Model for random hydrolysis and end degradation of linear polysaccharides:Application to the thermal treatment of mannan in solution

The kinetics for homogeneous hydrolysis of mannan is studied in a batch rea ctor at temperatures from 160 to 220 degrees C. A formate buffer ensures a pH of 3.8-4.0, measured at 25 degrees C. Samples are analyzed for oligosacc harides up to a degree of polymerization of 6 and also for the total amount of mannose after acid hydrolysis. A mathematical model with two reactions (1, random hydrolysis of the glucosidic bonds; 2, degradation of the reduci ng end of the molecule) describes accurately the time course of oligosaccha rides. Optimized rate constants follow closely an Arrhenius relationship, w ith the degradation having a higher activation energy (140 kJ/mol) than the hydrolysis (113 kJ/mol). The mathematical model has the advantage that pro duction of small molecules is independent of the initial chain-length distr ibution as long as the average initial chain length is some 5 times longer than the largest species measured. It can be applied to first-order depolym erization of other linear polymers with one link type in order to determine reaction rate constants or make predictions about molecular weight distrib ution on the base of known reaction rate constants.