In principle, cellulose, dissolved in an IL, can easily be converted also with other types of carboxylic acids, or their corresponding acid derivatives. Cellulose succi­nates and phthalates, i. e., dicarboxylic acid derivatives could be obtained by homogeneous derivatization in AMIMICl or BMIMCl with the aid of different acylation catalysts, e. g., DMAP, N-bromosuccinimide, or iodine [27—31]. Thereby, DMSO has partly been utilized as molecular co-solvent in order to guarantee homogeneous reaction conditions. The homogeneous preparation of cellulose glutarate by acylation in BMIMCl under ultrasound irradiation also has been reported as well [32]. ILs, partly in combination with DMF as co-solvent, have been utilized as reaction media for the preparation of 2-bromo and 2-chloro carboxylic acid ester of cellulose, which could be used as macro initiators for the grafting of poly(styrene) and different types of poly(methacrylate) chains onto the polysac­charide backbone [23—26].

Using AMIMCl as homogeneous reaction medium, a series of cellulose benzo­ates with high DS values in the range of 1.0-3.0 has been prepared that carried different moieties at the aromatic ring [20]. Also mixed cellulose derivatives, carrying benzoate groups (preferentially at the primary hydroxyl group) and

4- nitrobenzoate moieties (preferentially at the secondary hydroxyl groups) have been prepared in ILs by step-wise conversion with the corresponding acid chlo­rides. Highly substitute cellulose benzoates, as well as cellulose phenyl carbamates (see Sect. 5.2.3), exhibit high chiral resolving properties and could be utilized as stationary phase in liquid chromatography for separation of enantiomers [59]. Synthesis of these materials is usually performed under heterogeneous starting, e. g., by conversion of cellulose suspended in pyridine or other swelling agents [60]. However, homogeneous derivatization in ILs can facilitate preparation of tailor-made chromatography materials.

In general, cellulose esters are well soluble in ILs even at high DS values meaning that completely homogeneous esterification is feasible. However, in case of fatty acid ester with long, non-polar alkyl chains, the cellulose derivatives become increasingly hydrophobic upon advancing substitution, which renders the products insoluble in the reaction mixture. Cellulose laurates can be obtained in ILs but the derivatives rapidly precipitated upon derivatization of the polysaccharide [14]. Phase separation can be expected also for the higher homologue cellulose stearate although it has not been mentioned explicitly [19].

Compounds containing carboxyl groups are readily available but seldom used directly for esterification with polysaccharides due to their low reactivity. In case conversion into a more reactive acid chloride or anhydride is not feasible, activation of carboxyl groups with N, N’-carbonyldiimidazole (CDI) is a very versatile approach for obtaining cellulose esters under mild reaction conditions [61, 62]. Some cellulose derivatives, namely fuorates and water soluble oxy-carboxylic acid ester of cellulose with a broad range of DS from 0.1 to 3.0, have already been obtained in ILs by this procedure [21, 22]. As a first step of the homogeneous derivatization reaction, an imidazolide is formed in situ in the IL reaction medium, which reacts as active species with the polysaccharide backbone upon liberation of CO2 and imidazole as side products (Fig. 5.2). A carbodiimide has also been tested as coupling agent for homogeneous esterification of cellulose with stearic acid in an IL but only small DS values up to 0.16 could be achieved [36]. Another possibility for the activation of carboxyl groups for subsequent esterification with cellulose is the conversion with sulfonic acid chlorides (Fig. 5.2) [63, 64]. Moreover, reactive intermediates are generated by treating carboxylic acids with iminium chlorides, e. g., prepared in situ from DMF and oxalyl chloride (Fig. 5.2) [65]. The by-products liberated upon activation and subsequent esterification are either gaseous (CO, CO2, HCl) or may act as co-solvent (DMF). The last two approaches have been exploited for the

preparation of various polysaccharide esters but up to now not been adapted for the homogeneous derivatization of cellulose in ILs.