The increasing environmental awareness has led to the search for alternative reaction media to reduce or eliminate the use of organic solvents in polymer science. Deep eutectic solvents (DESs), a subclass of ionic liquids, have emerged as sustainable solvents for a plethora of chemical reactions. In this Trend Article, DES utilization in free-radical polymerizations will be discussed. Two main fields of application are reported. In the first section, monomers able to undergo free-radical polymerization while taking part in a DES, as hydrogen bond donor or ammonium salt, so called DES monomers, are presented. In the second section, the role of DESs as solvents, where the polymerization takes place, either in the same phase or in an emulsion, is described. Finally, the properties of the polymers resulting from these particular methods of synthesis are discussed with emphasis on their green aspects.
Progress in Polymer Science, 2018
(I) Types of DESs as reported by Abbott et al. (A) Phase diagram of a binary mixture exhibiting a eutectic point. (B) Chemical structure of hydrogen bonding in choline chloride/urea DES (1:2 molar ratio, respectively). (C) Spatial density functions of choline chloride/urea DES showing probabilistic 3D structures; yellow surfaces depict choline cations, purple surfaces represent urea molecules and green show chloride anions, Copyright 2016, Adapted with permission from The Royal Society of Chemistry. (D) Example of the DSC of a polymerizable DES (choline chloride/methacrylic acid, 1:2 molar ratio respectively) exhibiting a eutectic point with the corresponding melting point. (E) Chemical structure of hydrogen bonding in choline chloride/malic acid (1:1 molar ratio), described as either DES [3,15] or LTTM [12,132]. (F) Spatial density functions of choline chloride/malic acid showing probabilistic 3D structures; green isosurfaces correspond with chloride anions, orange isosurfaces are malic acid molecules and yellow isosurfaces are choline cations, Copyright 2017; Adapted with permission from American Chemical Society. G) Example of the DSC of a polymerizable DES/LTTM (choline chloride/acrylic acid, 1:2 molar ratio respectively) exhibiting a glass transition. (II) Schematic representation of room temperature ionic liquids. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)