The thiol-ene system

As reported by Reiser,a unique system of photoinitiated polymerization is based on thiols (mercaptons) with olefinic double bonds according to the following scheme  

It has been reported that if the average functionality is 2, cross-linking occurs in the system, and the reaction produces insoluble networks at comparatively low 

Reiser, Photoactive Polymers The Science and Technology of Resists, p. 145, John Wiley Sons, Hohoken, NJ (1989). 

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The most important attribute of the thiol-ene system is its insensitivity to oxygen thus, it is not inhibited by it. Another attractive feature is its very high cure speed. The disadvantage of the thiol-ene system is an unpleasant odor of the volatiles emitted from some polythiol compounds. ... 

Another example is shown by the thiol-ene system in which two different reactants A and B photochemically react to form larger molecules with concomitant reduction in solubility. Thus, in the presence of sensitizers/initiators, the multiple functional mercaptan (IV) will react with the multiple unsaturated oligomer (V) to form hardened addition products useful in letterpress platemalong (Figure 7). 

The thiol-ene system is used mostly in the production of letterpress printing plates. Since both components are liquid, they are mixed with appropriate sensitizers, and the mixture is coated on aluminium or steel plates and then covered with protective foils. Exposure through a negative film causes the exposure area to solidify. Air is then blown onto the exposed plate to remove the remaining liquid. The resultant relief plate is developed, washed, and re-exposed to give a strong letterpress plate ready for use. 

The thiol-ene type of cure is based on free-radical additions of thiols to unsaturated compounds. The unsaturated compounds are often, though not necessarily, allylic ftmctionalized resins, such as allyl ethers and allylic urethanes and ureas. The thiol-ene systems find application in poly(vinyl chloride) based flooring and in gaskets. 

There are a few other methods to crosslink VDF-based fluoroelastomers, Uke the thiol-ene system, but these are very rarely used. 

An FTIR spectrometer (Nicolet Magna-IR System 750) and a DTGS-KBr detector was used to obtain the IR spectra for the thiol-ene sample. Sixteen scans,... 

Thus, a semilogarithmic plot of the gel time as a function of 1/T should be linear, with the slope corresponding to the apparent activation energy. We have determined the gel times for a temperature range of 25°-50° C for a thiol-ene system consisting of stoichiometrically equivalent amounts of a trifunctional thiol, trimethylolpropane tris(2-mercaptoacetate), and a trifiinctional allyl monomer, triallyl isocyanurate. In this system, we also added 0.31% by weight of hydroquinone, to prevent premature polymerization, and 1.0% by weight of a commercial photoinitiator, Esacure TZT. 

The laser-initiated polymerization of styrene with maleic anhydride has been said to occur through either a singlet or triplet excimer (the authors surely mean exciplex ) of the anhydride as shown in Scheme 4. This study provides an interesting comparison between laser- and u.v.-initiated polymerization, and Table 2 shows clearly that the former is a very energy-efficient system. In the laser-initiated polymerization of a thiol-ene system, oxygen inhibition was not a significant problem and similar conclusions were reached on the energy efficiency of lasers. On a related note, the efficiency of the well-known benzo-phenone-triethylamine complex is apparently enhanced if carboxylic acids are added to the system. ... 

Thiol-Ene Photopolymerization The thiol-ene polymerization of suitable systems (10.84) in film is insensitive to oxygen. The reaction refers to the addition of a thiol to a double bond (e.g., vinyl, allyl, acrylate, and methacrylate) [310] and has led in these past years to a new revival of interest [311,312]. Thiol-vinyl ether or -allyl ether polymerization shows some following interesting features very fast process, low or even no oxygen inhibition effect and formation of highly cross-linked networks with good adhesion, and physical and mechanical properties. 

Unsaturated maleic/fumaric acid-containing polyesters (UPEs) dissolved in styrene, (2) acrylate/methacrylate systems, (3) thiol/ene systems, and (4) epoxide- or vinyl ether-containing systems. In the case of systems (l)-(3), free radical polymerizations are operative, while in case (4) cationic species are involved (see Chapter 10). Regarding thiol/ene systems, the mechanism of free radical thiol/aUcene polymerization outlined in Scheme 11.1 is assumed to be operative... 

Scheme 7 Photopolymerization of thiol-ene systems (step-growth addition reaction). The square represents the backbone of the polymer network and can be epoxy, polyester, polyurethane, or polyether. ...

Cyclization reactions are expected with the assumption fliat the probability of eyelization is proportional to the number of loeal pendant vinyl groups conneeted to the thiyl radical centers. The cyclization reaction is arbitrarily truncated at a critical size, N, in that only cycles with sizes less than N are allowed during gelation. The relative rate constant of cyclization with respect to propagation was calculated using the kinetic model and the experimental gelation data obtained for a thiol-ene system consisting of divinyl... 

The functional group conversion is taken as the independent variable, the reaction kinetics of the ring-free thiol-ene system is expected to be identical to that of usual step-growth reactions. In the rate expressions for intermolecular reactions in thiol-enes are first-order reactions overall. The cyclization rate in thiol-ene reactions is slower compared to that in step-growth reactions. This result arises from intramolecular chain transfer reactions reducing the probability of favorable intramolecular collisions between the functional groups. Some examples of thiols and olefins are listed in Table 3.3. 

A3 + B2 system), leading to branched or crosslinked materials iii) monomers incorporating furan and maleimide end groups (AiB or AB2 structures), which led to hyperbranched structures. In all cases, the thiol-ene reaction between 10-undecenoic acid and 2-furanmethanethiol was the main strategy to prepare the desired monomers. Corresponding DA polymerisations are discussed in Chapter 7. Scheme 6.5 illustrates the synthetic procedures adopted for all these monomers. 

However, these systems offer some important advantages when compared with traditional free-radical polymerisation processes simpler polymerisation kinetics milder reaction conditions insensitivity to oxygen inhibition [2, 3]. These characteristics have made the thiol-ene photo-polymerisation reaction the focus of extensive research, and its application in oleochemistry is of considerable importance in terms of polymer science and technology. 

Rather lhan allempting an uncritical comprehensive coverage of the large body of recent published work, the book purposely focuses on the originality of several systems in which novel me< hanisms are being explored successfully (e.g., application of the thiol ene reaction, metathesis, and Diels-Alder reactions to oils and their derivatives). Additional emphasis is placed on straightforward and technically viable processes that call upon utilisation of readily available precursors. 

Thiol-Ene Systems. Systems that combine thiols [such as a trithiol (16)] with ene comonomers, such as allyl ethers [like trimethylol propane diallyl ether (17)] or acrylates, were first considered in the 1970s however, because of their unpleasant odor, thiols were abandoned, and acrylates became the monomer of choice for industrial implementation. 

Electrons accelerated to 3-10 MeV are traveling at about 99% the speed of light. Approximately 50% of the beam energy is lost in hard collisions that remove electrons from host atoms and thereby produce ionized species. As a result of hard collisions, polymerization can occur by free-radical or ionic mechanisms. Materials that proceed via free-radical polymerization include acrylic/methacrylic systems, maleic and fumaric polyester resins, maleimides, and thiole-ene systems. Of these systems, resins based on acrylic and methacrylic ftinctionalities have been studied the most (116). The other major material system to be studied is that of epoxy polymerized via a cationic mechanism, which requires a diaryliodonium or triaryliodonium salt catalyst. 

In addition to acrylated systems, two other technologies bear mentioning. A mercaptan - olefin (thiol/ene) system was developed by W. R. Grace [11] in the early 70 s. This system is primarily used to make photopolymer printing plates. It is estimated that over half the newspapers in the U.S. are printed using plates produced by this process. High cost has limited growth of thiol/ene in other applications. 

In a similar way, Stenzel and coworkers (Chen et al., 2009) recently synthesized dendrimers by successive thiol-yne chemistry and esterification reactions. As for the thiol-ene case, it is an efficient process and does not require a metal catalyst. For the thiol-yne system, the reaction proceeds by reacting two equivalents of thiol with the alkyne, via a two-step process. Therefore, after the reaction, each alkyne functional group will be combined with two thiols, which is very attractive and advantageous in the synthesis of dendrimers. 

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