Atom transfer radical polymerization (ATRP) of styrene (St) and methyl methacrylate (MMA) initiated by various alkyl diethyldithiocarbamate (DC) initiators was successfully carried out in the presence of copper catalysts with nitrogen-based ligands. Well-controlled polymerizations with narrow molecular weight distribution (M-w/M-n < 1.1 (St) and M-w/M-n < 1.2 (MMA)) were achieved in both polymerizations. The polymerization rate followed first-order kinetics with respect to monomer conversion, and the molecular weight of the polymers increased linearly Lip to high conversion. Initiation efficiency of both polymerizations was strongly dependent on the structure of a DC. The results of H-1 NMR analysis of low-mass model compounds and chain extension confirmed that well-defined polystyrene bearing a DC group as the active chain end was obtained via ATRP of St with a DC initiator. Ligand structure and ligand/copper ratio also strongly affected the degree of control attained in the polymerization. Activation rate constants and equilibrium constants of ATRP with DC initiators and copper complexes were determined. The results of cyclic voltammetry with the (CuDC2)-D-II complex indicated that it has more negative reduction potential and, consequently, higher (pseudo)halidophilicity than those of (CuBr)-Br-II, or (CuCl2)-Cl-II with the same ligand Me6TREN.