Extensively deacetylated high molecular weight chitosan from the multistep ultrasound-assisted deacetylation of beta-chitin

• Ultrasound-assisted deacetylation (USAD) of beta-chitin is investigated.
• USAD promotes the random deacetylation of beta-chitin.
• USAD high molecular weight chitosan results after 50 min at 60 °C.
• Extensively deacetylated USAD chitosan results after three consecutive steps.

High intensity ultrasound irradiation was used to convert beta-chitin (BCHt) into chitosan (CHs). Typically, beta-chitin was suspended in 40% (w/w) aqueous sodium hydroxide at a ratio 1/10 (g mL−1) and then submitted to ultrasound-assisted deacetylation (USAD) during 50 min at 60 °C and a fixed irradiation surface intensity (52.6 W cm−2). Hydrogen nuclear magnetic resonance spectroscopy and capillary viscometry were used to determine the average degree of acetylation (DA‾) and viscosity average degree of polymerization (DPv‾), respectively, of the parent beta-chitin (DA‾ = 80.7%; DPv‾ = 6865) and USAD chitosans. A first USAD reaction resulted in chitosan CHs1 (DA‾ = 36.7%; DPv‾ = 5838). Chitosans CHs2 (DA‾ = 15.0%; DPv‾ = 5128) and CHs3 (DA‾ = 4.3%; DPv‾ = 4889) resulted after repeating the USAD procedure to CHs1 consecutively once and twice, respectively. Size-exclusion chromatography analyzes allowed the determination of the weight average molecular weight (Mw‾) and dispersity (Ð) of CHs1 (Mw‾ = 1,260,000 g mol−1; Р= 1.4), CHs2 (Mw‾ = 1,137,000 g mol−1; Р= 1.4) and CHs3 (Mw‾ = 912,000 g mol−1; Р= 1.3). Such results revealed that, thanks to the action of high intensity ultrasound irradiation, the USAD process allowed the preparation of unusually high molecular weight, randomly deacetylated chitosan, an important breakthrough to the development of new high grade chitosan-based materials displaying superior mechanical properties.