A series of well-defined three-arm star poly(epsilon-caprolactone)-b-poly(acrylic acid) copolymers having different block lengths were synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). First, three-arm star poly(epsilon-caprolactone) (PCL) (Mn = 24907830 g mol1; Mw/Mn = 1.191.24) were synthesized via ROP of epsilon-caprolactone (epsilon-CL) using tris(2-hydroxyethyl)cynuric acid as three-arm initiator and stannous octoate (Sn(Oct)2) as a catalyst. Subsequently, the three-arm macroinitiator transformed from such PCL in high conversion initiated ATRPs of tert-butyl acrylate (tBuA) to construct three-arm star PCL-b-PtBuA copolymers (Mn = 10,90019,570 g mol1; Mw/Mn = 1.141.23). Finally, the three-arm star PCL-b-PAA copolymer was obtained via the hydrolysis of the PtBuA segment in three-arm star PCL-b-PtBuA copolymers. The chain structures of all the polymers were characterized by gel permeation chromatography, proton nuclear magnetic resonance (1H NMR), and Fourier transform infrared spectroscopy. The aggregates of three-arm star PCL-b-PAA copolymer were studied by the determination of critical micelles concentration and transmission electron microscope. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013, 51, 1969-1975