Theoretical study of the reaction of chitosan monomer with 2,3-epoxypropyl-trimethyl quaternary ammonium chloride catalyzed by an imidazolium-based ionic liquid

• DFT calculations illustrate the mechanism of the graft reaction of chitosan with the epoxy compound.
• The polar environment of [Amim]Cl ionic liquid favors the graft reaction.
• The ionic liquid also serves a catalyst of the reaction to promote the ring-opening of the epoxy compound.

The molecular mechanism of the graft reaction of 2,3-epoxypropyl-trimethyl quaternary ammonium chloride with chitosan monomer was investigated by performing density functional theory (DFT) calculations. The calculated results show that the −NH2 group of chitosan monomer is more reactive than its −OH and −CH2OH groups, and the graft reaction on the −NH2 group is calculated to be exothermic by 20.5 kcal/mol with a free energy barrier of 42.6 kcal/mol. The reaction cannot benefit from the presence of the intruded water molecule, but can be considerably assisted by 1-allyl-3-methylimidazolium chloride ([Amim]Cl) ionic liquid. The reaction catalyzed by the ion-pair is calculated to be exothermic by 36.5 kcal/mol and the barrier is reduced to 29.3 kcal/mol, which are further corrected to 28.0 and 29.1 kcal/mol by considering the solvent effect of [Amim]Cl ionic liquid. Calculated results verified the experimental finding that imidazolium-based ionic liquids can promote the reaction of chitosan with epoxy compounds.