Furan–chitosan hydrogels based on click chemistry

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.