Synthesis of linear chitosan-block-dextran copolysaccharides with dihydrazide and dioxyamine linkers

Dihydrazide (ADH) and dioxyamine (PDHA) were assessed for their efficacy in coupling chitosan and dextran via their reducing ends. Initially, the end-functionalization of the individual polysaccharide blocks was investigated. Under non-reducing conditions, chitosan with a 2,5-anhydro-D-mannose unit at its reducing end exhibited high reactivity with both PDHA and ADH. Dextran, with a normal reducing end, showed superior reactivity with PDHA compared to ADH, although complete conversion with ADH could be achieved under reductive conditions with NaBH₃CN. Importantly, the oxime bond in PDHA conjugates exhibited greater stability against hydrolysis compared to the hydrazone bond in ADH conjugates. The optimal block coupling method consisted in reacting chitosan with an excess of dextran pre-functionalized with PDHA. The copolysaccharides could be synthesized in high yields under both reducing and non-reducing conditions. This methodology was applied to relatively long polysaccharide blocks with molecular weight up to 14,000 g/mol for chitosan and up to 40,000 g/mol for dextran. Surprisingly, block copolysaccharides did not self-assemble at neutral or basic pH; rather, they precipitated due to hydrogen bonding between neutralized amino groups of chitosan. However, nanoparticles could be obtained through a nanoprecipitation approach.