Poly(styrene)-poly(lactide) (PS-PLA), poly (tert-butyl styrene)-poly(lactide) (PtBuS-PLA) diblocks, and poly(tert-butyl styrene)-poly(styrene)-poly(lactide) (PtBuS-PS-PLA) segmented and tapered triblocks of controlled segment lengths were synthesized using nitroxide-mediated controlled radical polymerization. Well-defined PLA-functionalized macromediators derived from hydroxyl terminated TEMPO (PLA(T)) of various molecular weights mediated polymerizations of the styrenic monomers in bulk and in dimethylformamide (DMF) solution at 120-130 degrees C. PS-PLA and PtBuS-PLA cliblocks were characterized by narrow molecular weight distributions (polydispersity index (M-w/M-n) < 1.3) when using the PLA(T) mediator with the lowest number average molecular weight M-n = 6.1 kg/mol while broader molecular weight distributions were exhibited (M-w/M-n = 1.47-1.65) when using higher molecular weight mediators (M-n = 7.4 kg/mol and 11.3 kg/mol). Segmented PtBuS-PS-PLA triblocks were initiated cleanly from PtBuS-PLA diblocks although polymerizations were very rapid with PS segments similar to 5-10 kg/mol added within 3-10 min of polymerization at 130 degrees C in 50 wt % DMF solution. Tapering from the PtBuS to the PS segment in semibatch mode at a lower temperature of 120 degrees C and in 50 wt % DMF solution was effective in incorporating a short random segment of PtBuS-ran-PS while maintaining a relatively narrow monomodal molecular weight distribution (M-w/M-n approximate to 1.5). (C) 2008 Wiley Periodicals, Inc.