Highly active catalysts mediating atom transfer radical polymerization (ATRP) at low concentrations, therefore requiring no postpolymerization catalyst removal, are highly desirable for wide commercial applications of ATRP. We previously reported that CuBr ligated by a hexadentate ligand, N,N, N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), was highly active in ATRP, but in solution the activator existed as a binuclear [Cu2Br2(TPEN)] complex and had to rearrange to form it more active mononuclear pentadentate active center for catalysis. Herein, we prepared N,N,N'-tris(2-pyridylmethyl)-N'-methylethylenediamine (TPMEN), a pentadentate ligand equivalent to a TPEN with one pyridyl group removed. and used it as a ligand for copper halides to understand the effect of the catalyst Structure oil its catalysis in ATRP. TPMEN formed mononuclear complexes with both copper(I) and copper(II) halides. Cyclic voltammograms (CV) results showed that Cu-I/TPMEN-Cu-II/TPMEN couples had a more negative redox potential than that of those ligated by TPEN. (CuBr)-Br-I (or Cl)/TPMEN indeed showed a higher catalytic activity at low temperatures in ATRP of MA than copper halide/TPEN. However, they could not polymerize MA well at room temperature or higher because of their high reactivity toward the initiator (persistent radical effect). In the presence of a reducing agent, the catalyst mediated well-controlled activator-regenerated electron transfer (ARGET) ATRP of MA at catalyst/initiator molar ratios of 0.003-0.01 (22-75 ppm). Self-extension and block copolymerization of PMA confirmed the livingness of the polymerization.