## 1 Introduction  
In the past few decades, ionic liquids (IL) have garnered considerable attention in a variety of fields because of their excellent properties, such as vanishingly low vapor pressure, non-flammable, excellent chemical and thermal stability in the liquid state, polarity tunability, high ionic conductivity, wide electrochemical windows, _etc._[1; 2; 3; 4; 5; 6]. Meanwhile, poly(ionic liquid)s (PILs), derived from the polymerization of IL-based monomers and regarded as a distinct subclass of polyelectrolytes, can combine some unique characters of ILs with the practical properties of polymers, including processability, durability, mechanical stability. Therefore, PILs have also been collecting more and more interests in the fields of solid ion conductors, CO\({}_{2}\) sorbents, dispersants, porous materials, carbon precursors, potential desalination applications, _etc._[7; 8; 9; 10; 11].  
Recently, IL-derived stimuli-responsive systems, especially thermo-responsive behavior in different solvents, have been extensively investigated and considered as one of the promising \"intelligent\" materials [12; 13; 14; 15; 16]. In general, thermo-responsive phase behaviors of the polymer/solvent mixtures can be divided into two general classes: that with an upper critical solution temperature (UCST) and that with a lower critical solution temperature (LCST) depending on mixing behavior of the corresponding polymer solutions upon temperature change. To date, the fabrication of IL-based thermo-responsive phase behaviors can be achieved through IL/molecule liquid mixture, polymer/IL mixture, and homo-/copolymerization of IL. Several latest reviews have summarized the recent development on the design of ILs/PILs with thermo-responsive phase behaviors in water and organic solvents [17; 18; 19]. As cited in these reviews, some interesting thermo-responsive IL/PIL systems were masterly explored. Both [PERSON] and [PERSON] groups have contributed great in this field [20; 21; 22; 23]. In brief, the origin of the thermo-responsive phase behavior of ILs/PILs was inherently derived from the non-covalent interactions between IL and solvents. Given the structural diversity of the component ions, there are still many promising PIL materials that have great potential to present the thermo-responsive phase behavior. Moreover, these thermo-responsive PIL materials will provide the opportunity for discovering unprecedented phenomena and applications not previously realized for the classical neutral thermo-responsive polymers [24; 25].  
More recently, our group has demonstrated a facile synthesis strategy to prepare PIL-based nanogels via the conventional radical copolymerization of IL-based monomers and the cross-linkers ethylene glycol dimethacrylate (EGDMA) and divinylbenzene (DVB) in selective solvents [26; 27]. Interestingly, thermo-responsive nanogels could be obtained for the first time via the copolymerization of the geminal dicationic, 1,4-butanediyl-3,3'-bis-1-vinyl imidazolium halides, and the cross-linkers described previously under the same conditions [28]. These novel PIL-based thermo-responsive nanogels can reversibly form precipitates or macrogels in methanol with the temperature changing in the range of \(-15\) to \(25\)\({}^{\circ}\)C. Experimental studies revealed that the thermo-response of PIL-based nanogels was attributed to hydrogen bonding interactions between nanogels and methanol. Unfortunately, this strategy is infeasible when it comes to ILs which cannot form hydrogen bonding [26; 27]. Thus, it is highly desired to get insight into the rules of the thermo-responsive behavior, and develop a simple approach for the preparation of thermo-responsive nanogels based PIL. To achieve this goal, we will elucidate the relationship between the thermo-response of nanogels and the structure of ionic liquid-based monomers in this paper.