A series of well-defined triblock copolymers containing middle soft poly(n-butyl acrylate) (PBA) block and outer hard blocks of poly(alpha-methylene-gamma-butyrolactone) homopolymer (PMBL) or random poly (alpha-methylene-gamma-butyrolactone)-r-poly(methyl methacrylate) copolymer (PMBL-r-PMMA) were synthesized by atom transfer radical polymerization (ATRP). Phase separated morphologies of cylindrical or spherical hard block domains arranged in the soft PBA matrix were observed by atomic force microscopy and small-angle X-ray scattering. The mechanical and thermal properties of the copolymers were thoroughly characterized and their thermoplastic elastomer behavior was Studied. Dynamic mechanical analysis (DMA) showed for all PMBL-b-PBA-b-PMBL copolymers a very broad rubbery plateau range extending up to temperatures of 300 degrees C. Replacement of the PMBL hard block with the less brittle PMBL-r-PMMA resulted in an improvement of the tensile properties, without compromising the very good thermal stability of the materials. (C) 2009 Elsevier Ltd. All rights reserved.