On the kinetics of hydrolytic degradation of native cellulose

The kinetics of the hydrolytic degradation of celluloses the molecular properties of which still correspond largely to those of the native state are studied. The hydrolytic agent has been 0,5 mol/l aqueous solution of KHSO4. The degrees of polymerization were determined viscosimetrically in copper ethylene diamine (CuEn) as solvent under standard conditions. In order to convert viscosity‐average and weight‐average degrees of polymerization DPη and DPw in the corresponding number‐average DPn, a DPw/DPn calibration curve had been elaborated using polymer analogeous nitrates of a series of samples degraded under the same conditions. The results reveal that hydrolytic degradation of native cellulose takes place by two simultaneously occurring kinetic mechanisms, for which the following rate constants were derived: k1 = 0,12 h−1, k2 = 3,88·10−6 h−1, and k1 = 0,79 h−1; k2 = 38,35 · 10−6 h−1, corresponding to a degradation temperature of 40°C and 60°C, respectively, and related to an initial degree of polymerization DP0 = 13 000. The number of faster cleaving bonds per molecule (β1) and the number average of normally splitting bonds being between two “weak links” (β2) resulted in β1 ≈ 3,5 and β2 ≈ 3 600. The activation energies of the rapid and the slower reaction were found to be 20 kcal/mol (83,7 kJ/mol) and 24 kcal/mol (100,4 kJ/mol), respectively. Additional degradation experiments on the same, but pre‐swollen or pre‐reduced samples did not provoke any variations in the results. It can be excluded, therefore, that the simultaneous occurrence of two kinteic mechanisms is a consequence either of facts which involve diffusion control, or the existence of “stressed” bonds. The found relatively high activation energies rather permit to assume native modifications in the chemical structure of the cellulose molecule as the most probable reason for the faster bond splitting reaction.