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     

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     

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     

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     

Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

The ability to prepare high T_g low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (T_g) for several thiol–norbornene materials. Results indicate that T_gs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high T_g materials. In particular, as much as a 70 °C increase in T_g was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007     

Oxygen inhibition in thiol–acrylate photopolymerizations

The overall effects of oxygen on thiol–acrylate photopolymerizations were characterized. Specially, the choice of thiol monomer chemistry, functionality, and concentration on the extent of oxygen inhibition were considered. As thiol concentration was increased, the degree of oxygen inhibition was greatly reduced because of chain transfer from the peroxy radical to the thiol. When comparing the copolymerization of 1,6‐hexanediol diacrylate with the alkane‐based thiol (1,6‐hexane dithiol) to the copolymerization with the propionate thiol (glycol dimercaptopropionate), it was found that the propionate system was much more reactive and polymerized to a greater extent in the presence of oxygen. In addition, the functionality was considered where the glycol dimercaptopropionate was compared to a tetrafunctional propionate of similar chemistry (pentaerythritol tetrakis(mercaptopropionate)). Given the same thiol concentration, the higher functionality thiol imparted a faster polymerization rate, due to the increased polymer system viscosity, which limited oxygen diffusion and decreased the extent of overall oxygen inhibition. Thus, preliminary insight is provided into how thiol monomer choice affects the extent of oxygen inhibition in thiol–acrylate photopolymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2007–2014, 2006     

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     

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     

Divinyl‐End‐Functionalized Polyethylenes: Ready Access to a Range of Telechelic Polyethylenes through Thiol–Ene Reactions

Telechelic α,ω‐iodo‐vinyl‐polyethylenes (Vin‐PE‐I) were obtained by catalytic ethylene polymerization in the presence of [(C₅Me₅)₂NdCl₂Li(OEt₂)₂] in combination with a functionalized chain‐transfer agent, namely, di(10‐undecenyl)magnesium, followed by treatment of the resulting di(vinylpolyethylenyl)magnesium compounds ((vinyl‐PE)₂Mg) with I₂. The iodo‐functionalized vinylpolyethylenes (Vin‐PE‐I) were transformed into unique divinyl‐functionalized polyethylenes (Vin‐PE‐Vin) by simple treatment with tBuOK in toluene at 95 °C. Thiol–ene reactions were then successfully performed on Vin‐PE‐Vin with functionalized thiols in the presence of AIBN. A range of homobifunctional telechelic polyethylenes were obtained on which a hydroxy, diol, carboxylic acid, amine, ammonium chloride, trimethoxysilyl, chloro, or fluoroalkyl group was installed quantitatively at each chain end.     

A novel polymerization approach via thiol‐yne addition

The ability of thiyl radicals to add to terminal unsaturations in an efficient way made them considered being one of the click reactions. Recently, thiol‐yne addition reactions have been used extensively for the synthesis of crosslinked networks and dendrimers and postpolymerization functionalization protocols. Herein, we report a novel step‐growth type reaction for highly functional linear polymers using a monoalkyne and dithiol compound. First, we investigated the model reaction between 1‐octyne and 1‐octanethiol as well as 1,4‐butanedithiol compounds, which were initiated via self‐, thermal‐, and UV‐initiation; the UV‐initiation was found to be the most efficient method and completed within 2‐h reaction time. The same conditions were applied for the polymerization of four different functional alkynes bearing different functional groups with two dithiol compounds. All polymerizations resulted in highly functional linear polymers with number averaged molecular weights ranging from 5 to 30 kDa, except for propargylic acid and its methyl ester, where only oligomers formed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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.     

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     

Utilizing thiol–ene chemistry for crosslinked nickel cation‐based anion exchange membranes

Metal cation‐based anion exchange membranes (AEMs) are a unique class of materials that have shown potential to be highly stable AEMs with competitive conductivities. Here, we expand upon previous work to report the synthesis of crosslinked nickel cation‐based AEMs formed using the thiol–ene reaction. These thiol–ene‐based samples were first characterized for their morphology, both with and without nickel cations, where the nickel‐containing membranes demonstrated a disordered scattering peak characteristic of ionic clusters. The samples were then characterized for their water uptake, chemical and mechanical stability, and conductivity. They showed a combination of high water content and extreme brittleness, which also resulted in fairly low conductivity. The brittleness resulted from large water swelling as well as the need for each nickel cation to act as a crosslinker, necessary with the current nickel‐coordination chemistry. Therefore, increasing the ion exchange capacity (IEC) for these types of AEMs, important for enhancing conductivity, also increased the crosslink density. The low conductivity and brittleness seen in this work demonstrated the need to develop non‐crosslinking metal‐complexes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 328–339     

Alternating Copolymerization by Nitroxide‐Mediated Polymerization and Subsequent Orthogonal Functionalization

A novel method for the preparation of functionalized alternating copolymers is presented. Nitroxide‐mediated polymerization of hexafluoroisopropyl acrylate with 7‐octenyl vinyl ether provides the corresponding alternating polymer, which can be chemically modified using two orthogonal polymer‐analogous reactions. A thiol–ene click reaction followed by amidation provides dual‐functionalized alternating copolymers. The potential of this method is illustrated by the preparation of a small library (15 examples) of functionalized alternating copolymers.     

Thiol–Ene Click Chemistry

Following Sharpless′ visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chemistry communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, we review the radical‐mediated thiol–ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol–ene reaction is most frequently photoinitiated, particularly for photopolymerizations resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymerization are all reviewed.     

Embedding multiple site‐specific functionalities into polymer chains via nitrone‐mediated radical coupling reactions

A facile method to generate polymer materials with embedded functional groups at known and precise positions along the polymer backbone is described. In the presented approach, well‐defined bifunctional poly(isobornyl acrylate)s preformed via atom transfer radical polymerization (ATRP) containing α,ω‐bromo end groups are reactivated and subsequently coupled in a stepwise manner via the nitrone‐mediated radical coupling (NMRC) technique. The generated polymers contain on average four nitrone moieties at evenly spaced locations. The number of embedded functionalities, and thus, the size of the polymer is limited by disproportionation reactions occurring during the nitroxide termination sequence. Using the nitrone as a functional carrier, secondary functionalities can be incorporated into the polymer with ease. To exemplify such an approach, an alkyne‐functionalized nitrone is used to construct a multisegment structure via NMRC reactions followed by postmodification of the obtained polymers with 3‐mercaptopropionic acid via UV‐induced thiol‐yne reactions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Photoinduced thiol–ene crosslinking of globalide/ε‐caprolactone copolymers: Curing performance and resulting thermoset properties

The increasing demand for bioderived polymers led us to investigate the potential use of the macrolactone globalide in thermoset synthesis via the photoinduced thiol–ene reaction. A series of six lipase‐catalyzed poly(globalide‐caprolactone) copolyesters bearing internal main‐chain unsaturations ranging from 10 to 50 and 100 mol % were successfully crosslinked in the melt with equal amounts of thiol groups from trimethylolpropane‐trimercapto propionate affording fully transparent amorphous elastomeric materials with different thermal and viscoelastic properties. Three major conclusions can be drawn from this study: (i) high thiol–ene conversions (>80%) were easily attained for all cases, while maintaining the cure behavior, and irrespective of functionality at reasonable reaction rates; (ii) parallel chain‐growth homopropagation of the ene monomer is insignificant when compared with the main thiol–ene coupling route; and (iii) high ene‐density copolymers result in much lower extracted sol fractions and high T_g values as a result of a more dense and homogeneous crosslinked network. The thiol–ene system evaluated in this contribution serve as model example for the sustainable use of naturally occurring 1,2‐disubstituted alkenes in making semisynthetic polymeric materials in high conversions with a range of properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012.     

Dual Sulfide–Disulfide Crosslinked Networks with Rapid and Room Temperature Self‐Healability

Polymer‐based crosslinked networks with intrinsic self‐repairing ability have emerged due to their built‐in ability to repair physical damages. Here, novel dual sulfide–disulfide crosslinked networks (s‐ssPxNs) are reported exhibiting rapid and room temperature self‐healability within seconds to minutes, with no extra healing agents and no change under any environmental conditions. The method to synthesize these self‐healable networks utilizes a combination of well‐known crosslinking chemistry: photoinduced thiol‐ene click‐type radical addition, generating lightly sulfide‐crosslinked polysulfide‐based networks with excess thiols, and their oxidation, creating dynamic disulfide crosslinkages to yield the dual s‐ssPxNs. The resulting s‐ssPxN networks show rapid self‐healing within 30 s to 30 min at room temperature, as well as self‐healing elasticity with reversible viscoelastic properties. These results, combined with tunable self‐healing kinetics, demonstrate the versatility of the method as a new means to synthesize smart multifunctional polymeric materials.