The invention relates to a thermosensitive (poly(ethylene oxide) poly(propylene oxide))-acetal derivative, which can be used to functionalize chitosan, and to the process for the preparation thereof. The invention therefore also relates to a process for functionalizing chitosan by grafting this thermosensitive copolymer and optionally another thermosensitive polymer, poly(ethylene oxide). A subject of the invention is also a chitosan-(poly(ethylene oxide) poly(propylene oxide))-poly(ethylene oxide) derivative which has the advantage of being water-soluble at physiological pH. The invention therefore also relates to the aqueous compositions comprising this derivative and to the use of this composition for the manufacture of gel by heating.