Regioselective Bromination of the Dextran Nonreducing End Creates a Pathway to Dextran-Based Block Copolymers

Preparation of polysaccharide-based block copolymers with linear architectures is an important goal, opening up clear application potential and requiring significant advances in polysaccharide regio- and chemoselectivity. Herein we report a simple approach to prepare dextran-based block copolymers. Reaction with N-bromosuccinimide (NBS)/triphenyl phosphine (PPh₃) regioselectively brominates the sole primary alcohol of linear, unbranched dextran at C-6 of the nonreducing end monosaccharide. The resulting dextran, monofunctionalized with a terminal C-6 bromide, was coupled with various amine terminated polymers to prepare block copolymers, including dextran-b-poly(ethylene glycol), dextran-b-polystyrene, and dextran-b-poly(N-isopropylacrylamide). These renewable-based block copolymers, prepared in two selective, high-yielding steps from native linear dextran, exhibit various interesting properties. Dextran-b-poly(N-isopropylacrylamide) undergoes thermally induced micellization as revealed by dynamic light scattering (DLS), forming micelles upon exceeding 33 °C. We also observed microphase separation in dextran-b-polystyrene by using small-angle X-ray scattering (SAXS). Overall, this methodology provides a new, highly chemo- and regioselective, versatile route to diverse dextran-based block copolymers with useful properties, enabling drug delivery, compatibilization, and other applications.