Chemical Reactivity and the Influence of Initiators on the Epoxidized Vegetable Oil/Dicarboxylic Acid System

In a sustainable development context, epoxidized vegetable oils (EVO) have unlimited and promising future prospects as renewable and environmentally friendly feedstock. The only drawback to their use is their low and non-selective reactivity compared to the aromatic epoxides. Properly, a small optimized amount of “true” initiators can overcome this issue and also beneficially serve in properties such as glass transition, modulus, strength, elongation at break, and chemical resistance. This paper presents efforts to understand and identify the initiator’s effect to more accurately predict how to select a good initiator on EVO/dicarboxylic acid systems. A new bio-based reprocessable epoxy resin was prepared from epoxidized linseed oil (ELO) and 2,2′-dithiodibenzoic acid (DTBA). The evolution of the chemical structures and the reactions’ mechanisms have been systematically studied by in situ Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies and differential scanning calorimetry (DSC). A screening of 10 initiators was performed for the ELO/DTBA cross-linking reaction. The influence of the initiator’s structure, basicity, and nucleophilicity was assessed and ranked in terms of the kinetic response including the epoxy–acid reaction rate and the percentage of functional group consumption. An excellent effect achieved by imidazole as an initiator was demonstrated. An attempt has been proposed to corroborate the experimental values with the results of quantum chemistry calculations.