One of the unique trademarks of ILs, which results from the possibility to combine a huge number of potential anions and cations, is their broad diversity in terms of structural features as well as physical and chemical properties. Based on the number of IL cations and anions, which have been reported in literature for the dissolution of the polysaccharide, about 300 ILs can be generated that are likely to act as solvents for cellulose [8]. Nevertheless, the vast majority of studies related to the use of ILs for processing of cellulose or cellulosic biomass focused on only three of those, namely BMIMCl, AMIMCl, or EMIMAc (see Sect. 5.3.1). Although most of the derivatization reactions described above have been carried out more or less successfully in these ILs, they possess certain ‘intrinsic restrictions’. With respect to the increasing interest to use novel solvents for cellulose processing in commer­cial scales, it can be concluded that the development of task-specific IL based reaction media for the homogeneous cellulose chemistry bears huge potential and are going to be a focus of future research in this area.

‘Task-specific’ is not an absolute term but related to a particular ‘task’, i. e., homogeneous synthesis of different cellulose derivatives, and a “specific” feature that is enabled or prevented by a particular IL. In general, task-specific reaction media for the chemical modification of cellulose should be characterized by:

1. decreased viscosity

2. tunable hydrophobicity/hydrophobicity

3. efficient solvent recycling

4. the presence of specific chemical functionalities that prevent or catalyze specific derivatization reactions and

5. improved biocompatibility and/or biodegradability. The last two issues have scarcely been considered up to now.

First of all, even those ILs, used thus far for processing of cellulose, can become task-specific reaction media if they are applied consciously according to their specific advantages and disadvantages. This approach is limited to a certain extend and requires fundamental knowledge on the specific properties of ILs, e. g., viscos­ity and specific side reactions of anion and cation, with respect to the restrictions imposed by the particular derivatization reaction in which they are used as cellulose solvent and the subsequent recycling process. As an example, EMIMAc can be the reaction medium of choice for derivatization of cellulose at low temperatures. It is liquid at room temperature and shows lower viscosity then imidazolium chloride based ILs, which is beneficial in order to guarantee efficient mixing and mass distribution. However, it also needs to be considered that the acetate anion might act as catalyst or react with the derivatization reagents yielding unexpected reaction products (see Sect. 5.3.2) [37, 67]. Novel cellulose solvents can be obtained by systematic synthesis of ILs and low-melting organic salts with tailored properties based on the knowledge gained with common ILs, on one hand (Sect. 5.3.4.1). On the other, molecular co-solvents can be used to alter the properties cellulose/IL solutions towards specific applications (Sect. 5.3.4.2).