Polymeric ionic liquids are polymers that contain at least one ionic center on a polymer chain and the repeating units are structured like those of common ionic liquids. Polymeric ionic liquids are a kind of functional polymer materials that combine the properties of polymer and ionic liquid and the general structural formulas of polymeric ionic liquids are shown in Figure 1. Polymeric ionic liquids have the advantages of non-volatility, low melting point, wide liquid path, strong electrostatic field, wide electrochemical window, good electrical and thermal conductivity, high heat capacity, high stability, selective solubility and design ability. As a new kind of green solvent, polymeric ionic liquids are widely used in organic synthesis, electrical conduction, extraction and separation. In addition, monomers are low-molecular raw materials for the synthesis of polymeric ionic liquids, which are generally unsaturated, annular, or low-molecular compounds containing two or more functional groups. The designed structure of polymeric ionic liquids can be obtained by selecting appropriate monomers and corresponding polymerization methods. Moreover, commonly used monomers for the synthesis of polymeric ionic liquids include styrene, acrylate, acrylamide and others.
Figure 1 The general structural formula of polymeric ionic liquids.
According to the types of ions directly linked to the polymer chain, the polymeric ionic liquids can be divided into the following three categories.
- Polycationic type: In the structure formula of polycationic ionic liquids, cations are connected with molecular chains by covalent bonds, while anions are connected with cations by ionic bonds.
- Polyanion type: In the structure formula of polyanionic ionic liquid, anions are linked to molecular chains by covalent bonds, while cations are linked to anions by ionic bonds.
- Inner salt type: In the structure formula of the inner salt ionic liquid, both the cation and the anion are linked to the molecular chain by covalent bonds.
- Ion conductors: Taking the advantages of good thermal stability, high conductivity and wide electrochemical window, polymeric ionic liquids can be used as ion conductors. The polymeric ionic liquids can be used as electrolyte for lithium-ion batteries by further modification. In addition, in dye-sensitized solar cells field, polymeric ionic liquids can be applied to replace organic small molecules, which can improve the safety of dye-sensitized solar cells.
- Stabilizers: Nanomaterials have many unique properties, but high specific surface energy often leads to agglomeration of nanoparticles, thus losing practical value, so it is necessary to add stabilizers to prevent agglomeration of nanoparticles. Polymeric ionic liquids can be used as stabilizers of nanomaterials, including nanotubes, nano-polymers, metal nanoparticles and others.
- Others: Polymeric ionic liquids are also widely used in the field of adsorption separation and catalyst. For example, polymeric ionic liquids can replace organic solvents as reaction media and catalytic materials in catalytic reactions. In addition, polymer ionic liquid provides a new way for the recovery and separation of catalyst. Furthermore, in recent years, the preparation and application of polymeric ionic liquids have been extensively studied by taking advantage of the designability.