Kinetics and efficiency of chitosan reacetylation

Chitosan reacetylation kinetics and efficiency were studied in water–methanol (MeOH) mixtures. The polymer was dissolved using acetic acid and acetic anhydride was used for reacetylation. Combining second-order kinetics and acid–base dissociation equations of chitosan, and using acetic anhydride hydrolysis rates determined by conductivity measurements, reacetylation reaction rate constants of 187, 108, 46 min−1 M−1 were found in 0, 50 and 80% MeOH (v/v), respectively. Contrary to previous literature, it was found that improvement in reacetylation efficiency in the presence of MeOH is mainly due to an increase of acetic acid pKa by MeOH that limits the ionization of the polymer in the course of the reaction rather than to a decreased acetic anhydride hydrolysis rate, as previously thought. Based on these insights, the model developed in this study was able to predict the significantly reduced efficiency of the reaction for a large extent of reacetylation, without requiring any steric hindrance from the acetyl group. Conditions to maximize the reaction efficiency for a large extent of reacetylation were identified.