Synthesis of mixed chitin esters with long fatty and bulky acyl substituents in ionic liquid

This study revealed that mixed chitin esters with long fatty and bulky acyl substituents were efficiently synthesized by acylation using acyl chlorides in the presence of pyridine and N,N-dimethyl-4-aminopyridine in an ionic liquid, 1-allyl-3-methylimidazolium bromide (AMIMBr), at 100 °C for 24 h. A stearoyl group was selected as the first substituent, which was combined with different long fatty and bulky acyl groups as the second substituents. In addition to IR analysis of the products, which suggested progress of the acylation, ¹H NMR measurement was allowed for structural confirmation for high degrees of substitution (DSs) of the desired derivatives in CDCl₃/CF₃CO₂H solvents. Crystalline structures and thermal property of the products were evaluated by powder X-ray diffraction and differential scanning calorimetry measurements, respectively. All the products showed film formability by casting from solutions in chloroform or chloroform/trifluoroacetic acid solvents. The occurrence of halogen exchange between acyl chlorides and AMIMBr in the present system was speculated to produce highly reactive acyl bromides in situ, which efficiently reacted with hydroxy groups in chitin to obtain high DS products.