Diblock and triblock copolymers of poly-L-lactide (PLLA) and polystyrene (PS) were synthesized and the mechanical properties of these copolymers studied. Free radical polymerization of styrene in the presence of 2-mercaptoethanol as functional chain transfer agent produced mono-functionalized PS-blocks which were used as macroinitiators in the subsequent ring opening polymerization (ROP) of L-lactide to produce the diblock copolymers. Furthermore a a-?-bishydroxyl functionalized PS-block was synthesized by RAFT, which was then engaged as bifunctional initiator for the ROP of L-lactide to provide the triblock copolymers PLLA-PS-PLLA. Through the copolymerisation and high molar masses, it was possible to achieve an improved mechanical property profile, compared with pure PLLA, or the analogous blends of PLLA and PS. A weight fraction of PS of 10-30% was found to be the optimal range for improving the heat deflection temperature (HDT), as well as mechanical properties such as ultimate tensile strength or elongation at break. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012