Preparation of chitosan with a relatively narrow molecular weight distribution via synergistic regulation of crystalline structure and chain degradation by DES–NaOH

Chitosan exhibits high sensitivity of its solubility, rheological behavior, and performance consistency to molecular structure, among which an excessively broad molecular weight distribution (MWD) often leads to pronounced property fluctuations and poor reproducibility. However, effective control over MWD during conventional alkaline deacetylation remains challenging, and existing studies have largely focused on tuning molecular weight or degree of deacetylation (DD), with limited attention paid to MWD regulation. In this work, a controllable deacetylation strategy was established by combining strong alkali pre-soaking, an initial alkaline treatment stage, and a subsequent deep eutectic solvent (DES)–NaOH synergistic process, enabling the preparation of chitosan with a relatively narrow MWD. Compared with the water-substituted DES system (H₂O group) and the strong alkali system (NaOH group), chitosan obtained from the DES–NaOH synergistic system exhibited a lower MWD ( Đ ≈ 1.65) under identical chromatographic conditions. Structural characterization and thermal analysis indicate that DES partially disrupts the crystalline structure and modifies hydrogen-bond-related interactions, thereby potentially increasing the accessibility of acetyl groups to NaOH and facilitating deacetylation. Meanwhile, the presence of DES mitigates random alkali-induced glycosidic bond cleavage, reducing chain degradation and enabling a balance between deacetylation efficiency and polysaccharide backbone integrity. Overall, the DES–NaOH synergistic system provides a potential strategy for regulating chitosan MWD and contributes to understanding the relationship between processing conditions and structural evolution during polysaccharide deacetylation.