Variable-temperature NMR techniques were applied to monitor the gelation process of chitosan/glycerophosphate (CS/GP) thermosensitive hydrogel system. The results showed that the chemical shifts of protons in CS and phosphor in GP changed together while the temperature was increased. However, the protons in CS shown in ¹H NMR spectra moved to the high field, but the phosphor shown in ¹³P NMR moved to the low field. In addition, the chemical shift of H-2(D) in CS showed a turning point around the gelation temperature, indicating that the chemical environment of H-2(D) has been changed dramatically. Moreover, the chemical shift of H-2(D) in CS moved to higher field and the CS/GP system showed the lower gelation temperature with the increase of either GP content or pH value of the CS/GP mixture. Therefore, the gelation mechanism of CS/GP thermosensitive hydrogel was deduced as follows: with the increase of temperature, the electrostatic interaction between CS and GP is broken down, followed by the formation of hydrogen bonds between CS macromolecular chains, which induce the gelation.

Key words: smart hydrogel, chemical shift, ¹H NMR, ¹³P NMR, hydrogen bonding, electrostatic interaction