Advanced applications of ionic liquids in polymer science
Introduction
Ionic liquids (ILs), which have been widely promoted as “green solvents”, are attracting much attention for applications in many fields of chemistry and industry due to their chemical stability, thermal stability, low vapor pressure and high ionic conductivity properties. Over the last few years, ILs have been popularly used as solvents for organic synthesis, catalysis, and also been used as media for extraction processes [1], [2], [3], [4], [5], [6].
In recent years, ILs have been used in polymer science, mainly as polymerization media in several types of polymerization processes, including conventional free radical polymerization [7], living/controlling radical polymerizations (such as atom transfer radical polymerizations (ATRP) [8], [9], [10], [11], [12], [13], reversible addition-fragmentation transfer (RAFT)) [14], as well as in ionic and coordination polymerizations [15], [16]. When radical polymerizations are conducted in an IL, a significant increase of kp/kt ratio is normally observed in comparison to those carried out in other polar/coordinating solvents. As solvents for ATRP and RAFT, ILs facilitate separation of the polymer from residual catalyst and reduce the extent of side-reactions. Applications of ILs as solvents for polymerization processes have been reviewed by Kubisa [17] and Shen and Deng [10].
However, application of ILs in polymer science is not limited to traditional polymerization media. ILs are also investigated as components of the polymeric matrixes (such as polymer gels), as templates for porous polymers and as novel electrolytes for electrochemical polymerizations. This review focuses on recent developments and achievements of applications of ILs in the preparation of functional polymers.
Section snippets
Polymer gels based on ionic liquids
Ionic liquids have excellent ionic conductivity up to their decomposition temperature. This advantage enables them to play an important role in electrolyte matrixes. However, from the viewpoint of eliminating leakage, solid or quasi-solid ion-conductive electrolytes are generally preferred over fluidic materials. Therefore, it is desirable to convert IL-based electrolyte solutions into a solid or quasi-solid form. In recent years, the gelation of ILs by physical or chemical gelation has
From ionic liquids to porous polymer materials
Removal of ionic liquids from IL/polymer composites is a simple and convenient way to prepare porous polymers. Polymers with permanent porosity have been prepared by Winterton et al. through in situ polymerization in ionic liquid and subsequent removal of the ionic liquid from IL/polymer composites [85]. The produced polymers show pore sizes in the range of 290–570 nm.
Polyurea with exotic porous structures has been synthesized by the interfacial polymerization between hexane and a series of
Supported ILs
There are two important new application areas of supported ILs. In the first case, we have ionic liquid salts covalently linked to a polymer or to inorganic surfaces or particles, thereby supporting the organic salt. In such systems the character of the IL organic salt is changed to some extent, but many times the main features are retained. The second application area for supported ILs is where a polymeric membrane, porous matrix, particle, or bulk material absorbs an IL by solubilization or
Electrochemical polymerization in ILs
Ionic liquids are also ideal supporting electrolytes for the electrochemical generation of conjugated polymers because of their excellent oxidative and reductive stability, which allows access to potentials that cannot be provided in the smaller electrochemical windows of molecular solvent/electrolyte systems.
Naudin et al. reported the electrochemical oxidation of 3-(4-fluorophenyl) thiophene (FPT) in pure 1-ethyl-2,3-dimethylimidazolium bis((trifluoromethyl)sulfonyl) amide and
Ionic liquids as solvents for polymerization processes
The use of ionic liquids as solvents in polymerization instead of classical organic solvents offers not only some general advantages such as low volatility and nonflammability, but also markedly affect polymerization rate and degrees. In 2004, Kubisa [17] published an excellent review on the application of ionic liquids as solvents for polymerization processes. Since there is still a continuous increasing interest of polymerization processes in ionic liquid solvents, we provide a brief overview
Composites
A novel one-pot synthesis of conducting polymer-noble metal nanoparticle composites was developed by Pringle et al. [116], [118], based on their use of noble metals in IL solution as oxidizing agents. In this approach, gold or silver salts, such as gold chloride and silver nitrate, are solubilized in ILs, [EtMeIm][tf2N], and the metal ions oxidatively polymerize monomers such as pyrrole and terthiophene, as described above. The composites obtained, as analyzed by TEM on samples supported on
Conclusions
The use of ionic liquids in polymer science has quickly advanced from use as solvents and has become focused on using ionic liquids as functional additives to polymer chains or to hybrid materials. This functionality may be due to the solubility of an IL monomer, the charge density of the resulting material, or due to the basic mobility of the IL, as in the case of hybrid materials.
Very interesting initial applications for the incorporation of IL monomers into polymers have been published, but
Acknowledgments
This work was supported in part by U.S. Army Tank-Automotive and Armaments Command Cooperative Agreement No. DAAD19-03-2-0013, by U.S. Army Research Laboratory Contract W911QX-06-C-0102, and by AFOSR Grant No. FA9550-08-1-0431. F. Yan and J. Lu acknowledge the financial support provided by Natural Science Foundation of China (Nos. 20774063, 20874071, 20571054), the Program for New Century Excellent Talents in University (Grant NCET-07-0593), and Fok Ying Tung Education Foundation (No. 114022).
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