Chemical modification of chitosan: Synthesis and characterization of chitosan-crown ethers

Abstract

Four novel Schiff-type chitosan (CTS)-crown ethers were synthesized through a reaction between-NH2 in CTS or crosslinked chitosan (CCTS) and -CHO in 4′-formylbenzo-crown ethers, and four secondary-amino-type CTS-crown ethers were prepared through the reduced reaction of NaBH4, respectively. Their structures were characterized by elemental analysis, Fourier transform infrared (FTIR) spectra analysis, solid-state 13C-NMR analysis, and X-ray diffraction (XRD) analysis. The elemental analysis results showed that the percentage of nitrogen in all CTS-crown ethers were lower than that of CTS or CCTS. From the FTIR data of CTS, CCTS, and CTS-crown ethers I-VIII, we saw that the characteristic peaks of C=N, N-H, and Ar appeared and that the characteristic peaks of pyranoside in the chain of CTS or CCTS were not destroyed. The XRD spectra demonstrated that CTS-crown ethers I-VIII gave lower crystallinities than CTS or CCTS, which indicated that these compounds were considerably more amorphous than CTS or CCTS. In the solid-state 13C-NMR spectra, all of these CTS-crown ethers had a particular peak of aromatic at 128 or 129 ppm, and the greatest difference between Schiff-type CTS-crown ethers and secondary-amino-type CTS-crown ethers was that the Schiff-type CTS-crown ethers had the particular peak of C=N, which disappeared in secondary-type CTS-crown ethers. All these facts confirmed that the structures of CTS-crown ethers I-VIII were as expected.