Three tetrafunctional bromoxanthate agents (Xanthate(3)-Br, Xanthate(2)-Br-2, and Xanthate-Br-3) were synthesized. Initiative atom transfer radical polymerizations (ATRP) of styrene (St) or reversible addition fragmentation chain transfer (RAFT) polymerizations of vinyl acetate (VAc) proceeded in a controlled manner in the presence of Xanthate3-Br, Xanthate(2)-Br-2, or Xanthate-Br3, respectively. The miktoarm star-block copolymers containing polystyrene (PS) and poly(vinyl acetate) (PVAc) chains, PSn-b-PVAc4-n (n 1, 2, and 3), with controlled structures were successfully prepared by successive RAFT and ATRP chain-extension experiments using VAc and St as the second monomers, respectively. The architecture of the miktoarm star-block copolymers PSn-b-PVAc4-n (n 1, 2, and 3) were characterized by gel permeation chromatography and 1 H NMR spectra. Furthermore, the results of the cleavage of PS3-b-PVAc and PVAc2-b-PS2 confirmed the structures of the obtained miktoarm star-block copolymers. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5180-5188, 2010