Thiol-ene Polymerization Research Articles

Thiol-ene microfluidic devices for microchip electrophoresis: Effects of curing conditions and monomer composition on surface properties

Thiol-ene polymer formulations are raising growing interest as new low-cost fabrication materials for microfluidic devices. This study addresses their feasibility for microchip electrophoresis (MCE) via characterization of the effects of UV curing conditions and aging on the surface charge and wetting properties. A detailed comparison is made between stoichiometric thiol-ene (1:1) and thiol-ene formulations bearing 50% molar excess of allyls (“enes”), both prepared without photoinitiator or other polymer modifiers. Our results show that the surface charge of thiol-ene 1:1 increases along with increasing UV exposure dose until a threshold (here, about 200J/cm2), whereas the surface charge of thiol-ene 2:3 decreases as a function of increasing UV dose. However, no significant change in the surface charge upon storage in ambient air was observed over a period of 14 days (independent of the curing conditions). The water contact angles of thiol-ene 2:3 (typically 70–75°) were found to be less dependent on the UV dose and storing time. Instead, water contact angles of thiol-ene 1:1 slightly decrease (from initial 90 to 95° to about 70°) as a function of UV increasing exposure dose and storing time. Most importantly, both thiol-ene formulations remain relatively hydrophilic over extended periods of time, which favors their use in MCE applications. Here, MCE separation of biologically active peptides and selected fluorescent dyes is demonstrated in combination with laser-induced fluorescence detection showing high separation efficiency (theoretical plates 8200 per 4cm for peptides and 1500–2700 per 4cm for fluorescent dyes) and lower limits of detection in the sub-μM (visible range) or low-μM (near-UV range) level.

Fabrication of high aspect ratio (> 100:1) nanopillar array based on thiol-ene

Nanopillar arrays have attracted considerable attention lately based on confinement effects and large surface to volume ratios. In this study, we demonstrated an efficient low-cost method for fabrication of large-area high aspect ratio nanopillars based on thiol-ene. Anodic aluminum oxide (AAO) template with ordered deep hole array was employed as the master mold. The low-viscosity and highly rigid UV-curable thiol-ene polymer was used as the structure material. The deep holes were filled with the liquid thiol-ene by overcoming the surface tension. Upon curing under UV irradiation, the thiol-ene formed a rigid cross-linked polymer network. The thiol-ene nanopillars were achieved after releasing the AAO mold in the corrosive solution, due to the corrosive-resistance of the cross-linked polymer. Based on the novel method, we fabricated thiol-ene nanopillar array with the diameter of 296nm and aspect ratio of higher than 100:1. The experimental results demonstrated that the properties of thiol-ene polymer were excellent and the fabrication method was feasible.

Ester-free thiol–ene dental restoratives—Part A: Resin development

ObjectivesTo detail the development of ester-free thiol–ene dental resins with enhanced mechanical performance, limited potential for water uptake/leachables/degradation and low polymerization shrinkage stress. MethodsThiol-terminated oligomers were prepared via a thiol–Michael reaction and a bulky tetra-allyl monomer containing urethane linkages was synthesized. The experimental oligomers and/or monomers were photopolymerized using visible light activation. Several thiol–ene formulations were investigated and their performance ranked by comparisons of the thermo-mechanical properties, polymerization shrinkage stress, water sorption/solubility, and reactivity with respect to a control comprising a conventional BisGMA/TEGDMA dental resin. ResultsThe ester-free thiol–ene formulations had significantly lower viscosities, water sorption and solubility than the BisGMA/TEGDMA control. Depending on the resin, the limiting functional conversions were equivalent to or greater than that of BisGMA/TEGDMA. At comparable conversions, lower shrinkage stress values were achieved by the thiol–ene systems. The polymerization shrinkage stress was dramatically reduced when the tetra-allyl monomer was used as the ene in ester-free thiol–ene mixtures. Although exhibiting lower Young's modulus, flexural strength, and glass transition temperatures, the toughness values associated with thiol–ene resins were greater than that of the BisGMA/TEGDMA control. In addition, the thiol–ene polymerization resulted in highly uniform polymer networks as indicated by the narrow tan delta peak widths. SignificanceEmploying the developed thiol–ene resins in dental composites will reduce shrinkage stress and moisture absorption and form tougher materials. Furthermore, their low viscosities are expected to enable higher loadings of functionalized micro/nano-scale filler particles relevant for practical dental systems.

Hollow Nanospheres Array Fabrication via Nano-Conglutination Technology.

Hollow nanospheres array is a special nanostructure with great applications in photonics, electronics and biochemistry. The nanofabrication technique with high resolution is crucial to nanosciences and nano-technology. This paper presents a novel nonconventional nano-conglutination technology combining polystyrenes spheres (PSs) self-assembly, conglutination and a lift-off process to fabricate the hollow nanospheres array with nanoholes. A self-assembly monolayer of PSs was stuck off from the quartz wafer by the thiol-ene adhesive material, and then the PSs was removed via a lift-off process and the hollow nanospheres embedded into the thiol-ene substrate was obtained. Thiolene polymer is a UV-curable material via "click chemistry" reaction at ambient conditions without the oxygen inhibition, which has excellent chemical and physical properties to be attractive as the adhesive material in nano-conglutination technology. Using the technique, a hollow nanospheres array with the nanoholes at the diameter of 200 nm embedded into the rigid thiol-ene substrate was fabricated, which has great potential to serve as a reaction container, catalyst and surface enhanced Raman scattering substrate.

Semi-renewable latex via step-photopolymerization of dithiol and dienic terpenes

Abundance, low-cost and ene functionality are the key assets of terpenes in view of a large-scale use as renewable monomers in radical polymerization. However, their sterically hindered ene structure has been mostly ineffective in a chain-growth mechanism, leading to sluggish activity. To address this challenge, the alternative of a step-growth diene–dithiol linear photopolymerization is investigated here. Four non-modified dienic terpenes – geranyl acetate, linalool, limonene and geraniol – are reacted with ethylene glycol dithiol in various conditions. In bulk and solution, limited conversions prevent the formation of high-molecular-weight polymers. By contrast, a mini-emulsion containing the two difunctional comonomer yields higher conversions (>98%) in less than 10 min of ultraviolet irradiation, making the formation of polythioether chains possible. The authors assume that a thiol–ene polymerization confined in nanodroplets may drive high conversion thanks to a radical compartmentalization effect. The resultant aqueous polymer dispersions show an average diameter ranging from 100 to 140 nm, low glass transition temperature (less than −30°C), but only moderate molecular weights between 4 and 5 kDa for the fraction soluble in tetrahydrofuran. These promising results pave the way for a novel semi-synthetic and eco-efficient approach to polymer latexes based on natural terpene monomers.

Fabrication of hybrid soft-nanoimprint mold in benign ambient based on thiol–ene

Soft-nanoimprint lithography is a high-throughput and cost-effective lithographic technique with high resolution and low pressure, of which the crucial component is the mold. We demonstrate a method to fabricate hybrid soft-nanoimprint mold in benign ambient based on UV-curable thiol–ene polymer. Because of the special photopolymerization mechanism via “click chemistry”, thiol–ene can be cured completely at ambient conditions without inhabitation from oxygen or moisture. The mold can be fabricated in air not in vacuum or a nitrogen atmosphere to simplify the fabrication process and lower the cost. Based on this method, we fabricated a soft-nanoimprint hybrid mold in benign ambient, which is composed of rigid thiol–ene features with sub-60nm resolution on an elastic poly(dimethylsiloxane) (PDMS) substrate.

Nanobowl Array Fabrication via Conglutination Process Based on Thiol–Ene Polymer

The technique to fabricate nanostructures with high resolution is of crucial importance to nanosciences and nanotechnology. This paper presented a nonconventional method to fabricate metallic nanobowl arrays via the conglutination process, which relied on the high adhesive material to “stick” nanostructures. The ultraviolet (UV)-curable thiol-ene had low viscosity and was cured under UV light to form a cross-linked polymer with high Young's modulus and high surface energy via “click chemistry,” which was used as the adhesive material to firmly stick nanostructures from the mold. By using the method, a self-assembly polystyrene (PS) sphere template with a metallic layer was stuck by the cross-linked thiol-ene polymer from the substrate. After removing the PS spheres, the metallic nanobowl structure with the thiol-ene substrate was achieved. In this paper, we fabricated gold nanobowl arrays composed of nanoheaves with the average diameter of ca. 60 nm between nanobowls, which verified the feasibility of the fabrication method.

Rapid, Puncture-Initiated Healing via Oxygen-Mediated Polymerization.

Autonomously healing materials that utilize thiol-ene polymerization initiated by an environmentally borne reaction stimulus are demonstrated by puncturing trilayered panels, fabricated by sandwiching thiol-ene-trialkylborane resin formulations between solid polymer panels, with high velocity projectiles; as the reactive liquid layer flows into the entrance hole, contact with atmospheric oxygen initiates polymerization, converting the liquid into a solid plug. Using infrared spectroscopy, we find that formulated resins polymerize rapidly, forming a solid polymer within seconds of atmospheric contact. During high-velocity ballistics experiments, additional evidence for rapid polymerization is provided by high-speed video, demonstrating the immediate viscosity increase when the thiol-ene-trialkylborane resins contact atmospheric oxygen, and thermal imaging, where surface temperature measurements reveal the thiol-ene reaction exotherm, confirming polymerization begins immediately upon oxygen exposure. While other approaches for materials self-repair have utilized similar liquid-to-solid transitions, our approach permits the development of materials capable of sealing a breach within seconds, far faster than previously described methods.

Hexaarylbiimidazoles as visible light thiol–ene photoinitiators

ObjectivesThe aim of this study is to determine if hexaarylbiimidazoles (HABIs) are efficient, visible light-active photoinitiators for thiol–ene systems. We hypothesize that, owing to the reactivity of lophyl radicals with thiols and the necessarily high concentration of thiol in thiol–ene formulations, HABIs will effectively initiate thiol–ene polymerization upon visible light irradiation. MethodsUV–vis absorption spectra of photoinitiator solutions were obtained using UV–vis spectroscopy, while EPR spectroscopy was used to confirm radical species generation upon HABI photolysis. Functional group conversions during photopolymerization were monitored using FTIR spectroscopy, and thermomechanical properties were determined using dynamic mechanical analysis. ResultsThe HABI derivatives investigated exhibit less absorptivity than camphorquinone at 469nm; however, they afford increased sensitivity at this wavelength when compared with bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. Photolysis of the investigated HABIs affords lophyl radicals. Affixing hydroxyhexyl functional groups to the HABI core significantly improved solubility. Thiol–ene resins formulated with HABI photoinitiators polymerized rapidly upon irradiation with 469nm. The glass transition temperatures of the thiol–ene resin formulated with a bis(hydroxyhexyl)-functionalized HABI and photopolymerized at room and body temperature were 49.5±0.5°C and 52.2±0.1°C, respectively. SignificanceAlthough thiol–enes show promise as continuous phases for composite dental restorative materials, they show poor reactivity with the conventional camphorquinone/tertiary amine photoinitiation system. Conversely, despite their relatively low visible light absorptivity, HABI photoinitiators afford rapid thiol–ene photopolymerization rates. Moreover, minor structural modifications suggest pathways for improved HABI solubility and visible light absorption.

Panchromatic photoinitiators for radical, cationic and thiol-ene polymerization reactions: A search in the diketopyrrolopyrrole or indigo dye series

A series of diketopyrrolopyrrole derivatives (DPPs) and indigo derivatives (IDGs) are synthesized and applied as photoinitiators (PIs) for the cationic polymerization (CP) of epoxides, the free radical polymerization (FRP) of acrylates or the thiol-ene polymerization under different lights (purple, blue, green, yellow, red LEDs and green laser diode). The photochemical mechanisms involved in the presence of these PIs and additives (iodonium salt (Iod), N-vinylcarbazole (NVK), amine (MDEA) or 2,4,6-tris(trichloromethyl)-1,3,5-triazine (R-Cl)) are investigated by steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques. Real-time infrared spectroscopy studies reveal that DPP4/Iod/NVK is very efficient for both CP and FRP and, compared to two references (Eosin-Y and camphorquinone), displays noticeably higher polymerization efficiencies. Panchromatic curable formulations exhibiting an almost constant photosensitivity from the blue to the red are proposed.

Suspension “click” polymerizations: thiol-ene polymer particles prepared with natural gum stabilizers

The impact of different natural gum species employed as stabilizers in the suspension photopolymerization of water-borne thiol-ene polymer particles is reported. The focus of analysis was on how hydrocolloids impact the synthesis of polymer particles prepared using a “click” thiol-ene polymerization approach. The use of renewable and naturally occurring gums as particle stabilizers provide a shift toward greener methods of polymer particle synthesis. The natural gum species examined include gum Arabic, guar gum, and xanthan gum. In particular, gum Arabic was examined in thiol-ene particle synthesis due to inherent colloidal stabilizing properties that allow it to be utilized at concentrations comparable to synthetic stabilizers (1 wt% or greater). The use of these naturally occurring polysaccharides led to the development of spherical, cross-linked thiol-ene polymer particles in the size range of several micrometers to hundreds of micrometers. Polymer particle synthesis was completed within minutes via photopolymerization. Parameters examined include variation of natural gum species, natural gum concentration, monomer content, and stabilizer solution viscosity. Thiol-ene polymer particles can be successfully developed at very low stabilizer concentrations (0.005–0.5 wt%) due to the unique viscosity-enhancing properties of natural gum solutions compared to reactions stabilized by synthetic surfactant stabilizers.

Development of poly(ethylene glycol) hydrogels for salivary gland tissue engineering applications.

More than 40,000 patients are diagnosed with head and neck cancers annually in the United States with the vast majority receiving radiation therapy. Salivary glands are irreparably damaged by radiation therapy resulting in xerostomia, which severely affects patient quality of life. Cell-based therapies have shown some promise in mouse models of radiation-induced xerostomia, but they suffer from insufficient and inconsistent gland regeneration and accompanying secretory function. To aid in the development of regenerative therapies, poly(ethylene glycol) hydrogels were investigated for the encapsulation of primary submandibular gland (SMG) cells for tissue engineering applications. Different methods of hydrogel formation and cell preparation were examined to identify cytocompatible encapsulation conditions for SMG cells. Cell viability was much higher after thiol-ene polymerizations compared with conventional methacrylate polymerizations due to reduced membrane peroxidation and intracellular reactive oxygen species formation. In addition, the formation of multicellular microspheres before encapsulation maximized cell-cell contacts and increased viability of SMG cells over 14-day culture periods. Thiol-ene hydrogel-encapsulated microspheres also promoted SMG proliferation. Lineage tracing was employed to determine the cellular composition of hydrogel-encapsulated microspheres using markers for acinar (Mist1) and duct (Keratin5) cells. Our findings indicate that both acinar and duct cell phenotypes are present throughout the 14 day culture period. However, the acinar:duct cell ratios are reduced over time, likely due to duct cell proliferation. Altogether, permissive encapsulation methods for primary SMG cells have been identified that promote cell viability, proliferation, and maintenance of differentiated salivary gland cell phenotypes, which allows for translation of this approach for salivary gland tissue engineering applications.

High Refractive Index Polymer Composites Synthesized by Cross-Linking of Oxozirconium Clusters Through Thiol-Ene Polymerization

Transparent hybrid organic methacrylate capped zirconium oxo-clusters (ZOCs) with thiol-ene monomers. The vinyl capping groups potentially enable the ZOCs to become an integral part of the thiol-ene network. The ZOCs are reasonably soluble in the thiol-ene monomers yielding a loading of 1 wt. -%. The resulting polymer composites were highly transparent in the visible region and exhibited significant enhancement in the refractive indices with measured values ranging from 1. 714 to 1. 744. Moreover, dynamic mechanical analysis (DMA) revealed that all the composites exhibited good mechanical strength and were robust enough to undergo machining and/or polishing without fracture.

Novel panchromatic photopolymerizable matrices:N,N'-dibutylquinacridone as an efficient and versatile photoinitiator

N,N'-dibutylquinacridone (DBQA) is utilized here for the first time as a high-performance panchromatic photoinitiator for the cationic polymerization (CP) of epoxides, the free radical polymerization (FRP) of acrylates, the thiol-ene polymerization and the synthesis of interpenetrated polymer networks (epoxide/acrylate) under violet, blue, green and yellow lights (emitted from LED at 405 nm, 470 nm, 520 nm, or 594 nm, or laser diode at 532 nm). It confers a panchromatic character to the photopolymerizable matrices. Remarkably, the proposed DBQA based photoinitiating systems exhibit quite excellent efficiency (the final monomer conversion for multifunctional monomers at room temperature can reach 62% and 50% in CP and FRP, respectively) and appear as much more powerful than the camphorquinone or Eosin-Y containing reference systems for visible light. For green light, DBQA is much more reactive than the literature reference (Eosin-Y) and for blue light, a good reactivity is found compared with camphorquinone. The photochemical mechanisms are studied by molecular orbital calculations, steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1719–1727

Dynamic stiffening of poly(ethylene glycol)-based hydrogels to direct valvular interstitial cell phenotype in a three-dimensional environment

Valvular interstitial cells (VICs) are active regulators of valve homeostasis and disease, responsible for secreting and remodeling the valve tissue matrix. As a result of VIC activity, the valve modulus can substantially change during development, injury and repair, and disease progression. While two-dimensional biomaterial substrates have been used to study mechanosensing and its influence on VIC phenotype, less is known about how these cells respond to matrix modulus in a three-dimensional environment. Here, we synthesized MMP-degradable poly(ethylene glycol) (PEG) hydrogels with elastic moduli ranging from 0.24 kPa to 12 kPa and observed that cell morphology was constrained in stiffer gels. To vary gel stiffness without substantially changing cell morphology, cell-laden hydrogels were cultured in the 0.24 kPa gels for 3 days to allow VIC spreading, and then stiffened in situ via a second, photoinitiated thiol-ene polymerization such that the gel modulus increased from 0.24 kPa to 1.2 kPa or 13 kPa. VICs encapsulated within soft gels exhibited αSMA stress fibers (∼40%), a hallmark of the myofibroblast phenotype. Interestingly, in stiffened gels, VICs became deactivated to a quiescent fibroblast phenotype, suggesting that matrix stiffness directs VIC phenotype independent of morphology, but in a manner that depends on the dimensionality of the culture platform. Collectively, these studies present a versatile method for dynamic stiffening of hydrogels and demonstrate the significant effects of matrix modulus on VIC myofibroblast properties in three-dimensional environments.

Assessments of antibacterial and physico-mechanical properties for dental materials with chemically anchored quaternary ammonium moieties: Thiol–ene–methacrylate vs. conventional methacrylate system

ObjectiveFabrication of low shrinkage stress and strain dental resins containing highly available immobilized bactericidal moieties has been reported. The goal of this study is producing dental restorative materials with long-last antibacterial activity and reduced secondary caries.It is anticipated that antibacterial properties of quaternary ammonium moieties chemically immobilized in the backbone of dental resins is directly depended on accessibility of these functions. In the present study the antibacterial effect of a series of antibacterial monomers polymerized in a ternary thiol–ene–methacrylate system were compared with corresponding classical methacrylate system against Streptococcus mutans (an oral bacteria Strain). Physical and mechanical properties of dental materials obtained from these two systems were also evaluated and compared. MethodsThe viscosities of the resin matrixes were measured on a MCR 300 rheometer. Degree of conversion (DC%) of monomers was measured using FTIR spectroscopy. The shrinkage-strain of photocured resins was measured using the bonded-disk technique. A universal testing machine combined with a stress measurement device was utilized to measure the polymerization-induced shrinkage stress. Viscoelastic properties of the samples were also determined by dynamic mechanical thermal analysis (DMTA). Assessment of antibacterial properties was performed through agar diffusion test (AD) to confirm non-release behavior of chemically anchored moieties. Quantitative assay of antibacterial activity was evaluated through direct contact test (DCT) against S. mutans. Direct contact cytotoxicity assay with fibroblast cell line L-929 was also performed to find more insight regarding cytotoxicity of the antibacterial matrixes. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level=0.05). ResultsNeat methacrylate systems had significantly higher viscosity than thiol–ene–methacrylate analogous. The degree of conversion of methacrylate moieties in thiol–ene–methacrylate system was improved in comparison to conventional methacrylate system. Shrinkage stress and strain of thiol–ene–methacrylate system was lower than the neat methacrylate system. The thiol–ene-methacrylate systems show increased homogeneity and decreased glass transition temperature (Tg) and crosslink density (νc) in comparison to the neat methacrylate-based resins. The incorporated monofuctional quaternized monomer reduces degree of conversion, shrinkage stress and crosslink density of matrix. The results showed significant improvement in antibacterial activity and cytocompatibility of dental materials obtained from thiol–ene polymerization system. SignificanceIt was shown that with proper control of monomers molar ratio, significant improvement in antibacterial activity and cytocompatibility as well as acceptable mechanical properties can be attained for dental resins prepared through the application of thiol–ene polymerization methodology.

Polyanhydride nanoparticles by ‘click’ thiol–ene polymerization

The production of degradable polyanhydride-based nanoparticles using thiol–ene ‘click’ polymerizations is described.

Amino acid-incorporated polymer network by thiol-ene polymerization

Amino acid-incorporated polymer network by thiol-ene polymerization

Synthesis of degradable multi-segmented polymers via Michael-addition thiol–ene step-growth polymerization

Degradable multi-segmented poly(β-thioester) linear polymers and networks are synthesized via step-growth thiol–ene polymerization of diacrylates with telechelic dithiol polystyrene and poly(isobornylacrylate) precursor polymers.

Thermoresponsive antimicrobial wound dressings via simultaneous thiol-ene polymerization and in situ generation of silver nanoparticles

Thermoresponsive and antimicrobial wound dressingviathiol-ene polymerization reaction.