Solvent-free, one-pot mechanochemical synthesis of high-DS cellulose esters with tunable thermoplasticity

Cellulose esters represent promising thermoplastic biobased materials. However, their conventional synthesis often relies on the use of ionic liquids or substantial quantities of pyridine and acid anhydrides. This study presents an alternative approach in which cellulose was first converted to alkali cellulose using sodium hydroxide solution, followed by a mechanochemical reaction with acyl chlorides to obtain thermoplastic cellulose esters with a degree of substitution (DS) above 2.5. Hydrolysis of the acyl chloride was effectively suppressed by using conjugated and hydrophobic benzene-ring structures along with low reaction temperatures, which promoted efficient grafting onto cellulose even in the presence of water. The resulting para-substituted benzoyl cellulose esters exhibit clear thermoplasticity, with a measurable glass-transition temperature (Tg) and a thermal decomposition temperature approximately 30 °C higher than that of native cellulose. By varying the alkyl chain length attached to the rigid benzene ring, both Tg (from 175 °C for methyl to 151 °C for heptyl) and mechanical properties (strength and elongation) could be tuned. Moreover, external plasticization with triethyl citrate further lowered the Tg to 98 °C, significantly improving the melt-processability. This solvent- and catalyst-free mechanochemical method eliminates the need for pyridine or anhydrides, offering a scalable route to high-performance cellulose-based thermoplastics.