Synthesis of a chitosan derivative via conjugate addition-elimination with diethylethoxymethylenemalonate and its physicochemical properties

Abstract

Chitin, the second most abundant natural polymer after cellulose, is mainly sourced from fungal waste and crustacean exoskeletons. It can be deacetylated to yield chitosan, a biocompatible and renewable material with versatile applications due to its reactive amino groups, allowing modifications like the one presented here, namely the synthesis of a novel chitosan derivative (ChDEEM) through a conjugate addition-elimination reaction with diethylethoxymethylenemalonate (DEEM). The reaction was conducted under various conditions to optimize the degree of substitution (DS) of the chitosan derivative (ChDEEM). Stoichiometry (1:1.5, 1:1.75, 1:2 Ch:DEEM), temperature (60, 70, and 80 °C), and reaction time (1, 2, and 3 h) were varied. ChDEEM was characterized by a range of techniques including Fourier Transform Infrared Spectroscopy (FTIR), Carbon-13 Nuclear Magnetic Resonance (¹³C NMR), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Zeta Potential, Contact Angle (CA) and Elemental Analysis (EA). FTIR confirmed the successful modification, with a sharp peak at 805 cm⁻¹ corresponding to the CC bond stretch. ¹³C NMR analysis showed new chemical shifts (158, 166, 87 and 11 ppm), and in combination with EA, was used to estimate the degree of substitution (DS) as 0.225 and 0.291, respectively. While SEM revealed no significant morphological changes, TGA indicated a decrease in thermal stability and Zeta Potential suggested reduced colloidal stability. Conversely, contact angle measurements showed increased hydrophobicity and decreased surface energy. Finally, XRD analysis revealed a decrease in the crystallinity index (from 79 % to 38 %) of ChDEEM compared to chitosan, likely due to the incorporation of methylenemalonate groups.