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. 2025 Apr 3;5(3):174–183. doi: 10.1021/acspolymersau.5c00007

Photopolymerization Using Thiol–Epoxy ‘Click’ Reaction: Anionic Curing through Photolatent Superbases

Anzar Khan 1,*
PMCID: PMC12163949  PMID: 40519949

Abstract

Photoinduced anionic curing of epoxides by thiols offers many advantages over traditional (cationic and radical) photochemical cross-linking processes. This includes insensitivity to air and moisture, low volume shrinkage, good adhesion to substrates through β-hydroxy thioether linkages, and often no requirement for a postexposure baking step. Thus, interest in the thiol–epoxy ‘click’ reaction for photopolymerization purposes has been growing steadily. In this regard, photolatent catalysts have been developed with the capability to generate strong organic bases (superbases) under illumination from UV to the visible and near-infrared range. Besides bulk polymerizations, the base-catalyzed ring-opening reaction can also be harnessed for lithography purposes to fabricate micro- and nanosized patterns. Use of hydrophilic monomers can lead to the preparation of hydrogels. The cross-linked networks can be incorporated with photosensitive monomers to afford photoactive properties. Alternatively, the thioether linkages can be addressed through sulfur alkylation. This post-cross-linking modification reaction transforms the neutral thermosets into zwitterionic sulfonium/carboxylate or cationic sulfonium salts. The former endows the materials with antibiofouling properties, while the latter endows them with antibacterial surface properties. Postfabrication transesterification reactions within the material, on the other hand, bring vitrimer properties to the network and allow for object reshaping. The concepts of shape memory polymers and 3D printing have also been established. The aim of this Perspective is to review this nascent but growing area of research with the help of key literature examples.

Keywords: photoclick reaction, thiol−epoxy reaction, ‘click’ chemistry, photobase generators, superbases, anionic curing, photochemical cross-linking

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1. Introduction

Light, as a remote trigger for activation of a polymerization process, is an attractive synthetic tool in macromolecular science. − For instance, a formulation can be stored for a long period of time. When required, it can be coated on a surface and cured (cross-linked) with the help of light exposure. Since light stimulus can be controlled in space, curing can be achieved at selected areas within one substrate or at curved surfaces. Thus, a variety of photopolymerization methods have been developed in the past few decades. − They can be broadly classified into free-radical, cationic, and anionic processes. − Among these, the anionic approach has received the least attention. At first glance, it appears surprising, as base-catalyzed reactions are insensitive to oxygen and moisture and provide the opportunity of curing under ambient conditions. This eliminates the need for specialized chambers with controlled atmosphere and allows the fabrication process to be carried out on the benchtop and in the open. Furthermore, unlike cationic curing, a postexposure heating step is not always necessary for full curing. However, development of efficient photolatent bases has been the key issue in the field. Earlier works in which only primary and secondary alkylamines were generated upon photoexposure show that they are inadequate catalysts for efficient ring-opening reaction. , However, the situation has improved in the past decade. The key change in the molecular design is brought about by replacing the alkylamines with non-nucleophilic cyclic superbases such as aminidines and guanidines. These bases possess a higher basicity (by 2–3 pK aH units) compared to the alkylamines. For instance, in diazabicycloundecene (DBU), an imine is electronically conjugated to the tertiary amine. This enables delocalization of one nitrogen’s lone pair of electrons onto the other and allows for the protonated amidinium ion to be stabilized through resonance. This gives rise to a system with a pK aH of 12.5. The voluminous fused rings, on the other hand, hinder the molecule from engaging in sterically demanding situations (e.g., attacking carbon atoms in a substitution reaction). Overall, the electronic and steric factors create a non-nucleophilic strong organic base ideal for abstracting easily reached protons with appropriate acidity.

Thiols are mildly acidic with pK a values for the SH group ranging from 6 to 11. Such a proton can be abstracted by the superbases to create a thiolate anion (Figure ). In the presence of a three-membered strained epoxide ring, the anion can attack the least substituted, least hindered, and most electrophilic site to yield an alkoxide anion. The alkoxide anion, being a strong base, abstracts a proton from the thiols, the conjugate acid of the base catalyst, or is quenched by the moisture. Overall, this results in the formation of a β-hydroxy thioether linkage. This process is termed the thiol–epoxy reaction.

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Catalytic nature of the thiol–epoxy photoclick reaction (a). Chemical structure of the polythioether network and postsynthesis changes in its properties (b). Low material shrinkage due to the ring-opening reaction (c). Chemical structures of the photgenerated bases and photobase generator 1 (d,e).

The ‘click’ attributes − of this reaction are manifested under basic conditions. , It is observed to be highly efficient as thousands of epoxide units in a poly­(glycidyl methacrylate) scaffold can be converted into side-chain β-hydroxy thioethers in a few hours of reaction time under ambient conditions. The reaction is regioselective, , and it can be carried out under equimolar conditions. The reaction versatility is such that it can be performed in organic or aqueous solvents, including buffers typically used in biology studies, using a variety of organic and inorganic bases as catalysts. The reaction precursors can be obtained from commercial sources or prepared with synthetic ease. Finally, the β-hydroxy thioether linkages can be modified through esterification to yield cell-penetrating polymers for si-RNA delivery, while sulfur alkylation gives access to antibacterial polysulfonium salts. , Overall, thus, the influence of the thiol–epoxy ‘click’ reaction is growing in polymer science. In this Perspective, our aim is to discuss the development of photochemical thiol–epoxy ‘click’ reaction for polymer cross-linking purposes. The discussion does not include purely thermal curing and is limited to photoinitiated systems. Furthermore, off-stoichiometric systems and mixed thiol-based chemistries are not discussed due to an uncertain chemical structure of the network. −

2. Discussion

2.1. Photoclick Nature

NMR-tube experiments are a good method to gauge the efficiency of a chemical reaction (Figure ). For this, glycidyl methyl ether and bismercaptoethoxyethane were mixed in a 1:1 molar ratio in three different NMR tubes. In the first tube, which did not contain any photocatalyst, the reactants were irradiated with 365 nm light for 20 min. In the second tube, 1 wt % of the photocatalyst Cat1 was added, but no light irradiation was carried out. In the third tube, containing 1 wt % of Cat1 and the reactants, photoirradiation was carried out for 20 min. In the first two tubes, the precursors remained intact. In the third tube, a complete conversion of the epoxide and thiol functionalities into the anticipated β-hydroxy thioether compound was observed. Area integration analysis indicated that the ring-opening reaction was quantitative. This was a good indication that the ring-opening reaction is highly efficient. However, for cross-linking purposes, multifunctional precursors are employed. Thus, monomer M1 with four thiol groups (Figure ) was used with a 1:4 molar ratio with glycidyl methyl ether. Once again, the 4-fold reaction was quantitative upon photoirradiation only in the presence of the photocatalyst. No reaction occurred in the dark or in the absence of compound 3. These results are comparable to a list of photoclick reactions described by Tasdelen and Yagci. Similarly, recent results by Truong and co-workers are very encouraging in which glycidyl methacrylate and decanethiol are observed to undergo a quantitative conversion to the β-hydroxy thioether compound with preservation of the acrylate group in the presence of a photogenerated base under neat conditions. Such studies indicate that the photoreaction is as efficient as discussed above for reactions carried out under nonphotochemical conditions and can be described as a photoclick reaction.

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1H NMR of the precursors before irradiation (a,b) and after irradiation (c,d) in the presence of a photocatalyst (ref ).

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Chemical structures of the polymerization monomers.

2.2. Photochemical Cross-Linking

The application of the photoclick reaction on a monomer mixture, in which at least one carries multiple reactive sites (>2) and is referred to as a cross-linker, produces network polymers. Currently, two general strategies are applied to obtain such cross-linked photopolymers as discussed below.

2.2.1. Covalent Photocatalysts

In this approach, a superbase is covalently attached to a releasing group through a photocleavable linkage (Figure a). Typically, this is achieved by anchoring the imine nitrogen atom of the base onto the benzylic position of an arene. This arrangement disrupts the electronic communication between nitrogen atoms, isolates the system into ordinary alkyl amines, and reduces the pK aH of the base. Thus, formulations can be kept in the dark without initiating a cross-linking reaction. When needed, light exposure at an appropriate wavelength can cleave the C–N bond in a homolytic fashion. The carbon-centered radical abstracts the proton from the tertiary carbon atom activated by the adjacent nitrogen atoms, and the CN bond reforms to give the active catalyst, which can initiate the polymerization reaction by abstracting protons from the thiol monomers. For instance, as shown by Sangermano and co-workers, 2 wt % of Cat2 under irradiation with a UV beam using a medium pressure mercury lamp with a power intensity of 50 mW/cm2 could polymerize a 25 μm thick formulation containing monomers M2 and M3 into tack-free fully cross-linked film with full conversion of the reactive functionalities in 12 min of exposure time under ambient conditions. The authors also combined a base-catalyzed sol–gel process with the thiol–epoxy chemistry to prepare interpenetrating networks with silica exhibiting higher glass transition temperatures as compared to the pure thioether system. The content of silica precursors was found to be proportional to the glass transition temperature (T g). All of the systems showed strong adhesion to glass substrate as studied by crosscut and tape adhesion methods.

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Covalent approach to photocatalysts (a). Chemical structure of the thioxanthone photosensitizer (b). Chemical structures of photocatalysts 4 and 5 (c,d).

To modulate the spectral sensitivity, photosensitizers can be used. For instance, Schlögl and co-workers showed that use of thioxanthone (ITX) (Figure b) can allow for visible light irradiation (>400 nm). For instance, 5 wt % of Cat2 with 5 wt % of ITX leads to a near full thiol conversion in an exposure time of 8 min with light intensity of 1.8 J/cm2. The use of the ortho-nitrobenzyl ester (o-NBE)-based diepoxide monomer M4 renders the networks photosensitive. Upon exposure to UV light, the o-NBE group undergoes a cleavage reaction to give, among other photolysis products, carboxylic acid groups. The pristine surface displayed a water contact angle of 77°. It decreased to 60–62° upon photoactivation indicating a hydrophilic surface. The isoelectric point (IEP) of the network changed from 4.35 (indicating a neutral surface) to 1.2 (indicating an acidic surface) upon photocleavage reaction.

Similarly, the combination of Cat2, thioxanthone, and M5 was utilized by Schlögl and co-workers for 3D printing of objects and object reshaping through base-catalyzed transesterification reactions between β-hydroxy groups generated upon thiol–epoxy ‘click’ reaction and the ester groups of the thiol monomers M1 and M6 (Figure ). − In such a system, the cross-linked network features T g as well as a topology freezing transition temperature (T v). At T v, the thermoset can undergo a structural reorganization through exchange reactions within the volume of the cross-linked material. In the present context, a chosen area of a thermoset can be rendered a certain shape by exposing it to light, releasing the base only within the exposed area and keeping the system above T v for structural reorganization to occur through transesterification reactions.

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Digital photographs monitoring the reshaping process in ester-containing poly­(β-hydroxy thioether)­s. The schematic presentation on the right-hand side shows the concept behind the thiol–epoxy vitrimers (Reproduced from ref with permission from the Royal Society of Chemistry, Copyright 2022).

Li and co-workers combined the chemical structures of the thioxanthone photosensitizer and latent superbase to create a catalyst that could release the base upon exposure with blue light (405 nm) using LEDs (Figure a). Cat3 was used in 3 mol %, and the cross-linking was performed at the light intensity of 100 mW/cm2. The authors noted that the formulations were stable in the dark for 24 h, after which the viscosity increased. Furthermore, a few drops of methanol were necessary for the dissolution of Cat3 into the formulation. Finally, a two-step curing was carried out in which the first step was light exposure for 5 min during which the conversion of the reactive functionalities remained below 30%. Thus, after photoinitiation, the polymerization was completed at 90 °C for 50 min.

A unique new design is presented by Arimitsu and co-workers in which an amidine base is produced upon photocyclization of trans-o-coumaric acid derivative Cat4 (Figure c). Several monomers were investigated systematically, and epoxide oligomer M7 along with the rigid monomer M8 were found to be optimum to produce resins with high hardness (262 MPa) and good adhesion properties. The adhesion properties were studied on a silicon wafer with the help of a cross-cut method and acrylate-based radical curing, and anionically cured thiol-Michael-based coating materials were used for comparison purposes. The radically cured film showed no adhesion and floated as soon as it was cut. The thiol-Michael-type did not float. However, it could be peeled off by using a tape. The thiol–epoxy coating did not float after cut and resisted peeling off by tape. This indicates that the thioether motif alone is not sufficient to adhere on inorganic substrates. It is likely that the hydroxy group is necessary for good adhesion properties. The curing was achieved with 5 mol % of Cat4 in films of 26 μm thickness and an UV exposure intensity of 50 mW/cm2 with dosage of 1600 mJ/cm2. After the irradiation, the samples were left for 64 min, and the epoxy group conversion was measured to be 78% with the help of FT-IR analysis.

Recently, Truong and co-workers developed a new catalyst system Cat5 through which the irradiation wavelength could be significantly shifted to the red region (Figure d). In this design, methylene blue was covalently connected to tetramethylguanidine (TMG) via a photocleavable carbonyl linkage. Upon irradiation with red light (660 nm), the linkage breaks to release the base catalyst. Rheology data indicated that at 2 mol % of Cat5 and irradiation intensity of 54 mW/cm2 gelation of M1 and M9 occurred within 2–3 min. The results were found to be similar to the curing of thiol–acrylate Michael addition chemistry.

2.2.2. Ionic Photocatalysts

In this molecular design, the photolatent catalyst is in the salt form. For instance, an appropriate base is complexed with an acid molecule through acid–base interactions. The bimolecular complex can be regarded as supramolecular (versus the unimolecular covalent molecular design described above). It is a highly practical method to access the photolatent base, as it involves mixing of precursors, and the resulting salt is purified simply by a precipitation process. Although ionic in character, many complexes are well dissolved in typical thiol–epoxy formulations. When exposed to light, a decarboxylation reaction occurs, which removes the interactive site and releases the base in its free form. Often, this event is studied indirectly by detecting the free base produced using a pH indicator using UV–vis spectroscopy. 1H NMR spectroscopy can give a direct insight into this process as decarboxylation introduces a new proton in the system (Figure ). Alternatively, BPh4 can be complexed with a base to obtain photolatent salts which upon photorearrangement reaction release the active base compounds.

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Photolysis of catalyst 1 (a). 1H NMR showing the photolatent base before (b) and after (c) the photolysis reaction (ref ).

In 2013, Amritsu and co-workers established that xanthone acetic acid undergoes photoinduced decarboxylation with a quantum yield of 38% at 365 nm in polystyrene films. Thus, it becomes a good candidate for being an acid component in an ionic photocatalyst. Complexation with triazabicyclodecene (TBD) gives rise to a photolatent base Cat6 (Figure ) which could produce 14 μm thick cross-linked films from monomers M10 and M11 upon an exposure dose of 1000 mJ/cm2. The catalyst was used in 10 mol % relative to the epoxide monomer M10, and the light source irradiated the sample at 365 nm with an intensity of 10 J/cm2. In the same year, He and Nie established the utility of Cat7, a tetraphenylborate complex of TBD (TBD·HBPh4), in the present context. Thioxanthone (2 wt %) was used as the photosensitizer, and the irradiation was in the range of 320–500 nm with the help of a high-pressure mercury lamp with light intensity of 20 mW/cm2. With 4 wt % of catalyst, complete conversion of the monomers M1 and M5 was observed with 10 min of reaction time. The polymerization-induced shrinkage was also calculated and found to be nearly 2% versus 8% induced by thiol/acrylate chemistry. In a later study, thioxanthone acetic acid was applied by Li and Liu in conjunction with TBD (Cat8), diazabicycloundecene (DBU) (Cat9), and cyclohexyl amine (CA) (Cat10) in the present context, and the results were compared with TBD·HBPh4 (Cat7). At 2 mol % concentration of the photobase generators, the epoxide conversion was 82, 71, 50, and 69% in TBD, DBU, CA, and TBD·HBPh4, respectively. The use of TBD·HBPh4 (3 wt %) is also seen in the preparation of centimeter thick composite films containing additives such as glass fibers and carbon nanotubes by Li and co-workers in a two-step process. In the first step, formulations of M7 and M11 were exposed for 20 min with 385 nm LED light with an intensity of 196 mW/cm2. This released the base in the system. Then, the formulations were heated to 90 °C for full curing. The authors argued that such a procedure allows for a time window in which the formulation can be transferred, for instance, for molding purposes, before the thermal second step locks the shape in place by material cross-linking. The use of TBD (TBD·HBPh4) and xanthone acetic acid systems is further seen in the works of Liu and co-workers in the preparation of yne/epoxy hybrid networks.

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Chemical structures of the photolatent ionic catalysts.

Frigoli and Lalevee presented a new acid component based on a bisthiophene structure (Cat11-12). Due to the electron-rich chromophore, absorption of the DBU/DBN-based photobase generators could be observed in the range of 325–450 nm. Thus, 385 and 405 nm LED lights could be used for photoactivation processes. Using Cat11, 100 μm thick films containing 6 wt % were irradiated at 385 nm with intensity of 500 mW/cm2 for 49 min to obtain the best epoxy (M3) conversion of 75%. Cat12 was found to be less efficient as 90 min of irradiation led to 65% epoxy conversion.

To further tune spectral sensitivity toward longer wavelengths, Tang and co-workers prepared azocarbazole-based carboxylates and complexed them with TBD and TMG to furnish Cat13 and Cat14. The azobenzene chromophore utilized featured a donor–acceptor arrangement of the nitro and carbazole amine groups. Thus, the maximum absorption could be observed in the 440–450 nm range, and 455 nm LED light with an intensity of 50 mW/cm2 could be used for photopolymerization purposes. A 30 min exposure led to nearly 60% epoxy conversion in a 5 wt % TBD system as compared to 50% in a TMG system. The authors also studied Cat7 (TBD·HBPh4) which led to 30% conversion and was concluded to be the least optimum catalyst in the present system.

To further red-shift the photopolymerization wavelength, Li and co-workers presented a powerful design through incorporation of lanthanide-based upconversion nanoparticles in the thiol–epoxy formulation. Such nanoparticles can convert near-infrared light into UV–vis light by absorbing two or more photons. Thus, the photolysis of typical photobase generators described above becomes possible. The authors employed thioxanthone-based initiator Cat15. Due to the thermal heat effect of the NIR light, the exposures were carried out intermittently. The irradiation wavelength was 980 nm with an intensity of 8.9 W/cm2. In 2 min of irradiation time, the gel content was observed to be 100%. As discussed earlier, the preparation of thick samples becomes a possibility using long wavelength light. This was demonstrated by preparing a 11.8 cm thick sample with 10 min of irradiation (29.7 W/cm2) and 1 h of thermal treatment at 80 °C. The authors also demonstrated shape memory properties of the thioether networks (Figure ). The materials can be deformed into a temporary shape (S) by heating above T g and then locking the shape by cooling. Upon reheating above T g, the shape recovers fully in 150 s. This process could be repeated 10 times with full recyclability.

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Digital photos of the samples cured by using upconversion nanoparticles under irradiation with 365 (a) and 980 (b) nm light. The shape recovery process can be achieved thermally at 75 °C (c) and photochemically using a 980 nm laser (d) (Reproduced from ref with permission from the Royal Society of Chemistry, Copyright 2022).

2.3. Fabrication of Hydrogels

The examples discussed thus far relate to an overall hydrophobic network formation. Use of hydrophilic monomers can lead to a change in the network properties such that it can become polar and can hold large amounts of water. Such hydrogel materials can find use in biorelevant applications. − The thiol–epoxy ‘click’ chemistry was first applied for the thermal synthesis of hydrogels in 2013. In 2018, the photochemical route was investigated for the first time. For this, the commercially available biguanide salt system Cat16 with exposure at 254 nm and light intensity of 2 mW/cm2 resulted in gel formation in 15 min while using high molecular weight polyethylene glycol cross-linker M12 (number-average molecular weight = 10 kDa) and a relatively small polyethylene glycol-based B2 monomer M9 (number-average molecular weight = 2 kDa). Due to the long length of polymer chains between the potential cross-linking points, the material could hold 800% water of its own weight. A change of precursors to M1 and M13 which are insoluble in water necessitated use of an organic cosolvent such as tetrahydrofuran. A 10% aqueous tetrahydrofuran solvent is efficient in this regard as water accelerates the thiol–epoxy reaction. In aqueous systems, the efficiency is such that a very small amount of the photocatalyst (0.025 molar ratio to the thiol group) is required, and the gelation occurs within a reasonable time frame (a maximum of 30 min). If the gelation is required in nonaqueous systems, then acetonitrile could also be used as a solvent. A preliminary study was also carried out to replicate a simple disc-like pattern from a hard silicon master with a width of approximately 700 nm and height of approximately 200 nm into the network from precursors M1 and M9, chosen due to their good mechanical properties, in their hydrated state. Overall, photochemical gelation was observed to be as good as gelation under ambient and thermal conditions. An important aspect of this hydrogel study is that it established for the first time that the thioether linkages of the cross-linked material can be alkylated to access sulfonium salts with antibacterial capacity (Figure ).

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Confocal laser microscopy images of the pristine thioether (a) and alkylated sulfonium hydrogel surfaces with E. coli colonies (b) (ref ).

2.4. Bulk Properties and Soft Lithography Applications

Soft lithography is a replica-molding technique involving elastomeric (soft) materials. In 2020, the full scope of the thiol–epoxy ‘click’ photopolymerization was established for soft lithography purposes. For this, ketoprofen salts of DBU and DBN (Cat17) were applied. The DBU salt Cat1 was found to be more efficient. Initially, photochemical cross-linking was carried out in the bulk using 1 wt % Cat1 and illumination at 365 nm with light intensity of 50 mW/cm2 (Figure ). All monomer combinations could produce millimeter thick large surface area films or centimeter sized cubic samples made in ice-tray molds. The bulk samples were important in characterizing the chemical nature and mechanical properties of the cross-linked materials. By varying the nature of the monomers, the elastic modulus could be varied from low values of 3–4 MPa, indicating soft materials, to 21 MPa, indicating stiff materials. Another strategy to meet this goal was to incrementally increase the amount of M15 in a mixture of M17 and M1 to bridge the stiffness gap between 6 and 21 MPa. The hydrophilic ethylene oxide containing monomers gave rise to hydrophilic surfaces with a low water contact angle, while siloxane-based monomers led to hydrophobic surfaces with high water contact angle. Finally, material shrinkage was studied and was found to be in the range of 4–5% as compared to 17% in radically prepared networks.

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Digital pictures of millimeter (a) and centimeter (b) thick bulk polymerization samples from thiol–epoxy photoclick chemistry (ref ).

The photopatterning studies began with a less demanding system known as micromolding in capillaries (MIMIC) (Figure a–d). In this technique, a polydimethylsiloxane (PDMS)-based elastomeric mold is brought in contact with a solid substrate, and the prepolymer mixture is introduced into the micrometer-sized empty spaces through capillary forces. The photopolymerization then transforms the monomer mixture into a cross-linked solid. The PDMS mold is then peeled off to reveal the patterned photopolymer. Using this method, simple line patterns, as well as shapes with acute angles, could be produced with high fidelity over large surface areas. In the case of soft materials (entry 1, Table ), the patterns could be lifted from the substrate to give free-standing materials. They wrinkled and stretched without breaking and indicated a good degree of material cross-linking to enable structural integrity.

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Scanning electron micrographs (SEM) of star (a, height ca. 1 μm) and grid (b, line width of ca. 10 μm, height ca. 1 μm) patterns. Freestanding films obtained from patterns b and c, respectively, are shown underneath (c and d). SEM micrographs of patterns produced using imprint lithography using PDMS templates (feature height of ca. 1 μm) (e–h). SEM showing a silicon master (i) with cylindrical cavities (diameter: ca. 200 nm, height: ca. 200 nm) and replication using precursor combinations of M14 and M17 (j), M1 and M16 (k), and M1 and M17 (l). SEM showing an i-PAO master template (a, diameter ca. 200 nm, height ca. 12 nm) (m) and upon replication with the precursor combination of M1 and M9 (n). The bottom images show corresponding surface topologies as studied with the help of atomic force microscopy (o and p) (ref ).

1. Details of Cross-Linking Time under Ambient and Photochemical Conditions, Contact angle (CA), and Young’s Modulus (Y) for the Thiol–Epoxy Photopolymers .

SH Monomer Epoxy Monomer Ambient Conditions Photochemical Irradiation CA (°) Y (MPa)
M14 M16 25 min 25 min 61 3.3
M14 M17 15 min 15 min 90 6.0
M1 M9 2 min 5 min 71 7.4
M1 M16 1 min 5 min 74 8.2
M1 M15 10 min 5 min 96 6.0
M1 M17 10 min 15 min 91 21.4
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a

Reference .

Imprint lithography was applied next (Figure e–l). In this technique, a mold is stamped into a prepolymer droplet and polymerized. For micrometer-sized shapes, all the monomers were applicable. However, for smaller sizes (diameter and length of 500 nm), low-modulus materials (entries 1–3, Table ) could not be used to produce shape-persistent features. Stiffer networks were more reliable in replicating the features with considerable fidelity. The cavity sizes could be reduced to 250 nm.

The feature sizes were reduced further by using inverse aluminum oxide templates with height and diameter of features of 12 and 200 nm, respectively (Figure m–p). Here, low modulus materials could not keep their depth after photopolymerization, and stiffer networks were necessary. The ability to produce and retain feature sizes in the range of 12 nm was remarkable and reflected on the practicality of the present chemistry in photopolymerization applications.

The photopatterned features could be modified by sulfur alkylation chemistry (Figure a,b). For this, the patterned surfaces could be made emissive in the blue region (λemission = 420 nm) using iodoacetyl amino ethyl amino naphthalene sulfonic acid. The use of bromoacetic acid as an alkylating reagent produced a zwitterionic surface carrying sulfonium/carboxylate groups (Figure c,d). X-ray photoelectron spectroscopy (XPS) characterization indicated that the functionalization efficiency was approximately 43%. To assess the antibiofouling properties, bovine serum albumin (BSA) tagged with fluorescein was used as a biofouling agent. The unfunctionalized thioether and functionalized sulfonium/carboxylate patterns were exposed to BSA. After 24 h of exposure, the unfunctionalized pattern turned green (λemission = 515 nm) because of adsorption of BSA. The functionalized pattern surface, however, could resist the adsorption of BSA successfully and remained completely nonfluorescent.

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Fluorescence microscopy images [(a) = bright field, (b) = λexcitation = 330–385 nm] of the pattern after sulfur alkylation with the emissive dye. Bright field (c) and fluorescence (d) microscopy images of a pattern cut into two parts (scale bar = 100 μm). One part was kept as made, and the other was modified with bromoacetic acid. The two parts were then exposed to BSA (pH = 7.4). The left part of each image shows the unmodified part, and the right part shows the modified part. A line is drawn to show the cut edge of the material (ref ).

3. Conclusions and Outlook

The thiol–epoxy ‘click’ reaction can be performed under photochemical conditions to furnish cross-linked polymer networks. In general, TBD and DBU-based systems are found to be superior to all other photolatent bases, while it is also clear that aqueous conditions accelerate the ring-opening reaction, as already predicted by Sharpless and co-workers, and require a very small amount of the photobase generator to produce networks of good mechanical properties. The covalent and noncovalent approaches are of similar value. However, photodecarboxylation of ionic systems is often described as a negative aspect as it generates CO2 gas which eventually leaves the cross-linked network and may leave behind free volume. Such voids may cause scattering or compromise mechanical stability of the network. While reasonable, the issue of degassing is not limited to CO2 but involves all small molecular photolysis products that can diffuse out of the network. A possible new line of investigations would be to explore the design of inimers in which initiators also carry cross-linkable sites and get covalently incorporated into the system. The produced CO2 and other small molecules can also be minimized by using a very small amount of the photobase generator as seen in the hydrogel preparation. A shift to aqueous systems would favor such a research goal. For this, water-soluble epoxide and thiol monomers must be developed.

In comparison to radically cured networks, the thiol–epoxy networks offer the advantage of low shrinkage. However, due to a step-growth polymerization mechanism, the thiol–epoxy system requires a longer curing time, while the curing time is similar to the other step-growth and anionic mechanisms such as the thiol-Michael addition chemistry. One advantage of the thiol–epoxy system over the other systems is enhanced adhesion to hard substrates due to the β-hydroxy thioether motif.

One aspect that remains unexploited in the present context is the redox properties of the thioether linkages. It is known that oxidants such as hydrogen peroxide and meta-chloroperbenzoic acid can transform hydrophobic thioether groups into hydrophilic sulfoxide and sulfones. Thus, the redox chemistry of the thioethers could be harnessed to tune the hydrophilicity of the photopolymerized materials. Furthermore, cleavage of the sulfones to sulfonic acid can potentially be engaged in achieving material degradation and recycling purposes.

A red shift in the curing wavelength is certainly a future goal, with promising results obtained using upconversion nanoparticles. The work on hydrogels is the least studied in the present context and can be explored further. The monomers from renewable resources must be pursued in the future to enhance the sustainability of the synthesis. Finally, postfabrication modification of networks through sulfur alkylation chemistry is a valuable future route to enhancing the scope and applicability of the present materials.

Acknowledgments

Financial support from Ministry of Research, Innovation and Digitalization under Romania’s National Recovery and Resilience Plan PNRR-III-C9-2023-I8 program, Project code 108/31.07.23., is acknowledged.

The author declares no competing financial interest.

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