The preparation of O-methyl- and O-ethyl-celluloses having controlled distribution of substituents

O-Methyl- and O-ethyl-celluloses having controlled distributions of substituents were prepared by alkylation of 6-O-triphenylmethyl ("trityl") cellulose in dimethyl sulfoxide (Me₂SO) and subsequent detritylation of the alkylated products. Water-free alkylation reactions failed to give complete substitution of the hydroxyl groups at both the C-2 and C-3 positions. The addition of a small amount of water to the tritylcellulose solution in Me₂SO improved the efficiency of the alkylations, yielding 2,3-di-O-substituted-6-O-tritylcelluloses which in turn gave 2,3-di-O-alkylcelluloses on detritylation with HCl gas. Repeated further alkylation of the 2,3-di-O-alkylcelluloses in Me₂SO furnished products with high degrees of substitution at the C-6 position. The distribution of substituents in the alkyl cellulose ethers was determined by acid hydrolysis, reduction, and gas-chromatographic separation of the partially alkylated alditol acetates. The distribution of methyl and ethyl groups in the polymers prepared by repeated alkylation systematically changed with each alkylation step.