Functionalisation of the non-reducing end of chitin by selective periodate oxidation: A new approach to form complex block polysaccharides and water-soluble chitin-based block polymers

Most polysaccharides used in polysaccharide-based block copolymers are attached to the second block through the reducing end, due to the few and highly polysaccharide specific non-reducing end (NRE) functionalisation methods available. Chitin oligomers, prepared by nitrous acid degradation of chitosan (AnM) can, however, be selectively oxidised by periodate since they only possess a single vicinal diol in the NRE residue. Here, we show that both aldehydes formed after oxidation are highly reactive towards bifunctional oxyamines and hydrazide linkers. Sub-stochiometric amounts of linkers resulted in conjugation of AnM oligomers through both chain termini to yield a discrete distribution of ‘polymerised’ oligomers. Such chitin-based block polymers were, in contrast to chitins of the same chain lengths, water-soluble. Oxidised AnM oligomers, functionalised at both termini can also enable the preparation of more complex block polysaccharides such as ABA- or ABC-type. [Display omitted] •Chitin oligomers were prepared by nitrous acid degradation of chitosan.•The non-reducing ends were selectively oxidised by periodate to yield dialdehydes.•The 2,5-anhydro-D-mannose residues at the reducing end were resistant to periodate.•The dialdehydes were highly reactive towards dioxyamines and dihydrazides.•Water-soluble chitin-based block polymers were prepared.