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.

Thiol–Ene Click Chemistry

The use of photoinitiated polymerization is continuously growing in industry as reflected by the large number of applications in not only conventional areas such as coatings, inks, and adhesives but also high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology. In this Perspective, the latest developments in photoinitiating systems for free radical and cationic polymerizations are presented. The potential use of photochemical methods for step-growth polymerization is also highlighted. The goal is, furthermore, to show approaches to overcome problems associated with the efficiency, wavelength flexibility, and environmental and safety issues in all photoinitiating systems for different modes of activation. Much progress has been made in the past 10 years in the preparation of complex and nano-structured macromolecules by using photoinitiated polymerizations. Thus, the new and emerging applications of photoinitiated polymerizations in the field of biomaterials, surface modific...

Photoinitiated Polymerization: Advances, Challenges, and Opportunities

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.

Thiol-yne click chemistry: A powerful and versatile methodology for materials synthesis

Nach Sharpless’ visionarer Charakterisierung verschiedener idealisierter Reaktionen als Klick-Reaktionen hat man innerhalb der Materialwissenschaften und Synthesechemie grose Anstrengungen unternommen, um solche Reaktionen zu finden und zu implementieren. In diesem Aufsatz diskutieren wir die radikalvermittelte Thiol-En-Reaktion, die uber all die Merkmale einer Klick-Reaktion verfugt: hohe Effizienz, einfache Durchfuhrung, keine Nebenprodukte, hohe Reaktionsgeschwindigkeiten, hohe Ausbeuten. Daruber hinaus besteht die Moglichkeit, die Thiol-En-Reaktion photoinitiiert auszufuhren, was insbesondere fur Photopolymerisationen zur Synthese extrem einheitlicher Polymernetzwerke genutzt wird. Der Reaktionsmechanismus wird nach dem aktuellen Kenntnisstand erlautert, und zentrale Anwendungen der Thiol-En-Reaktion in der Material- und Molekulsynthese, der Biofunktionalisierung, der Polymersynthese und der Oberflachen- und Polymermodifizierung werden zusammengefasst.

Thiol-En-Klickchemie†

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 inhibition in thiol–acrylate photopolymerizations

Photoinduced frontal polymerization was performed on a variety of multifunctional (meth)acrylates, and evaluation of the effect of monomer structure on the ability to sustain a traveling front was made. Photo-DSC was used to measure the photopolymerization rate of each monomer at 25 °C. The results were correlated with their ability to initiate and sustain a traveling front polymerization. The start time and the front velocity of each (meth)acrylate that could initiate/sustain a front photolytically were measured. The results clearly demonstrate that the molecular weight per double bond is directly related to the front velocity. Trimethylolpropane triacrylate (TMPTA), which exhibited the fastest front velocity, was evaluated in depth to determine the effect of varying the photoinitiator and thermal initiator on the frontal polymerization rate. When a multifunctional thiol was added to TMPTA, the start time was dramatically reduced and the velocity increased for low thiol concentrations. However, the veloc...

UV-Induced Frontal Polymerization of Multifunctional (Meth)acrylates

The kinetics of vinyl acrylate photopolymerization

Thermal frontal polymerization is a process in which a localized reactionpropagates through an unstirred system by the coupling of the thermal diffusion andthe Arrhenius kinetics of an exothermic polymerization. A trithiol was found to affectthe front velocity and the time for inducing a front upon exposure to UV light fortrimethylolpropane triacrylate polymerization fronts with either kaolin or calciumcarbonate ﬁller present. The addition of trithiol and ﬁller both decreased the frontvelocity. VV C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8091–8096, 2008 Keywords: addition polymerization; frontal polymerization; kinetics (polym.);radical polymerization INTRODUCTION Frontal polymerization is a process in which thereaction propagates directionally through thereaction vessel. To date three main types of fron-tal polymerizations have been reported: thermalfrontal polymerization (TFP), which uses anexternal energy source to initiate the front 1–4 ;photofrontal polymerization in which the localizedreaction is driven by an external UV sourceapplied continuously

The Effect of a Trithiol and Inorganic Fillers on the Photo-Induced Thermal Frontal Polymerization of a Triacrylate

Photoinduced free radical polymerization using self-initiating monomers

The photocuring rates of acrylate monomers initiated by abstraction type photoinitiators (benzophenone, isopropylthioxanthone, and N-methylmaleimide) and a series of structurally different tertiary amine combinations are reported. Photo-DSC results confirm that transferable hydrogens on tertiary amines are essential for efficient acrylate polymerization. Laser flash photolysis experiments were carried out to define the electron/proton transfer reactions which occur between excited triplet states of the photoinitiators and tertiary amines. In the case of N-methylmaleimide, an intermediate radical anion was detected for amines with no readily transferable proton. This confirms that the photoreduction of the triplet state of N-substituted maleimides by tertiary amines occurs by an electron/proton transfer when the tertiary amine electron donor has a transferable proton.

Charles Nason

Papers

UV-Induced Frontal Polymerization of Multifunctional (Meth)acrylates

The kinetics of vinyl acrylate photopolymerization

The Effect of a Trithiol and Inorganic Fillers on the Photo-Induced Thermal Frontal Polymerization of a Triacrylate

Photoinduced free radical polymerization using self-initiating monomers

Effect of Amine Structure on Photoreduction of Hydrogen Abstraction Initiators