New route of deacetylation of alpha- and beta-chitins by means of freeze--pump out--thaw cycles

1st and 2nd heterogeneous deacetylations of alpha- and beta-chitins in a multistep process by means of freeze-pump out-thaw (FPT) cycles in the presence of 50% (w/v) NaOH, for temperatures ranging from 80 to 110 degrees C, were compared to the classical method using argon as the medium atmosphere. It was clearly demonstrated that FPT cycles extensively improved the reaction effectiveness by opening the crystalline structure of the two chitins and made them more permeable to alkaline solutions. The acetylated groups being more accessible, the preexponential factors of the reaction consequently enhanced, whereas the activation energies slightly increased, attesting to a more sensitive reaction to temperature. Compared to the usual method that mainly led to block copolymers for intermediate DAs, the statistical copolymers proceeded by FPT cycles improved the solubility of the samples and strongly modified the reaction mechanisms. For the very first time, the quasi-full absence of dioxygen in the reaction medium allowed us to estimate the non-oxidative degradation of the copolymer in alkaline conditions, independently of the oxydo-reductive process. Thus, heterogeneous deacetylation by means of FPT cycles exhibited all of the advantages of a reaction in homogeneous conditions (high solubility of the samples produced, reaction independent of the starting crystalline structure) without the drawbacks (low efficiency, severe degradation). It allowed us to generate fully deacetylated chitosans with the highest molecular weights never published in the literature.