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     

Photo‐induced thiol‐ene polysulfide‐crosslinked materials with tunable thermal and mechanical properties

Photo‐induced thiol‐ene crosslinked polymeric networks have been extensively explored in constructing a variety of new materials with enhanced mechanical properties for optical, biomedical, and sensing applications. Toward the broad applications, however, tunable mechanical properties are greatly desired. Here, an effective approach utilizing high‐molecular‐weight methacrylate copolymers having pendant thiol and vinyl groups (MCPsh and MCPenes) to modulate thermal and mechanical properties of photo‐induced thiol‐ene crosslinked materials is reported. The MCP copolymers are synthesized by an industrially friendly polymerization method, followed by post‐modification including either a facile coupling reaction or reductive cleavage. Upon UV irradiation, thiol‐ene reactive blends of MCPsh and MCPenes yield highly crosslinked materials through the formation of flexible sulfide linkages. These polysulfide‐crosslinked materials based on rigid MCP backbones exhibit enhanced mechanical properties. Further, their thermal and mechanical properties are tuned by modulating monomer compositions of MCPs as well as varying numbers of pendant SH or vinyl groups (i.e., extent of crosslinking densities). This approach is versatile and effective for development of high performance polymeric materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3060–3068     

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     

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     

Step‐growth thiol–thiol photopolymerization as radiation curing technology

This report introduces a novel UV‐curing technology based on thiol–thiol coupling for polydisulfide network formation. Beginning with a model tris(3‐mercaptopropionate) trithiol monomer and xanthone propionic acid‐protected guanidine as photobase generator, a comprehensive characterization based on spectroscopic techniques supports the reaction of thiols into disulfides without side reactions. The best experimental conditions are described as regards to film thickness, irradiance, emission wavelength, and atmosphere composition. The results shed light on a step‐growth photopolymerization mechanism involving two steps: first, the formation of thiyl radicals by thiolate air oxidation or/and thiol photolysis, and second, their recombination into disulfide. By varying thiol functionality and structure, oligomer chain length and monomer/oligomer ratio, the network architecture can be finely tuned. The molecular mobility of the polydisulfide network is crucial to high thiol conversion rates and yields as revealed by ¹H T₂ NMR relaxation measurements. Ultimately, spatial control enables the formation of a photopatterned poly(disulfide) film, used as next‐generation high refractive index photoresist. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 117–128.     

Synthesis of water‐soluble allyl‐functionalized oligochitosan and its modification by thiol–ene addition in water

A novel, straightforward and versatile chemical pathway has been studied to functionalize water‐soluble chitosan oligomers. This metal‐free methodology is based on the epoxy‐amine reaction of the allyl glycidyl ether with chitosan, followed by thiol‐ene radical coupling reaction of ω‐functional mercaptans, using 4,4′‐Azobis(4‐cyanovaleric acid) as a free radical initiator. Both reactions were entirely carried out in water. In a preliminary step, chitosan depolymerization was carried out using H₂O₂ in an acetic medium under 100 W microwave irradiation, optimizing the yield of water‐soluble oligomers. Functionalization by six different thiols bearing alcohol, carboxylic acid, ester, and amino groups was then performed, leading to a range of functional oligochitosans with different grafting efficiencies. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 39–48     

Facile synthesis of dopa‐functional polycarbonates via thiol‐Ene‐coupling chemistry towards self‐healing gels

Since extraction of the naturally occurring mussel‐foot proteins is expensive and time‐consuming, routes towards synthetic analogues are continuously being explored. Often, these methods involve several protection and deprotection steps, making the synthesis of synthetic analogues time‐consuming and expensive as well. Herein, we show that UV‐initiated thiol‐ene coupling between a thiol‐functional dopamine derivative and an allyl‐functional aliphatic polycarbonate can be used as a fast and facile route to dopa‐functional materials. Different thiol‐to‐allyl ratios and irradiation protocols were used and it was found that nearly 50% of the allyl groups could be functionalized with dopa within short reaction times, without the need of protecting the catechol. It is also demonstrated herein that the dopa‐functional polymers can be used to form self‐healing gels through complexation with Fe³⁺ ions at increased pH. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2370–2378     

High‐Tg and low‐dielectric epoxy thermosets based on a propargyl ether‐containing phosphinated benzoxazine

A propargyl ether‐containing benzoxazine (4) was prepared from a potassium carbonate‐catalyzed nucleophilic substitution of propargyl bromide and a phenolic OH‐containing benzoxazine (3) , which was prepared from 1‐(4‐hydroxyphenyl)‐1‐(4‐aminophenyl)‐1‐(6‐oxido‐6H ‐dibenz <c,e><1,2> oxaphosphorin‐6‐yl)ethane (1) by a three‐step procedure. The curing reactions of (4) were monitored by IR and DSC. A reaction mechanism was proposed based on the observation. Benzoxazines (3) and (4) were applied as epoxy curing agents. The microstructure and the structure‐property relationship of the resulting thermosets are discussed. The double‐strand structure in (4) ‐cured epoxy thermosets afforded higher crosslinking density, and led to higher thermal properties. In addition, the (4) ‐cured epoxy thermosets possess half the amount of highly polar hydroxyl groups than those of the (3) ‐cured epoxy thermosets, resulting in a lower dielectric constant, dissipation factor, and water absorption. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1359–1367     

Triallyl monomer bearing adamantane‐like core derived from naturally occurring myo‐inositol: Synthesis and polyaddition with dithiols

This paper deals with a triallyl monomer bearing a rigid adamantane‐like core derived from myo‐inositol, a naturally occurring cyclic hexaol. The core structure of the monomer can be readily constructed by orthoesterification of myo‐inositol. The polyaddition of the triallyl monomer with dithiols based on the thermally induced radical thiol‐ene reaction gives the corresponding networked polymers. These networked polymers exhibit much higher thermal stability than the comparative networked polymers obtained from a triallyl monomer bearing less rigid cyclohexyl core. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1193–1199     

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     

Cure kinetics modeling and thermomechanical properties of cycloaliphatic epoxy‐anhydride thermosets modified with hyperstar polymers

Hyperstar polymers (HSPs) with hyperbranched aromatic polyester core and arms consisting of block copolymers of poly(methyl methacrylate) and poly(hydroxyethyl methacrylate) have been used as polymeric modifiers in cycloaliphatic epoxy‐anhydride formulations catalyzed with tertiary amines, with the purpose of enhancing the impact strength of the resulting materials without compromising other thermal and mechanical properties.> In this work, the effect of these polymeric modifiers on the curing kinetics, processing, thermal‐mechanical properties and thermal stability has been studied using thermal analysis techniques such as DSC, TMA, DMA, and TGA. The morphology of the cured materials has been analyzed with SEM. The curing kinetics has been analyzed by isoconversional procedures and phenomenological kinetic models taking into account the vitrification during curing, and the degradation kinetics has been analyzed by means of isoconversional procedures, summarizing the results in a time‐temperature‐transformation (TTT) diagram. The results show that HSPs participate in the crosslinking process due to the presence of reactive groups, without compromising significantly their thermal‐mechanical properties. The modified materials show a potential toughness enhancement produced by the formation of a nano‐grained morphology. The TTT diagram is shown to be a useful tool for the optimization of the curing schedule in terms of curing completion and safe processing window, as well as for defining storage stability conditions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1227–1242     

Synthesis and functionalization of dendron‐polymer conjugate based hydrogels via sequential thiol‐ene “click” reactions

Fabrication and functionalization of hydrogels from well‐defined dendron‐polymer‐dendron conjugates is accomplished using sequential radical thiol‐ene “click” reactions. The dendron‐polymer conjugates were synthesized using an azide‐alkyne “click” reaction of alkene‐containing polyester dendrons bearing an alkyne group at their focal point with linear poly(ethylene glycol)‐bisazides. Thiol‐ene “click” reaction was used for crosslinking these alkene functionalized dendron‐polymer conjugates using a tetrathiol‐based crosslinker to provide clear and transparent hydrogels. Hydrogels with residual alkene groups at crosslinking sites were obtained by tuning the alkene‐thiol stoichiometry. The residual alkene groups allow efficient postfunctionalization of these hydrogel matrices with thiol‐containing molecules via a subsequent radical thiol‐ene reaction. The photochemical nature of radical thiol‐ene reaction was exploited to fabricate micropatterned hydrogels. Tunability of functionalization of these hydrogels, by varying dendron generation and polymer chain length was demonstrated by conjugation of a thiol‐containing fluorescent dye. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 926–934     

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     

Comparative studies of polymer‐dispersed liquid crystal films via a thiol‐ene click reaction

In this work, the thiol‐ene click reaction is employed to fabricate polymer‐dispersed liquid crystal (PDLC) films by photoinitiated polymerization. The PDLC films are prepared by systematic variation of key conditions: variety and content of ‐ene monomer, liquid crystal (LC) content, curing time, and curing light intensity. We find that both the morphologies and electro‐optic properties of these films are adjustable. When increasing the length of alkyl main chain of ‐ene monomers, the driving voltages reduce, but in turn, the contrast ratio decreases. Increasing ‐ene monomer content raises the driving voltages as well as the response time, and the increase of LC content lowers the driving voltages but has a negative effect on the contrast ratio. The changes to the curing conditions (both curing time and UV light intensity) can be used to modify the driving voltages, response time, and contrast ratios of PDLC films. These comparative studies will elucidate new insights in commercial applications of intelligent PDLC films.     

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     

Evaluation of thiol‐ene click chemistry in functionalized polysiloxanes

Polysiloxanes are commonly used in a myriad of applications, and the “click” nature of the thiol‐ene reaction is well suited for introducing alternative functionalities or for crosslinking these ubiquitous polymers. As such, understanding of the thiol‐ene reaction in the presence of silicones is valuable and would lead to enhanced methodologies for modification and crosslinking. Here, the thiol‐ene reaction kinetics were investigated in functionalized oligosiloxanes having varying degrees of thiol functionalization (SH), π–π interactions (from diphenyls, DP), and ene types (C?C). In the ene‐functionalized oligomers, π–π interactions were controlled through the use of dioctyl repeats (DO). The polymerization rate and rate‐limiting steps were determined for all systems containing an allyl‐functionalized oligomer, and rates ranging from 0.10 to 0.54 mol L^?1 min^?1 were seen. The rate‐limiting step varied with the oligomer composition; examples of rate‐limited propagation (5:3:2 C?C:DP:DO/1:1 SH:DP) or chain transfer (5:3:2 C?C:DP:DO/3:1 SH:DP) were found in addition to cases with similar reaction rate constants (5:2:3 C?C:DP:DO/1:1 SH:DP). None of the siloxanes were found to exhibit autoacceleration despite their relatively high viscosities. Instead, the allyl‐, vinyl‐, and acrylate‐functionalized siloxanes were all found to undergo unimolecular termination based on their high α scaling values (0.98, 0.95, and 0.82, respectively) in the relation R_p ∝ R_i^α. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

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     

UV‐curable thio‐ether‐urethane network with tunable properties

A photoinitiating system composed of a sensitizer (isopropylthioxanthone, ITX) and a photobase generator (triazabicyclodecene tetraphenylborate TBD.HBPh4) was successfully applied to the photopolymerization of a thiol‐isocyanate‐epoxy mixture. The final polymer network was considered in term of the compositional ratio between the thiol‐isocyanate and thiol‐epoxy coupling reactions. In parallel with structural investigation by spectroscopic technics, the relationships between the compositional ratio and kinetics of photopolymerization, thermomechanical properties and hardness were investigated. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3119–3126