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The homogeneous atom transfer radical polymerization (ATRP) of styrene using solubilizing 4,4‘-dialkyl substituted 2,2‘-bipyridines yielded well-defined polymers with M_w/M_n ≤ 1.10. The polymerizations exhibited an increase in molecular weight in direct proportion to the ratio of the monomer consumed to the initial initiator concentration and also exhibited internal first-order kinetics with respect to monomer concentration. The optimum ratio of ligand-to-copper(I) halide for these polymerizations was found to be 2:1, which tentatively indicates that the coordination sphere of the active copper(I) center contains two bipyridine ligands. The exclusive role for this copper(I) complex in ATRP is atom transfer, since at typical concentrations that occur for these polymerizations (≈10^-7−10^-8 M), polymeric radicals were found not to react with the copper(I) center in any manner that enhanced or detracted from the observed …