Natural deep eutectics: expanding green solvents for thermally-/photo-induced polymerization of N-isopropylacrylamide toward key components for sustainable production of semi-natural polymers

Abstract

Natural deep eutectic solvents (NADESs), which consist of natural components, including organic acids, amino acids, and sugars, have attracted attention as environmentally friendly solvents. Herein, we describe the design and synthesis of natural deep eutectic monomers (NADEMs), which were prepared by reacting N-isopropylacrylamide (NIPAM) with naturally occurring solid components (thymol, menthol, 1-tetradecanol, and coumarin) and the corresponding NADEM-based polymers. Initially, the effects of NADESs comprising natural compounds (thymol/(±)-menthol: Thy/Men (1/1 and 1/2), 1-tetradecanol/(±)-menthol: Tdc/Men (1/2), and thymol/coumarin: Thy/Cou (1/1 and 2/1)) were investigated on the polymerization of a solid amido-containing monomer, NIPAM. We examined the thermally-/photo-induced free radical and reversible addition–fragmentation chain-transfer (RAFT) polymerization of NIPAM using NADES as a polymerization solvent; the characteristics and usefulness of five NADESs were evaluated. High monomer conversion was accomplished within 1 min by photo-induced free radical polymerization of NIPAM in NADES using LED-UV light. The photo-induced RAFT polymerization could be well controlled. An efficient polymer purification process that excluded volatile organic solvents was also realized. Four NIPAM-based NADEMs were developed by mixing NIPAM and natural compounds. Among them, NIPAM/Men and NIPAM/Tdc NADEMs exhibited quantitative monomer conversion within 10 s during photopolymerization, affording high-molecular-weight products with good mechanical properties. Controlled photo-induced RAFT polymerization of NIPAM/Tdc (1/1) and NIPAM/Men (1/1) could be accomplished. Successful integration of natural deep eutectics (NADESs and NADEMs) and green photo-induced polymerization allows for the expansion of polymeric deep eutectics with unique intrinsic properties and potential applications.