Kinetic studies of acid-catalyzed hydrolysis of mixed cellulose ethers

The rate of acid-catalyzed hydrolysis of O-ethyl-O-methyl-cellulose (EMC) and O-methoxyethyl-O-methyl-cellulose (MEMC), respectively, has been studied. By a two-step depolymerization procedure and monitoring of all individual substitution patterns, rate constants were determined for glucosyl residues with each particular substitution pattern. Two MCs of DS 1.27 (MC1) and 1.95 (MC2) have been perethylated and permethoxyethylated and submitted to TFA hydrolysis in water/acetone in a heating block at 120 °C. EMC1 has also been hydrolyzed without acetone addition, under microwave irradiation, and after partial mechanical degradation by treatment with a sonotrode. For all hydrolyses, a slow (k-a) and, after about 10% conversion, a faster phase of hydrolysis (k-b) was found. Rate constants k-b were in the range of 2–4 × 10⁻⁴ s⁻¹ for all peralkylated celluloses. In contrast to previously studied MC, hydrolysis in water/acetone was faster than in water, indicating the influence of solution state and the macromolecular character. No difference was observed for sonotrode pretreated EMC1, while microwave instead of heating block treatment accelerated hydrolysis by a factor of 30 under the chosen conditions. Selectivity with respect to the substitution patterns, expressed as the standard deviation of the individual rate constants k _i (i = position of methyl), was higher in the slow initial phase (23–28%), while ranging between 9 and 18% in the main phase of hydrolysis with lower values for MEMC compared to EMC and, in case of EMC1, for aqueous TFA compared to acetone-containing mixtures. Randomness in hydrolytic cleavage is favored by water as solvent and apparently by microwave heating.