Thiol-ene Polymerization Research Articles

Alpha-Amylase Immobilization on Epoxy Containing Thiol-Ene Photocurable Materials

Thiol-ene polymerization is a versatile tool for several applications. Here we report the preparation of epoxide groups containing thiol-ene photocurable polymeric support and the covalent immobilization of alpha-amylase onto these polymeric materials. The morphology of the polymeric support was characterized by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) coupled with SEM was used to explore the chemical composition. The polymeric support and the immobilization of the enzyme were characterized by FTIR analysis. SEM-EDS and FTIR results showed that the enzyme was successfully covalently attached to the polymeric support. The immobilization efficiency and enzyme activity of alpha-amylase were examined at various pH (5.0-8.0) and temperature (30-80 degrees C) values. The storage stability and reusability of immobilized alpha-amylase were investigated. The immobilization yield was 276 +/- 1.6 mg per gram of polymeric support. Enzyme assays demonstrated that the immobilized enzyme exhibited better thermostability than the free one. The storage stability and reusability were improved by the immobilization on this enzyme support. Free enzyme lost its activity completely within 15 days. On the other hand, the immobilized enzyme retained 86.7% of its activity after 30 days. These results confirm that alpha-amylase was successfully immobilized and gained a more stable character compared with the free one.

Preparation of Reactive Three‐Dimensional Microstructures via Direct Laser Writing and Thiol‐ene Chemistry

Three-dimensional microstructures are fabricated employing the direct laser writing process and radical thiol-ene polymerization. The resin system consists of a two-photon photoinitiator and multifunctional thiols and olefins. Woodpile photonic crystals with 22 layers and a rod distance of 2 μm are fabricated. The structures are characterized via scanning electron microscopy and focused ion beam milling. The thiol-ene polymerization during fabrication is verified via infrared spectroscopy. The structures are grafted in a subsequent thiol-Michael addition reaction with different functional maleimides. The success of the grafting reaction is evaluated via laser scanning microscopy and X-ray photoelectron spectroscopy. The grafting density is calculated to be close to 200 molecules μm(-2) .

Thiol-ene polymer based fast photo-curable gate insulator for organic field effect transistors

We report the synthesis and properties of fast photo-cured polymer dielectrics used as top gate insulators in organic field-effect transistors (OFETs). Two thiol functionalized compounds and a vinyl ether monomer were used to formulate thiol-ene polymers. The kinetic of the photo-curing of these formulations was determined by monitoring the curing process by FT-IR spectroscopy. Guided by measurements on metal–insulator–metal and metal–insulator–semiconductor structures, we determined the composition of the thiol-ene polymer and the optimal time of exposure to UV-irradiation to fabricate high performance poly(3-hexylthiophene)-based OFETs. We cured a mixture of a mercapto-functionalized polysiloxane produced with sol–gel process, 1,4-cyclohexanedimethanol divinyl ether and irgacure 127 in 15 s. The produced transistors have high on/off ratios of 105. The OFETs showed a field effect mobility of 10−2 cm2 V−1 s−1 and low threshold voltage of −3 V.

Thiol-containing polymeric embedding materials for nanoskiving

This paper describes the characterization of new embedding resins for nanoskiving (ultramicrotomy) that contain thiols. Nanoskiving is a technique to produce nanoscale structures using an ultramicrotome to section thin films of materials (e.g., gold) embedded in polymer. Epoxies are used typically as embedding resins for microtomy. Epoxies, however, do not adhere well to gold or other smooth metallic structures that are used commonly for nanoskiving. Thiol–ene and thiol–epoxy polymers provide improved adhesion to gold due to the thiol functional group. In addition, the thiol–ene polymers can be prepared within minutes using photopolymerization, which allows for rapid prototyping. Two commercial thiol-containing adhesives were evaluated as resins in addition to several formulations of commercially available monomers. The important physical and mechanical properties for microtomy of these unconventional embedding resins were characterized and the properties were compared to commercial epoxy resins. Gold nanowires were fabricated using nanoskiving of gold films embedded in these unconventional resins. These studies show that a 3 : 4 mixture of thiol (pentaerythritol tetra(3-mercaptopropionate)) and ene (triallyl-1,3,5-triazine-2,4,6-trione) works very well as a resin for nanoskiving and provides improved adhesion and reduced preparation time relative to epoxies.

Redox initiation of bulk thiol–ene polymerizations

The unique formation-structure-property attributes and reaction behavior of the thiol-ene "click" reaction have been explored extensively for photochemically and thermally initiated reactions but have been much less explored for redox initiation. Therefore, the objective of this work is to characterize fully the impact of the initiation system, monomer structure, degree of functionalization, and inhibitor level on the redox-mediated thiol-ene polymerization rate and behavior. Moreover, this study confirms the ability of redox initiation to achieve full conversion of desired thiol-ene "click" products for small molecules in solution. For the multifunctional thiol-ene systems, polymerization rate was shown to be comparable to photo- and thermally initiated systems, but with the additional advantages of unlimited depth of cure and mild reaction conditions. Additionally, the network properties of the redox-initiated thiol-ene systems were on par with a photocured material formulated with identical monomers and radical initiating potential. Lastly, control over the polymerization rate and preceding induction period was garnered from the concentration of inhibitor included in the reaction mixture. The mechanism of action of quinone inhibition in redox-mediated thiol-ene polymerizations is shown to depend on both the presence of an aniline reducing agent and the concentration of inhibitor, with quinone concentrations in great excess of oxidizing agent concentrations actually leading to heightened polymerization rates when aniline is present.

Rapid photochemical surface patterning of proteins in thiol–ene based microfluidic devices

The suitable optical properties of thiol-ene polymers combined with the ease of modifying their surface for the attachment of recognition molecules make them ideal candidates in many biochip applications. This paper reports the rapid one-step photochemical surface patterning of biomolecules in microfluidic thiol-ene chips. This work focuses on thiol-ene substrates featuring an excess of thiol groups at their surface. The thiol-ene stoichiometric composition can be varied to precisely control the number of surface thiol groups available for surface modification up to an average surface density of 136 ± 17 SH nm(-2). Biotin alkyne was patterned directly inside thiol-ene microchannels prior to conjugation with fluorescently labelled streptavidin. The surface bound conjugates were detected by evanescent wave-induced fluorescence (EWIF), demonstrating the success of the grafting procedure and its potential for biochip applications.

UV cured thiol-ene flame retardant hybrid coatings

In this study, bis-(triethoxysilylpropyl) phenyl phosphamide (BESPPA) was synthesized by the reaction between dichlorophenyl phosphine oxide and 3-aminopropyltriethoxysilane. The chemical structure of the BESPPA was characterized with Fourier transform infrared and NMR techniques. Flame retardant, BESPPA and sol–gel precursor containing hybrid materials were prepared by thiol-ene polymerization with the aim to improve their final thermal and flame retardant properties. The thermal stabilities of the phosphorous/sol–gel containing UV-cured hybrid materials were investigated by thermogravimetric analysis. The results showed that the addition of sol-gel precursor and BESPPA into the organic network also improves the thermal-oxidative stability of the hybrid materials. The flame-retardant properties of the UV-cured hybrid materials were also studied. Furthermore, the phosphorus–silicon synergistic effect on LOI enhancement and increasing flame retardancy of the UV-cured hybrid materials were demonstrated. An LOI enhancement from 20.7 to 26.5 is observed for organic resins containing both phosphorus and silicon. The surface morphology was also characterized by scanning electron microscopy (SEM). SEM studies indicated that inorganic particles were dispersed homogenously throughout the organic matrix.

Efficient Curing of Vinyl Carbonates by Thiol‐Ene Polymerization

Vinyl carbonates have recently been identified as a suitable alternative to (meth)acrylates, especially due to the low irritancy and cytotoxicity of these monomers. The drawback of some vinyl carbonates containing abstractable hydrogens arises through their moderate reactivity compared with acrylates. Within this paper, we use the thiol-ene concept to enhance the photoreactivity of vinyl carbonates to a large extent to reach the level of those of similar acrylates. Mechanical properties of the final thiol-ene polymers were determined by nanoindentation. Furthermore, low toxicity of all components was confirmed by osteoblast cell culture experiments.

Mussel-Inspired Thiol–Ene Polymer Networks: Influencing Network Properties and Adhesion with Catechol Functionality

In this work, we report the synthesis of photocurable, ternary polymer networks prepared by incorporating dopamine acrylamide (DAm) into a cross-linked thiol–ene network based on pentaerythritol triallyl ether (APE) and pentaerythritol tetra(3-mercaptopropionate) (PETMP). We systematically evaluate the effect of DAm, in the nonoxidized catechol form, on photopolymerization kinetics and thermal, thermomechanical, and mechanical properties of the modified thiol–ene networks. We show that while DAm only affects photopolymerization kinetics at high concentrations, the presence of the catechol moiety significantly increases the glass transition temperature of the networks across the compositional range (0–50 mol % DAm) due to hydrogen bonding interactions between the catechol and various hydrogen bonding acceptors (ethers, amides, esters) within the network, despite an obvious decrease in cross-link density with increasing concentration of the monofunctional acrylamide. Similarly, trends in the mechanical properties of the DAm–APE–PETMP networks reflect the interplay that exists between competing network design parameters, including the catechol hydrogen bonding interactions and the decrease in cross-link density derived from the use of a monofunctional acrylamide—both of which simultaneously influence network properties. Additionally, we report improved macroscopic adhesion of DAm–APE–PETMP coatings to a range of substrates—including glass, aluminum, steel, and marble—by systematically varying the amount of DAm in the network. Adhesion of the DAm–APE–PETMP coatings were evaluated using two methods, including pull-off and crosshatch adhesion tests.

Polymer Microspheres Prepared by Water-Borne Thiol-Ene Suspension Photopolymerization.

Thiol-ene polymerizations are shown to be possible in a water-borne suspension-like photopolymerization and yield spherical particles that have diameters in the range of submicrometers to hundreds of micrometers. This is the first report of such colloidal thiol-ene polymerizations. Thiol-ene polymerization offers unique conditions not commonly associated with a water-borne polymerization including a step-growth polymerization mechanism along with photoinitiation under ambient conditions. Example polymerizations of a triene, 3,5-triallyl-1,3,5-triazine-2,4,6 (1N,3H,5H)-trione (TTT), and a tetrathiol, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), with the photoinitiator 1-hydroxycyclohexyl phenyl ketone, surfactant sodium dodecyl sulfate (SDS), and a cosolvent (chloroform or toluene) are discussed. Various experimental parameters were examined such as surfactant concentration, homogenization energy, cosolvent species, and cosolvent amount in order to develop an understanding of the mechanism of microsphere formation. It is demonstrated that particle size is dependent on homogenization energy, with greater mechanical shear yielding smaller particles. In addition, higher concentrations of surfactant or solvent also produced smaller spherical particles. These observations lead to the conclusion that the particles are formed via a suspension-like polymerization.

Imprint Lithography with Degradable Elastomeric Polyanhydrides

A photocurable, degradable polyanhydride cross-linked elastomer that can be used as a stamp in imprint lithography applications has been developed. The degradable stamp materials are based on polyanhydrides synthesized using thiol-ene polymerization. In this study, curing the monomers 4-pentenoic anhydride and pentaerythritol tetrakis(3-mercaptopropionate) on a master mold yields low modulus, elastomeric, degradable polyanhydride polymer stamps that are a negative of the master. These stamps can be then used as a sacrificial template during the fabrication of a replica of the master, and can be readily degraded away from the replica using water. The resultant imprinted materials exhibited excellent uniformity over a large area. Compared with other conventional imprint lithography stamp materials, the thiol-ene polymerized polyanhydrides are degradable, master mold safe, show great release properties, have fast cure rates, are relatively low cost, and can be fabricated onto variety of substrates and materials.

Cell-mediated delivery of glucocorticoids from thiol-ene hydrogels

Thiol-ene-based poly(ethylene glycol) (PEG) hydrogels provide a unique functional platform for the sustained and localized delivery of bioactive small molecules like glucocorticoids. As a proof of concept, the synthetic glucocorticoid Dexamethasone (Dex) was conjugated to the N-terminus of a matrix metalloproteinase(MMP)-degradable peptide, which was then easily co-polymerized into PEG gel scaffolds by a thiol-ene polymerization mechanism. The conjugated Dex was locally sequestered until released by cleavage of the MMP-degradable peptide tether triggered by cell-secreted MMPs, and was only available for uptake by local co-encapsulated cells. Elevated alkaline phosphatase (ALP) activities and calcium deposition levels were observed for human mesenchymal stem cells (hMSCs) that were encapsulated in PEG hydrogels functionalized with 10μM of a Dexamethasone-conjugated peptide (Dex-peptide). The cellular responses stimulated by the tethered Dex lasted for over 21days. Using co-culture experiments, hMSCs encapsulated in hydrogels with the MMP-degradable Dex-peptides had elevated levels of ALP activity and calcium deposition, whereas no elevated cellular responses were observed in co-cultured hMSCs surrounding the gel. Moreover, modifying the peptide sequence to alter its susceptibility to cleavage and/or changing the Dex-peptide loading further regulated the hMSC response to Dex at different levels and on different time scales. Collectively, these results demonstrate a tunable system for the delivery of glucocorticoids in a localized and cell-dictated manner.

Small peptide functionalized thiol–ene hydrogels as culture substrates for understanding valvular interstitial cell activation and de novo tissue deposition

A thiol–ene polymerization platform was used to synthesize peptide functionalized poly(ethylene glycol) hydrogels, which were initially characterized and compared to theoretical predictions of Young’s modulus via a theoretical crosslinking density equation presented herein. After thorough characterization, this material system’s utility for answering specific biological hypotheses was demonstrated with the culture and observation of aortic valvular interstitial cells (VICs). Specifically, these materials were used to better understand the role of substrate elasticity and biochemical functionality on VIC α-smooth muscle (αSMA) expression and secretory properties (i.e. de novo extracellular matrix (ECM)). The Young’s moduli of the hydrogels varied from 28kPa (activating, 90% myofibroblasts) to 4kPa (non-activating, 15% myofibroblast), and the biochemical functionality was tailored by incorporating three small adhesive peptide sequences, RGDS, VGVAPG and P15. To promote VIC adhesion, a basal [RGDS] of 0.8mM was used in all formulations, while the [VGVAPG] or [P15] were varied to be lower than, equal to or higher than 0.8mM. The substrates with 1.2mM VGVAPG and all gels with P15 led to significantly higher αSMA expression for both stiff and soft substrates, as compared to 0.8mM RGDS alone. Importantly, all gel conditions αSMA expression were significantly lower than tissue culture poly(styrene) (TCPS; ∼4- to 10-fold difference). The ECM produced decreased significantly as the total integrin-binding peptide concentration increased, but was significantly higher than that produced on TCPS. This easily tailored material system provides a useful culture platform to improve the fundamental understanding of VIC biology through isolating specific biological cues and observing VIC function.

Preparation and characterization of boron containing thiol–ene photocured hybrid coatings

In this study, flame retardant, 4-vinylphenyl boronic acid containing coatings were prepared by thiol–ene polymerization. The photopolymerization kinetics of thiol–ene based formulations was investigated by real time infrared spectroscopy. The experiments showed that as the amount of boronic acid was increased, flame retardant properties of the coatings were also increased. Tensile test results showed that boronic acid groups strengthened the polymeric network. DSC results revealed that the addition of thiol causes Tg values of the coatings to decrease. On the other hand acid groups improved the hydrophilicity of the coatings.

Structurally Diverse Polyamides Obtained from Monomers Derived via the Ugi Multicomponent Reaction

The combination of the Ugi four-component reaction (Ugi-4CR) with acyclic diene metathesis (ADMET) or thiol-ene polymerization led to the formation of poly-1-(alkylcarbamoyl) carboxamides, a new class of substituted polyamides with amide moieties in the polymer backbone, as well as its side chains. 10-Undecenoic acid, obtained by pyrolysis of ricinoleic acid, the main fatty acid of castor oil, was used as the key renewable building block. The use of different primary amines, as well as isonitriles (isocyanides) for the described Ugi reactions provided monomers with high structural diversity. Furthermore, the possibility of versatile post-modification of functional groups in the side chains of the corresponding polymers should be of considerable interest in materials science. The obtained monomers were polymerized by ADMET, as well as thiol-ene, chemistry and all polymers were fully characterized. Finally, ortho-nitrobenzylamide-containing polyamides obtained by this route were shown to be photoresponsive and exhibited a dramatic change of their properties upon irradiation with light.

On the Polymerization Behavior of Telomers: Metathesis versus Thiol–Ene Chemistry

The potential of butadiene ditelomers for the synthesis of polymers has been investigated for the first time following two different approaches: acyclic diene metathesis (ADMET) polymerization and thiol–ene polyaddition. The feasibility of both step-growth polymerization methods has been investigated by focusing on the particular polymerization behavior of these unusual monomers. It has been shown that ring-closing metathesis of the studied ditelomers predominates in the first steps of ADMET, followed by oligomerization and double bond isomerization. On the other hand, during thiol–ene polyaddition, additional isomerization reactions, converting allyl ether moieties to vinyl ether moieties, were observed. Generally, the thiol–ene polymerization approach led to higher molecular weight polymers with better characteristics and interesting material properties.

Biocompatible “click” wafer bonding for microfluidic devices

We introduce a novel dry wafer bonding concept designed for permanent attachment of micromolded polymer structures to surface functionalized silicon substrates. The method, designed for simultaneous fabrication of many lab-on-chip devices, utilizes a chemically reactive polymer microfluidic structure, which rapidly bonds to a functionalized substrate via"click" chemistry reactions. The microfluidic structure consists of an off-stoichiometry thiol-ene (OSTE) polymer with a very high density of surface bound thiol groups and the substrate is a silicon wafer that has been functionalized with common bio-linker molecules. We demonstrate here void free, and low temperature (< 37 °C) bonding of a batch of OSTE microfluidic layers to a silane functionalized silicon wafer.

Surface Modification of Silicate Glass Using 3-(Mercaptopropyl)trimethoxysilane for Thiol–Ene Polymerization

A thiol-ene polymerization was accomplished on silicate glass slides to graft a series of homopolymers and copolymers using 3-(mercaptopropyl)trimethoxysilane (MTS) as both a silane coupling agent and initiator. MTS was initially covalently bonded to an acid cleaned glass surface via a classical sol-gel reaction. Poly(acrylic acid) (PAA), poly(acrylamide) (PAAm), poly(methyl acrylate) (PMA), poly(acrylamido-2-methyl-propanesulfonic acid) (PAMPS), and the copolymer poly(AA-co-AAm-co-MA-co-AMPS) were grafted from the thiol group of MTS. The surface chemistry of the MTS modified slides and polymer grafts was characterized with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Surface texture was evaluated with tapping mode atomic force microscopy (TM-AFM). The Owens-Wendt-Kaelble (OWK) and Lifshitz-van der Waals acid-base (LW-AB) methods were used to evaluate surface energies by sessile drop contact angle method. The synthetic approach demonstrated a facile, rapid method for grafting to glass surfaces.

Glass fiber reinforced high glass transition temperature thiol–ene networks

Although thiol–ene polymers have highly desirable processing properties the networks usually are limited to having characteristically low glass transition temperatures with low strength. This study is one of the first studies to examine a thiol–ene polymer thermoset matrix, having many industrial advantages compared to conventional polymer matrices, reinforced with continuous E-glass fibers. In order to control the interphase, a mercapto functional sizing of 1 wt% is applied to the glass fibers. The resulting composites of 12 vol% fibers are comparable to glass fiber reinforced polyesters in terms of strength with Young’s modulus. This work contributes to the furthering of thiol–ene ultra-violet cure systems, with their range of advantageous properties, for use in a broader scope of applications by way of creating a stronger material based on a novel class of thermoset matrix.

Cellulose acetate/poly(methyl methacrylate) interpenetrating networks: synthesis and estimation of thermal and mechanical properties

IPN-type composites consisting of cellulose acetate (CA) and poly(methyl methacrylate; PMMA) were successfully synthesized in film form. In this synthesis, a mercapto group (SH)-containing CA, CA-MA, was prepared in advance by esterification of CA with mercaptoacetic acid, and then intercomponent cross-linking between CA-MA and PMMA was attained by thiol–ene polymerization of methyl methacrylate (MMA) onto the CA-MA substrate. For comparison, polymer synthesis was also attempted to produce a semi-IPN type of composites comprising CA and cross-linked PMMA, via copolymerization of MMA and ethylene glycol dimethacrylate as cross-linker in a homogeneous system containing CA solute. Thermal and mechanical properties of thus obtained polymer composites were investigated by differential scanning calorimetry, dynamic mechanical analysis, and a tensile test, in correlation with the mixing state of the essentially immiscible cellulosic and methacrylate polymer components. It was shown that the specific IPN technique using thiol–ene reactions usually resulted in a much better compatibility-enhanced polymer composite, which exhibited a higher tensile strength and even an outstanding ductility without parallel in any film sample of CA, PMMA, and their physical blends.