Mechanochemical and aging-based SN2 method to access CO2-responsive, high-amine-loading chitosan

Tertiary amines in water exhibit CO2-responsiveness through the equilibrium between the neutral form and the tertiary ammonium bicarbonate salt. This phenomenon makes it possible to use CO2 to trigger reversible property changes such as a change in the osmotic pressure (π), which is useful for forward osmosis, a membrane-based water separation technique. Chitosan, the deacetylated derivative of the abundant nitrogen-containing natural polymer chitin, is a great biopolymer carrier of desired functionalities through chemical modification. Herein, we report a mechanochemical and aging-based method to functionalize chitosan with a tertiary amine in a one-pot SN2-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 proceeded well with catalytic amounts of liquid and only stoichiometric amounts of reagent, a favourable improvement upon past reports, which all used excess reagent. It was also scalable and demonstrated on a range of molecular weights. We also showed that O-functionalization was effective on chitin up to a DS of 0.15. The 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.