New photo‐induced thiol‐ene crosslinked films based on linear methacrylate copolymer polythiols

Thiol‐ene radical addition by photolysis is a highly efficient click reaction of sufhydryl groups with reactive enes that has been extensively explored as a promising means to construct multifunctional materials. Here, photo‐induced thiol‐ene crosslinked films composed of linear methacrylate copolymer polythiols (MCPsh) are reported. Well‐defined MCPsh copolymers were prepared by thiol‐responsive cleavage of pendant disulfide linkages positioned in the corresponding methacrylate copolymers with narrow molecular weight distribution which were synthesized by a controlled radical polymerization method. With a commercially available multifunctional acrylate as a model ene, photo‐induced thiol‐ene radical polyaddition of these polythiols is competitive to free‐radical homopolymerization of acrylates, yielding crosslinked films exhibiting rapid cure, uniform network, and enhanced mechanical properties; these properties are required for high performance coating materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2860–2868.     

Thiol‐ene photopolymerization of polymer‐derived ceramic precursors

The liquid, ceramic precursor monomer VL20 was copolymerized with a thiol monomer in a traditional radical thiol‐ene photopolymerization. Polymerization occurred via addition of the thiol functional group to the vinyl silazane functional group in a 1:1 ratio consistent with a step‐growth polymerization. Gelation occurred at a high conversion of functional groups (70%) consistent with an average molecular weight and functionality of 560 and 1.7, respectively, for VL20 monomers. Initiatorless photopolymerization of the thiol‐VL20 system also occurred upon irradiation at either 365 or 254 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1752–1757, 2004     

Influence of the alkene structure on the mechanism and kinetics of thiol–alkene photopolymerizations with real‐time infrared spectroscopy

The effect of the chemical structure on the reactivity of alkenes used in thiol–ene photopolymerizations has been investigated with real‐time infrared spectroscopy. Model studies of thiol–ene photoreactions with various monofunctional hydrocarbon alkenes and the monofunctional thiol ethyl‐3‐mercaptopropionate have been performed to identify and understand structure–reactivity relationships. The results demonstrate that terminal enes react very rapidly with thiol, achieve complete conversion, and are independent of the aliphatic hydrocarbon substituent length. Disubstitution on a single carbon of a terminal ene significantly reduces the reactivity, whereas substitution on the carbon α to the terminal ene has a minimal influence on the reactivity. Internal trans enes display reduced reactivity and a lower overall conversion and deviate from the standard thiol–ene reaction mechanism because of steric strain induced by 1,3‐interactions. The reactivity and conversion of internal trans enes decrease as the substituents on the ene become larger, reaching a minimum when the substituent size is greater than or equal to that of propyl groups. Internal cis enes react rapidly with thiol; however, they undergo a fast isomerization–elimination reaction sequence generating the trans ene, which proceeds to react at a reduced rate with thiol. The reactivity of cyclic enes is dictated by ring strain, stereoelectronic effects, and hydrogen abstractability. The reactivity trends in the model studies have been used to explain the photopolymerization mechanism and kinetics of a series of multifunctional thiol–ene systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6283–6298, 2004     

Photopolymerization of ternary thiol–ene/acrylate systems: Film and network properties

Photocurable, ternary‐component mixtures of a 1:1 molar multifunctional thiol–ene (trithiol and triallyl ether) blend and a 16‐functional acrylate based monomer have been photopolymerized, and the final film properties of the ternary crosslinked networks have been measured. The photopolymerization kinetics, morphology, and mechanical and physical properties of the films have been investigated with real‐time infrared, atomic force microscopy, and dynamic mechanical analysis. The photopolymerization process is a combination of acrylate homopolymerization and copolymerizations of thiol with allyl ether and acrylate functionalities. The tan δ peaks of the photopolymerized ternary systems are relatively narrow and tunable over a large temperature range. The morphology is characterized by a distinct phase‐separated nanostructure. The photocured thiol–ene/acrylate ternary systems can be made to exhibit good mechanical properties with enhanced energy absorption at room temperature by the appropriate selection of each component concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 822–829, 2007.     

Orthogonal multifunctionalization of aliphatic polycarbonate via sequential Michael addition and radical‐thiol‐ene click reactions

Aliphatic polycarbonate (PC) copolymer is synthesized by ring opening copolymerization of acrylate‐ and allyl‐functional cyclic carbonate monomers. The post‐polymerization functionalization of the resulting copolymer is performed quantitatively using a variety of thiol compounds via sequential Michael addition and photo‐induced radical thiol‐ene click reactions within relatively short reaction time at ambient temperature. This metal‐free click chemistry methodology affords the synthesis of biocompatible PC copolymer with multifunctional groups. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1581–1587     

Photopolymerization behavior in nanocomposites formed with thiol–acrylate and polymerizable organoclays

Because of the inherent characteristics of the thiol–ene step growth mechanism in preparation of thiol–ene photopolymer clay nanocomposites, the ratio between thiol and ene functional groups at and near the organoclay surfaces may have a significant effect on the polymerization behavior. This study investigates the influence of monomer composition and the type of polymerizable organoclay on thiol–acrylate photopolymerization behavior in preparation of photocurable clay nanocomposite systems. To this end, two types of polymerizable organoclays with acrylate or thiol functional group on the clay surfaces were compared in monomer compositions with different polarity and functionality. Real‐time infrared spectroscopy was used to characterize polymerization behavior in conjunction with photo‐DSC. The degree of clay exfoliation was evaluated using small angle X‐ray scattering and correlated with photopolymerization behavior. Higher chemical compatibility of components induced enhanced clay exfoliation resulting in increase in photopolymerization rate. By affecting the stoichiometric ratio of functional groups in the clay gallery, thiolated organoclays enhance thiol–ene reaction, whereas acrylated organoclays encourage acrylate homopolymerization. In addition, inducing more propagating thiyl radicals on the organoclay surfaces by increasing functionality of thiol monomer also facilitates thiol–ene copolymerization, whereas the increase of acrylate functionality reduces final thiol conversion. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Effect of introducing a cationic system into a thiol‐ene photopolymerizable formulation

Hybrid materials derived from a thiol‐ene and cationic polymerization were obtained from concomitant polymerization. The hybrid materials were cured by both photopolymerization and thermally induced polymerization. The kinetics of the photopolymerization were measured using time resolved‐IR and optical pyrometry. The nucleophilic character of the polysulfide obtained initially in the thiol‐ene polymerization inhibited the development of the cationic photoinitiated polymerization of epoxy monomers. Besides, the epoxide groups underwent a proton catalyzed addition reaction with the thiols to form new sulfides groups in the reaction mixture. It is proposed that the formed sulfides can terminate the growing polyether chains forming dormant species like trialkylsulfonium salts. These salts promote the thermal polymerization of the epoxy monomer in a post treatment, producing hard and transparent materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4829–4843, 2007     

Ultrathin gradient films using thiol‐ene polymerizations

The application of surface‐attached, thiol‐ene polymer films for controlling material properties in a gradient fashion across a surface was investigated. Thiol‐ene films were attached to the surface by first depositing a thiol‐terminated self‐assembled monolayer and performing a thiol‐ene photopolymerization reaction on the surface. Property gradients were created either by creating and modifying a gradient in the surface thiol density in the SAM or by changing the polymerization conditions or both. Film thickness was modified across the substrate by changing either the density of the anchoring thiol functional groups or by changing the reaction conditions such as exposure time. Thicker films (1–11 nm) were obtained by polymerizing acrylate polymer brushes from the surface with varying exposure time (0–60 s). The two factors, that is, the surface thiol density and the exposure time, were combined in orthogonal directions to obtain thiol‐ene films with a two‐dimensional thickness gradient with the maximum thickness being 4 nm. Finally, a thiol‐acrylate Michael type addition reaction was used to modify the surface thiol density gradient with the cell‐adhesive ligand, Arg‐Gly‐Asp‐Ser (RGDS), which subsequently yielded a gradient in osteoblast density on the surface. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 7027–7039, 2006     

Living radical photopolymerization induced grafting on thiol–ene based substrates

The formation of reactive substrates with iniferter‐mediated living radical photopolymerization is a powerful technique for surface modification, which can readily be used to facilitate the incorporation of a variety of surface functionalities. In this research, the photopolymerization kinetics of novel bulk thiol–ene systems have been compared with those of typical acrylate and methacrylate systems when polymerized in the presence of the photoiniferter p‐xylene bis(N,N‐diethyl dithiocarbamate) (XDT). In the presence of XDT, the thiol–ene systems photopolymerize more quickly than the traditional acrylate and methacrylate systems by one to two orders of magnitude. Fourier transform infrared spectroscopy has been used to monitor the photografting kinetics of various monomers on dithiocarbamate‐functionalized surfaces. Furthermore, this technique has been used to evaluate surface‐initiation kinetics and to emphasize the influence of bulk substrate properties on grafting kinetics. Finally, photopatterning has been demonstrated on a dithiocarbamate‐incorporated thiol–ene substrate with conventional photolithographic techniques. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2134–2144, 2005     

An efficient nonisocyanate route to polyurethanes via thiol‐ene self‐addition

A novel and efficient strategy for the synthesis of nonisocyanate polyurethanes has been developed via thiol–ene self‐photopolymerization. An aliphatic thiol–ene carbamate monomer (allyl(2‐mercaptoethyl)carbamate, AMC) was synthesized by a one‐step synthesis procedure, from cysteamine and allyl chloroformate. The urethane group was therefore incorporated directly into the monomer precursor, avoiding the problems associated to toxic isocyanates. AMC was successfully stabilized with the radical inhibitor pyrogallol (1% wt). In addition, the use of phenyl phosphonic acid as coadditive allowed its stabilization for lower concentrations of pyrogallol (0.1% wt). AMC was directly transformed into thermoplastic polyurethane (TPU) through thiol–ene photopolymerization by UV‐irradiation at 365 nm. The obtained TPU presented semi‐crystalline nature and very high thermal stability (T_5% ～325 °C). It was found that high concentrations of pyrogallol decreased the reaction rate and final conversion of photopolymerization. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3017–3025     

Photopolymerization of thiol‐ene systems based on oligomeric thiols

Thiol oligomers were copolymerized with a triallyl ether by a photoinduced polymerization process. These oligomeric thiol‐ene systems comprise the same components as a photopolymerized thiol‐ene‐acrylate ternary system, yet the photopolymerized networks have much lower glass transition temperatures. An investigation into the effect of oligomeric thiol design on network formation was conducted by analyzing the reaction kinetics and thermal/mechanical properties of the thiol‐ene networks. Real‐time FTIR analysis shows that total conversion is >90% for all thiols investigated. Photo‐DSC analysis shows that the maximum exotherm rate is roughly equivalent for all of the thiols when the equivalent weight of the thiol is taken into account. As would be expected, the glass transition temperature and tensile strength increase with thiol functionality and lower thiol equivalent weight for thiols with functionality from 2 to 4. Films made using the oligomeric thiols have essentially the same glass transition temperatures and tensile modulus values regardless of thiol design. These results distinguish the method for generation of networks consisting of an initial Michael reaction of thiols and acrylates followed by a photoinitiated copolymerization with a multifunctional ene from the traditional photolysis of the corresponding thiol‐ene‐acrylate ternary systems with no Michael reaction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 14–24, 2009     

Synthesis and photopolymerization of novel multifunctional vinyl esters

Novel mono‐ and multifunctional vinyl ester monomers containing thioether groups were synthesized via an amine‐catalyzed Michael addition reaction between vinyl acrylate and multifunctional thiols. Using photo‐differential scanning calorimetry and real‐time Fourier transform infrared (RTIR) spectroscopy, the polymerization kinetics and oxygen inhibition of the homopolymerizations of the vinyl ester monomers were investigated. The effect of the vinyl ester and thioether group on acrylate/vinyl ester and thiol/vinyl ester copolymerizations was determined using real‐time IR spectroscopy to monitor polymerization rates of acrylate, vinyl, and thiol groups simultaneously. Polymerization of the vinyl esters used was found to be relatively insensitive to oxygen inhibition. We propose that the thioether group is responsible for reducing oxygen inhibition by a series of chain transfer/oxygen‐scavenging reactions. In polymerization of a acrylate/vinyl ester mixture both in nitrogen and in air, the vinyl ester monomer significantly enhances the polymerization rates and the conversion of the acrylate double bonds via plasticization of the crosslinked matrix and reduction of inhibition by oxygen. Ultimately, the vinyl ester monomer is incorporated into the polymer network. Thiol/vinyl ester free‐radical copolymerization is much faster than either thiol/allylether copolymerization or vinyl ester homopolymerization. The electron‐rich vinyl ester double bonds ensure rapid copolymerization with thiol. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4424–4436, 2004     

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions as a route to well‐defined mono and bis end‐functionalized poly(N‐isopropylacrylamide)

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions have been used as a facile and quantitative method for modifying end‐groups on an N‐isopropylacrylamide (NIPAm) homopolymer. A well‐defined precursor of polyNIPAm (PNIPAm) was prepared via reversible addition‐fragmentation chain transfer (RAFT) polymerization in DMF at 70 °C using the 1‐cyano‐1‐methylethyl dithiobenzoate/2,2′‐azobis(2‐methylpropionitrile) chain transfer agent/initiator combination yielding a homopolymer with an absolute molecular weight of 5880 and polydispersity index of 1.18. The dithiobenzoate end‐groups were modified in a one‐pot process via primary amine cleavage followed by phosphine‐mediated nucleophilic thiol‐ene click reactions with either allyl methacrylate or propargyl acrylate yielding ene and yne terminal PNIPAm homopolymers quantitatively. The ene and yne groups were then modified, quantitatively as determined by ¹H NMR spectroscopy, via radical thiol‐ene and radical thiol‐yne reactions with three representative commercially available thiols yielding the mono and bis end functional NIPAm homopolymers. This is the first time such sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions have been used in polymer synthesis/end‐group modification. The lower critical solution temperatures (LCST) were then determined for all PNIPAm homopolymers using a combination of optical measurements and dynamic light scattering. It is shown that the LCST varies depending on the chemical nature of the end‐groups with measured values lying in the range 26–35 °C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3544–3557, 2009     

Postfunctionalization of polyoxanorbornene via sequential Michael addition and radical thiol‐ene click reactions

We demonstrated the successful postfunctionalization of poly(oxanorbornene imide) (PONB) with two types of double bonds using sequential orthogonal reactions, nucleophilic thiol‐ene coupling via Michael addition and radical thiol‐ene click reactions. First, the synthesis of PONB with side chain acrylate groups is carried out via ring‐opening metathesis polymerization and nitroxide radical coupling reaction, respectively. Subsequently, the resulting polymer having two different orthogonal functionalities, main chain vinyl and side chain acrylate, is selectively modified via two sequential thiol‐ene click reactions, nucleophilic thiol‐ene coupling via Michael addition and photoinduced radical thiol‐ene. The orthogonal reactivity of two diverse double bonds, vinyl and acrylate functionalities, for the abovementioned consecutive thiol‐ene click reactions was first demonstrated on the model compound. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Three‐dimensional microfabrication of protein hydrogels via two‐photon‐excited thiol‐vinyl ester photopolymerization

Engineering three‐dimensional (3D) hydrogels with well‐defined architectures has become increasingly important for tissue engineering and basic research in biomaterials science. To fabricate 3D hydrogels with (sub)cellular‐scale features, two‐photon polymerization (2PP) shows great promise although the technique is limited by the selection of appropriate hydrogel precursors. In this study, we report the synthesis of gelatin hydrolysate vinyl esters (GH‐VE) and its copolymerization with reduced derivatives of bovine serum albumin (acting as macrothiols). Photorheology of the thiol‐ene copolymerization shows a much more rapid onset of polymerization and a higher end modulus in reference to neat GH‐VE. This allowed 2PP to provide well‐defined and stable hydrogel microstructures. Efficiency of the radical‐mediated thiol‐vinyl ester photopolymerization allows high 2PP writing speed (as high as 50 mm s^?1) with low laser power (as low as 20 mW). MTT assays indicate negligible cytotoxicities of the GH‐VE macromers and of the thiol‐ene hydrogel pellets. Osteosarcoma cells seeded onto GH‐VE/BSA hydrogels with different macromer relative ratios showed a preference for hydrogels with higher percentage of GH‐VE. This can be attributed both to a favorable modulus and preferable protein environment since gelatin favors cell adhesion and albumin incurs nonspecific binding. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4799–4810     

Oxygen‐mediated polymerization initiated by oltipraz‐derived thiones

A pyrrolopyrazine‐thione derived from oltipraz, a compound that has been investigated as a chemopreventive agent, affords radicals in the presence of thiols and oxygen via a redox cycle, an attribute that suggests its suitability as an initiator for oxygen‐mediated polymerization. Here, we explore the utilization of this pyrrolopyrazine‐thione, generated in situ from a precursor, as an initiator for the radical‐mediated thiol–ene polymerization. While the pyrrolopyrazine‐thione was shown to be capable of generating radicals in the presence of atmospheric oxygen and thiol groups, the reaction extents achievable were lower than desired owing to the presence of unwanted side reactions that would quench radical production and, subsequently, suppress polymerization. Moreover, we found that complex interactions between the pyrrolopyrazine‐thione, its precursor, oxygen, and thiol groups determine whether or not the quenching reaction dominates over those favorable to polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1373–1382     

The influence of a radical structure on the kinetics of photopolymerization

A radical initiation ability of new initiating systems in photopolymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate has been investigated and presented. The evaluation of alkyltriphenyl‐ and tetraalkylborates, iodonium salts, N‐alkoxypyridinium salts, maleimides, phthalimides, 1,3,5‐triazine derivatives and others as a free radical source in combination with suitable photosensitizer for radical polymerization of triacrylate is described. It is assumed that the photochemical decomposition of a coinitiator molecule results in formation of free radicals, which further initiate polymerization. The order of activity of free radical sources on kinetic of photopolymerization was also presented. Different initiator activity can be explained by the difference in the decomposition rate constant and the reactivity of radicals formed toward the double bond of monomer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1575–1589     

Silicon‐based mercaptans: High‐performance monomers for thiol‐ene photopolymerization

Ester‐free silane and siloxane‐based thiol monomers were successfully synthesized and evaluated for application in thiol‐ene resins. Polymerization reaction rates, conversion, network properties as well as degradation experiments of those thiol monomers in combination with triallyl‐1,3,5‐triazine‐2,4,6(1H,3H,5H)‐trione (TATT) as ene component were performed and compared with formulations containing the commercially available mercaptopropionic ester‐based thiol pentaerythritol tetra‐3‐mercaptopropionate. Kinetic analysis revealed appropriate reaction rates and conversions reaching 90% and higher. Importantly, storage stability tests of those formulations clearly indicate the superiority of the synthesized mercaptans compared with pentaerythritol tetra‐3‐mercaptopropionate/TATT resins. Moreover, photocured samples containing silane‐based mercaptans provide higher glass transition temperatures and withstand water storage without a significant loss in their network properties. This behavior together with the observed excellent degradation resistance of photocured silane‐based thiol/TATT formulations make these multifunctional mercaptans interesting candidates for high‐performance applications, such as dental restoratives and automotive resins. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 418–424     

Efficient initiation of radical‐mediated thiol‐ene chemistry with photoactive silica particles

This work aims at the design of photoactive inorganic filler by covalently attaching tri(methoxy)silyl‐functionalized bis(acyl)phosphane oxides (TEMSI²‐BAPO) onto silica particles. The immobilization of the photoactive groups is evidenced by spectroscopic measurements and thermal gravimetric analysis. The modified particles are then incorporated into thiol–ene resins to study the efficiency of the Norrish type I photofragmentation reaction of the covalently bound TEMSI²‐BAPO derivatives. The photopolymerization kinetics of the thiol–ene system is monitored by FT‐IR spectroscopy and photo‐DSC upon prolonged exposure with UV‐light and compared with the results achieved with IRGACURE 819 (free BAPO). Rapid curing and high conversion yields are obtained evidencing the high efficiency of the photoactive particles. In addition, negative‐toned patterns are inscribed in thin thiol–ene films by photolithographic processes and characterized by microscopic techniques demonstrating the versatile applicability of the photoactive particles. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 894–902