5.2.1.1 Cellulose Esters of Short C2 to C6 Carboxylic Acids

Cellulose esters of short chain carboxylic acids, in particular acetates, acetate- propionates, and acetate-butyrates, are of huge commercial importance and found in many applications in the form of fibers, films, coatings, and additives [50, 51]. Nowadays, these derivatives are exclusively prepared by heterogeneous pro­cesses. However, homogeneous esterification in ILs is considered as a commer­cially attractive alternative. Various patents have been published, e. g., by Eastman Chemical Company, that describe the preparation of cellulose esters and mixed esters in imidazolium — and ammonium based ILs, including the synthesis of the solvents and their recycling subsequent to the esterification [52—55].

Conversions of cellulose with carboxylic acid chlorides and anhydrides proved to be very efficient when performed homogeneously in an IL. The reactions are usually performed at elevated temperature (>80 °C), thus the adverse effect of high viscosity of cellulose/IL solutions is less pronounced. The esterification of cellulose proceeds completely homogeneous even up to a complete derivatization of all hydroxyl groups. Using relatively small amounts of acetic anhydride or chloride (3-5 mol equivalents, Table 5.2), highly functionalized cellulose acetates with DS up to 3 can be obtained within short reaction times (0.5-8 h) [12—14]. Using BMIMCl as homogeneous reaction medium, acetylated derivatives could even be obtained from bacterial cellulose that is usually difficult to dissolve and to chem­ically modify in other cellulose solvents due to its high degrees of crystallinity and polymerization [15].

In analogue to the homogeneous acetylation in ILs, homologues cellulose esters of higher carboxylic acids, from propionates up to hexanoates, have been prepared in various ILs using the corresponding anhydrides [16—18, 56]. The DS values of cellulose esters were found to decrease successively when increasing the number of carbon atoms in the acyl moiety from 2 to 4 but to increase again with further prolongation of the alkyl chain up to 6 carbon atoms [17]. Cellulose pentanoates

Reaction conditions Product

aAMIMCl: 1-allyl-3-methylimidazolium chloride, BMIMCl: 1-butyl-3-methylimidazolium chloride bBC: bacterial cellulose (DP: 6,493), CH: cellulose from corn husk (DP: 530), DIP: dissolving pulp (DP: и 650), MC: microcrystalline cellulose (DP: 286) c+: soluble, —: insoluble, n. a.: no information available dAcetic acid derivative used

eMolar ration of acetylation reagent to anhydroglucose units fAdditionally, 2.5 mol equivalent pyridine

and hexanoates slightly exceeded the DS values of cellulose acetates, prepared under identical reaction conditions. In contrast, the corresponding propionates and butyrate had slightly lower DS values. Cooperative interaction of the long chain anhydrides with the partially substituted, i. e., lipophilic, cellulose chain have been postulated to explain this unexpected finding.

The efficiency of the esterification of cellulose in ILs can be increased by adding pyridine (stoichiometric amounts) or 4-dimethylaminoaminopyridine (DMAP; cat­alytic amounts) [14, 18]. Moreover, microwave assisted esterification can yield products with an increased DS in comparison to products prepared under conven­tional heating [17, 56]. Within a microwave field, ILs rapidly heat due to their ionic nature [57]. Thus, the irradiation must be controlled by monitoring power input, pulse length and — interval, and maximum temperature. Efficient mixing is also

crucial, especially in case of highly viscous cellulose/IL solutions, in order to avoid local ‘hot-spots’ of extremely high temperature. Otherwise, degradation of cellu­lose and carbonization might occur. In addition to microwave assisted cellulose derivatization in ILs, esterification with the aid of ultrasound irradiation has been reported [32].

Parallel conversion of cellulose, dissolved in an IL, with two different carboxylic acid anhydrides yields mixed cellulose esters (Table 5.3) [16, 17, 58]. The product properties (e. g., hydrophobic/hydrophilic character) can be tailored by variation of ester moieties, their partial DS values, and the overall amount of substituents, attached to the cellulose backbone. As already pointed out, the reactivity of carboxylic acid anhydrides is dependent on the length of the alkyl chain [17]. In addition to reaction temperature, time, and amount of acylation reagent, the sequence of adding the two different anhydrides (simultaneous vs. step-wise) is consequently of huge importance. Considering the huge commercial importance of mixed cellulose esters, in particular acetate/propionates, acetate/butyrates, and propionate/butyrates, further comprehensive studies are required in order to eval­uate the individual effect of reaction parameters on the product composition, including the distribution of ester moieties within the AGU and along the polymer chain. Moreover, choice of the IL and its recycling subsequent to the reaction (see Sect. 5.3.3) are going to be important issues.