The living radical polymerization of methyl methacrylate initiated from aromatic sulfonyl chlorides and catalyzed by the new catalytic systems CuSBu/bpy CuSPh/bpy and CuC=CPh/bpy (bpy = 2,2'-bipyridine) is described. For a target degree of polymerization of 200, lowering the ratio of catalyst to sulfonyl chloride group from 1/1 to 0.25/1 mol/mol decreases the values of the experimental rate constant of polymerization from 5.12 X 10(-2) 2.4 X 10(-2), and 1.87 X 10(-2) min(-1) to 1.8 X 10(-3), 4.9 x 10(-3). and 4.2 X 10(-3) min(-1) for CuSBu, CuSPh, and CuC=CPh, respectively, whereas the corresponding initiator efficiency increases from 62 to 99%. The external orders of reaction in the catalyst are 0.79 for CuSPh, 0.88 for CuC=CPh, and 1.64 for CuSBu. A mechanistic interpretation that involves the in situ generation of, most likely, the real catalyst CuCl, starting from combinations of CuSBu, CuSPh, and CuC=CPh and sulfonyl chloride or alkyl halide growing species, is suggested.