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.     

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     

Thiourethane‐based thiol‐ene high Tg networks: Preparation,thermal, mechanical,and physical properties

Thiourethane‐based thiol‐ene (TUTE) films were prepared from diisocyanates, tetrafunctional thiols and trienes. The incorporation of thiourethane linkages into the thiol‐ene networks results in TUTE films with high glass transition temperatures. Increases of T_g were achieved by aging at room temperature and annealing the UV cured films at 85 °C. The aged/annealed film with thiol prepared from isophorone diisocyanate and cured with a 10,080‐mJ/cm² radiant exposure had the highest DMA‐based glass transition temperature (108 °C) and a tan δ peak with a full width at half maximum (FWHM) of 22 °C, indicating a very uniform matrix structure. All of the initially prepared TUTE films exhibited good physical and mechanical properties based on pencil hardness, pendulum hardness, impact, and bending tests. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5103–5111, 2007     

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     

Synthesis of new fluorinated allyl ethers for the surface modification of thiol–ene ultraviolet‐curable formulations

Thiol–ene photocurable systems based on a trifunctional thiol [trimethylolpropane tris‐(3‐mercaptopropanoate)] and two different multifunctional allyl ethers (trimethylolpropane triallyl ether and Boltorn U2, an allyl functional dendritic polyester) were examined. To these systems, small amounts (<1 wt %) of fluorinated allyl ethers were added for the modification of their surface properties. Two new fluorinated allyl ethers, 1H,1H‐perfluoro‐1‐heptylallyl ether and 1H,1H‐perfluoro‐1‐decylallyl ether, were synthesized for this purpose by allylation of the corresponding 1H,1H‐perfluoro alcohols. The fluorinated monomers, despite their very low concentrations, caused sharp changes in the surface properties of the films and in the solvent resistance without any changes in the curing conditions and bulk properties. Completely hydrophobic surfaces were obtained (as a result of the selective enrichment of the fluorinated monomers on the film surfaces) that depended on the monomer structure and its concentration. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2583–2590, 2002     

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     

Synthesis of polybenzoxazine precursors using thiols: Simultaneous thiol–ene and ring‐opening reactions

A new class of polybenzoxazine precursors of combined molecular structure of benzoxazine in the open and ring form has been developed. Thermally curable benzoxazine networks were obtained by simultaneous photoinduced thiol‐ene and Catalytic Opening of the Lateral Benzoxazine Rings by Thiols (COLBERT). The thiol‐ene reactions were initiated by photolysis of a free radical photoinitiator, 2, 2‐dimethoxy‐2‐phenyl acetophenone (DMPA), and the competing COLBERT reaction was triggered by thiol compound, 1,2‐ethanedithiol, present in the reaction mixture. The extent of reactions was evaluated by conducting experiments both under UV irradiation and in dark using model benzoxazines. The precursor soft films (pre‐P(B‐ala‐DTE)) were prepared by irradiating solutions of diallyl functional benzoxazine (B‐ala), 1,2‐ethanedithiol and DMPA. The obtained pre‐P(B‐ala‐DTE) films were then cured through thermally activated ring opening reaction of remaining benzoxazine groups yielding a more rigid and tough film. Thermal and mechanical properties of the films were investigated by DSC, DMA and TGA and compared with a typical polybenzoxazine, P(B‐ala). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Thiol‐Ene Cationic and Radical Reactions: Cyclization,Step‐Growth,and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol‐ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step‐growth polymerization between a dithiol and divinyl ether. p‐Toluenesulfonic acid (PTSA) induced a cationic thiol‐ene reaction to generate a thioacetal in high yield, whereas 2,2′‐azobisisobutyronitrile resulted in a radical thiol‐ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high‐dilution conditions, the cationic and radical reactions resulted in 16‐ and 18‐membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step‐growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.     

Multifunctional linear methacrylate copolymer polyenes having pendant vinyl groups: Synthesis and photoinduced thiol–ene crosslinking polyaddition

UV‐induced thiol‐ene crosslinked films composed of linear methacrylate copolymers having pendant enes (MCPenes) are reported. An approach involving a combination of controlled radical polymerization to synthesize well‐controlled pendant hydroxyl containing copolymers (MCP_OHs) with the following facile carbodiimide coupling of the formed MCP_OHs with enes allows for the synthesis of well‐controlled MCPenes with narrow molecular weight distribution. The density of the pendant enes in MCPenes are easily modulated by varying monomer ratios in the feed. Under UV irradiation, the resulting MCPenes undergo thiol‐ene polyaddition reactions with polythiols to form crosslinked films with a uniform network. The results from thermal and mechanical analysis suggest these properties are tuned by adjusting the densities of pendant enes in MCPenes and the amount of thiols in the reactive mixtures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 572–581     

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     

Novel photocurable monomers: The synthesis of difunctional vinyl ethers with a phosphonate group and difunctional 1‐propenyl ethers with a phosphonate group and their photoinitiated cationic polymerization

Phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were prepared by the regioselective addition reaction of glycidyl vinyl ether (GVE) or 1‐propenyl glycidyl ether with diaryl phosphonates with quaternary onium salts as catalysts. The reaction of GVE with bis(4‐chlorophenyl) phenylphosphonate gave bis[1‐(4‐chlorophenoxy methyl)‐2‐(vinyloxy)ethyl]phenylphosphonate in a 68% yield. The structures of the resulting phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were confirmed by IR and ¹H NMR spectra and elemental analysis. Photoinitiated cationic polymerizations of the resulting phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were investigated with photoacid generators. The polymerization of vinyl ether groups and 1‐propenyl ether groups of the obtained monomers proceeded very smoothly with a sulfonium‐type cationic photoinitiator, bis[4‐(diphenylsulfonio)phenyl]sulfide‐bis(hexafluorophosphate), upon UV irradiation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3105–3115, 2005     

Thiol‐terminated polyisobutylene: Synthesis,characterization, and derivatization

Thiol‐terminated polyisobutylene (α,ω‐PIB‐SH) was synthesized from thiourea and α,ω‐bromine‐terminated PIB in a three‐step, one‐pot procedure, using a cosolvent system of 1:1 (v:v) heptane:dimethylformamide. The initial alkylisothiouronium salt was produced at 90 °C. Aqueous base hydrolysis at 110 °C resulted in thiolate chain ends, which were re‐acidified to form telechelic PIB‐SH. ¹H and ¹³C NMR confirmed thiol functionality and complete terminal halogen conversion. Thiol‐based “click” reactions were used to demonstrate PIB‐SH utility. Alkyne‐terminated PIB was synthesized by a phosphine‐catalyzed thiol‐ene Michael addition with propargyl acrylate. Reaction of this product with 6‐mercaptohexanol produced tetrahydroxy‐functional PIB by a sequential thiol‐ene/thiol‐yne procedure. ¹H NMR confirmed the structures of both products. PIB‐SH was reacted with isocyanates in the presence of base to produce polythiourethanes. A model reaction used phenyl isocyanate in THF with catalytic triethylamine. Similar conditions were used to produce PIB‐based thiourethanes with and without a small‐molecule chain extender. Increased molecular weights and thiol group conversion were observed with GPC and ¹H NMR, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

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     

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     

Thiol–ene polymerizations using imide‐based monomers

New diene and dithiol monomers, based on aromatic imides such as benzophenone‐3,3′,4,4′‐tetracarboxylic diimide were synthesized and used in thiol‐ene polymerizations which yield poly(imide‐co‐thioether)s. These linear polymers exhibit limited solubility in various organic solvents. The molecular weights of the polymers were found to decrease with increasing imide content. The glass transition temperature (T_g) of these polymers is dependent on imide content, with T_g values ranging from ?55 °C (with no imide) up to 13 °C (with 70% imide). These thermal property improvements are due to the H‐bonding and rigidity of the aromatic imide moieties. Thermal degradation, as studied by thermogravimetric analysis, was not significantly different to the nonimide containing thiol‐ene polymers made using trimethyloylpropane diallyl ether and 3,5‐dioxa‐1,8‐dithiooctane. It is expected that such monomers may lead to increased glass transition temperatures in other thiol‐ene polymer systems as these normally exhibit low glass transition temperatures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4637–4642     

Novel soluble fluorinated poly(ether imide)s with different pendant groups: Synthesis,thermal, dielectric,and optical properties

Three types of new bis(ether dianhydride) monomers, [4,4′‐(2‐(3′‐methylphenyl)‐1,4‐phenylenedioxy)‐diphthalic anhydride (4a)], [4,4′‐(2‐(3′‐trifluoromethylphenyl)‐1,4‐phenylenedioxy)‐diphthalic anhydride (4b)], and [4,4′‐(2‐(3′,5′‐ditrifluoromethylphenyl)‐1,4‐phenylenedioxy)‐diphthalic anhydride (4c)] were prepared via a multistep reaction sequence. Three series of soluble poly(ether imide)s (PEIs) were prepared from the obtained dianhydrides by a two‐step chemical imidization method. Experimental results indicated that all the PEIs had glass transition temperature in the range of 200–230 °C and the temperature of 5% weight loss in the range of 520–590 °C under nitrogen. The PEIs showed excellent solubility in a variety of organic solvents due to introduction of the bulky pendant groups and were capable of forming tough films. The casting films of PEIs (80–91 μm in thickness) had tensile strengths in the range from 88 to 117 MPa, tensile modulus from 2.14 to 2.47 GPa, and elongation at break from 15 to 27%. The casting films showed UV‐Vis absorption edges at 357–377 nm, low dielectric constants of 2.73–2.82, and water uptakes lower than 0.66 wt %. The spin‐coated films of PEIs presented a minimum birefringence value as low as 0.0122 at 650 nm and low optical absorption at the optical communication wavelengths of 1310 and 1550 nm. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3281–3289, 2010     

A versatile approach to the synthesis of polyphosphazene derivatives via the thiol–ene reaction

The thiol–ene radical addition reaction has been successfully used to synthesize polyphosphazene derivatives. Poly[bis(allylamino)phosphazene] with pendant allyl groups was reacted with different thiol reagents under UV irradiation. These thiol reagents include 1‐pentanethiol, 3‐mercaptopropionic acid, 3‐mercapto‐1,2‐propane‐diol, and 2,3,4,6‐tetra‐O‐acetyl‐1‐thio‐β‐D ‐glucopyranose. ¹H NMR analyses confirm that the allyl polyphosphazene can be quantitatively modified by the mercaptans. In total, 100% conversion of the allyl groups was reached in <60 min toward the first three mercaptans, whereas about 80% conversion of the allyl groups was reached after 120‐min reaction toward the thioglucose. This method is a facile route for the synthesis of functional polyphosphazenes without the needs for protection/deprotection procedures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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     

The power of thiol‐ene chemistry

As a tribute to Professor Charlie Hoyle, we take the opportunity to review the impact of thiol‐ene chemistry on polymer and materials science over the past 5 years. During this time, a renaissance in thiol‐ene chemistry has occurred with recent progress demonstrating its unique advantages when compared with traditional coupling and functionalization strategies. Additionally, the robust nature of thiol‐ene chemistry allows for the preparation of well‐defined materials with few structural limitations and synthetic requirements. To illustrate these features, the utility of thiol‐ene reactions for network formation, polymer functionalization, dendrimer synthesis, and the decoration of three‐dimensional objects is discussed. Also, the development of the closely related thiol‐yne chemistry is described. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 743–750, 2010