ILs are considered as non-derivatizing cellulose solvents, i. e., the dissolution of cellulose is not due to chemical conversion of the polysaccharide [13]. Nevertheless, ILs are not necessarily chemically inert. Both cation and anion can participate in the course of chemical derivatization reactions of cellulose or react with the dissolved polysaccharide. In addition, the effect of typical IL impurities needs to be considered.

The proton at position C-2 of 1,3-dialkylimidazolium based ILs is rather acidic; the pKa-values are estimated to be about 21-24, and can be abstracted with bases to yield N-heterocyclic singlet carbenes [83, 84]. These reactive species act as nucle­ophilic intermediates in the catalytic cycles of many organic reaction, which is the reason for surprisingly high yields and/or unexpected products frequently observed for reactions performed in ILs [85, 86]. The presence of carbenes and their influence on the derivatization of cellulose in ILs needs to be considered, in particular when bases are utilized. In addition, it has been reported for low-molecular weight cellulose mimics and later on for cellulose as well that the carbene species attach to the reducing end-group in its open-chain aldehyde form (Fig. 5.6) [87, 88]. Although the conversion is accelerated in the presence of bases, it also occurs upon dissolution of cellulose in pure imidazolium acetates. To a

R: H orCH

Fig. 5.6 Reaction scheme for side reactions observed during dissolution and chemical derivati­zation of cellulose in typical imidazolium based ionic liquids (A+X_) [67, 87, 88, 91] certain extent, these particular ILs undergo self-deprotonation as result of the relatively high basicity of their anion [89]. The equilibrium is further shifted by subsequent reaction of the carbenes formed with the cellulose end-group. To avoid the effect of reactive carbene intermediates, imidazolium based ILs that were
methylated at C-2 have been proposed as cellulose solvents [14]. However, the methyl group can also be deprotonated to a certain extend [90].

In addition to the IL’s cation, the anion can undergo specific side reactions. The conversion of cellulose, dissolved in an imidazolium chloride IL, with furoyl, tosyl, and trityl chloride as well as with SO3 complexes yields the expected cellulose derivatives 8, 16, 19, and 15 (see Table 5.1) [33, 35]. In contrast, only acetylated products could be obtained when the same derivatization reactions were carried out in an EMIMAc [33, 67]. This unexpected finding has been attributed to the formation of mixed anhydrides of the anion, which is present in high concentrations and not surrounded by a solvent cage, with the reagents applied (Fig. 5.6). These intermediates subsequently react with cellulose and transfer the acetyl group. If well understood, the chemical reactivity of ILs is not necessarily a drawback of this class of cellulose solvents. As an example, the acetate anion acts as a catalyst for the ring opening of oxiranes, which could be exploited for the efficient hydroxyalkylation of cellulose in ILs [37]. Moreover, ILs can become valuable for performing derivatization reactions in which other homogeneous cellulose reaction media, such as DMA/LiCl and DMSO/TBAF, show specific side reactions and cannot be employed.

ILs often contain certain impurities, derived from the synthesis, such as unreacted educts, side products, inorganic salts, and organic acids [92]. These compounds can affect the dissolution and chemical derivatization of cellulose in ILs. N-Methylimidazole is a starting material for the synthesis of imidazolium salts, the most frequently applied type of ILs in cellulose research, and one of their major impurities. This heterocyclic base acts as catalyst, e. g., for the silyation of cellulose. Thus, highly silylated products could be obtained using common reagent grade ILs (90-95 % purity) that contain traces of N-methylimidazole (0.1-0.5 wt%), whereas no significant derivatization could be achieved when ILs of high purity (>99 %) were applied as reaction media [41].

Even hydrophobic ILs can adsorb rather high amounts of moisture from humid air atmosphere [93]. Thus, the ubiquitous presence of water should not be neglected when using ILs for processing of cellulose. However, handling under protecting gas and strictly anhydrous conditions is not necessary for most applications. Water can directly influence chemical derivatization reactions. Most reagents applied for chemical modification of cellulose are prone to hydrolysis, which leads to an apparent decrease in reaction efficiency. Water also promotes the chain degradation of cellulose especially at high temperatures or if the ILs applied contain acidic impurities [94, 95].

In addition to the influence of water on chemical reactions, water affects the solubility and state of dissolution of cellulose in ILs. In large excess, the protic non-solvent acts as ‘precipitation agent’, i. e., cellulose is regenerated from ILs upon pouring the solution into five to ten times the volume of water. It has been reported that cellulose solutions in some ILs tolerate rather high amounts of water of up to 20 wt% [96]. However, even traces of water can alter the state of dissolution of cellulose in ILs before any ‘macroscopic changes’ can be detected. It has been demonstrated that the intrinsic viscosity of cellulose/EMIMAc solutions, which is directly correlated with the size and conformation of the dissolved polysaccharide chains, first increases with increasing water content up to a maximum of 10 wt% and then decreases again until finally reaching the solubility limit [97]. Based on this finding it was concluded that a ‘micro-gel’, i. e., agglomerates of polymer coils, is formed upon the addition of water to cellulose/IL solutions. This phenomenon has significant influence on the rheological flow behavior of these solutions and might also affect the chemical derivatization of cellulose as well as the processing into cellulosic fibers by spinning processes.

Pure ILs are commonly regarded as highly thermostable with a broad liquid range; some individual representatives of this class withstand temperatures up to 400 °C [10]. Under practical lab-conditions, however, decomposition may already occur at much lower temperatures, in particular in the presence of impurities [98]. For common cellulose/IL solutions, onset temperatures (ron) for the chemical decompo­sition and liberation of gaseous compounds around 180-220 °C have been observed by means of differential scanning — and reaction calorimetry [99, 100]. The values changed only slightly upon the addition of additives such as silver or charcoal. In contrast, cellulose solutions in NMMO, employed for the production of cellulosic fibers on a technical scale, are significantly less stable (Ton « 130-160 °C) [101, 102]. Moreover, stabilizers are required in order to prevent autocatalytic thermal runaway reactions. Usually, dissolution, shaping, and chemical derivatization of cellulose in ILs is performed below 130 °C, i. e., cellulose/IL solutions are safe to handle at typical processing temperatures. However, it has been noted that the thermostability is reduced significantly when using recycled ILs. This is an indication that already below Ton, degradation products are formed. The thermal decomposition of imidazolium-based ILs, which proceeds by a dealkylation mechanisms inverse to the synthesis, yields 1-alkylimidazoles (Fig. 5.6) [103, 104]. These primary products can further decompose, e. g., into imidazole, and/or condensate with other fragments formed [91]. These compounds are highly basic and can significantly affect chemical derivatization of cellulose in ILs. Although they might initially be formed only in small amounts, these heterocyclic degradation products cannot be removed simply by evaporation. Thus, they might accumulate during multiple recycling sequences.