This study explores the solvation behavior of organic polymers in ionic liquids, focusing on the thermal responsiveness and structural transitions of poly(2-isopropyl-2-oxazoline) (PiPrOx). We discovered that PiPrOx undergoes a lower critical solution temperature (LCST)-type phase separation at elevated temperatures in the ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim][BF₄]), followed by crystallization. This dual thermal response behavior, first demixing due to LCST-type phase separation, then undergoing an ordering process through crystallization, is an intriguing example of how polymer chain conformations and intermolecular interactions can give rise to complex phase behavior in non-aqueous media. This finding is particularly significant because it demonstrates that the self-organization of polymers, a phenomenon extensively studied in aqueous solutions, can also be achieved in ionic liquids, which offer a fundamentally different solvation environment. In contrast to water, ionic liquids possess negligible vapor pressure, high thermal stability, and tunable interactions through ion pair design. These properties make them promising solvents for studying thermoresponsive polymers beyond conventional systems. One of the key implications of this study is that the crystallization behavior of PiPrOx is tightly linked to its conformation in solution state. Our data suggest that the polymer chains adopt a trans-rich extended conformation in the dissolved state, which becomes stabilized upon heating. As the solution reaches its LCST, phase separation occurs due to entropically driven solvation breakdown. This is then followed by the formation of crystalline domains, reflecting a decrease in conformational entropy and an increase in local ordering. The featured image highlights this dual-phase transition behavior and was selected as the outside front cover for the issue of Chemical Communications in which the article appears. The illustration depicts PiPrOx chains aligned in a trans-rich extended conformation within the ionic liquid matrix. Upon heating, these chains undergo LCST-type demixing and subsequently crystallize, forming ordered structures within the phase-separated domains. This visual captures the core essence of our research: the emergence of molecular order in an ionic liquid environment driven by subtle thermodynamic balances. By demonstrating LCST-type phase separation followed by crystallization of a synthetic polymer in an ionic liquid, this work provides new insights into polymer solubility, conformational transitions, and the role of enthalpy-entropy interplay in non-aqueous systems. It may pave the way for designing responsive soft materials and functionalized polymers with controllable order–disorder transitions under non-volatile, non-flammable conditions.

Created at: 2025-06-14 03:07:11 +0900 Updated at: 2026-09-29 09:57:33 +0900