Unexpected increase in water solubility by the introduction of hydrophobic units into imidazolium-based polymeric ionic liquids with carboxylate counteranions

Water solubility of polymers is affected by hydrophobic or hydrophilic moieties introduced into polymer chains. In this study, the introduction of hydrophobic units into polymeric ionic liquids (ILs) unexpectedly resulted in an increase in the water solubility of polymers. A vinyl ether (VE) homopolymer bearing imidazolium-type IL moieties with nonanoate counteranions and poly(isobutyl VE) [poly(IBVE)] are both insoluble in water, while statistical copolymers of these VEs dissolved in water. Moreover, statistical copolymers with appropriate IBVE contents exhibited lower critical solution temperature (LCST)-type thermoresponsive behavior. The water solubility of an upper critical solution temperature (UCST)-type thermoresponsive polymeric IL with 2-naphthoate counteranions was also increased by the introduction of hydrophobic units.