Synthesis of Biobased Polyols by Thiol-Ene Coupling from Vegetable Oils

A model study of the radical addition of 2-mercaptoethanol onto oleic acid was performed under mild conditions (generation of radicals under UV light at room temperature without any photoinitiator). To evaluate the efficiency and the robustness of thiol-ene reaction, experimental parameters were varied, such as the irradiation intensity (ranging from 0.5 to 15.0 W/cm(2)), the thiol/double bond ratio (ranging from 1.2/1 to 5.0/1), the solvent/double bond ratio (ranging from 0/1 to 500/1), and the number of double bonds per chain. It was especially shown that the higher the content of polyunsaturated fatty chains, the lower the rate of 2-mercaptoethanol grafting. Best yields were reached in 1 h reaction for a thiol/ene ratio of 3/1, without solvent, using maximum UV intensity. Side reactions, identified by NMR, FT-IR, LC-MS, and iodine titration, were upon esterification between the carboxylic acid function of oleic acid and the hydroxyl function of 2-mercaptoethanol, disulfide formation, double-bond isomerization, and inter- and intramolecular bond formation. Optimized conditions were then applied to vegetable oil functionalization. 2-Mercaptoethanol was photochemically grafted onto vinyl groups of rapeseed oil by thiol-ene addition to yield biobased polyols. Those polyols showed an average functionality of 3.6. The presence of byproducts was also confirmed; they were however found to exhibit hydroxyl functions and thus could be included in a polymer network. Finally, the functionalized rapeseed oil was used to synthesize polyurethanes with 1,6-hexamethylene diisocyanate and methylene dipheny-1-4,4'-diisocyanate. The thermal properties of elastomeric products were found to be similar to those from a commercial polyol (Desmophen 1150).