The separation and recycling of catalyst and cocatalyst from the products and solvents are of critical importance. In this work, a class of functionalized ionic liquids (ILs) were designed and synthesized, and by tuning the hydrophilicity and hydrophobicity of cation and anion, respectively, these ILs could reversibly transfer between water and organics triggered upon undergoing a temperature change. From a combination of multiple spectroscopic techniques, it was shown that the driving force behind the transfer was originated from a change in conformation of the PEG chain of the IL upon temperature variation. By utilizing the novel property of this class of ILs, a highly efficient and controllable CuI-catalyzed cycloaddition reaction was achieved wherein the IL was used to entrain, activate, and recycle the catalyst, as well as to control the reaction.