Thiol-Based Click Reactions

Scheme 1.10 Thiol-based click reactions used for the synthesis of...

Figure 8.6 Synthesis of the graft or comb polymers by thiol-based click reactions.

Figure 8.10 The "arm-first" technique used to fabricate star polymers via thiol-based "click" reaction.

Figure 8.12 Synthesis of qrclodextrin-centered star polymers via thiol-based click" reaction. 8.2.4...

Figure 8.13 Cyclization of linear polymers by thiol-based click reaction to form cyclic polymer.

Figure 8.14 Synthesis of cyclic poly(lactide) by thiol-based click reaction.

Renewable diisocyanates can be synthesised for the preparation of PU using the thiol-ene click reaction. This approach was first described by the research team of Cramail [27]. They reacted the prepared diisocyanates with commercially available and fatty acid-based diols to obtain partially and fully bio-based PU. These materials were obtained with values of weight average molecular weight of 6-38 kDa and displayed fair thermal stability with no significant weight loss below 235 °C. Scheme 6.6 shows the approach adopted in this study for the synthesis of the novel diisocyanate. 

In this context, vegetable oils and their fatty acids are key structures because they offer a wide range of possibilities for polymer syntheses based on renewable resources. Thiol-ene and thiol-yne click reactions applied to vegetable oils and their fatty acids constitute a promising strategy to meet those goals. This combination is relatively new and, despite the important contributions achieved, many more are needed. 

Ordered mesoporous silica containing triazine moieties (Figure 3), based on the SBA-15 structure, has also been reported as an efficient and a reusable catalyst for the synthesis of chromenes 20 xmder solvent-free reaction conditions (Scheme 12) [75]. The catalyst was prepared from 3-mercaptopropyl-grafted SBA-15 by thiol-ene click reaction with 2,4,6-triallyloxy-l,3,5-triazine. The characterization data of the solid demonstrated the total disappearance of C=C bonds in triazine moiety. This feature strongly suggested the formation of bridges between the triazine and the mesopore walls. 

Fringuelli and coworkers have explored the use of InCl3 (Lewis acid), TsOH (Bronsted acid), n-Bu3P (Lewis base), K2C03 (Bronsted base), and so on, as catalysts in solvent-less conditions for thiol-epoxide click reactions [51]. 

Figure 8.1 Example of reactions carried out in thiol-based click" for the fabrication of polymeric materials.

In a typical procedure, the thiol-epoxy click reaction near quantitative yields can be employed as a polymerization reaction for preparing linear polymer chains substituted with free hydroxyl groups. Khan et al. [16] reported that the linear polymer can be easily constructed by step-growth polymerization of the commercially available diepoxide (AA-type) and bis-sulfur (BB-type) monomers using lithium hydroxide (base catalysts) as the polymerization catalyst. (Figure 8.3)... 

Here, it should be pointed out that the thiol-epoxy click reaction is a feasibility implemented in many important biosynthetic and biomedical applications because the base-catalyzed reactions can be easily carried out in water and in solvent-free conditions with high yield. 

The ability of thiol-ene click chemistry to retain high efficiency under the most benign reaction conditions was exempHfied by Ritter and Bardts, in the preparation of hydrogels based on methacryhc acid [224]. In this study, poly(methacrylic acid) polymers were modified with cysteamine and aUyl amine to afford complementary thiol-ene reactive polymer chains. Solutions of the polymer pairs in water were mixed in the presence of a radical initiator, and the formation of a highly elastic gel was observed within only 2 h at room temperature. The authors noted that this approach might have broad application for a rapid and straightforward access to hydrogel materials for which the properties could easily be tailored by the choice of the amine modifiers. 

Gandini and co-workers described a unique double click strategy related to the preparation of monomers based on vegetable-oil derivatives bearing furan heterocycles appended through thiol-ene click chemistry, and their subsequent polymerisation via the Diels-Alder (DA) polycondensation between furan and maleimide complementary moieties (i.e., a second type of click chemistry). Details about the DA reaction, its mechanism, applications and the reason why it is classified as a click reaction can be found in Chapter 7. 

There are many reports on azide-alkyne click polymerisation (a subject not discussed in the present chapter) but research on thiol-yne click polymerisation is in its early stages. New reaction types, novel catalyst systems other than those existing (i.e., photon, heat, organic base, transition-metal complexes) and new functionalities of the ensuing polymers are waiting to be developed [51]. 

Click Reaction on SAMs SAMs provide ideal molecularly defined bases to study reactions in two dimensions [464]. SAMs formed from alkanethiols and aromatic thiols have well-ordered structures, and allow a wide range of functionalities to... 

Problem 12.14 Discuss a possible method of synthesizing an asymmetric telechelic polymer based on polystyrene (DP 50) with a carboxylic group at one end and a hydroxyl group at the other, using combined thiol-ene and CuAAC click reactions. 

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