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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.

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.

The photolytic decomposition in air of a polymide film based on a dianhydride and a diarylamine with hexafluoroisopropylidene 6F bridging groups is extremely rapid leading to efficient chain cleavage and subsequent photooxidative decomposition. Relatively short photolysis times with an unfiltered medium pressure mercury lamp are required to give a clean photoablation process of the 6F-6F polyimide films in the air. IR difference spectroscopy shows the appearance of anhydride groups on photolysis in air indicating a photooxidation process via decomposition of the arylimide linkage. A photophysical and photochemical analysis of several model N-arylphthalimides indicates that both solvent polarity and electron withdrawing/donating substituents can greatly after all the efficiency of the photolysis process.

Ordering of the medium in which free radical polymerization is conducted can, under appropriate circumstances, alter the kinetics of the chain-growth process, resulting in enhanced rates. Since in many systems medium changes occur at higher conversion, it is important to take into account any alterations in the polymerization rate induced by such phenomena. We have found that exotherms initiated by individual laser pulses can be useful in identifying and providing detailed information concerning rate acceleration in liquid crystalline systems

The effect of polymer matrices on a photo-induced rearrangement process has been shown to be dependent upon whether the photoreactive group is attached to a polymer backbone, or free. If diphenylcarbonate is simply embedded in a polymer matrix, the rearrangement process is independent of whether the host film is above or below its glass transition. However, if the diphenylcarbonate group is incorporated as part of a polycarbonate backbone, the Fries rearrangement process is significantly reduced for photolyses conducted at temperatures well below the glass transition of polycarbonate. The utility of fluorescence spectroscopy in identification of the initial salicylate type photo-Fries type rearrangement product of polycarbonate is also demonstrated. The broad, structureless fluorescence spectrum with peak maximum at 470 nm produced by photolysis of polycarbonate films for short time periods is assigned to emission from phenyl salicylate type photoproducts.

Polyimide films made from diarylanhydrides with oxygen, carbonyl, and hexafluoroisopropylidene 6F bridging groups are photolabile when irradiated with unfiltered light from a medium pressure mercury lamp. The presence of oxygen is necessary to the photolytic degradation process, which results in the ultimate oxidative ablation of thin polyimide films. Films based on the 6F dianhydride containing a hexafluoroisopropylidene hinge group are the most unstable. However, photolysis of 6F dianhydride based polyimides in an inert atmosphere with either the unfiltered medium pressure mercury lamp or an electron beam results in no detectable changes even after long exposure times.

Copolymers of acrylamide containing less than 0.5 mol% N-[(1-pyrenylsulfonamido)ethyl]acrylamide were studied in aqueous solution. The copolymer prepared by a surfactant technique was shown to possess some inherent blockiness or short runs of the pyrenesulfonamide label in dilute solution. Intermolecular associations were observed above a critical concentration. The copolymer prepared by solution polymerization showed random incorporation of the pyrenesulfonamide comonomer and no intermolecular association tendency over the concentration range studied

Pyrene fluorescence spectra and lifetimes have been employed to study the pH-induced conformational transitions of maleic acid-ethyl vinyl ether copolymers that have been hydrophobically modified with varying amounts of 4-butylaniline (4-BA). Results indicate that those copolymers containing up to 50 mol% 4-BA (relative to the total number of maleic acid groups) undergo a transition from a highly collapsed ‘hypercoil’ conformation at low degrees of ionization (α) to an open, hydrated conformation at high α. In contrast, a copolymer containing 70 mol% 4-BA moieties forms compact, hydrophobic microdomains over the entire range of α. Fluorescence quenching of the pyrene probe by Tl + and nitromethane indicates that the accessibility of these microdomains is related to α and/or the degree of hydrophobe incorporation. Diffusion coefficients and effective coil diameters of these polymers as determined via dynamic light scattering measurements indicate an increasing degree of intramolecular association with increasing hydrophobe content at high α, while fluorescence measurements indicate no such interactions. It is suggested that, though the pyrene probe may associate with the hydrophobic regions of these polymers, it is not adequately ‘protected’ from the bulk aqueous solution for fluorescence enhancement to occur.

Based on analysis of the photochemistry of a model compound, N -phenylphthalimide, the photolysis of a polyimide derived from 4,4′-oxydianiline (ODA) and a diarylanhydride with a hexafluorinated isopropylidine bridging group (6F) is proposed to proceed via formation of a phthalic anhydride type photoproduct. The quantum yield for product formation of N -phenylphthalimide is quite low ( −3 ) indicating a rather inefficient first step in the overall oxidative degradation process. However, based on the efficiency of the photo-oxidation of the primary photoproducts, a process leading to the ultimate failure and complete ablation of the 6F-ODA films in air upon exposure to the unfiltered output of a mercury lamp is postulated.

The photo-Fries rearrangement has been observed for the aryl cinnamate chromophore in both polyaryl cinnamates and small-molecule models. Laser flash photolysis of solutions of aryl cinnamates and the corresponding polymers leads to phenoxy and cinnamoyl radicals from the cleavage of the CO ester bond. The formation of these radicals has been verified by their independent generation by flash photolysis of phenols (e.g. p-pentyloxyphenol) and aldehydes (e.g. p-methoxycinnamaldehyde). The effects of triplet sensitizers and quenchers on the formation of these transient intermediates and on the formation of the photo-Fries rearrangement products are reported. It is concluded that these products arise from either the aryl cinnamate singlet state or a short-lived upper triplet state.

Polyimides based on a hexafluorinated dianhydride decrease in thermal and thermal-oxidative stability upon photolysis with an unfiltered medium pressure mercury lamp source in air. Additionally, the glass transition of photolyzed polyimides containing the hexafluorinated moiety is lowered with increasing photolysis time. By contrast, pyromellitic-dianhydride-based polymers are relatively thermally stable even after prolonged photolysis under similar conditions.

The photodegradation of aromatic dianhydride based polyimide films with perfluorinated chromophores in the dianhydride moiety is characterized by significant weight loss and chain cleavage. A conventional polyimide film based on pyromellitic dianhydride yields little decomposition under similar conditions of photolysis. Emission changes on photolysis of polyimide solutions demonstrates and efficient rearrangement to a yield a highly fluorescent species which can be used as an effective tool for following the degradation process.

The fluorescent probes were utilized to investigate the microenvironment created by a water-soluble polyphosphazene copolymer in aqueous solution. The combination of intensity, wavelength shift, and vibrational analysis measurements of the three small molecule probes is used to postulate an environment at high pH (with no added electrolyte) which is similar in polarity to methanol. In the absence of excess small molecule electrolyte, negatively charged probes are repelled by the charge on the polymer. Addition of small molecule electrolyte results in solubilization of the negatively charged probes. However, pyrene, even in the absence of added electrolyte (NaCl) is solubilized. Investigation of pyrene fluorescence quenching demonstrates the ability of the polyphosphazene copolymer to bind positively charged ions (TI+) and repel negatively charged ions (I-). Neutral quenchers such as nitromethane readily penetrate the microenvironment of the polymer. (jes)

Copolymers with acrylic acid and methacrylic acid (MAA) are studied in aqueous solutions. An increase in excimer emission relative to monomer emission (I E /I M ) is observed upon increasing solution pH. The hydrophobic character of both the naphthyl chromophores and the pendent methyl groups of MAA and the effective decoupling of the naphthyl chromophores from the polymer backbone via a spacer linkage are responsible for this behavior. The effects of NaCl, urea, and Triton X-100 surfactant indicate that hydrophobic interactions among naphthyl chromophores are responsible for formation of an intramolecular «hypercoil» at high pH

It is shown that addition of benzophenone to a system containing a polymerizable diacrylate monomer and an N-substituted maleimide results in a very rapid polymerization rate. Laser flash photolysis results show thar benzophenone sensitizes the triplet state of N-methylmaleimide. Additionally, a very large rate constant for quenching the excited triplet state of N-methylmaleimide by an amine synergist suggest that an electron transfer process is operative

The photoinitiated polymerization of 1,6-hexanedioldiacrylate (HDDA) has been investigated using a variety of N-substituted maleimides to initiate the polymerization process upon exposure to light from a mercury lamp. When readily abstractable hydrogens are available, the polymerization is rapid. A maleimide with a carbonate group attached gave the greatest exotherm for HDDA polymerization.

Bismaleimides containing between 2 and 12 methylene spacer units have been synthesized and characterized using laser flash photolysis spectroscopy. In the case of the monofunctional N-methylmaleimide, the 1,4-biradical was observed only at maleimide concentrations high enough to effectively quench the triplet state, indicating that the 1,4-biradical is formed from self-quenching of the triplet excited state. The transient spectral features and the transient lifetimes of bismaleimides containing 2 and 9 methylene spacer units closely resemble those of N-methylmaleimide and are attributed to triplet-state transients. N,N′-Alkylenebismaleimides with 3 and 6 methylene spacer groups exhibit a transient with a very short lifetime attributed to a singlet 1,4- biradical species formed from intramolecular interaction. The absence of triplet spectra in these latter bismaleimides indicate that the 1,4-biradical is formed directly from the excited singlet state. The large difference in the lifetime of the 1,4-biradical formed in monofunctional maleimide solutions and bismaleimide solutions is reflective of differences in the biradical spin-state multiplicity. The bismaleimide with a 12 methylene spacer group exhibits transient spectra corresponding to both a triplet state of the maleimide and a 1,4-biradical formed by exo interaction of two maleimide groups. As a demonstration of the ability of acrylate monomers to react with the short-lived singlet biradicals, it is shown that the photopolymerization of a multifunctional acrylate monomer is readily initiated by the 1,4-biradical formed from the bismaleimides with 3 and 6 methylene spacer groups, presumably by direct reaction of the biradical with the acrylate.

The effect of solvent on the formation of excimer from copolymers of 2-(1-naphthylacetyl)ethylacrylate (NAEA) with both acrylamide (AM) and N-vinyl pyrollidone (VP) in dilute solution was investigated. A minimum in excimer intensity was observed for each of the copolymers at approximately 2.20 × 104 (J/m3)0.5. It is suggested that the relatively high content of NAEA in the copolymers (between 30 and 60%) governs the overall solubility parameter of the polymer.

A liquid crystalline monomer was photopolymerized under a variety of conditions by both free radical and cationic processes. Comparison of the polymerization rates and the polymer formed by the two mechanisms gives insight into polymerization in ordered phases. The polymerization exotherms generated by the free radical polymerization in the ordered cholesteric nematic and isotropic media were compared to those generated for a similar system upon polymerization in smectic and isotropic media.

A trimolecular liquid crystalline mixture of monomers was photopolymerized while being subjected to macroscopic orientation via surface contact or continuous application of a magnetic field. Polymerization kinetics were not influenced by the macroscopic orientation, and crosslinked networks were rapidly produced. Both IR dichroism measurements and x-ray diffraction analysis indicated that oriented networks were generated when polymerization was conducted from a nematic phase.

The photochemical behavior of thin films and solutions of main chain nematic liquid crystalline trans-stilbene dicarboxylate polyesters (SDA) is discussed. Observation of spectral differences between solutions and thin films indicates the presence of chromophore aggregation. Hypochromic effects attributed to aggregation of chromophores become more pronounced upon thermal annealing above 40 degree(s) C. Polymer solutions and thin films (above 40 degree(s) C) exhibit the ability to undergo a reversible photocycloaddition reaction.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The synthesis and photochemistry of comb-like thermotropic liquid-crystalline (L.C.) polymers containing UV-sensitive cinnamate ester chromophores incorporated as part of the pendant groups is described. The methacrylate monomer was polymerized using AIBN as a free radical initiator. The acrylate monomer was resistant to polymerization under the same reaction conditions. The principle photochemical reactions on photolysis (313 nm) of solutions of the polymer as well as freshly cast films are photocycloaddition and photo-Fries rearrangement of the aryl cinnamate chromophore. Aggregation of chromophores was studied in films as a function of phase type at different temperatures. These results are compared with those obtained from main chain L.C. polyaryl cinnamates.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thiol–ene click chemistry was adapted to easily and rapidly modify exo-olefin polyisobutylene with an array of thiol compounds bearing useful functionalities, including primary halogen, primary amine, primary hydroxyl, and carboxylic acid.

Radical mediated thiol-yne polymerization reactions complement the more well-known thiol-ene radical polymerization processes, with the added advantage of increased functionality. In one system studied, the rate constant for the addition of the thiol to the vinyl sulfide created by the initial reaction of the thiol with the alkyne is three times faster than the initial reaction. When hydrocarbon based dialkynes and dithiols were copolymerized, the resulting thiol-alkyne networks containing only hydrocarbon and sulfide linking groups exhibited refractive index values tunable above 1.65, with the refractive index directly related to the sulfur content. The thiol-yne reaction was also found to be useful in functionalizing thiol-terminated polymer chain ends via sequential Michael thiol-ene addition followed by the thiol-yne reaction: the result is the dual functionalization of the polymer chain end. A thermally responsive polymer hydrogel network was formed when an yne terminated water-soluble homopolymer was polymerized with a tetrafunctional thiol.

The merits of thiol-click chemistry and its potential for making new forays into chemical synthesis and materials applications are described. Since thiols react to high yields under benign conditions with a vast range of chemical species, their utility extends to a large number of applications in the chemical, biological, physical, materials and engineering fields. This critical review provides insight into emerging venues for application as well as new mechanistic understanding of this exceptional chemistry in its many forms (81 references).

In order to obtain oriented liquid crystalline films which maintain the alignment of monomeric species from whence they are derived, it is necessary to induce rapid, isothermal polymerization processes. In recent studies, monofunctional and difunctional mesogenic methacrylate monomers with nematic and smectic phases have been aligned and polymerized to yield macroscopically oriented polymers. In some cases, the orientation is maintained even after the polymer is heated above its clearing temperature. By utilizing a thin-foil calorimeter, we have succeeded in measuring polymerization rates in macroscopically oriented liquid crystalline monomers. Although the macroscopic structure of the aligned system may be locked-in during polymerization, this does not necessarily result in enhanced rates compared to non-oriented samples. The experimental techniques used to measure the polymerization exotherms of oriented liquid crystals are described. Initial results which illustrate the effect of macroscopic alignment are discussed with respect to polymerization kinetic rates. Implications for polymerization of oriented liquid crystals are briefly discussed in the context of optical properties of the final films.

A previous review published over a decade ago introduced the concepts inherent to the measurement of exotherms resulting from photopolymerization.1 Since then, the photocuring field has grown at an extremely rapid rate. Applications range from the curing of systems comprising acrylated oligomers and low-molecular-weight acrylates (both monofunctional and difunctional) for the production of highly cross-linked films to the selective hardening of microencapsulated spheres for generation of colored images by a latent dye transfer process.2–10 No matter what the final application, one universal challenge faces anyone who desires to formulate, develop, and successfully process a photocurable resin: how can the rate, degree of cure, and ultimately the kinetics of the photopolymerization process responsible for the curing and production of highly cross-linking networks be evaluated on a rather routine basis by a rapid method that is accurate and readily repeatable. Of course, just because one is able to measure the various kinetic parameters associated with the photopolymerization of functionalized formulations, there is no assurance that the physical properties desired for a particular application can be attained. However, it is highly unlikely that photocurable systems that show extremely low rates of polymerization and reduced overall conversion efficiency can be employed as useful formulations for practical applications under normal circumstances.

The photo-oxidation of certain polyimides has been found to be extremely efficient provided that the photolysis is conducted in the presence of oxygen. The apparent rate of decomposition depends on the chemical composition of the components composing the polyimide backbone as well as the spectral properties of the light source. In the case of polyimides which are made from a hexafluorinated dianhydride and a hexafluorinated diamine, the photo-oxidation process is extremely rapid. The photolysis mechanism is quite complicated, but analysis of model compound photochemistry by conventional methods indicates that a variety of products, which themselves are photolabile, are generated in the primary photochemical process.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

During the past few years we have conducted extensive analysis of the laser-initiated polymerization of acrylate type monomers which polymerize by a free-radical mechanism. In this paper we report on the dependence of the rate of polymerization of a variety of monofunctional and multifunctional monomers on the intensity of pulses produced from an excimer laser. In the case of multifunctional systems, the rates of certain monomers show an absence of dependence on the laser intensity over a selected range while others are highly dependent on the laser intensity over a broad range. It appears that the ability to exhibit rapid rates when high intensities are employed is influenced by the mechanism of the macrogelation process which occurs during the polymerization. Also, the photoinitiator type and concentration can play a major role in the dependence of the rate on the pulse light intensity.

Thin films of a liquid crystalline polyaryl cinnamate have been irradiated at (lambda) > 400 nm in the presence of ketocoumarin triplet sensitizers. The aryl cinnamate chromophore disappears upon irradiation with concomitant loss of solubility attributed to cross-linking by interchain cyclobutane photoproduct formation. Competing loss of sensitizer absorption also occurs during irradiation. Up to ca. 80% of the aryl cinnamate chromophores in amorphous films can be destroyed using high weight percentages (>= 5%) of sensitizer. At low concentrations of sensitizer (0.5 wt.%), approximately 80 cinnamate chromophores are photolyzed for each sensitizer molecule initially present, despite the competing photolysis of sensitizer during the course of irradiation. The ketocoumarins are insoluble in the glassy (frozen) nematic films of the polymer and thus are ineffective in sensitizing photolysis in this more organized phase.

The photodecomposition of four polymer series containing the hexafluoroisopropylidene (6F moiety was studied. We report herein the photostability of a poly(ether ketone) series when subjected to UV light in air. Replacement of isopropylidene with 6F results in decreased photostability for the PEKs as evidenced by changes in molecular weight and IR and UV-VIS spectra. Results for polyamides, Polyimides, and polyarylates are also under study.

A novel convergent route to 3-arm star polymers is described that takes advantage of RAFT-synthesized homopolymers serving as masked macromolecular terminal thiol-containing materials capable of undergoing thiol-ene click reactions.

Photocurable Coatings Electron-Beam Curing of Polymeric Materials Depth of Cure Profiling of UV-Cured Coatings Modified Kubelka-Munk Optical Treatment of Photon Flux in a Coating and Its Relationship to UV Curing as Measured with a UV N101 Cure Tester Time-Resolved Laser Spectroscopy of Synergistic Processes in Photoinitators of Polymerization Photochemistry and Photopolymerization Activity of Water-Soluble Benzophenone Initators Cationic Photoinitiators: Rearrangement Reactions from Direct Irradiation of Diarylhalonium Salts Novel Photoinitiator for Modern Technology Coreactive Photoinitiators from Surface Polymerization Effect of Monomer Structure on Concurrent Grafting During Radiation Curing Photoinitiated Cross-Linking of Polyethylenes and Diene Copolymers Nonacrylate Curing Mechanisms: Photoiniated Cross-Linking Using Amine-Ene Reaction Photoinitiated Cross-Linking of Norbornene Resins with Multifunctional Thiols Acrylated Melamines in UV-Curable Coatings Coatings Curable with Low-Emission Radiation Polymeric Coatings Containing Chlorendic Anhydride: Decomposition Products of Chlorendic Anhydride Photochemical Approaches to Ordered Polymers Comparison of Thermal, Mechanical, and Spectroscopic Techniques for Characterization of Radiation-Cured Adhesives Mechanical Properties of UV-Cured Coatings Containing Multifunctional Acrylates Structure-Performance Relationships of Urethane Acrylates Enthalpy Relaxation in UV-Cured Epoxy Coatings Temperature Effect on the Phase Transformation of UV-Curable Systems Microcolumn Imaging: Simulation of the Microcapsule Imaging System Photolysis Studies of Bisphenol-A-Based Model Compounds: Effect of Decomposition Products on the UV Stability of Bisphenol-A-Based Epoxy Coatings Photooxidative Stability and Photoyellowing of Electron-Beam-and UV-Cured Multifunctional Amine-Terminated Diacrylates Radiation-Induced Cationic Curing of Vinyl-Ether-Functionalized Urethane Oligomers UV Cure of Epoxysiloxanes and Epoxysilicones UV Cure of Epoxy-Silicone Monomers UV-Induced Polymerization of Highly Filled Epoxy Resins in Microelectronics Laser-Initiated Polymerization of Multifunctional Acrylates: Repetition Rate Effects on Percent Conversion UV-Radiation and Laser-Induced Polymerization of Acrylic Monomers High-Energy-Radiation-Induced Cationic Polymerization of Vinyl Ethers in the Presence of Onium Salt Initiators Structure-Property Behavior of Caprolactone-Allyl Glycidyl Ether Copolymers Cross-Linked by Electron-Beam Irradiation Electron-Beam Exposure of Organic Materials: Radiation Curing of Perfluorinated Acrylates Molecular Weight Dependence of E-Beam Resist Sensitivity Electron-Initiated Graft Modification of Polyolefins