Starch esterification using deep eutectic solvents as chaotropic agents and reaction promoters

Abstract

Starch derivatizations require harsh chemicals as solvents or catalysts, which can negate the green nature of starch-based materials. Often, these reaction systems require several hours to achieve moderate degrees of substitution (DS), drastically reduce molar mass and can result in undesired crosslinking compromising solubility. Here, we report an efficient and sustainable starch esterification with acetic anhydride (AA) and the optimization of the process avoiding initiators in terms of molar ratios, time, and temperature. Furthermore, chromatographic (HPSEC-MALS-dRI) and spectroscopic (FTIR, ¹H NMR, 2D HSQC and HMBC, Solid State CP/MAS ¹³C NMR, TG-IR) tools were implemented to elucidate the chemical composition and structure of the resulting starch acetates and potential intermediate side reaction products over the course of the reaction. Different combinations of choline chloride (ChCl) with several hydrogen bond acceptors (urea, tartaric, malonic, and malic acids) were used as both chaotropic solvents and reaction promoters for starch acetylation. The reaction system comprising 1 : 1 molar ChCl : urea and AA showed good miscibility at 100 °C after 30 min, representing a seemingly homogeneous reaction system while better preserving starch molar mass. Side products emerging from solvent-reagent interactions, such as starch carbamate and acetylurea, were identified. Reaction optimization resulted in no side products, fast reaction rates (36 min), high DS (2.87) and starch loads (20 wt%), and increased reaction throughput and atom economy. Catalyst-free starch acetylation reduced hygroscopicity and increased glass transition and degradation temperatures (162 °C and 397 °C, respectively), while generally keeping a relatively higher molar mass (1.5–2.9 × 10⁶ Da) than traditional starch acetates.