Thiol-ene Polymerization Research Articles

Flame retardant thiol–ene photocured coatings

The low polymerization tendency of allyl compounds can be enhanced by thiol–ene polymerization systems. In this study, allyldiphenyl phosphine oxide (ADPPO) containing thiol–ene based photocured coatings were prepared with the aim to improve their final thermal and flame retardant properties. The photopolymerization kinetics of thiol–ene based formulations were investigated by real-time infrared spectroscopy. The incorporation of ADPPO into the polymer chains was confirmed by monitoring the conversion of acrylate double bonds and measuring the gel content of the crosslinked films. The experiments showed that as the amount of ADPPO was increased, thermal and flame retardant properties of the coatings were also increased.

Photopolymerized thiol-ene systems as shape memory polymers

In this study we introduce the use of thiol-ene photopolymers as shape memory polymer systems. The thiol-ene polymer networks are compared to a commonly utilized acrylic shape memory polymer and shown to have significantly improved properties for two different thiol-ene based polymer formulations. Using thermomechanical and mechanical analysis, we demonstrate that thiol-ene based shape memory polymer systems have comparable thermomechanical properties while also exhibiting a number of advantageous properties due to the thiol-ene polymerization mechanism which results in the formation of a homogeneous polymer network with low shrinkage stress and negligible oxygen inhibition. The resulting thiol-ene shape memory polymer systems are tough and flexible as compared to the acrylic counterparts. The polymers evaluated in this study were engineered to have a glass transition temperature between 30 and 40°C, exhibited free strain recovery of greater than 96% and constrained stress recovery of 100%. The thiol-ene polymers exhibited excellent shape fixity and a rapid and distinct shape memory actuation response.

Synthesis and physical properties of a main-chain chiral smectic thiol-ene oligomer

The synthesis and characterisation of a main-chain chiral smectic liquid crystalline thiol-ene polymer system is described. The chemical structure is designed based on the structure of W317, a mesogen with a large electroclinic effect in the Smectic A* (SmA*) phase. The novel thiol-ene monomer K0902 has a large surface electroclinic effect with an unexpected small electroclinic effect in the SmA* phase. The corresponding thiol-ene oligomer poly-K0902 is characterised by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), polarised optical microscopy (POM), X-ray diffraction (XRD) and shows an expected I–SmA*–Glass phase sequence, but the electroclinic angle is relatively small.

Investigation of thiol-ene and thiol-ene–methacrylate based resins as dental restorative materials

Objectives The objective of this work was to evaluate thiol-norbornene and thiol-ene–methacrylate systems as the resin phase of dental restorative materials and demonstrate their superior performance as compared to dimethacrylate materials. Methods Polymerization kinetics and overall functional group conversions were determined by Fourier transform infrared spectroscopy (FTIR). Flexural strength and modulus were determined with a 3-point flexural test. Polymerization-induced shrinkage stress was measured with a tensometer. Results Thiol-ene polymer systems were demonstrated to exhibit advantageous properties for dental restorative materials in regards to rapid curing kinetics, high conversion, and low shrinkage and stress. However, both the thiol-norbornene and thiol-allyl ether systems studied here exhibit significant reductions in flexural strength and modulus relative to BisGMA/TEGDMA. By utilizing the thiol-ene component as the reactive diluent in dimethacrylate systems, high flexural modulus and strength are achieved while dramatically reducing the polymerization shrinkage stress. The methacrylate–thiol-allyl ether and methacrylate–thiol-norbornene systems both exhibited equivalent flexural modulus (2.1 ± 0.1 GPa) and slightly reduced flexural strength (95 ± 1 and 101 ± 3 MPa, respectively) relative to BisGMA/TEGDMA (flexural modulus; 2.2 + 0.1 GPa and flexural strength; 112 ± 3 MPa). Both the methacrylate–thiol-allyl ether and methacrylate–thiol-norbornene systems exhibited dramatic reductions in shrinkage stress (1.1 ± 0.1 and 1.1 ± 0.2 MPa, respectively) relative to BisGMA/TEGDMA (2.6 ± 0.2 MPa). Significance The improved polymerization kinetics and overall functional group conversion, coupled with reductions in shrinkage stress while maintaining equivalent flexural modulus, result in a superior overall dental restorative material as compared to traditional bulk dimethacrylate resins.

Silica-metalloporphyrins hybrid materials: preparation and catalysis to hydroxylate cyclohexane with molecular oxygen

Three types of novel silica-metalloporphyrins hybrid materials, Si-S-APTCPPFe, Si-S-APTCPPMn and Si-S-APTCPPCo, were prepared at room temperature by sol–gel method involving a thiol-ene polymerization reaction of 5-(4-allyloxy)phenyl-10,15,20-tri(4-chlorophenyl)porphyrin (APTCPP) with 3-mercaptopropyltrimethoxysilane (MPS). The hybrid materials were characterized by XRD, SEM, FT–IR, UV–Vis and TG, and were investigated as catalysts for the aerobic oxidation of cyclohexane. It is found that these hybrid materials are more efficient catalysts than the analogous non-supported metalloporphyrins for cyclohexane hydroxylation in metalloporphyrin–O2–ascrobate system and the metal ion in the porphyrins significantly affected the catalytic efficiencies of these hybrid materials.

Thiol–ene UV-curable coatings using vegetable oil macromonomers

Abstract Allyl, acrylate, and vinyl ether derivatives of castor oil were synthesized and blended with multifunctional thiols for evaluation as thiol–ene ultraviolet (UV) curable systems. The UV cured films were characterized and evaluated via ASTM tests, gel content, and dynamic mechanical analysis (DMA) as a function of time. Fourier transform infrared spectroscopy and DMA studies established that property advancement resulted from auto-oxidation, continued thiol–ene polymerization, or a combination of both mechanisms.

Novel liquid-crystalline mesogens and main-chain chiral smectic thiol-ene polymers based on trifluoromethylphenyl moieties

The synthesis and characterization of a series of novel liquid-crystalline molecules containing a trifluoromethylphenyl core or a 2,3-bis(trifluoromethyl)phenyl core is described. All trifluoromethylphenyl-containing compounds (W530, W551, and W558) have mesomorphic properties while 2,3-bis(trifluoromethyl)phenyl-containing compounds (W555, W556, W557) are room-temperature crystals without mesophases. Thereafter, based on the chemical structure of W558, to pursue a mesogen with a large electroclinic effect in the SmA* phase for development into electromechanical actuators, a chiral smectic thiol-ene monomer (K0901) is synthesized and polymerized in organic solvents. The polymer is characterized by GPC, DSC, PLM, XRD and shows an expected I–SmA* –Glass phase sequence though the electroclinic effect is relatively small.

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

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

AbstractThe ability to prepare high Tg 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 (Tg) for several thiol–norbornene materials. Results indicate that Tgs 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 Tg materials. In particular, as much as a 70 °C increase in Tg 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

Holographic polymer dispersed liquid crystals (HPDLCs) containing triallyl isocyanurate monomer

The morphology and corresponding performance of holographic polymer dispersed liquid crystals (HPDLCs) based on thiol–ene polymer are dependent on a number of factors including the gel point conversion of the polymer, polymerization kinetics, and extent of liquid crystal (LC) phase separation. Previous research of HPDLC reflection gratings made from thiol–allyl ether polymer indicates that increasing polymerization rate in systems with moderate gel point conversion can improve diffraction efficiency (DE). This work examines HPDLC reflection gratings that contain the ene monomer triallyl isocyanurate (TATATO). In HPDLCs, thiol–TATATO polymerization is two times faster than the thiol–ene polymerization of triallyl ether. By substituting TATATO for triallyl ether, the LC droplet size within HPDLC reflection gratings decreases from 100nm to 25nm. The dramatic reduction in LC droplet size for thiol–TATATO HPDLCs increases baseline transmission from 55% in thiol–triallyl ether HPDLCs to 90% at 450nm. Unfortunately, the DE of thiol–TATATO HPDLCs is only approximately 10% due to poorly defined lamellae in the grating morphology. As determined with real-time IR (RTIR) spectroscopy, thiol–TATATO HPDLCs have significantly faster LC demixing kinetics in comparison to thiol–allyl ether HPDLCs. During holographic photopolymerization, the increased rate of LC demixing causes formation of LC droplets throughout the grating. The low DE of thiol–TATATO HPDLCs can be improved by mixing TATATO and allyl ether monomer. The morphology of ternary thiol–ene HPDLC formulations containing TATATO and allyl ether has a well-defined grating structure due to increased LC solubility in the system, an average LC droplet size of 50nm, and baseline transmission of nearly 85% at 450nm.

Effects of Modulus and Surface Chemistry of Thiol-Ene Photopolymers in Nanoimprinting

Thiol-ene photopolymers were studied as patternable resins for nanocontact molding imprint lithography. Photopolymerizable thiol and ene monomer mixtures were used, and after molding, patterned thiol-ene polymer features the size and shape of the original molds were replicated. Adhesion and release were examined and controlled by manipulating the surface chemistry of the substrate and mold. A direct correlation between cured thiol-ene polymer modulus and pattern fidelity was observed.

The effect of functionalized nanoparticles on thiol–ene polymerization kinetics

Functionalized silica nanoparticles with thiol or acrylate groups were synthesized, and the effect of the functionalized particles on the thiol–ene photopolymerization kinetics was investigated by real-time FTIR spectroscopy. To explain the effect of the nanoparticles on thiol–ene polymerization kinetics, the thiol–ene polymerization at the interface of the particle and the bulk monomer was studied in conjunction with the effects of light intensity and viscosity changes caused by the nanoparticles. These results are compared with corresponding nanofilled acrylate systems. Nanoparticles were not found to significantly affect the polymerization of acrylate-based nanocomposites regardless of the functional group type attached to the particle surface while significant changes in polymerization kinetics were observed with thiol–ene based nanocomposites. The thiol–ene polymerization rate decreases with increasing particle content for small amounts of particle loadings due to a stoichiometric imbalance of thiol and ene groups at the particle surface. However, the polymerization rates increase with larger particle loadings because of polymerization viscosity enhancements. Thiol–ene based nanocomposites exhibit higher final conversions than acrylate systems, reduce the oxygen inhibition relative to acrylate polymerizations, and still react rapidly to form highly crosslinked, hard, high glass transition temperature materials.

Comprehensive Examination of the Morphology of Holographic Polymer Dispersed Liquid Crystal Reflection Gratings Written in Thiol-ene Polymer Hosts

Comprehensive Examination of the Morphology of Holographic Polymer Dispersed Liquid Crystal Reflection Gratings Written in Thiol-ene Polymer Hosts

Investigations of step-growth thiol-ene polymerizations for novel dental restoratives

The goal of this work was to investigate the feasibility of formulating novel dental restorative materials that utilize a step-growth thiol-ene photopolymerization. Particularly, we are aiming to significantly reduce the polymerization shrinkage and shrinkage stress while retaining adequate physical properties as compared to current dimethacrylatre-based systems. The thiol-ene system is composed of a 4:3 molar mixture of triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetramercaptopropionate (PETMP). The simultaneous measurement of shrinkage stress and functional group conversion was performed. Solvent extraction of unreacted monomers and dynamic mechanical analysis on the polymer networks that were formed were also studied. Flexural strength was measured for both filled and unfilled PETMP/TATATO and Bis-GMA/TEGDMA systems. Photopolymerization of PETMP/TATATO occurs at a much higher rate, with the maximum polymerization rate six times faster, than Bis-GMA/TEGDMA cured under the identical conditions. The results from the simultaneous measurement of shrinkage stress and conversion showed that the onset of shrinkage stress coincides with the delayed gel point conversion, which is predicted to be 41% for the 3:4 stoichiometric PETMP/TATATO resin composition. The maximum shrinkage stress developed for PETMP/TATATO was about 0.4 MPa, which was only approximately 14% of the maximum shrinkage stress of the Bis-GMA/TEGDMA system. Adequate flexural strength and flexural modulus values were obtained for both filled and unfilled PETMP/TATATO systems. The dramatically reduced shrinkage stress, increased polymerization rate, significance increased functional group conversion, and decreased leachable species are all benefits for the use-of thiol-ene systems as potential dental restorative materials.

Memory effects in polymer stabilized ferroelectric liquid crystals, and their dependence on the morphology of the constituent molecules

The origin of distinct memory states (multistable, bistable, monostable) was verified in thiol-ene polymer stabilized ferroelectric liquid crystal system. The stabilization is obtained by the polymerized ene in the interlayer space. The multistability and the resolution of memory states has strong correlation with the dichroic ratio of thiol. As the thiol lengthens, thiols are more intersticed between ferroelectric liquid crystal molecules in the layer and the multistability and the resolution were also enhanced, consequently. In addition, it was found that the binding of thiol to the ene through polymerization was essential for the realization of multistable memory states. The cone mode viscosity was decreased as the thiol lengthens, and this also seems to promote multistability and resolution of memory.

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

AbstractThe 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

Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.

Electro-optical and Morphological Properties of Bragg Transmission Gratings Written In Holographic Polymer Dispersed Liquid Crystals by Thiol-ene Photopolymerization

AbstractHolographic Bragg transmission gratings formed via the anisotropic phase separation of nanosized LC droplets (H-PDLC) were fabricated using thiol-ene photopolymerization. Using coherent UV laser light and a single prism, electrically switchable transmission gratings in the Bragg regime were written. The performance of the thiol-ene-based gratings were compared with those of multifunctional acrylate-based gratings written under similar conditions. Optical and electro-optical measurements suggest that thiol-ene polymers offer promise as hosts for improved H-PDLC performance. Interesting differences in the diffraction efficiencies for s– and p-probe beams are noted for the two matrices. Morphology studies by TEM and SEM exhibit striking differences in droplet shape and uniformity. These differences are speculated to be due to differences in polymer MW growth due to the step-growth propagation mechanism for thiol-enes as compared to the chain-growth propagation mechanism in multifunctional acrylates. The response times of the thiol-ene gratings were ten times slower than those of acrylates.

Photoinitiated Bulk Polymerization of Liquid Crystalline Thiolene Monomers

The photoinitiated bulk polymerization of liquid crystalline thiolene monomers based on a phenyl benzoate core was investigated and the structure of the corresponding polymers analyzed. We show that polymerization occurs substantially in bulk, and can be initiated on demand by UV-irradiation. The liquid crystalline thiolene polymers exhibit a nematic mesophase, and the molecular weight, molecular weight distribution and overall conversion are characteristic for polymers obtained by a step-growth polymerization. Using NMR spectroscopy, we show that anti-Markovnikov structures are predominantly formed, with a small fraction of the structures displaying different configurations that can be attributed to thiol olefin addition−cooxidation processes or to homopolymerization of the olefinic moiety. The conversions of the thiol and vinyl groups were monitored using time-resolved Raman spectroscopy and dissimilar partial reaction rates were observed.

Formation of a host nanostructure for ferroelectric liquid crystals using thiol-ene polymers

Liquid crystal displays are a subject of intense research interest because of their application to high definition display devices. Recently, polymer stabilized ferroelectric liquid crystals (PSFLCs) have been investigated due to the enhanced electro-optic properties of FLCs. We have utilized thiol-ene photopolymerizations to form a PSFLC system. Thiol-ene photopolymerizations are radical reactions, which proceed via a step growth reaction mechanism. During the polymerization, the polymer network structure is trapped into place due to the rapid transition from low molecular mass monomers and oligomers to high molecular mass polymer. This aspect is evidenced by phase transition data for the FLC, which indicates that the monomer and polymer are not phase separated from the FLC. Infrared dichroism shows that both monomer and polymer are ordered in parallel with the smectic layers of the FLC. Small angle X-ray scattering (SAXS) data show that both monomer and polymer are swelling the smectic layers. Thus, a polymer nanostructure is produced that serves as an ordered, stabilizing host for the FLC.