Oligosaccharide-Reactive Dye Macromolecular Conjugates: A Green Strategy for Reducing Salt and Alkali in Cotton Dyeing

Abstract Conventional reactive dyeing of cotton requires high doses of electrolyte and alkali, leading to severe environmental burdens from saline and alkaline wastewater. We developed a class of macromolecular dyes featuring a multianchor design on an oligosaccharide skeleton, which enhances substantivity to cotton and greatly reduces the use of auxiliary chemicals. In this study, saccharide-based macromolecular dyes were synthesized via covalent grafting of reactive dyes onto cellobiose with five catalysts: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), betaine, choline chloride, triethylamine, and trimethylamine. Dye uptake comparison confirms that DBU is the best catalyst for the synthesis, and spectroscopic characterizations confirm the covalent linkage between the reactive dye and cellobiose. These cellobiose-based macromolecular dyes show markedly improved cotton substantivity, allowing efficient dyeing under mild conditions (salt ≤ 40 g/L, alkali ≤ 10 g/L). Compared with conventional Reactive Red 195 (wet rubbing fastness: grade 2–3), the cellobiose-based dye CB-RR195 yields a wet rubbing fastness of grade 4–5, thus achieving both low-salt, eco-friendly dyeing and enhanced rubbing fastness. This strategy is broadly applicable to various oligosaccharides. Among the tested derivatives, the oligosaccharide-based dye with the highest hydroxyl group density achieved a maximum uptake of 69.4%. Cotton fabrics dyed with this formulation displayed deep color intensity, improved hydrophilicity, enhanced moisture permeability, and retained mechanical strength. Theoretical calculations reveal that the dye–saccharide interaction energy increases monotonically with the hydroxyl count. By drastically cutting auxiliary chemical inputs while maintaining dyeing performance, this work offers a practical and sustainable pathway toward low-salt, low-alkali textile processing, aligning with the principles of green chemistry and circular economy.