Poly(styrene) macromercaptanes (M-n = 1900, 3600, and 6100 g mo(-1), PDI approximate to 1.2) derived from thiocarbonyl thio capped polymers prepared via reversible addition fragmentation chain transfer polymerization were employed to initiate the ring opening polymerization (ROP) of ox-lactide under conditions of organo-catalyis employing 4,4-dimethylaminopyridine. Poly(styrene)-block-poly(lactide) polymers of number average molecular weights up to 25,000 g mol(-1) (PDI approximate to 1.2 to 1.6) were obtained and characterized via multiple detection size exclusion chromatography (SEC) using refractive index as well as UV detection. In addition, diffusion ordered nuclear magnetic resonance and liquid chromatography at critical conditions (of both polystyrene as well as poly(lactide) were employed to assess the copolymers' structure. Furthermore, it was demonstrated that polyethylenes capped with a thiol moiety can also be readily chain extended in a ROP employing D,L-lactide, evidenced via NMR and high temperature SEC. This study indicates that the direct use of macromercaptantes is indeed a methodology to switch from a radical to a ROP process. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 803-813, 2011