Cinnamoylated Chitosan Derivatives Prepared via Reactive Extrusion: Structure–Property Relationships and Antifungal Activity

Abstract Objective: The aim of this work was to prepare cinnamoylated chitosan derivatives through solid-state reactive extrusion using cinnamic acid (CA) and cinnamoyl chloride (CC) as acylating agents, and to establish the relationships between the modification pathway, degree of substitution (DS), aggregate behavior, and antifungal activity. Methods: Mechanochemical modification was performed under solvent-free conditions in a co-rotating twin-screw extruder at 100°C for the CS–CA system and 60°C for the CS–CC system. The products were purified, fractionated in 2% acetic acid, and characterized using FT-IR spectroscopy, elemental analysis, and dynamic light scattering (DLS). Antifungal properties of the derived films were evaluated against Aspergillus niger on an agarized nutrient medium by measuring growth inhibition zones. Results and Discussion: FT-IR and elemental analysis revealed that modification with cinnamic acid occurs via both ionic (acid-base) interactions and covalent amide bond formation, whereas cinnamoyl chloride predominantly forms covalent amide bonds, resulting in covalent degrees of substitution of 0.14–0.34. DLS measurements showed that hydrophobic cinnamoyl modification promotes polymer self-organization, increasing supramolecular aggregate sizes from 555 nm (unmodified chitosan) up to 760–1300 nm for the derivatives. Increasing incorporation of cinnamoyl fragments progressively decreased derivative solubility in acidic media. Conclusions: N-Acylated chitosan derivatives with cinnamoyl fragments were successfully synthesized via a green, technologically efficient reactive extrusion method. The chemical nature of the acylating agent determines the precise structural modification pathway. Modified films exhibited distinct fungistatic activity against Aspergillus niger, with the most pronounced antifungal effect observed in the derivative featuring the highest degree of covalent substitution.