**1.1 Ionic liquids (ILs) and their properties**

The melting points of ILs, also known as low-melting-point organic salts, are usually below 100°C; ILs are composed of organic cations and inorganic/organic anions, as shown in **Figure 1** [1, 2]. The number of possible cation-anion combination has been estimated to be >106 [3]. ILs are most commonly abbreviated by writing the abbreviation/formula of the cation and anion in square brackets (without charges); e.g., [bmim][PF6], [bmim][Tf2N] and [emim][Cl] are the abbreviations for 1-butyl-3-methyl-imidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide and 1-ethyl-3-methylimidazolium chloride, respectively. Owing to their high chemical and thermal stability, low volatility, and low toxicity, ILs have attracted much attention for applications in chemistry and industry. In addition, the properties of ILs include high conductivity, wide electrochemical window, low flammability, ability to dissolve organic and inorganic solutes and gases, and recyclability. As far as vapour pressures are concerned, several

**Figure 1.**

*Typical cationic and anionic components of ILs.*

ILs can be vaporised under a high vacuum at 200–300°C and then recondensed [4]; however, ILs indeed have negligible vapour pressures at near ambient conditions. Thus, for general reactivities or processes, they may be considered as low-volatile reaction media. ILs are generally chemically stable reaction media, but this cannot be taken as granted. The proton at the C(2)-position of imidazolium cation is acidic, and under basic conditions, deprotonation leading to carbene is possible [5]. ILs are considered as rational designable solvents that can be easily tuned by using various combinations of cations and anions to achieve ILs exhibiting appropriate properties and achieving practical applications for a desired task (so-called task-specific ILs), making it possible to introduce ILs into specific synthesis processes [6].

Because of these special properties, ILs have emerged as novel and exciting reaction media in their own right. Every year, an increasing number of papers are being published on the applications of ILs for enhancing reactivities or processes in both chemical research and industry. So far, free-radical polymerisation, polycondensation and ionic polymerisation have been successfully carried out using ILs as the reaction media. In the step polymerisation field, there is a huge interest for highperformance polymers (HPPs). Despite improved synthesis methods and commercial availability of various ILs for replacing typical organic reaction media, they are still more expensive than typical organic reaction media. Therefore, the application of ILs as reaction media for enhancing reactivities or processes is limited.

#### **1.2 High-performance polymers (HPPs) and their categories**

HPPs are also known as high-temperature polymers, special engineering plastics, advanced engineering materials, and heat-resistant polymers [7]. They are defined as polymers that can retain the desirable properties when exposed to very harsh conditions, including, but not limited to, a high-temperature, a high-pressure, and corrosive environment. They are well known for outstanding thermal stability and service temperatures, good mechanical properties, dimensional and environmental stability, high resistance to most chemicals, gas barrier and electrical properties, etc., under extreme conditions, even at elevated temperatures [8, 9]. To better understand the reason for their strength, one must consider the bond strength that can be quantified by bond dissociation energy. First, the higher the bond dissociation energy, the harder it is to break the polymer chain, and thus the better the resistance of the polymer to harsh environment. The bond energies of C–C and C〓C bonds are 83 and 145 kcal mol<sup>−</sup><sup>1</sup> , respectively; thus, it is harder to break a C〓C bond than a C–C bond. Most HPPs contain more C〓C bonds than C–C bonds. Similarly, the bond energies of C–H and C–F bonds are 99 and 123 kcal mol<sup>−</sup><sup>1</sup> ,

**101**

*Progress in Ionic Liquids as Reaction Media, Monomers and Additives in High-Performance…*

respectively; some of the C–H groups are also replaced with C–F groups. The resonance stabilisation is enhanced by adding aromatic components along the backbone, and it is estimated that the incorporation of resonance-stabilised units

improves their resistance and stability; thus, they can retain the desirable properties

HPPs include polysulphones (PSFs), polyimides (PIs), polyaryletherketones (PAEKs), poly(arylene sulphide)s (PASs), polyarylates (PARs), liquid crystalline polymers (LCPs), fluoroplastics (PVDFs), *p*-hydroxybenzoic acid polymers, poly(naphthalene), poly(oxadiazole), and high-temperature nylon (HTN). HPPs can be divided into amorphous polymers, semi-crystalline polymers, and LCPs; e.g., PSFs are described as amorphous polymers and polyetheretherketones are semi-crystalline polymers. The applications of HPPs span across aerospace, defence, weaponry, energy, electronics, automotive, construction, nuke industry, membrane technologies, etc. In recent years, new HPPs and materials containing HPPs with enhanced application potential in more fields have been reported, including materials obtained by the chemical modification and blending of HPPs containing ILs.

The aim of this article is to review the recent progress in the field of ILs as reaction media, monomers and additives in the synthesis, chemical modification and physical processing of HPPs based on recent literatures, with the main emphasis on possible advantages, limitations and importance of the work. The article is structured as follows: section 2 focuses on progress in IL application in HPPs. Section 2.1 focuses on ILs as reaction media in the synthesis of HPPs, including PIs, PSFs, and PAEKs, and synthesis of HPPs in ILs under microwave (MW) irradiation. Section 2.2 focuses on ILs as monomers for the chemical modification of HPPs. The last part of Section 2 focuses on ILs as additives for the physical processing of HPPs, including membranes, microcapsules, electrolytes, nanocomposites (NCs) and grease.

Most HPPs are synthesised by step polymerisation reactions. Step polymerisation is one of the main polymerisation reactions for preparing polymers, usually requiring elevated temperatures, high-boiling-point reaction solvents, high vacuum and the removal of small molecules to reach the equilibrium. Therefore, it seems to be suitable to introduce ILs into step polymerisation owing to their intrinsic properties as described above. In 2002, high-molecular-weight aromatic PIs and polyamides were synthesised for the first time, obtaining polymers with inherent viscosities from 0.52 to 1.35 dL/g in ILs 1,3-dialkylimidazolium bromides [10]. The use of ILs as novel solvents for the synthesis of other HPPs has been reported, such

In 1908, Jones et al. first synthesised PIs, but it was difficult to process and fabricate them [14]. Until the early 1960s, Du Pont, USA, made a substantial progress in the processing of PIs; thus, PIs were developed and widely utilised in various applications [15]. These polymers are known as HPPs and possess outstanding

to the bond strength. Such a molecular structure of HPPs

*DOI: http://dx.doi.org/10.5772/intechopen.86472*

**2. Progress in IL application in HPPs**

as PAEK [3, 11] and PSF [12, 13].

*2.1.1 Synthesis of PIs in ILs*

**2.1 ILs as reaction media for synthesis of HPPs**

can add 40–70 kcal mol<sup>−</sup><sup>1</sup>

**1.3 Overview**

under very harsh conditions.

*Progress in Ionic Liquids as Reaction Media, Monomers and Additives in High-Performance… DOI: http://dx.doi.org/10.5772/intechopen.86472*

respectively; some of the C–H groups are also replaced with C–F groups. The resonance stabilisation is enhanced by adding aromatic components along the backbone, and it is estimated that the incorporation of resonance-stabilised units can add 40–70 kcal mol<sup>−</sup><sup>1</sup> to the bond strength. Such a molecular structure of HPPs improves their resistance and stability; thus, they can retain the desirable properties under very harsh conditions.

HPPs include polysulphones (PSFs), polyimides (PIs), polyaryletherketones (PAEKs), poly(arylene sulphide)s (PASs), polyarylates (PARs), liquid crystalline polymers (LCPs), fluoroplastics (PVDFs), *p*-hydroxybenzoic acid polymers, poly(naphthalene), poly(oxadiazole), and high-temperature nylon (HTN). HPPs can be divided into amorphous polymers, semi-crystalline polymers, and LCPs; e.g., PSFs are described as amorphous polymers and polyetheretherketones are semi-crystalline polymers. The applications of HPPs span across aerospace, defence, weaponry, energy, electronics, automotive, construction, nuke industry, membrane technologies, etc. In recent years, new HPPs and materials containing HPPs with enhanced application potential in more fields have been reported, including materials obtained by the chemical modification and blending of HPPs containing ILs.

### **1.3 Overview**

*Solvents, Ionic Liquids and Solvent Effects*

*Typical cationic and anionic components of ILs.*

**Figure 1.**

ILs can be vaporised under a high vacuum at 200–300°C and then recondensed [4]; however, ILs indeed have negligible vapour pressures at near ambient conditions. Thus, for general reactivities or processes, they may be considered as low-volatile reaction media. ILs are generally chemically stable reaction media, but this cannot be taken as granted. The proton at the C(2)-position of imidazolium cation is acidic, and under basic conditions, deprotonation leading to carbene is possible [5]. ILs are considered as rational designable solvents that can be easily tuned by using various combinations of cations and anions to achieve ILs exhibiting appropriate properties and achieving practical applications for a desired task (so-called task-specific ILs),

Because of these special properties, ILs have emerged as novel and exciting reaction media in their own right. Every year, an increasing number of papers are being published on the applications of ILs for enhancing reactivities or processes in both chemical research and industry. So far, free-radical polymerisation, polycondensation and ionic polymerisation have been successfully carried out using ILs as the reaction media. In the step polymerisation field, there is a huge interest for highperformance polymers (HPPs). Despite improved synthesis methods and commercial availability of various ILs for replacing typical organic reaction media, they are still more expensive than typical organic reaction media. Therefore, the application

HPPs are also known as high-temperature polymers, special engineering plastics, advanced engineering materials, and heat-resistant polymers [7]. They are defined as polymers that can retain the desirable properties when exposed to very harsh conditions, including, but not limited to, a high-temperature, a high-pressure, and corrosive environment. They are well known for outstanding thermal stability and service temperatures, good mechanical properties, dimensional and environmental stability, high resistance to most chemicals, gas barrier and electrical properties, etc., under extreme conditions, even at elevated temperatures [8, 9]. To better understand the reason for their strength, one must consider the bond strength that can be quantified by bond dissociation energy. First, the higher the bond dissociation energy, the harder it is to break the polymer chain, and thus the better the resistance of the polymer to harsh environment. The bond energies of C–C and C〓C

bond than a C–C bond. Most HPPs contain more C〓C bonds than C–C bonds. Similarly, the bond energies of C–H and C–F bonds are 99 and 123 kcal mol<sup>−</sup><sup>1</sup>

, respectively; thus, it is harder to break a C〓C

,

making it possible to introduce ILs into specific synthesis processes [6].

of ILs as reaction media for enhancing reactivities or processes is limited.

**1.2 High-performance polymers (HPPs) and their categories**

**100**

bonds are 83 and 145 kcal mol<sup>−</sup><sup>1</sup>

The aim of this article is to review the recent progress in the field of ILs as reaction media, monomers and additives in the synthesis, chemical modification and physical processing of HPPs based on recent literatures, with the main emphasis on possible advantages, limitations and importance of the work. The article is structured as follows: section 2 focuses on progress in IL application in HPPs. Section 2.1 focuses on ILs as reaction media in the synthesis of HPPs, including PIs, PSFs, and PAEKs, and synthesis of HPPs in ILs under microwave (MW) irradiation. Section 2.2 focuses on ILs as monomers for the chemical modification of HPPs. The last part of Section 2 focuses on ILs as additives for the physical processing of HPPs, including membranes, microcapsules, electrolytes, nanocomposites (NCs) and grease.
