Thiol-ene Reactions Applied to Polymer Synthesis

The mechanism of this type of polymerisation reaction follows typical step-growth polymerisation rules, and the diene dithiol (AA BB) ratio is therefore a very important parameter [19]. This observation implies that, in contrast to some of the grafting reactions mentioned above, the dithiol compound cannot be used in excess to compensate for the lower reaction rates with internal alkenes. Hence, these polymerisations are limited to the use of structures bearing terminal double bonds [19]. If this rule is applied to vegetable oils, 10-undecenoic acid and its derivatives are the only species that fulfill this criterion. 

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. 

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. 

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. 

PA can also be obtained via the thiol-ene click reaction from a,ro-diene monomers bearing amide groups in the main backbone or as side moieties via the Ugi four-component reaction [36, 37]. In one such study [38], the authors used 10-undecenoic acid as carboxylic acid, 10-undecen-l-al as an aldehyde, and different primary amines and isonitriles to synthesise a,C0-diene monomers, and then applied them to polymerisation reactions. The main focus of this specific work was the ADMET mechanism, though thiol-ene photopolymerisation was also implemented to prepare linear aliphatic PA with Mn values of 3-9 kDa and Tg values of 1-10 °C. 

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