It should be pointed out that it is unlikely to find one particular IL that is the most suited cellulose solvent for all kinds of chemical derivatization reactions. Instead, the reaction media should be adapted to the specific requirements of a particular reaction. The synthesis of novel ILs for the use in cellulose chemistry involves the design of new cations and anions as well as the combination of both to an IL. Thereby, two major limitations need to be considered. It is self-evident that the new ILs (or low melting salts) must be able to dissolve reasonable amounts of cellulose (at least 5-10 %) without serve degradation of the polysaccharide. More­over, dissolution should be possible at moderate temperatures (<130 °C), which implies that the polysaccharide solvents needs to be liquid in that area. Strictly speaking, the term IL describes compounds with a melting point below 100 °C. However, it has been demonstrated that also several organic salts with slightly higher melting points in the range of 100-140 °C, henceforward designated as low-melting salts, can either directly or in combination with certain co-solvents be useful solvents for derivatization of cellulose.

The dissolution mechanism of cellulose in ILs is still not understood yet and is matter of controversial discussions; as is the question why cellulose is actually insoluble in most molecular solvents (see [8, 110] and references therein). How­ever, it has been demonstrated that in particular the anion has a very important role. It was found by means of solvatochromic experiments that cellulose is only soluble in ILs with a strong hydrogen acceptor capacity (в > 0.8) [111, 112]. This param­eter is mainly determined by the nature of the anion. Thus variation and structure design of the IL’s anion is restricted to a certain extent if the ability to dissolve cellulose needs to be retained. Most cellulose dissolving ILs applied so far for chemical derivatization of the polysaccharide contained either chloride or acetate as relatively strong hydrogen bond acceptor ions. Although chloride base ILs proofed to be efficient solvents for cellulose, their use is limited by the high melting points and viscosities. ILs bearing acetate anions are usually less viscous and liquid at room temperature but show some specific side reactions (see Sect. 5.3.2). Following the initial reports on this class of cellulose solvents, several task-specific ILs with alternative anions and beneficial properties have been reported that could be used for dissolution of cellulose. However, up to now only the minority has been studied as homogeneous reaction media.

In addition to acetate, other carboxylates have been proposed as anionic species in cellulose dissolving ILs (Fig. 5.8). In particular imidazolium formates have been reported to possess lower viscosities compared to the corresponding chloride and acetate analogues [113]. With respect to the specific side reactions of the acetate anion, these low viscous ILs might be useful reaction media, e. g., for derivatization reactions that do not tolerate high temperatures. Several low melting ammonium formates have been prepared that could dissolve some polysaccharides including cellulose [71]. These low melting salts could successfully be applied also for the preparation of carboxymethyl cellulose with a rather high DS of about 1.6 and a non-statistic functionalization pattern.

Imidazolium- and ammonium salts with alkylphosphonate or dialkylphopshate as anions possess high hydrogen bond acceptor capacities and could be utilized as cellulose solvents [114, 115]. These ILs are liquid at room temperatures and their viscosities are in the range of 100-500 mPa s, which is comparable to the corresponding acetates. It has been proposed that substitution of oxygen by sulfur will result in a decreased viscosity due to the reduced symmetry of the anion [116]. First results on the use of phosphate ILs as homogeneous reaction media indicate that choice of the cation is crucial for the derivatization reaction. Whereas tetraalkylammonium dialkylphosphates could be used as solvents for the homoge­neous preparation of cellulose esters and mixed ester, the corresponding imidazolium salts appear to be less suitable for the derivatization of cellulose because gelation of the reaction mixture occurs shortly after the addition of derivatization reagents [35, 54].

Tailoring the nature of the IL cation, e. g., by choosing among different general types of cations or by modifying length, degree of branching, and flexibility of side chains, offers access to a very broad structural diversity (Fig. 5.8). Nevertheless, up to now cellulose research has been focused mainly on 1-alkyl-3-methylimi — dazolium based ILs, bearing mostly ethyl, butyl, or allyl in the side chain. The cation has a significant effect on the ILs physical properties, e. g., melting point and viscosity, as well as its chemical reactivity. If and to what extent it is also directly involved in the cellulose dissolution mechanism is matter of current scientific discussion [8]. However, it appears that the choice of potential cations is less restricted compared to the limitation in terms of the possible anions.

Regarding commonly applied 1,3-dialkylimidazolium IL, task-specific solvents with interesting properties can be generated be substituting also the other positions of the aromatic ring [117]. The possibility to suppress deprotonation and carbene formation by methylation at position 2 has already been described (see Sect. 5.3.2). Another possibility is to increase flexibility of the side chains by introducing oxy-alkyl groups that also facilitate additional hydrogen bond interactions [56]. Replacing the butyl group in BMIMCl by a 2-methoxyethyl group seems to improve the solubility of cellulose and also resulted in an increased reactivity for acetylation reactions performed in this IL. Further studies are required to fully understand the reasons for these improvements, in particular because the opposite effect was observed when changing from a heptyl — to a 2-(2-ethoxyethoxy)ethyl substituted IL.

Taking into account the specific side reactions of dialkylimidazolium ILs, quaternary ammonium salts can be expected to become more important as solvents for dissolution and chemical modification of cellulose. A certain degree of cation asymmetry is required to obtain ILs with low melting points [118]. Despite that limitation, quaternary ammonium salts exhibit a much broader structural diversity than imidazolium ILs because in principle four side chains can be tailored.

Moreover, they can easily be prepared by complete alkylation of primary, second­ary, and tertiary amines, precursors that are inexpensive and readily available.

Several pyrolidinium, piperidinium, and morpholinium ILs with different anions have been reported for the use as cellulose solvents [115, 119]. However, compre­hensive comparison of their dissolution power as well as chemical and physical properties in comparison to ILs derived from aromatic or acyclic amines are scarcely found in literature. Especially morpholinium salts are of considerable scientific interest due to their relationship with NMMO. Low melting triethyl — and tributylmethylammonium formates could be obtained from the commercially available methyl carbonates by conversion with formic acid [71]. In the presence of a small excess of acid, these salts could be used as solvents for the homogeneous carboxymethylation of cellulose. Cellulose dissolving ammonium ILs with rather low viscosities (30-220 mPa s at 25 °C) have been obtained by introducing a cyclohexyl side chain to decrease symmetry of the cation [120]. The ILs carrying either acetate or alkoxyacetates could dissolve 1-9 % cellulose, which might be sufficient for specific applications. The solubility could be increased by adding DMSO.

Ether functionalized IL cations are expected to be better biodegradable, less toxic, and less viscous than their aliphatic analogues [121]. Several quaternary di — and trimethylammonium acetates bearing one or two alkoxy side chains have been prepared that showed melting points of 30-40 °C. Compared to imidazolium acetate, the viscosities of these ILs were slightly higher but they could dissolve similar amounts of cellulose and might consequently be interesting alternative reaction media for the derivatization of the polysaccharide [122]. Mono — and bicationic ammonium ILs with longer alkoxy groups and acetate as counter ion could be derived from poly(ethylene glycols) (PEG) and PEG monomethylethers [123]. According to the same approach, imidazolium and piperidinium acetates could be prepared as well. Depending on the number of C2H4O-units, some of these PEG functionalized ILs could dissolve approximately 8-12 % cellulose.