Insights into the Interaction between Amylose-Rich Corn Starch and Maleic Anhydride in Imidazolium Chloride-Based Ionic Liquids

Starch maleates with a high degree of maleic anhydride (MA) substitutions (DS) are in-demand for producing advanced composites. High-DS maleates are synthesized in imidazolium chloride-based ionic liquids because they could sustain starch-MA esterification, the precise mechanism of which, however, is largely unknown. In this study, we mapped chemical shifts of imidazolium cations before and after MA was added to each of the three starch-[C2mim]Cl, starch-[Amim]Cl, and starch-[C4mim]Cl systems. [C4mim]+ and [Amim]+ cations were observed to form substantial H-bonds with hydroxyls of starch. These hydroxyls could further associate with MA, liberating the bound cations. The liberated [Amim]+ cations, unlike [C4mim]+ cations, could readily donate their C2–H protons to activated MA, providing high-DS maleates (DSNMR ≈ 0.1, DStitration ≈ 1.20). The insights gained from this study would benefit the development of maleates from a wide range of biopolymers and expedite the screening of ionic liquids with varying cation–anion combinations for high-DS maleate synthesis.