Synthesis and Carboxymethylation of Organo-Soluble Trifluoroacetates and Formates of Cellulose

Abstract

Soluble cellulose intermediates are converted into carboxymethyl cellulose (CMC) with a high degree of substitution (DS_CMC) of up to 2.2 by removing the solubilizing ester groups in an one-step synthesis in dimethylsulfoxide (DMSO) with suspended solid NaOH powder as a base and monochloroacetate within 4 hours at 70°C. The reaction is a pseudohomogeneous conversion, i.e., it proceeds under temporary inhomogeneity of the substrate (cellulose). As confirmed by means of HPLC analysis, these CMCs contain a significantly higher amount of both 2,3,6-tri-O-carboxymethylated and unsubstituted units in the polymer chain than those obtained in a slurry of cellulose in isopropanol/water (totally heterogeneous conversion) at comparable DS_CMC. In contrast to heterogeneously prepared samples, the CMCs show a distribution of carboxymethyl groups within the anhydroglucose unit in the order O-6 > O-3 > O-2 as revealed by means of ¹³C- and ¹H-NMR studies. As intermediates for subsequent carboxymethylation, organo-soluble cellulose trifluoroacetates (CTFA) and cellulose formates (CF) have been synthesized and studied. The CFs were obtained by a new, convenient synthesis method consisting of the acylation of cellulose with mixtures of formic acid and phosphorus oxychloride within 4 hours at room temperature. The CFs with a degree of substitution (DS_CF) of 2.2 are soluble in DMSO and DMF, and they show a complete substitution of the primary OH groups as confirmed by ¹³C-NMR studies and HPLC analysis after permethylation and chain degradation. The CTFAs (DS_CTFA = 1.5) prepared by acylation of cellulose with a mixture of trifluoroacetic acid/trifluoroacetic anhydride also show a preferred substitution of the C-6 position.