Specific hydroxide ion catalysis of the endwise depolymerization of cellulose

Abstract

The endwise depolymerization (unzipping reaction) of hydrolyzed cotton cellulose (x = 200) in water under a nitrogen atmosphere was followed at 98/sup 0/C at several alkalinities in the pH range of 8.0-10.5. The observed apparent first-order rate constant k/sub 1/ was invariable at low alkalinity (k/sub 1/ = k/sub 0/), while above pH 8.5, k/sub 1/ increased with pH. The data conform with the expression rate = (k/sub 0/ + K/sub OH/-(OH/sup -/))(SH) = (0.3 + 2.1 x 10/sup 3/ (OH/sup -/))(SH), where (SH) denotes substrate concentration. The specific hydroxide ion catalysis is considered to involve ionization of the anomeric hydroxyl group at the reducing chain end that leads to elimination of the glucosidic oxygen atom bearing the polymer chain from C4 of the terminal D-glucose residue. In this initiation process, the glucosidic oxygen is eliminated as an anion so that rapid propagation of the unzipping along the polymer chain may occur. Thus, entire chains will depolymerize. The kinetic chain length ..nu.. is defined as the ratio k/sub 1/:k/sub t/ where k/sub t/ is the pseudofirst-order rate constant for chain terminations, and a value of ..nu.. approximately 100 D-glucose residues was found at all the alkalinities investigated.