Synthesis and Physicochemical Properties of Cardanol-Bonded Mixed Chitin Esters for Biobased Thermoplastic Materials

In this study, a cardanol-bonded chitin was synthesized for the first time via acylation of chitin dissolved in an ionic liquid, 1-allyl-3-methylimidazolium bromide, with an activated cardanol, prepared from 3-pentadecylphenoxy acetic acid and 1,1'-carbonyldiimidazole. Although the introduction of cardanol substituents induced no crystallization of the chitin chains and the appearance of a glass transition temperature (T_g), the resulting derivative did not exhibit useful properties, such as thermoplasticity for melt-press processing, due to strong interaction among the cardanol substituents. Therefore, the remaining hydroxy groups were further acylated using several fatty acyl chlorides in a LiCl/DMAc solvent to afford a series of cardanol-bonded mixed chitin esters entirely composed of naturally occurring moieties. The chemical structures and degrees of substitution (DSs) of the products were characterized by IR and ¹H NMR spectroscopies. The obtained cardanol-bonded mixed chitin esters exhibited thermoplastic behavior owing to the reduced interaction among the cardanol substituents, accompanied by the appearance of T_gs, which enabled the film formation by melt-pressing at 150 °C at 15 MPa. The thermal properties of the derivatives, evaluated by differential scanning calorimetric analysis, were systematically discussed in accordance with structures and DSs of the substituents.