Thiol-Ene Additions

The fluorinated network has been elaborated starting from 3,3,4,4,5,5,6,6,7,7, 8,8,8-tridecafluorooctyl acrylate (AcRf6) by free-radical co-polymerization with ethylene glycol dimethacrylate in the presence of DCPD as the initiator. In this case, the PDMS network could not be synthesized by thiol-ene addition... 

Scheme 30.2 The free radical chain reaction mechanism between a thiol and an unactivated carbon-carbon double bond to form the anti-Markovnikov thiol-ene addition product. Adapted from Ref [22].

PI with pendant ftmctional groups was successfully synthesized by thiol-ene addition reaction (Figure 11) and was further modified to serve as macroinitiator for the ATRP of styrene or tert-butyl acrylate (tBA) to give graft copolymers with V -shaped side chains (Pl-g-PSi and PI- -PtBA2 or PI- -PAA2 (by hydrolysis of PI-g-PtBA2)). ... 

Plant oils and their derivatives are attractive for building renewable polymers due to their renewability, worldwide availability, and their comparatively low price (65). In order to achieve a more sustainable production of polymers from these resources the recent methods of synthesis go in the direction of catalytic transformations. In particular, olefin metathesis and thiol-ene additions have been used for synthetic purposes. These methods have been reviewed recently (65,66). 

Renewable monomers derived from vanillin and fatty acids have been studied with various polymerization techniques, such as ADMET, thiol-ene addition, and polycondensation (91). The synthesis of such monomers is shown schematically in Figure 4.17. 

The radical addition of 2-mercaptoethanol onto oleic acid, from rapeseed oil, has been done by the photochemical generation of radicals without any photoinitiator (20). The 2-mercaptoethanol is photochemically grafted onto vinyl groups of rapeseed oil by a thiol-ene addition to yield polyols. The higher the content of polyunsaturated fatty chains, the lower the rate of 2-mercaptoethanol grafting was found. 

The first publication on the thiol-ene reaction of a vegetable-oil derivative dates to 1957, when Koenig and Swern published the free-radical addition of mercaptoacetic acid to oleic acid, methyl oleate, methyl ricinoleate and 10-undecenoic acid. That is, the synthesis of derivatives of a,(0-dicarboxylic oleic acid through thiol-ene addition [6]. 

Also, following this same approach, the research team of Cramail reported a series of novel fatty acid-based monoester- and diester-containing diols synthesised from sunflower and ricin oils using a series of reactions (including the thiol-ene addition). In sequence, the polycondensation reactions of the diols with isophorone diisocyanate, in the presence of dibutyltin dilaurate as the catalyst, were carried out and led to materials with Mn values <35 kDa [23]. 

Renewable polyamides (PA) can also be prepared taking advantage of the synthesis of thiol-ene monomers. Meier and co-workers [30] studied the thiol-ene addition of cysteamine hydrochloride to the double bonds of methyl 10-undecenoate, methyl oleate, and methyl erucate (Scheme 6.8), which yielded heterodifunctional monomers at good yields. Ensuing monomers were homopolymerised, copolymerised with each other, as well as copolymerised with adipic acid and 1,6-hexamethylene diamine to yield copolyamides with TBD as the catalyst. The produced PA were obtained with Mn values of 4-19 kDa. 

Application of the of thiol-ene addition reaction to unsaturated plant oils and their fatty-acid derivatives represents an interesting possibility of developing technologies for the production of sustainable chemicals. In this sense, the synthesis of monomer structures with intrinsic potential to be applied to the design of new materials using this approach is very important. However, this reaction is also used as a direct polymerisation method, and its application in vegetable oils is well-established. The next section aims to illustrate the most recent advances reported on this subject. 

The thiol-ene addition reaction associated with oleochemistry is a very versatile tool for the polymerisation of a,ro-diene monomers bearing, for example, ester, ether, and anhydride functional groups in the main chain. Due to the availability of specific monomers, the AA/BB approach is the choice for conducting polymer syntheses most of the time. 

Authors stated that 2.5 mol% of azobisisobutyronitrile (AIBN) was used as an initiator to obtain complete conversion of the monomer after 2 h, at 80 °C. Low temperatures are favourable for thiol-ene additions. However, for polymerisations, the reaction temperature should be increased to avoid crystallisation of the polymer during polymerisation [33]. Gel permeation chromatography (GPC) analyses showed that fatty acid-based polyesters with Mn = 12 kDa (reactions 2 and 3, Scheme 6.11) could be synthesised via thiol-ene polymerisation. However, this approach was less suitable for the polymerisation of reactions 1 and 3, Scheme 6.11, which led to a Mn value of only 5 kDa. The high reactivity of the anhydride functionalities towards nucleophiles (in this case the thiol groups) caused the scission of the monomer or polymer backbone via thioester formation, leading to a decrease in molecular weight [33]. 

Scheme 6.12 Copolymerisation of different ratios of derivatives of fatty acids and a derivative of ferulic acid via the thiol-ene addition with 1,4-butanedithiol [34]...

Firdaus and Meier [35] investigated the synthesis of monomers derived from vanillin and fatty acids to be used in a comparative study of ADMET, thiol-ene addition, and polycondensation-type polymerisation reactions. The highest value of Mn was obtained with the ADMET polymerisation (50 kDa). Ffowever, the thiol-ene approach also led to materials with good values Mn = 10-16 kDa, Tg ranging from -37 to -22 °C, and T , = 36-68 C. 

Kolb and Meier [43] prepared a malonate derivative of methyl 10-undecenoate, which was polymerised further with 1,6-hexanediol using titanium (IV) isopropoxide as a catalyst. This polymalonate, bearing a C9 aliphatic side chain with terminal double bonds, was then subjected to grafting by ruthenium-catalysed cross-metathesis reactions with acrylates or thiol-ene addition reactions. This functionalisation enabled a subsequent Passerini multi-component reaction [44] using the pendant carboxylic-acid moiety of the modified polymers that resulted from the thiol-ene addition of 3-mercaptopropionic acid into the initial double bonds of the polymer. 

Cadiz and co-workers [48] synthesised two ROx monomers, 2-nonyl-2-oxazoline and 2-(9-decenyl)-2-oxazoline, from undecenoic acid and decanoic acid, respectively, and then copolymerised them with different feed ratios hy cationic ROR Thiol-ene addition of 2-mercaptoethanol was used to produce a set of polyoxazoline-polyols with quantitative transformation of double bonds. Polyols obtained in this way were reacted with methylene-bis(phenylisocyanate) to yield a series of amorphous and semi-crystalline PU networks. 

However, thiol-ene addition forming yS-products usually predominates. Hence only this addition will be considered in further discussions on TEC reactions. The TEC reaction ocurs by either radical or nucleophilic mechanisms (see Fig. 12.4), depending on the reaction conditions and the imsaturated substrates. 

Click reactions are an attractive means to synthesize SCNPs due to their high efficiency, high functional group tolerance, and mild reaction conditions. Copper-mediated azide-allgme cycloaddition, thiol-ene addition, thiol-yne addition, and amine-isocyanate addition" click reactions have been used to synthesize cross-links and form SCNPs. 

We are often asked if there are more orthogonal click reactions like the CuAAC and the thiol-ene addition lurking out there. The answer is a confident yes . However, when people want to replace the azide group with another 1,3-dipole, this is hard to imagine, assuming one demands identical, or better reactivity parameters. The simple reason is that the other dipoles are not adequately invisible in the acid-base world. Hence, the azide functionality should remain a rich source of new reactivity discoveries for many years. 

Furthermore, a dialdehyde, a diisocyanide, and a carboxylic acid were applied in the Passerini one-pot polymerization (Scheme 4c). For this combination of reactants, Li et al. used adipaldehyde, 1,6-diisocyanohexane, and undecanoic acid, resulting in polyamides with functional side groups [35]. Optimization of the reaction conditions revealed that a IM chloroform solution of bifunctional components and a 2.2-fold excess of the carboxylic acid at 40°C gave the best results, yielding polymers with of up to 16.6 kDa in a yield of 90%. Here, m-alkene- or a)-alk5me-functionalized carboxylic acids were used as monomers in order to allow post-polymerization modifications via thiol-ene addition or copper-catalyzed azide-alkyne click chemistry (CuAAC). 

A similar approach was followed by Meier and coworkers, which allowed variation of the isocyanide component [54]. In this case, stearic acid served as onset for the chain elongation via Passerini-3CR along with 10-undecenal and various isocyanides. 10-Undecenal, because of its terminal double bond, enabled the subsequent thiol-ene addition of 3-mercaptopropionic acid with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. The newly introduced carboxylic acid provided reactivity for further sequences of Passerini-3CRs and thiol-ene... 

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