Branched polyethylenimine (b-PEI), an amine-rich polymer, can obtain thermosensitivity by a simple acylation reaction. The resulting N-acylated b-PEI derivatives showed a similar lower critical solution temperature (LCST) transition as their linear correspondent, poly(N-allcyloxazoline). Three acyl groups (propionyl, and isobutyryl, and n-butyryl) were introduced and resulted in LCSTs ranging from 10 degrees C to 90 degrees C depending on the structure and environment. The hydrophobicity of N-acylated b-PEI can be controlled by varying the acyl group and degree of acylation. Because the LCST transition is determined by the delicate balance between the hydrophobicity and hydrophilicity of the polymers, an increase of the hydrophobicity in N-acylated b-PEI lowers the transition temperature. Also, N-acylated b-PEI contains tertiary amines as well as unacylated primary or secondary amines which can be protonated during acidification from a pH of 7.4 to a pH of 5.5. The LCST transition was observed at elevated temperatures due to the increase of hydrophilicity by the protonation in the acidic environment. The LCST was also influenced by the salt concentration. A decrease of the LCST was observed as the NaCl concentration increased, probably due to the dominance of the salting-out effect. The very simple introduction of thermosensitivity into pre-existing polymers can be easily applied for the development of various dual or multiple signal-sensitive polymer systems. (C) 2011 Elsevier Ltd. All rights reserved.