Twenty-five years of cellulose chemistry: innovations in the dissolution of the biopolymer and its transformation into esters and ethers

The anniversary of the journal “Cellulose” is an opportunity to review innovations that were introduced during the past 25 years. Of these, from our perspective, the development of solvents that dissolve cellulose physically, i.e., without formation of covalent bonds is most relevant. The reasons are that cellulose can be regenerated from these media in different shapes and transformed into many important derivatives. Twenty-five years is a long time-span! As the volume of information on the applications of the above-mentioned solvents in cellulose chemistry is extensive, we made choices to reach a balance between the amount of material covered and the length of the review. Consequently, we focus on cellulose derivatization under homogeneous reaction conditions to produce selected derivatives. We dwell on the latter because a comprehensive discussion was recently published on derivatization under heterogeneous and homogeneous conditions (Heinze et al. in Cellulose derivatives, Springer, Cham, pp 259–292, 2018a). The derivatives selected are esters of organic acids, ionic and nonionic ethers because of their tremendous commercial and scientific importance. Cellulose derivatization in homogeneous media is advantageous because of much better control of product properties relative to those obtained under the heterogeneous counterparts. These properties include degree of substitution in the anhydroglucose unit and along the biopolymer back-bone, and regioselectivity. Thus, novel cellulose derivatives were prepared that are not accessible under heterogeneous conditions. The requirement to dissolve cellulose physically is to disrupt hydrogen bonding and hydrophobic interactions. Thus, the solvents employed to dissolve cellulose are usually composed of strong electrolytes whose cations and anions interact preferentially with cellulose. These electrolytes are used pure or as solutions in water or dipolar aprotic solvents. Salient examples include LiCl/N,N-dimethylacetamide, tetra(n-butyl)ammonium fluoride·3H₂O/dimethyl sulfoxide, ionic liquids, salts of quaternary amines and super-bases. We discuss briefly the essentials of each solvent in terms of its mechanism of cellulose dissolution and show the most relevant results regarding its application for obtaining esters and ethers and back the discussion with relevant references. This information is summarized at the end of the review. We hope that this historical perspective shows the innovations made since the first publication of “Cellulose” and points out to future possibilities—with potential industrial application—of this renewable raw material and its biocompatible and biodegradable derivatives.