Chitosan is the most important derivative of chitin, a polysaccharide found
in the exoskeleton of shellfish like shrimp and crab. It is a product of deacetylation
of chitin under alkaline conditions or enzymatic hydrolysis in the presence
of chitin deacetylase. Both chitin and chitosan are linear polysaccharides and are
chemically defined as copolymers consisting of varying amounts of β-(1→4)-
linked 2-acetamido-2-deoxy β-D-glucopyranose (GlcNAc) and 2-amino-2-deoxy-
β-D-glucopyranose (GlcN). The difference between chitin and chitosan lies in the
content of GlcNAc and GlcN units. Chitin samples contain a high content of Glc-
NAc units. Due to excellent properties of chitosan, such as biocompatibility, biodegradability,
hydrophilicity, non-toxicity, cationicity, ease of modification, film
forming ability, affinity to metals, protein and dyes, etc., this polymer has found
applications in medicine and pharmacy, as food additive, antimicrobial agent, in
paper and textile industry, in environmental remediation and other industrial areas.
The presence of functional groups, reactive amino and hydroxyl groups, in chitosan
backbone makes it suitable candidate for chemical modification. Chemical modification
of chitosan to generate new polymers with useful physicochemical properties
and distinctive biological functions is of key interest because it would not change
the fundamental skeleton of the polymer. In this article the main three methods of
chitosan modification: substitution reactions, reactions leading to the chain elongation
and/or molecular weight increasing and methods of depolymerization are
shortly characterized. Moreover, the selected methods of chitosan modification, i.e.
quaternization, alkylation, acylation, carboxyalkylation, phosphorylation, sulfation,
graft copolymerisation, crosslinking and depolymerization are discussed in more
detail. A special attention is drawn to chitosan crosslinking with low and high molecular
compounds. Chitosan modification by covalent and ionic crosslinking allows
to obtain polymer materials with improved mechanical and chemical resistance and
suitable for example for chitosan hydrogel membranes formation.
Keywords: chitosan, chitosan modification, chitosan derivatives, crosslinking
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