This article reports on the synthesis of a new pH-sensitive amphiphilic A(2)B mikto-arm star-shaped aliphatic copolyester [with A = poly(epsilon-caprolactone) and B tertiary amine-bearing poly(epsilon-caprolactone)] with two hydrophobic arms and one hydrophilic arm when protonated at pH = 5.5. First, the ring-opening polymerization of epsilon-caprolactone (epsilon CL) was initiated by an aliphatic diol substituted by an alkyne. The copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) was use to convert the alkyne into a hydroxyl group prone to initiate the ring-opening copolymerization of gamma-bromo-epsilon-caprolactone (gamma Br epsilon CL) and epsilon CL. After the substitution of the bromide atoms into azide functions, the N,N-dimethylprop-2-yn-1-amine was grafted onto the azide bearing PCL arm by CuAAC, with the purpose to make the B arm hydrophilic at low pH. The precursors of the A(2)B copolymers were characterized by H-1 NMR, SEC, and MALDI-TOF. As expected, the A(2)B copolyester was soluble into water at pH = 5. The formation of polymersomes in water at pH 5 was assessed by DLS and TEM analyses. The effects of the architecture and the molecular weight of the A(2)B copolymers on the formation of polymersomes were investigated. Moreover, the versatility of our approach was demonstrated by the synthesis of an AB(2) star-shaped copolyester. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1552-1563, 2011