Mechanistic investigation of photoinduced ATRP in deep eutectic solvent for highly efficient synthesis of poly(methyl methacrylate)

Abstract

This work investigates the behavior of photoinduced atom transfer radical polymerization (ATRP) of methyl methacrylate in deep eutectic solvents (DES). Recently, several works have found that eosin Y (EY) is a highly efficient organic photocatalyst to reduce X-Cu^II/L complex under green light irradiation for photoinduced ATRP. Nevertheless, the effect of solvents on this dual photoredox/Cu ATRP has not been systematically explored before. This study compares the photoinduced ATRP mediated by EY in dimethyl sulfoxide (DMSO) and a DES composed of tetrabutylammonium chloride (TBACl) and ethylene glycol. The polymerization rate in DES is significantly faster than that in DMSO. Nearly full monomer conversion was achieved within 3 h in DES, but 24 h was required for the polymerization in DMSO. More intriguingly, the initiation mechanism in DES is similar to that of initiators for continuous activator regeneration (ICAR) ATRP. EY mainly acts as a type II photoinitiator in DES to gradually provide radicals that regenerate the Cu^I/L activator. On the other hand, the photoredox efficiency between excited EY and X-Cu^II/L is higher in DMSO than in DES. As a result, EY behaves as a photocatalyst in the conventional solvent, DMSO. Finally, we also demonstrate that the fast photoinduced ATRP in DES exhibits high oxygen tolerance and proceeds smoothly in an open-to-air environment. In short, our results reveal that DES enhances the polymerization rate and governs the initiation mechanism of photoinduced ATRP with EY.