Photoinduced Thiol-ene Research Articles

A MOF-membrane based on the covalent bonding driven assembly of a NMOF with an organic oligomer and its application in membrane reactors.

A UiO-66-MOF-based membrane UiO-66-TEM (1) was prepared by the assembly of methacrylamide-decorated UiO-66-NH-Met with a thiol side chain-attached polysiloxane (PSI-SH) via a photoinduced thiol-ene click reaction. The obtained membrane 1 can be a platform to support Au nanoparticles (Au NPs) to generate an Au-MOF-polymer composite membrane Au@UiO-66-TEM (2). 1 and 2 can be used to build highly efficient continuous flow-through membrane reactors for Knoevenagel condensation of 4-nitrobenzaldehyde with malononitrile and 4-nitrophenol (4-NP) reduction at ambient temperature, respectively.

Stimuli-responsive nanoparticles from ionic cellulose derivatives.

Stimuli-responsive nanoparticles (NPs) based on sustainable polymeric feedstock still need more exploration in comparison with NPs based on synthetic polymers. In this report, stimuli-responsive NPs from novel ionic cellulose derivatives were prepared via a facile nanoprecipitation. Cellulose 10-undecenoyl ester (CUE) with a degree of substitution (DS) of 3 was synthesized by esterification of cellulose with 10-undecenoyl chloride. Then, CUE was modified by photo-induced thiol-ene reactions, in order to obtain organo-soluble ionic cellulose derivatives with DSs of ∼3, namely cellulose 11-((3-carboxyl)ethylthio)undecanoate (CUE-MPA), cellulose 11-((2-aminoethyl)thio)undecanoate (CUE-CA), cellulose 11-(2-(2-(diethylamino)ethyl)thio)undecanoate (CUE-DEAET) and cellulose 11-(2-(2-(dimethylamino)ethyl)thio)undecanoate (CUE-DMAET). CUE-MPA could be transformed into NPs with average diameters in the range of 80-330 nm, but these NPs did not show particular stimuli-responsive properties. Moreover, the dropping technique resulted in smaller NPs than a dialysis technique. Stable NPs with average diameters in the range of 90-180 nm showing pH-responsive and switchable sizes were obtained from CUE-DEAET and CUE-DMAET possessing tertiary amines using nanoprecipitation. Thus, altering the terminal functional groups will be a new approach to prepare stimuli-responsive cellulose-derived polymeric NPs.

Thiol-ene cross-linking and functionalisation of Polydimethylsiloxane for biomedical applications

Event Abstract Back to Event Thiol-ene cross-linking and functionalisation of Polydimethylsiloxane for biomedical applications Khai Duong Quang Nguyen1, 2*, William Valentine Megone1, 2, Dexu Kong1, 2* and Julien Gautrot1, 2* 1 Queen Mary, University of London, Institute of Bioengineering, United Kingdom 2 Queen Mary, University of London, School of Engineering and Materials Science, United Kingdom Silicone or Polydimethylsiloxane (PDMS) is an important class of organo-silicon synthetic materials based on molecular chains of alternating silicon and oxygen atoms. This hybrid material has been used in a wide range of biomedical applications due to its unique characteristics, including physiological inertness, low toxicity, its similar elasticity to soft biological tissue, good thermal and oxidative stability, optical transparency, high permeation to oxygen and the ease of fabrication[1],[2]. However, as a result of its extremely low glass transition temperature and weak intermolecular forces, PDMS exhibits very low mechanical properties at room temperature and requires a cross-linking to decrease its compliance. One of the well-known chemistry to cross-link PDMS is the use of hydrosilylation addition, which requires an expensive metal catalyst as well as poisoning by traces of compounds containing sulphur, nitrogen, phosphorous or tin[3]. Therefore, to overcome the current drawbacks of platinium-catalysed curing of silicones, photo-induced thiol-ene chemistry is a particularly good candidate for efficiently linking PDMS chains. The reaction is versatile with various choices of -thiol and -ene functional groups and kinetically fast with a simple requirement under mild condition, i.e. room temperature and little limitations to oxygen or moisture presence[4]. This project explores the use of photo initiated thiol-ene chemistry to cross-link the side-chain thiol-functionalised PDMS with the telechelic vinyl PDMS cross-linkers at ambient temperature and generate substrates with storage modulus varying from Pa to sub MPa. Various factors have been studied, which includes ene:thiol ratios, cross-linker’s chain length and UV radiation intensity, to assess their influence on the network properties and gelation kinetics using Rheology, swelling and tensile mechanical characterisation. It has been shown that the curing reaction is rapid and very efficient with gelation obtained within 1-2 seconds, while mechanical properties are easily altered by the combination of ene:thiol ratios and the length of cross-linker. In addition, toxicity and cell behaviour on substrates with low and high stiffness were also characterised. There is no significant toxicity observed for both types of stiffness and the cells were spreading nicely on those substrates. Furthermore, by introducing the excess of thiol- moieties on the surface of the cross-linked substrate, we investigate the possibility of attaching different biological molecules (e.g. peptides) via a spacing backbone, i.e. PEG or polyoxazoline. The ability of post surface-modification allows us to use this system for various biomedical applications such as studying cell behaviour, micro-patterning and for microfluidics systems.

Multifunctional allyl-terminated hyperbranched poly(ethyleneimine) as component of new thiol–ene/thiol–epoxy materials

A new allyl terminated hyperbranched poly(ethyleneimine) was synthesized and characterized and then used in different proportions as multifunctional macromonomer in tetrathiol–diglycidyl ether of bisphenol A formulations. The curing process had a two-stage character and was composed by two click reactions: a first photoinduced thiol–ene addition followed by a thermal thiol–epoxy reaction. The thiol–ene reaction was catalyzed by a radical initiator and the thiol–epoxy curing by tertiary amines. The evolution of the first part of the curing was studied by photo-DSC and FTIR and the results compared with those obtained in a photoirradiation chamber, which was used to prepare samples for thermomechanical tests. These studies showed that the thermal thiol–epoxy process prematurely began during the photoirradiation because the presence of amines in the PEI structure accelerated this process. The thiol–epoxy reaction was more extensively produced when the proportion of the poly(ethyleneimine) increased in the formulation. The overlapping between both processes was greater in the photoirradiation chamber than in the photo-DSC. The intermediate material was completely cured by thermal treatment in an oven. The need of adding 1-methylimidazole as catalyst to complete the thiol–epoxy reaction was derived from the calorimetric studies. The materials prepared were characterized by thermogravimetry and thermomechanical analysis.

Antibacterial Networks Based on Isosorbide and Linalool by Photoinitiated Process

In this study, we used photo-induced thiol-ene reactions to design bio-based cross-linked networks from diallyl derivative isosorbide, a crude monoterpene as linalool and a trithiol. The aim of this study is to prepare new antibacterial UV-cured networks by using a thiol-ene formulation with covalent immobilization of monoterpenes as antibacterial agents and without any organic solvent thanks to the solvent-like properties of linalool. The challenge was to demonstrate the advantage of using linalool as a chemical platform molecule and, at the same time, as an antibacterial agent. The incorporation of linear linalool to the rigid isosorbide moiety generates flexible networks. Different networks were prepared by varying the mass ratio of linalool (from 0 to 100% w/w) mixed into the coating and then characterized. Their antibacterial activities were investigated in vitro against two pathogenic bacteria strains: Escherichia coli and Staphylococcus aureus. The results have shown a promising antiadherence for S. aureus without any diffusion of linalool that will allow potential applications of terpenes coatings for antibacterial adhesion.

Photo-patterned natural rubber surfaces with tunable tribological properties

The present work highlights the use of UV induced thiol–ene click chemistry for designing elastomer surfaces with tailored friction properties. A two-step surface modification strategy based on two consecutive photoinduced thiol–ene reactions has been developed which enables controlled and patterned immobilization of micro-scaled inorganic particles onto diene-rubber surfaces. The influence of the coupled particles on both surface topography and surface roughness is determined by microscopic techniques whilst tribological studies are carried out to characterize the friction properties. The results give evidence that the attachment of selected micro-scaled particles provides a distinctive increase in surface roughness and a considerable decrease in the coefficient of friction. Both are influenced by the amount of particles immobilized onto the elastomer surface. The application of photolithographic techniques further provides elastomer materials with precisely and spatially controlled tribological properties. Whilst elastomer surfaces with randomly attached particles exhibit isotropic coefficients of friction, the tribological properties of micro-patterned elastomers are anisotropic.

A facile route to multifunctional cage silsesquioxanes via the photochemical thiol–ene reaction

Starting from octavinylsilsesquioxane (OVS), several multifunctional cage silsesquioxanes were successfully prepared by the photo–induced thiol–ene reaction with the corresponding commercially available thiols. These cage compounds were fully characterized by FTIR, NMR, ESI–MS and element analysis.

Photoinduced development of antibacterial materials derived from isosorbide moiety.

A straightforward method for immobilizing in situ generated silver nanoparticles on the surface of a photoactivable isosorbide-derived monomer is developed with the objective to design a functional material having antibacterial properties. The photoinduced thiol-ene mechanism involved in these syntheses is described by the electron spin resonance/spin trapping technique. The resulting materials with or without silver nanoparticles (Ag NPs) were used as films or as coatings on glass substrate. The surface of the synthesized materials was characterized by X-ray photoelectron spectroscopy and scanning electron microscopy, and their thermal and mechanical properties were evaluated by dynamic-mechanical thermal tests, differential scanning calorimetry, thermogravimetric analyses, along with pencil hardness, nanoindentation, and scratch resistance tests. The photoinduced formation of Ag NPs is also confirmed by UV spectrophotometry. Finally, a primary investigation demonstrates the antibacterial properties of the isosorbide-derived material against Staphylococcus aureus and Escherichia coli, as well as its cytocompatibility toward NIH 3T3 fibroblastic cells.

A rapid and facile preparation of novel macroporous silicone-based cryogels via photo-induced thiol-ene click chemistry.

Novel poly(dimethylsiloxane) based macroporous gels (cryogels) were prepared via a simple and rapid photo-induced thiol-ene click reaction at low temperatures. The cryogels formed were able to float on a water surface and selectively remove oils or organic solvents with excellent recyclability.

Metal-free synthesis of E-vinylphosphonates via photoinduced alkyne hydrophosphonylation and thiol promoted isomerization sequence

The alkyne hydrophosphonylation is an atom economical method for the synthesis of vinylphosphonates that are compounds of both biological and synthetic relevance. Recent approaches based on the use of metal catalysts have only partially solved the problems of the reaction selectivity as well as of economical and large amount production. We report in this Letter on a metal-free approach that leads to E-vinylphosphonates in high yields. The approach is based on a two-step one-pot synthesis, namely: (a) photoinduced anti-Markovnikov addition of a H-phosphonate to alkynes to give mixtures of E- and Z-vinylphosphonates, and (b) cysteine promoted photoinduced conversion of the crude mixtures of adducts into E-isomers (58–92% overall yields).

Soft hydrogels from tetra-functional PEGs using UV-induced thiol–ene coupling chemistry: a structure-to-property study

In this work, photo-induced thiol–ene coupling (TEC) was used to produce well-defined poly(ethylene glycol) (PEG)-based hydrogels. PEGs of four different molecular weights (2k, 6k, 10k, and 20k) were functionalized with G1-allyl dendrons using anhydride chemistry to produce tetra-functional TEC crosslinkable PEGs. The tetra-functional PEGs were subsequently crosslinked with a tri-functional thiol in ethanol to form hydrogels. The synthesized hydrogels were characterized with respect to swelling behaviour, rheological properties and hydrolytic degradation. It was found that the molecular weight of the PEG chain greatly influences the final properties of the hydrogel, where a higher molecular weight of PEG gives an increased weight swelling ratio from 240% for PEG-2k hydrogels to 1400% for PEG-20k hydrogels, as well as decreased elastic moduli, with Young's moduli ranging from 106 MPa to 6 MPa, for PEG-2k and PEG-20k hydrogels, respectively. It was also found that the hydrolytic stability in alkaline conditions (pH 10) decreased when the molecular weight of PEG in the hydrogels increased.

Low Modulus Dry Silicone-Gel Materials by Photoinduced Thiol–Ene Chemistry

The curing behavior of telechelic vinyl-functionalized poly(dimethylsiloxane) (PDMS) with poly(dimethylsiloxane-co-propylmercaptomethylsiloxane) using photoinitiated radical thiol–ene polyaddition was studied, by means of rheology, mechanical analysis of the cured elastomeric products, and high resolution magic angle spinning NMR (HR-MAS). Postpolymerization modification of a hydroxy-functionalized PDMS-derivative (OH-PDMS) yielded a telechelic thiol-functionalized PDMS-derivative, which was subsequently used as chain extender for the preparation of dry silicone-gel materials with elastic moduli between 30 and 500 kPa. The rate of thiol–ene polyaddition of the chain extender proved to be similar to that of the cross-linking process using multifunctional PDMS-based thiols. HR-MAS analysis of the loosely cross-linked thiol–ene PDMS networks and their fluoride-solubilized counterparts proved a highly efficient cross-linking with an optimal cure at 1:1 thiol to ene stoichiometric ratios. Using mechanical anal...

A simple method for the quantification of molecular decorations on silica particles

A simple, rapid quantitative approach to determining attachment density on silica nanoparticles has been demonstrated using attenuated total reflectance Fourier transform infrared spectroscopy and verified by thermogravimetric analysis. A very high attachment of approximately 5 attachments per nm2 has been achieved through photoinduced thiol–ene click reaction of 11-bromo-1-undecene with a thiol functionalized silica nanoparticle formed from mercaptopropyltrimethoxysilane as the sole precursor. Attachment density with concentration of alkene and reaction time is shown to be highly nonlinear and appears to be limited by accessability of thiols on the surface of the particle. This method opens the opportunity to form nanoparticles with controlled functionality including multifunctional particles, which have been produced in this work.

Photoinduced thiol-ene polymerization reaction for fast preparation of macroporous hybrid monoliths and their application in capillary liquid chromatography.

Hybrid monoliths with a macroporous structure were prepared within a few minutes via a photoinduced thiol-ene polymerization reaction, the surfaces of which showed hydrophobic character. The monolithic column demonstrated good separation performance towards alkylbenzenes, peptides, proteins and BSA tryptic digest in cLC.

Thiol-Containing Polysilsesquioxane Liquid and Photocurable Sulfur-Containing Transparent Organic–Inorganic Hybrid Monoliths Obtained via Cosolvent-Free Hydrolytic Polycondensation

Abstract Cosolvent-free hydrolytic polycondensation of trimethoxy(3-sulfanylpropyl)silane under acidic conditions was found to yield thiol-containing polysilsesquioxane (PSQ) liquid. The resultant PSQ liquid is stable at room temperature and serves as a precursor of sulfur-containing transparent organic–inorganic hybrid monoliths prepared via ultraviolet (UV) photoinduced thiol–ene reaction. The factors that determine the UV transparency of the hybrids were investigated. Sulfur-containing organic–inorganic hybrids showing good transparency in the UV spectral region are obtained using a vinyl compound, which is transparent around the UV absorption edge of thiol groups.

Monocatenary, branched, double-headed, and bolaform surface active carbohydrate esters via photochemical thiol-ene/-yne reactions

An original and versatile method for the synthesis of a range of novel mannose-based surfactants was developed via metal-free photo-induced thiol-ene/-yne ‘click’ reactions. This light-mediated hydrothiolation reaction involving a thiolated mannose was successfully applied to terminal and internal alkenes, dienes, and alkynes, leading to monocatenary, branched, double-headed, and bolaform amphiphilic carbohydrate esters, respectively. A surface activity study showed that these new compounds possess valuable properties and display specific behavior at the air–water interface. It also demonstrated the greater flexibility of the thioether moiety in the spacer of the surfactants produced via a thiol-ene reaction in comparison with the triazole heterocyclic rings in similar glucose-based surfactants synthesized elsewhere by the alkyne–azide 1,3-dipolar addition.

Polythioethers by Thiol‐Ene Click Polyaddition of α,ω‐Alkylene Thiols

The straightforward synthesis of a series of poly(thioether)s by photoinduced thiol-ene click polyaddition of α,ω-alkylene thiols is reported. It is found that linear and telechelic poly(thioether)s can be directly obtained from α,ω-alkylene thiols with, for example, alkyl chain length of m = 1,2,3, and 9. The reaction proceeds without additives such as (radical) initiators or metal compounds and can simply be carried out by UV-irradiation of the bulk monomer or monomer solution. Ex situ kinetic studies reveal that the reaction proceeds by a typical a step-growth polyaddition mechanism. As the homologue series of poly(thioether)s are now synthetically accessible, new direct pathways to tailored poly(alkyl sulphoxide)s and poly(alkyl sulfone)s are now possible.

Functionalization of Metal–Organic Frameworks through the Postsynthetic Transformation of Olefin Side Groups

For the first time, the adaptability of the C=C double bond as a versatile precursor for the postsynthetic modification (PSM) of microporous materials was extensively investigated and evaluated. Therefore, an olefin-tagged 4,4'-bipyridine linker was synthesized and successfully introduced as pillar linker within a 9,10-triptycenedicarboxylate (TDC) zinc paddle-wheel metal-organic framework (MOF) through microwave-assisted synthesis. Different reactions, predominately used in organic chemistry, were tested, leading to the development of new postsynthetic reactions for the functionalization of solid materials. The postsynthetic oxidation of the olefin side groups applying osmium tetroxide (OsO₄) as a catalyst led to the formation of a microporous material with free vicinal diol functionalities. The epoxidation with dimethyldioxirane (DMDO) enabled the synthesis of epoxy-functionalized MOFs. In addition to that, reaction procedures for a postsynthetic hydroboration with borane dimethyl sulfide as well as a photoinduced thiol-ene click reaction with ethyl mercaptan were developed. For all of these PSMs, yields of more than 90% were obtained, entirely maintaining the crystallinity of the MOFs. Since the direct introduction of the corresponding groups by means of pre-synthetic approaches is hardly possible, these new PSMs are useful tools for the functionalization of porous solids towards applications such as selective adsorption, separation, and catalysis.

Bioinspired Water-Enhanced Mechanical Gradient Nanocomposite Films That Mimic the Architecture and Properties of the Squid Beak

Inspired by the water-enhanced mechanical gradient character of the squid beak, we herein report a nanocomposite that mimics both the architecture and properties of this interesting natural material. Similar to the squid beak, we have developed nanocomposites where the degree of cross-linking is controlled along the length of the film. In this study, we utilized tunicate cellulose nanocrystals as the nanofiller that are functionalized with allyl moieties. Using photoinduced thiol-ene chemistry, we have been able to cross-link the CNC nanofiller. In the dry state where strong CNC interactions can occur, only a small mechanical contrast is observed between the cross-linked and uncross-linked samples. However, when the films are exposed to water, which "switches off" the noncovalent CNC interactions, a significant mechanical contrast is observed between the same films. For example, at 20 wt % CNC (in the dry film), an increase in wet modulus from 60 to 300 MPa (∼500% increase) is observed after photoirradiation. Furthermore, we show that the wet modulus can be controlled by altering the UV exposure time which allows access to mechanical gradient films.

Synthesis of S-Linked N-Acetylneuraminic Acid Derivatives via Photoinduced Thiol-ene and Thiol-yne Couplings

Thio-linked mono- and bivalent mimetics of α(2→3) and α(2→6)-linked sialosides were prepared by photoinduced hydrothiolation of alkenes and alkynes with the 2-mercapto sialic acid. Thiosialylation of 6-O-allyl- or 3-O-allyl-substituted galactose derivatives has been carried out by the thiol–ene click reaction. Double thiosialylation has also been achieved via thiol–yne chemistry using propargylated galactose derivatives as the alkyne components and the peracetylated 2-mercapto sialic acid as the thiol.