The similarities and differences between sulfonyl chloride and alkyl halide initiators for metal-catalyzed living radical polymerizations are discussed. The differences in the rates of formation, reactivities, and reactions of primary radicals derived from sulfonyl halides and alkyl halides demonstrated the design principles for mono-sulfonyl and multisulfonyl chlorides that provided quantitative initiation and higher rates of initiation than of propagation. Multifunctional initiators with two, three, four, six, and eight sulfonyl chloride groups that produced perfect star polymers in 95% conversions were designed and synthesized on the basis of these principles.