Thiol-ene polymerization

The process may be used to form linear polymers. Nuyken and Volkel described a method for tclcchclic production, based on the radical initiated reaction of difunctional transfer agents with dienes e.g. divinyl benzene (13), dimethacrylate esters). However, currently the most common use of thiol-ene 

One may envisage polymerizations analogous to the thiol-cnc process using other bis- or multi transfer agents (e.g. radical-induced hydros lylation between bis-silanes and dienes). However, none has been described or achieved significance. 

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Even in the absence of added transfer agents, all polymerizations may be complicated by transfer to initiator (Sections 3.2.10 and 3.3), solvent (Section 6.2.2.5), monomer (Section 6.2.6) or polymer (Section 6.2.7). The significance of these transfer reactions is dependent upon the particular propagating radicals involved, the reaction medium and the polymerization conditions. Thiol-ene polymerization consists of sequential chain transfer and reinitiation steps and ideally no monomer consumption by propagation (Section 7.5.3). 

Thiol-ene polymerization was first reported in 1938.220 In this process, a polymer chain is built up by a sequence of thiyl radical addition and chain transfer steps (Scheme 7.17). The thiol-ene process is unique amongst radical polymerizations in that, while it is a radical chain process, the rate of molecular weight increase is more typical of a step-growth polymerization. Polymers ideally consist of alternating residues derived from the diene and the dithiol. However, when dienes with high kp and relatively low A-, monomers (e.g. acrylates) are used, short sequences of units derived from the diene are sometimes formed. 

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. 

Mathematical models of the frontal copolymerization process were developed, studied and compared with experimental data in [67, 90]. An interesting observation was that the propagation speed of the copolymerization wave was not necessarily related to the propagation speeds in the two homopolymerization processes, in which the same two monomers were polymerized separately. For example, the propagation speeds in the homopolymerization processes could be 1 cm/min in each, but in the copolymerization process, the speed could be 0.5 cm/min. Mathematical models of free-radical binary frontal polymerization were presented and studied in [66, 91]. Another model in which two different monomers were present in the system (thiol-ene polymerization) was discussed in [21]. A mathematical model that describes both free-radical binary frontal polymerization and frontal copolymerization was presented in [65]. The paper was devoted to the linear stability analysis of polymerization waves in two monomer systems. It turned out that the dispersion relation for two monomer systems was the same as the dispersion relation for homopolymerization. In fact, this dispersion relation held true for W-monomer systems provided that there is only one reaction front, and the final concentrations of the monomers could be written as a function of the reaction front temperature. 

There are also a t riety of other parameters that may lead to alterations of the initiation, propagation, and termination reactions, adding even more complexity to the reartion behavior. For instance, oxygen is well known to inhibit radical polymerizations due to its high reactivity toward radicals and can influence network behavior with sample depth. Although a standard radical mechanism with an initiator is discussed in this section, other techniques (e.g., thiol-ene polymerizations) will be discussed in subsequent sections. 

Decker, C. and Nguyen thi viet, T Photo-cross-linking of functionalized rubbers 9 thiol-ene polymerization styrene-butadiene block copolymers. Polymer, 2000, 41, 3905-3912. 

KHl 06] Khire V.S., Harant A.W., Watkins A.W. et al, Ultrathin Patterned Polymer Films on Surfaces Using Thiol-Ene Polymerizations ,... 

Khire, V.S., Harant, A.W., Watkins, A.W., Anseth, K.S., Bowman, C.N. 2006. Ultrathin patterned polymer films on surfaces using thiol-ene polymerizations. Macromolecules 39 5081-5086. 

As with all condensation reactions, thiol-ene polymerizations are sensitive to the stoichiometry of the thiol- and ene-containing monomer substrates. Typically, the best results are obtained when there is a molar equivalent concentration of thiol and double bonds present. Recently, there has been a considerable effort to extend the photoinduced radical addition of thiols to alkynes (the so-called thiol-yne reaction). 

Thiols can be used in two ways with free-radical polymerization. Thiols react with electron-rich enes (allyl ethers) via a step-growth mechanism to create a polymer only if both ene and thiol have functionalities of at least two. The allyl ethers cannot homopolymerize. If thiols are present, the acrylate can homopolymerize and copolymerize with the thiol. Pojman et al studied frontal thiol-ene polymerization using pentaer-ythrytoltriallyl ether (PTE) and trimethylolpropanetris (3-mercaptopropionate) (95%) (TTl). Not surprisingly, the front velocity was a maximum at a 1 1 thiol ene ratio (Figure 35). ... 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multifunctional mono-mers. " The thiol-ene polymerization proceeds by a step growth addition mechanism that is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

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