Thiol–ene coupling of 1,2-disubstituted alkene monomers: The kinetic effect of cis/trans-isomer structures

The free-radical induced reaction between a tri-functional thiol (2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto propionate) and two 1,2-disubstituted alkenes (methyl oleate and methyl elaidate) has been investigated under photochemical conditions. The photoreaction was monitored via time-resolved FTIR, Raman and NMR spectroscopy to provide insights about the kinetics and efficiency in end-product formation. The information collected was subjected to numerical modelling using the GEPASI software using pre-established literature values for the rate coefficients in order to verify the proposed reaction scheme. The results confirm the thiol–ene reaction mechanism showing a very fast cis/trans-isomerization (<1.0 min) when compared with the total disappearance of unsaturations, indicating that the rate-limiting step controlling the reaction is the hydrogen transfer from the thiol involved in the formation of product. High thiol–ene conversions can be obtained at reasonable rates without major influence of side-reactions when performed in bulk indicating that this reaction is suitable for network forming purposes with monounsaturated fatty acid methyl esters derivatives. The kinetic and mechanistic information collected provides a basis for the design of new thiol–ene systems aiming at material and coating applications.