Cross-linking sago starch with thermo-reversibility: Transesterification in supercritical carbon dioxide (CO ₂ ) followed by diels-alder chemistry approach

This study investigates the synthesis of thermo-reversible biopolymers from sago starch utilizing supercritical CO ₂ (scCO ₂ ) solvent instead of conventional organic solvents. The process involved a two-step reaction: (1) transesterification of starch with methyl 2-furoate in scCO ₂ , followed by (2) a cross-linking reaction utilizing Diels-Alder chemistry with 1, 1′-(methylenedi-4, 1-phenylene)bismaleimide (BM), where the solid starch granules were used in both reaction steps. The research demonstrated that the degree of substitution (DS) of the starch ester was maximized at specific parameters: 10 MPa pressure, a K ₂ CO ₃ catalyst ratio of 0.3 mol/mol anhydroglucose unit (AGU), and a temperature of 100°C. The resulting cross-linked products exhibited thermo-reversible behavior, as confirmed by changes in degree of cross-linking (DC) values with annealing temperature. The degree of cross-linking (DC) was found to be directly influenced by the annealing temperature, with the maximum and minimum values observed at 50°C and 150°C, respectively. The relationship between annealing temperature and degree of cross-linking suggests that the cross-linked starch product possesses thermoplastic properties, allowing for potential recycling and reprocessing, a significant improvement over conventional cross-linked starch. Furthermore, the final product demonstrated enhanced thermal stability compared to both native and esterified starches, which is a desirable characteristic for various industrial applications.