Novel solvents for cellulose: Use of dibenzyldimethylammonium fluoride/dimethyl sulfoxide (DMSO) as solvent for the etherification of the biopolymer and comparison with tetra(1-butyl)ammonium fluoride/DMSO

Abstract

Solutions of some quaternary ammonium fluoride hydrates in dimethyl sulfoxide (DMSO) are efficient solvents for the dissolution and derivatization of cellulose. Examples are the hydrated fluorides of the following quaternary ammonium ions: dibenzyldimethyl- (BMAF-0.1H2O), tetraallyl- (TAAF-H2O), and tetra(1-butyl)- (TBAF-3H2O). The tetramethyl compound (TMAF) does not dissolve cellulose. Here we report on using solution of (BMAF-0.1H2O) in DMSO for the synthesis of industrially important ethers of cellulose, namely, allyl-, benzyl-, and sodium carboxymethyl cellulose in the presence of dispersed solid NaOH. The degree of substitution of the products were similar to those obtained with (TBAF-3H2O)/DMSO, in spite of the large difference in the water content of both electrolytes. Based on literature data, we attribute this similarity to the stability toward hydrolysis of the reactant halides and the produced cellulose ethers, and the reduced activity of the NaOH-bound water. In order to probe the interactions in the biopolymer/R4NF/DMSO system, we have carried out quantum chemical calculations on systems composed of cellobiose, methylated at positions O-4 and O-1′ (Cell-M, model for cellulose), DMSO, and each of the above-mentioned R4NF. These calculations showed that the most stable aggregate involves F−-mediated hydrogen bonding of DMSO to the hydroxyls of Cell-M. The inefficiency of TMAF/DMSO is attributed to strong hard acid–hard base interaction between the electrolyte ions.