Mechanochemical and Aging-Based S_N2 Method to Access CO₂-Responsive, High-Amine-Loading Chitosan

Tertiary amines in water exhibit CO₂-responsiveness, enabling CO₂-triggered reversible property changes. Chitosan is a great biopolymer carrier of desired functionalities through chemical modification. Herein, a mechanochemical and aging-based method is reported to functionalizing chitosan with a tertiary amine in a one-pot S_N2-type aminoalkylation reaction. The reaction afforded chitosan with a high degree of substitution (DS), up to 1.67 per sugar unit, doubling the previously reported DS achieved with solvothermal methods. Unique to this method is its ability to functionalize both the amine and the primary alcohol groups of chitosan. The reaction proceeds well with catalytic amounts of liquid and only stoichiometric amounts of reagent, a favorable improvement upon past reports, which all use excess reagent. It is also scalable and demonstrated on a range of molecular weights. O-functionalization is shown to be effective on chitin up to a DS of 0.15. The osmotic pressure (π) of aqueous solutions of the aminoalkylated chitosan reversibly increases 47% upon carbonation. This work showcases how mechanochemistry can deliver greatly improved DS on the functionalization of recalcitrant substrates, such as chitinous biomass, with reduced process mass intensity and energy use compared to past works to deliver a CO₂-responsive material based on biomass.