The atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate with differently substituted model and macroinitiators was performed, in order to evaluate qualitatively the structural features which affect the efficiency of the initiation step. It was found that substituents which stabilize radicals enhance the dissociation of the halogen atom and lead to high initiation efficiencies of the model initiators and macroinitiators. The phenyl group and the oxycarbonyl group result in the efficient initiators la and 3a for the initiation of both styrene and methyl methacrylate. Further, it was shown that the poly(ethylene oxide) moiety in the initiator, because of its low solubility, e.g. in styrene/poly(styrene) mixtures, leads to phase separation that effects the reactivity of the initiating group.