A new amphiphilic Y-shaped copolymer, comprised of hydrophobic Poly(trimethylene carbonate) (PTMC) and hydrophilic Poly(N,N-dimethylamino-2-ethyl methacrylate) (PDMAEMA), was designed and synthesized by a combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP) using a new heterofunctional initiator, Br-Init-(OH)2, bearing one initiation site for ATRP and two for ROP. At first, a new trifunctional core molecule bearing hydroxyl group and bromine moieties, Br-Init-(OH)2, was synthesized via protection followed by esterification reaction of 5-ethyl-5-hydroxymethyl-2,2-dimethyl-1,3-dioxane with 2-bromoisobutyryl bromide and deprotection. In the presence of trifunctional core molecule, Br-Init-(OH)2, target Y-shaped miktoarm star copolymers, (PTMC)(2)-b-PDMAEMA, were successfully synthesized by sequence conducting the ROP of TMC and ATRP of DMAEMA. The Y-shaped copolymers were characterized by H-1 NMR and GPC measurements. Subsequently, the self-assembly behavior of these copolymers was investigated by dynamic light scattering method and transmission electron microscopy, which indicated that these amphiphilic Y-shaped copolymers can self-assemble into micelles and possess distinct pH-dependent size in aqueous milieu. The results indicate that the amphiphilic Y-shaped copolymers had the pH-responsive properties similar to the expected PDMAEMA. POLYM. ENG. SCI., 51:776-784, 2011. (C) 2011 Society of Plastics Engineers