the depth of the initial network and leading to the formation of a highly cross-linked skin layer. This approach leads to the formation of well-defined, persistent, reproducible wrinkles on the surface of the material with wavelength and amplitude of 8.50 ± 1.60 and 1.41 μm, respectively, for a polymer with a 1280 μm total film thickness. Control over the wavelength and amplitude of these wrinkles using the resin film thickness is further demonstrated by studying the surface profiles using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Additionally, this novel technique allows the spatial selectivity of wrinkle formation with a wrinkled area that is only 8 μm wider than the photomasked area. This strategy represents a unique approach to generate photodirected wrinkling on the entire surface of the elastomer in one single step.

Modifying Surface Resistivity and Liquid Moisture Management Property of Keratin Fibers through Thiol–Ene Click Reactions

This paper reports on a new method for improving the antistatic and liquid moisture management properties of keratinous materials. The method involves the generation of thiols by controlled reduction of cystine disulfide bonds in keratin with tris(2-carboxyethyl) phosphine hydrochloride and subsequent grafting of hydrophilic groups onto the reduced keratin by reaction with an acrylate sulfonate or acrylamide sulfonate through thiol-ene click chemistry. The modified substrates were characterized with Raman spectroscopy and scanning electron microscopy and evaluated for their performance changes in liquid moisture management, surface resistivity, and wet burst strength. The results have revealed that the thiol-acrylate reaction is more efficient than the thiol-acrylamide reaction, and the keratinous substrate modified with an acrylate sulfonate salt exhibits significantly improved antistatic and liquid moisture management properties.

Degradable thiol-acrylate hydrogels as tunable matrices for three-dimensional hepatic culture

A degradable poly(ethylene glycol)-diacrylate (PEGDA) hydrogel system was developed using simple macromer formulations and visible light initiated thiol-acrylate photopolymerization. In addition to PEGDA, other components in this gelation system include eosin-Y as a photo-sensitizer, bi-functional thiol (dithiothreitol, DTT) as a dual-purpose co-initiator and cross-linker, and N-vinylpyrrolidone (NVP) as a co-monomer. Gelation was achieved through a mixed-mode step-chain growth polymerization mechanism under bright visible light exposure. Increasing photo-sensitizer or NVP concentrations accelerated photo-crosslinking and increased final gel stiffness. Increasing bi-functional thiol content in the prepolymer solution only increased gel stiffness to some degree. As the concentration of thiol surpassed certain range, thiol-mediated chain-transfer events caused thiol-acrylate gels to form with lower degree of cross-linking. Pendant peptide, such as integrin ligand RGDS, was more effectively immobilized in the network via a thiol-acrylate reaction (using thiol-bearing peptide Ac-CRGDS. Underline indicates cross-linkable motif) than through homo-polymerization of acrylated peptide (e.g., acryl-RGDS). The incorporation of pendant peptide comes with the expense of a lower degree of gel cross-linking, which was rectified by increasing co-monomer NVP content. Without the use of any readily degradable macromer, these visible light initiated mixed-mode cross-linked hydrogels degraded hydrolytically due to the formation of thiol-ether-ester bonds following thiol-acrylate reactions. An exponential growth relationship was identified between the hydrolytic degradation rate and bifunctional thiol content in the prepolymer solution. Finally, we evaluated the cytocompatibility of these mixed-mode cross-linked degradable hydrogels using in situ encapsulation of hepatocellular carcinoma Huh7 cells. Encapsulated Huh7 cells remained alive and proliferated as time to form cell clusters. The addition of NVP at a higher concentration (0.3%) did not affect Huh7 cell viability but resulted in reduction of cell metabolic activity, which was accompanied by an elevated urea secretion from the encapsulated cells.

Visible light cured thiol-vinyl hydrogels with tunable degradation for 3D cell culture

We report here a synthetically simple yet highly tunable and diverse visible light mediated thiol-vinyl gelation system for fabricating cell-instructive hydrogels. Gelation was achieved via a mixed-mode step-and-chain-growth photopolymerization using functionalized 4-arm poly(ethylene glycol) as backbone macromer, eosin-Y as photosensitizer, and di-thiol containing molecule as dual purpose co-initiator/cross-linker. N-vinylpyrrolidone (NVP) was used to accelerate gelation kinetics and to adjust the stiffness of the hydrogels. Visible light (wavelength: 400–700nm) was used to initiate rapid gelation (gel points: ∼20s) that reached completion within a few minutes. The major differences between current thiol-vinyl gelation and prior visible light mediated photopolymerization are that: (1) the co-initiator triethanolamine (TEA) used in the previous systems was replaced with multifunctional thiols and (2) mixed-mode polymerized gels contain less network heterogeneity. The gelation kinetics and gel properties at the same PEG macromer concentration could be tuned by changing the identity of vinyl groups and di-thiol cross-linkers, as well as concentration of cross-linker and NVP. Specifically, acrylate-modified PEG afforded the fastest gelation rate, followed by acrylamide and methacrylate-functionalized PEG. Increasing NVP concentration also accelerated gelation and led to a higher network cross-linking density. Further, increasing di-thiol peptide concentration in the gel formulation increased hydrogel swelling and decreased gel stiffness. Due to the formation of thiol-ether-ester bonds following thiol-acrylate reaction, the gels degraded hydrolytically following a pseudo first order degradation kinetics. Degradation rate was controlled by adjusting thiol or NVP content in the polymer precursor solution. The cytocompatibility and utility of this hydrogel system were evaluated using in situ encapsulation of human mesenchymal stem cells (hMSC). Encapsulated hMSCs remained alive (>90%) throughout the duration of the study and the cells were differentiated down osteogenic lineage with varying degrees by controlling the rate and mode of gel degradation.

Thiol–epoxy/thiol–acrylate hybrid materials synthesized by photopolymerization

Thiol–epoxy/thiol–acrylate hybrid networks with systematic variations (100/0, 75/25, 50/50, 25/75, 0/100, w/w) were prepared by free-radical photoinitiator (ITX) and photo-base generator (TBD·HBPh4) induced photopolymerization. The enhanced spectral sensitivity of TBD·HBPh4 at long wavelengths (320–500 nm) with aid of ITX was capable of in situ generation of a strong base (TBD) to achieve the relatively fast and essentially quantitative thiol–epoxy click reaction. The ITX/TBD·HBPh4 photoinitiating system, which generated both the free radicals and the strong base upon UV exposure, could induce thiol–epoxy/thiol–acrylate hybrid polymerizations. The kinetics investigated with real-time IR indicated that the thiol–acrylate reactions were faster and more efficient than the thiol–epoxy reactions (>95% conversion in a matter of seconds and minutes, respectively), and the thiol conversion increased with an increase in the epoxy concentration. The incorporation of the thiol–epoxy reaction offered several advantages: the polymerization shrinkage decreased with the increase in the thiol–epoxy content due to the low shrinkage factor for the thiol–epoxy system (every mole of epoxy group polymerized produces 6.3 mL of shrinkage). The glass transition temperature of the thiol–epoxy/thiol–acrylate hybrid networks progressively increased as a function of the thiol–epoxy content, also resulting in enhanced mechanical and physical properties. This work will be helpful to make optical and electronic devices with low shrinkage and stress, decreased residual monomers, and improved mechanical properties.

Multifunctional thiols as additives in UV-cured PEG-diacrylate membranes for CO 2 separation

The utility of incorporating only 20 mol% of multifunctional thiols into UV-cured poly(ethyleneglycol)diacrylate (PEGDA) membranes for CO 2 separation is presented. Five different multifunctional thiols were investigated, spanning a range of functionalities from 2 to 4 and also different rigidities. All thiol-modified networks exhibited lower modulus values according to both small-strain and large-strain mechanical testing. This decrease in modulus was primarily attributed to a decrease in crosslink density via the step-growth thiol–acrylate reaction. Decreased crosslink density was more pronounced for dithiol additives which formed a “train-like” topology of connected PEGDA segments. Because of this more linear topology, dithiol-modified networks exhibited increases in gas permeability and strain at break. CO 2 selectivity of each thiol-modified network was nearly comparable to the unmodified PEGDA network. Each network rapidly reached 100% conversion under a nitrogen atmosphere. However in air, the pure acrylate network reached negligible conversion while the networks modified with tri and tetrathiols reached noticeably higher conversions. Outdoor sunlight curing revealed that all thiol-modified networks were tack-free after only several minutes depending on thiol functionality. In contrast, the pure acrylate membrane remained tacky and uncured on the surface indefinitely. This study emphasizes the importance of thiol monomer selection in diminishing oxygen inhibition in UV-initiated acrylate polymerization as well as marked improvement in permeability for this specific PEGDA network via the incorporation of dithiols.