Ionic liquid (IL)-based polyelectrolytes (PILs), referred to as polymeric ILs, polymerised ILs, or poly(IL)s are a new subclass of polymer materials. They are distinct from conventional polyelectrolytes due to their unique physico-chemical properties originated from a dense packing of ILs in the macromolecular architecture. Mixtures of PILs and solvents, in particular, water have attracted a great deal of interest especially in terms of their compatibilities depending on temperature, namely, thermoresponsiveness of PIL/solvent mixtures. Apart from static compatibility, such as the compatibility of PILs with solvents, which do not change largely by a temperature change, there are mainly two types of dynamic phase changes, an upper critical solution temperature (UCST)-and a lower critical solution temperature (LCST)-type phase behaviour. Some PILs dissolved in solvents homogenise upon heating; this behaviour is classified as UCST behaviour. On the other hand, only in the last two years have PIL/water mixtures with LCST been discovered. This article summarises rapidly growing studies on the design of thermoresponsive PIL systems with water or organic solvents. The hydrophobicity/hydrophilicity balance of the starting IL monomers features the phase behaviour of the resulting polyelectrolytes, and some IL monomers that show thermoresponsive phase behaviour in solvents were found to maintain their thermoresponsiveness even after the polymerisation. Based on their unique combination of properties derived from an ionic and thermoresponsive nature, these thermoresponsive PILs will attract considerable interest, and their wide applications are expected in the fields of separation, sensing and desalination.

Phosphonium cations bearing different alkyl chains were coupled with several common anions so as to prepare ionic liquids (ILs) with diverse hydrophobicity. A temperature-driven phase behaviour of the mixture of various ILs and water has been examined. A few ILs were found to exhibit temperature-sensitive lower critical solution temperature (LCST)-type phase transition after mixing with water. The phase separation temperature (T c ) of the IL/water mixtures depended strongly on the hydrophobicity of the component ions as well as mixing ratio. The number of water molecules per ion pair in the IL phase (m water ) increased dramatically upon cooling. The temperature dependence of this parameter was found to be useful to predict the possibility of the ILs to show the LCST-type phase behaviour after mixing with water. Since the value of m water depended on the ion structure, especially on the hydrophobicity, the T c was accurately set out by suitably mixing two ILs with different hydrophobicity.

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Through the study to analyse the required factors for zwitterion (ZI)-type ionic liquids to show LCST-type phase transition with water, we found that ammonium based ZIs having adequate hydrophilicity showed both LCST- and UCST-type phase transitions in a very narrow range of temperature after mixing with water.

Both water soluble lysine- and aspartic acid-based amino acid ionic liquids (AAILs) were mixed to prepare polar AAILs showing phase separation with water.

The phase separation temperature of the tetrabutylphosphonium trifluoroacetate-water mixture, which undergoes a lower critical solution temperature-type phase transition, is highly sensitive to the hydrophilicity of added salts; this system can be used to compare the hydrophilicity of the added ions.

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A new ammonium-type zwitterion (ZI), N,N-dihexyl-N-monopentyl-3-sulfonyl-1-propaneammonium (N665 C3S) with adequate hydrophobicity showed reversible and highly temperature-sensitive lower critical solution temperature (LCST)-type phase transitions after being mixed with pure water. Generally for such compounds, those with longer alkyl chains were immiscible with water and those with shorter chains were miscible with water, regardless of temperature. A slightly more hydrophobic ZI than N665 C3S showed LCST-type phase behavior with water when it was mixed with equimolar amounts of a Brønsted acid such as trifluoromethanesulfonic acid (HTfO). The phase-transition temperature of the ZI/Brønsted acid mixed aqueous solution was controllable by water content.