Unconventional methods in cellulose functionalization

This review paper gives an overview of synthesis paths recently studied for the preparation of unconventional, i.e. not commercially produced, cellulose derivatives with alternative functional groups and patterns of functionalization. A classification of solvents that can be applied as medium for homogeneous reactions is given and the different types of solvents, namely both aqueous and non-aqueous non-derivatizing and derivatizing ones, are evaluated. Carbonic and sulfonic acid esters of the polymer, deoxycellulose derivatives, ionic and non-ionic ethers of cellulose as well as silylated cellulosics are included. Moreover, exotic products, e.g. hydrophobically-modified water-soluble as well as regioselectively functionalized cellulosics are critically reviewed. Among the synthesis pathways discussed are conversions in homogeneous phase using different aqueous and non-aqueous solvents, the reaction via organo-soluble intermediates, the functionalization of cellulose in reactive microstructure obtained by induced phase separation and by applying a certain degree of accessibility as well as protecting group technique with cellulose. A number of new synthesis tools for the derivatization of cellulose are reviewed, e.g. new in situ activating agents for cellulose esterification like N,N-dicyclohexylcarbodiimide, N,N-carbonyldiimidazole and p-toluenesulfonyl chloride. Furthermore, nucleophilic displacement reactions, offering the possibility to design advanced cellulose materials by reactions at the C-atoms, as well as oxidation reactions are described. Selected methods appropriate for structure analysis of cellulose derivatives both on the level of the repeating unit and along the polymer chain are briefly reviewed. Emphasis is placed on the application of NMR spectroscopy including two-dimensional methods and of chromatographic techniques after specific sample pretreatment as enzymatic and acidic partial or complete depolymerization. Some comments on structure–property relationships of the cellulose derivatives are also given.