Multifunctional initiators, derived from cyclotriphosphazenes, cyclosiloxanes, and organic polyols, were used in the synthesis of styrenic and (meth)acrylic star polymers by atom transfer radical polymerization (ATRP). Conditions were identified in each system which provided linear first-order kinetics for polymers with narrow, monomodal molecular weight distributions. Molecular weight measurements relative to linear polystyrene standards showed that the star polymers had lower molecular weights than theoretically predicted. Triple detection SEC measured on poly(n-butyl acrylate) samples demonstrated that the absolute molecular weight matched the theoretical value-the smaller relative chain length was due to lower hydrodynamic volumes of the star-branched polymers relative to Linear analogues. Kinetic arguments were used to demonstrate that each alkyl halide moiety bound to the initiators was participating in ATRP. Well-defined poly(methyl acrylate) stars of molecular weights M-n > 500 000 and low polydispersity (M-w/M-n < 1.2) have been prepared. Star-block copolymers with a soft poly(methyl acrylate) core and a hard poly(isobornyl acrylate) shell were also synthesized.