Substitution patterns of cellulose ethers - Influence of the synthetic pathway

Commercial cellulose ethers are usually prepared under heterogeneous reaction conditions. In contrast, this contribution also describes the derivatization under homogeneous conditions in N-methylmorpholine-N-oxide monohydrate (NMMNO*H2O) and under heterogeneous conditions after converting native cellulose to amorphous cellulose. Amorphous cellulose is prepared by dissolving cellulose in NMMNO*H2O followed by precipitation in different media. The degree of order and the porosity of the regenerated cellulose is significantly influenced by the content of water in the precipitating agent. The differences are described by measurements using wide angle X-ray scattering, solid-state 13C-NMR, mercury porosimetry, and water/liquid retention values. Three synthetic pathways (heterogeneous, heterogeneous with amorphous cellulose and homogeneous) are compared regarding the structure-property relationship of the cellulose ethers formed. Carboxymethylation, hydroxyethylation, hydroxypropylation and sulfoethylation are considered in detail. The choice of synthetic pathway has a significant influence on the degree of substitution (DS), the distribution of substituents on the level of the anhydroglucose unit (AGU), solubility behavior, and the viscosity of aqueous solutions. In general an increasing solubility and an increasing viscosity are observed from heterogeneous to heterogeneous with amorphous cellulose to homogeneous reaction conditions. There is a remarkable difference between the heterogeneously produced cellulose ethers with a DS distribution C2 C6 > C3 and the strictly homogeneous etherification in NMMNO*H2O/organic solvent systems with a DS distribution of C3 > C2 C6. This high regioselectivity at the secondary OH-groups of the AGU may be caused by the strong solvation behavior of NMMNO*H2O and thereby a protecting function at the C6-OH-group.