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The functionalization of polymeric substances is of great interest for the development of innovative materials for advanced applications. For many decades, the functionalization of chitosan has been a convenient way to improve its properties with the aim to prepare new materials with specialized characteristics. In the present article, we summarize the latest methods for the modification and derivatization of chitin and chitosan, trying to introduce specific functional groups under experimental conditions, which allow a control over the macromolecular architecture. This is motivated because an understanding of the interdependence between chemical structure and properties is an important condition for proposing innovative materials. New advances in methods and strategies of functionalization such as click chemistry approach, grafting onto copolymerization, coupling with cyclodextrins and reactions in ionic liquids are discussed.

a b s t r a c t In this study, chitosan was chemically modified by reductive amination in a two-step process. The syn- thesis of N-(furfural) chitosan (FC) was confirmed by FT-IR and 1 H NMR analysis, and the degrees of substitution were estimated as 8.3 and 23.8%. The cross-linkable system of bismaleimide (BM) and FC shows that FC shared properties of furan-maleimide chemistry. This system produced non-reversible hydrogel networks by Diels-Alder cycloadditions at 85 ◦C. The system composed of BM and FC (23.8% substitution) generated stronger hydrogel networks than those of FC with an 8.3% degree of substitution. Moreover, the FC-BM system was able to produce hydrogel microspheres. Environmental scanning elec- tron microscopy revealed the surface of the microspheres to be non-porous with small protuberances. In water, the microspheres swelled, increasing their volume by 30%. Finally, microspheres loaded with methylene blue were able to release the dye gradually, obeying second-order kinetics for times less than 600 min. This behavior suggests that diffusion is governed by the relaxation of polymer chains in the swelled state, thus facilitating drug release outside the microspheres.

The modification of polymers by click chemistry has increased abruptly over the past years. In this study, furan groups were attached onto chitosan chain via reaction of 6-azido-6-deoxy chitosan and furfuryl propargyl ether. With this purpose, 6-azido-6-deoxy chitosan was synthesized by bromination and the subsequent nucleophilic substitution with sodium azide on the C6 hydroxyl groups of a previously amino-protected N-phthaloyl chitosan. Then, 6-azido-6-deoxy chitosan was reacted with furfuryl propargyl ether by the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction, resulting in an o-substituted furan–chitosan derivative with a degree of substitution of 10 %. The structure of all derivatives was analyzed and confirmed by means of infrared and nuclear magnetic resonance spectroscopies. Clicked chitosan with furan side groups was cross-linked with a bismaleimide to produce a polymer network via Diels–Alder reaction at 75 °C. The system presented a sol–gel transition with some syneresis. The gelation process was monitored by the evolution of the viscoelastic properties of the reaction mixture. The generated chitosan–furan–maleimide polymer network exhibited the typical pattern of a soft polymer hydrogel, in which both moduli were almost frequency independent with values lower than 10 Pa. These weak mechanical properties were interpreted as a consequence of the polymer degradation, which took place during the N-phthaloyl deprotection procedure.