A kinetic and mechanistic study of copper-based catalysts in the ARGET-ATRP of multifunctional natural molecules: The case of methacrylated eugenol

Herein we report on a kinetic study of Cu-based catalysts employed in the ARGET-ATRP of the methacrylic derivative of eugenol, namely eugenyl methacrylate (EuMA). Polymerizations were carried out in solution in presence of catalytic systems formed in situ by CuBr2 and nitrogen-ligands such as BiPy, PMDETA, HMTETA and Me6TREN. The formation of insoluble polymers, due to secondary reactions responsible of cross-linking, was observed with CuBr2/BiPy and CuBr2/HMTETA systems; Me6TREN- and PMDETA-based catalyst proved, instead, to be capable of generating linear polymers. For the latter, first order kinetics occurred for monomer conversion up to ca 50 %, whilst higher conversion led to deviations from the linear trend. This suggested the direct involvement of the allyl group in the termination reactions, which was convincedly demonstrated by comparing kinetic results for EuMA and the corresponding di-hydrogenated monomer (DEuMA). At EuMA conversion above 50 %, the side reactions lead to inactivation of the PMDETA-based catalytic system via reducing agent consumption rather than to the formation of insoluble/crosslinked polymers. Electronic structure calculations provided the energy profile for all possible side reactions. Among these, the radical chain transfer to the allyl group through hydrogen abstraction, as well as the attack of the propagating methacrylic radical to the allyl group, contributed to rationalizing the experimental behavior of the three copper-catalyst systems employed in this work. This study demonstrates that modulating the kinetic of polymerization by properly selecting ligands and reaction temperatures represents a useful strategy towards the reduction of undesired secondary reactions of molecules with sensitive functional groups such as bio-derived phenols; moreover, such preserved functional groups would serve as possible post-functionalization sites (i.e. epoxidation) allowing for the preparation of new materials with tailored properties.