The exo-substituted eta(4)-cyclopentadiene CpCo(I) complexes were found to be the actual catalyst for the cobaltocene-catalyzed ATRP. Cobaltocene is just a precatalyst, which can be converted to the catalyst by the reaction with halide initiator. As a new kind of ATRP catalyst, the exo-substituted eta(4)-cyclopentadiene CpCo(I) complexes can catalyze the living radical polymerization of MMA, styrene, and the block copolymerization of MMA with styrene. (C) 2008 Elsevier Ltd. All rights reserved.