Efficient Visible-Light-Driven RAFT Polymerization Mediated by Deep Eutectic Solvents under an Open-to-Air Environment

Abstract

In this work, we investigate the effects of deep eutectic solvents (DESs) on a facile reversible addition–fragmentation chain-transfer (RAFT) polymerization driven by visible light. A DES composed of tetrabutylammonium chloride (TBACl) and ethylene glycol served as a nonvolatile medium for a variety of monomers, including methyl methacrylate, methyl acrylate, dimethylacrylamide, and styrene. We first employed the polymerization-through pathway to overcome oxygen inhibition in an open-to-air environment using methyl methacrylate as the model compound. The photoiniferter polymerization using trithiocarbonates as the chain transfer agent (CTA) in DESs exhibited enhanced polymerization rates and achieved a narrow molecular weight distribution. Interestingly, even dithiobenzoate, the CTA with low efficiency for photoiniferter polymerization, exhibited a nearly 4.5-fold increase in the apparent polymerization rate constants when compared to the reaction in dimethyl sulfoxide. Moreover, the stability of the CTAs was increased in DESs for efficient control of molecular weight distribution and preservation of functional chain ends. We then surveyed photocatalysts for photoelectron/energy transfer (PET) RAFT polymerization and found that eosin Y was compatible with DESs. The PET-RAFT polymerization using dithiobenzoate in DESs showed even higher polymerization rates than the photoiniferter polymerization and enabled the polymerization under natural sunlight. Besides methyl methacrylate, other monomers also exhibited an increased polymerization rate in DESs, comparable to reactions in ionic liquids. These results suggest that DESs are effective and green media to facilitate photo-induced RAFT polymerization without additives or tedious degassing processes.