We introduced upper critical solution temperature (UCST) properties into an amine-rich polymer, branched polyethylenimine (b-PEI) by a simple sulfopropylation reaction. The phase transition behavior of the N-sulfopropylated b-PEI (PS-PEI) derivatives was influenced by the degree of sulfopropylation, concentration, molecular weight, ionic strength, and pH. In particular, the phase transition temperature of PS-PEI was strongly dependent upon pH. As the pH decreased, the protonable amine residues of PS-PEI became positively charged and interacted strongly with the negatively charged sulfonate residues, exhibiting the increase of the UCST phase transition temperature. (c) 2013 Elsevier Ltd. All rights reserved.