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     

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     

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     

Kinetics of thiol–ene and thiol–acrylate photopolymerizations with real‐time fourier transform infrared

We used real‐time Fourier transform infrared to monitor the conversion of both thiol and ene (vinyl) functional groups independently during photoinduced thiol–ene photopolymerizations. From these results, the stoichiometry of various thiol–ene and thiol–acrylate polymerizations was determined. For thiol–ene polymerizations, the conversion of ene functional groups was up to 15% greater than the conversion of thiol functional groups. For stoichiometric thiol–acrylate polymerizations, the conversion of the acrylate functional groups was roughly twice that of the thiol functional groups. With kinetic expressions for thiol–acrylate polymerizations, the acrylate propagation kinetic constant was found to be 1.5 times greater than the rate constant for hydrogen abstraction from the thiol. Conversions of thiol–acrylate systems of various initial stoichiometries were successfully predicted with this ratio of propagation and chain‐transfer kinetic constants. Thiol–acrylate systems with different initial stoichiometries exhibited diverse network properties. Thiol–ene systems were initiated with benzophenone and 2,2‐dimethoxy‐2‐phenylacetophenone as initiators and were also polymerized without a photoinitiator. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3311–3319, 2001     

Thiol‐isocyanate‐acrylate ternary networks by selective thiol‐click chemistry

Thiol‐isocyanate‐acrylate ternary networks were formed by the combination of thiol‐isocyanate coupling, thiol‐acrylate Michael addition, and acrylate homopolymerization. This hybrid polymerization reaction sequence was preferentially controlled by using phosphine catalyst systems in combination with photolysis. The reaction kinetics of the phosphine/acrylate thiol‐isocyanate coupling reactions were systematically investigated by evaluating model, small molecule reactions. The thiol‐isocyanate reaction was completed within 1 min while the thiol‐acrylate Michael addition reaction required ～10 min. Both thiol‐isocyanate coupling and thiol‐acrylate Michael addition reactions involving two‐step anionic processes were found to be both quantitative and efficient. However, the thiol‐isocyanate coupling reaction was much more rapid than the thiol‐acrylate Michael addition, promoting initial selectivity of the thiol‐isocyanate reaction in a medium containing thiol, isocyanate, and acrylate functional groups. Films were prepared from thiol‐isocyanate‐acrylate ternary mixtures using 2‐acryloyloxyethylisocyanate and di‐, tri‐, and tetra‐functional thiols. The sequential thiol‐isocyanate, thiol‐acrylate, and acrylate homopolymerization reactions were monitored by infrared spectroscopy during film formation, whereas thermal and mechanical properties of the films were evaluated as a function of the chemical composition following polymerization. The results indicate that the network structures and material properties are tunable over a wide range of properties (T_g ～ 14–100 °C, FWHM ～ 8–46 °C), while maintaining nearly quantitative reactions, simply by controlling the component compositions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3255–3264, 2010     

Thiol–enes: Chemistry of the past with promise for the future

The photopolymerization of mixtures of multifunctional thiols and enes is an efficient method for the rapid production of films and thermoset plastics with unprecedented physical and mechanical properties. One of the major obstacles in traditional free‐radical photopolymerization is essentially eliminated in thiol–ene polymerizations because the polymerization occurs in air almost as rapidly as in an inert atmosphere. Virtually any type of ene will participate in a free‐radical polymerization process with a multifunctional thiol. Hence, it is possible to tailor materials with virtually any combination of properties required for a particular application. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5301–5338, 2004     

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     

High‐throughput kinetic analysis of acrylate and thiol‐ene photopolymerization using temperature and exposure time gradients

In this work, a high‐throughput technique for evaluating photopolymers is developed to enable simultaneous measurement of the effects of temperature in combination with exposure time. Temperature and exposure time gradients were produced in orthogonal directions on a single sample, and subsequently sampled using Fourier transform infrared (FTIR) spectroscopy. The technique developed here allows for photopolymerization kinetics to be analyzed rapidly over a large range of industrially relevant temperatures, giving insight into the role temperature and the polymer's glass transition temperature have in dictating the photopolymerization kinetics. In the 70/30 wt % hexyl acrylate and hexanediol diacrylate system, conversion in samples below the glass transition temperature (T_G) was 66 ± 2% after 12 s, significantly lower than the 93 ± 4% conversion at 12 s for samples polymerized at temperatures above the T_G. In addition, a thiol‐ene system was analyzed to study the effect of temperature on the ene homopolymerization in allyl ether monomers, which leads to incomplete thiol conversion in stoichiometrically balanced systems. At a 60% thiol conversion, the allyl ether‐ene conversion at all temperatures is 65 ± 3% irrespective of initial formulation temperature, indicative of the homopolymerization behavior being nearly independent of temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1502–1509, 2008     

Synthesis and thiol–ene photopolymerization of allyl‐ether functionalized dendrimers

Well‐defined, allyl‐ether functional, first‐generation dendrimers have been synthesized. The convergent growth approach was utilized, using the anhydride of the allyl‐ether terminated building block. Three different core moieties were used: trimethylolpropane, trisphenol, and ditrimethylolpropane. The coupling reactions proceeded in good yields and all compounds were characterized by NMR, MALDI‐TOF, and SEC. The allyl‐terminated dendrimers were crosslinked by thiol–ene chemistry, using a multifunctional thiol, TriThiol, to give clear and smooth films. The photopolymerization was conducted in the presence of a photoinitiator, Irgacure 651, and no traces of either allyl‐ether groups or thiols were observed by FT‐Raman after cure. All crosslinked films were characterized with respect to mechanical (DMA) and thermal (DSC) properties. It was found that homogeneous networks were formed and that the core functionality and structure had little effect on the network properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1339–1348, 2008     

Synthesis and characterization of thiol–ene functionalized siloxanes and evaluation of their crosslinked network properties

Three types of linear thiol‐functionalized siloxane oligomers and three types of ene‐functionalized oligomers were synthesized and subsequently photopolymerized. Within each type of thiol‐functionalized oligomer, the ratio of mercaptan repeat units to nonreactive phenyl repeat units was varied to manipulate both the crosslink density and the degree of secondary interactions through π–π stacking. Similarly, the repeat units of the three ene‐functionalized oligomers are composed of allyl‐functional monomers, benzene‐functional monomers, and octyl‐functional monomers in varying ratios of benzene:octyl but with a constant fraction of allyl moieties. The structural composition of the siloxane oligomers plays a pivotal role in the observed material properties of networks formed through thiol–ene photopolymerization. Networks with a high concentration of thiol functionalities exhibit higher rubbery moduli, ultimate strengths, and Young's moduli than networks with lower thiol concentrations. Moreover, the concentration of functionalities capable of participating in secondary interactions via hydrogen bonding or π–π stacking directly impacts the network glass transition temperature and elasticity. The combination of low crosslink density and high secondary interactions produces networks with the greatest toughness. Finally, the fraction of octyl repeats correlates with the hydrophobic nature of the network. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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     

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.     

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     

One‐pot enzymatic route to tetraallyl ether functional oligoesters: Synthesis,UV curing,and characterization

An enzymatic one‐pot route in bulk was used to synthesize tetraallyl ether (tAE) functional oligomers based on divinyl adipate, 1,4‐butanediol and trimethylolpropane diallyl ether. By using lipase B from Candida antarctica as catalyst and varying the stoichiometric ratio of monomers, it was possible to reach targeted molecular weights (from 1300 to 3300 g mol^?1) of allyl‐ether functional polyesters. The enzyme catalyzed reaction reached completion (>98% conversion based on all monomers) within 24 h at 60 °C, under reduced pressure (72 mbar) resulting in ～90% yield after filtration. The tAE‐functional oligoesters were photopolymerized, without any purification other than removal of the enzyme by filtration, with thiol functional monomers (dithiol, tetrathiol) in a 1:1 ratio thiol‐ene reaction. The photo‐initiator, 2,2‐dimethoxy‐2‐phenylacetophenone, was used to improve the rate of reaction under UV light. High conversions (96–99% within detection limits) were found for all thiol‐ene films as determined by FT‐Raman spectroscopy. The tAE‐functional oligoesters were characterized by NMR, MALDI, and SEC. The UV‐cured homopolymerized films and the thiol‐ene films properties were characterized utilizing DSC and DMTA. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Depth characterization by confocal raman microscopy of oxygen inhibition in free radical photopolymerization of acrylates: Contribution of the thiol chemistry

The oxygen inhibition of acrylate photopolymerization using visible light was depth characterized by confocal Raman microscopy. The sample thickness was found to influence the depth conversion profile. With increasing sample thickness, the conversion at the surface was increased and the oxygen‐affected layer (OAL) decreased, up to a limit where the profiles became independent of the thickness. The addition of a thiol in the acrylate mixture reduced the OAL and the conversion in this region increased. This effect was noticeable even at low concentration of thiol. Real‐time infrared spectroscopy (RT‐FTIR) experiments pointed out that for low thiol content, this beneficial effect is not only attributable to the thiol–ene process—oxygen insensitive—but also to the homopolymerization of acrylates which is enhanced. Homopolymerization and thiyl radical addition were found to have the same impact on the overall mechanism. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Dendrimers in thiol‐ene crosslinked networks and the effect of subsequent generations on thermoset properties

A series of well‐defined allyl‐ether functionalized polyester dendrimers has been synthesized via the divergent approach using traditional esterification reactions. Two commercially available trifunctional thiols, trimethylolpropane tri(3‐mercaptopropanoate) (TRIS) and ethoxylated trimethylolpropane tri(3‐mercaptopropanoate) (ETTMP), were incorporated with the dendrimers into the thiol‐ene polymer networks. The thiol‐ene reactions were conducted at room temperature and cured by UV light without the addition of photoinitiatior. Highly crosslinked films were obtained and characterized with respect to mechanical (DMA) and thermal (DSC and TGA) properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 589–601, 2009     

Antimicrobial Thiol–ene–acrylate Photosensitive Resins for DLP 3D Printing

Designing digital light processing (DLP) 3D printable photosensitive resins with antibacterial properties is especially vital because of their potential applications in various biomedical fields. In this contribution, a thiol–ene–acrylate ternary system with reduced volume shrinkage and fast photopolymerization rate was chosen as the antibacterial 3D printing matrix resin. Two quaternary ammonium salt‐type antibacterial agents (QAC and SH‐QAC) with different molecular weight were designed and prepared, which can participate in the curing of matrix resin to achieve contact antibacterial effect. The effects of antibacterial agent content on the photopolymerization kinetics and on thermal and mechanical properties were discussed in detail. When the amount of added QAC is 4wt%, the antibacterial rate is almost 100% for Escherichia coli and Staphylococcus aureus, and when the amount of SH‐QAC is 10wt%, the antibacterial rate against S. aureus is also essentially 100%. Both antibacterial photosensitive resins have been successfully applied in DLP technology to fabricate tooth model with high precision.     

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     

Synergistic effects in hybrid free radical/cationic photopolymerizations

Multifunctional alkyl glycidyl ether and oxetane monomers are usually deemed to be poorly reactive and are consequently of limited use for high speed photocuring applications. However, these monomers can be made to undergo exceedingly rapid exothermic photopolymerization when combined with a multifunctional acrylate monomer and a corresponding free radical photoinitiator. Under optimum UV irradiation conditions, these hybrid photopolymerizations take place rapidly and substantially without an induction period. A mechanism was proposed on the basis of thermal acceleration of the cationic ring‐opening polymerizations induced by the fast exothermic free radical acrylate photopolymerization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3759–3769, 2007     

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