A mechanistic comparison of the ATRP and SET-LRP is presented. Subsequently, simulation of kinetic experiments demonstrated that, in the heterolytic outersphere single-electron transfer process responsible for the SET-LRP, the activation of the initiator and of the propagating dormant species is faster than of the homolytic inner-sphere electron-transfer process responsible for ATRP. In addition, simulation experiments suggested that in both polymerizations the rate of deactivation is similar. In SET-LRP, the Cu(II)X-2/L deactivator is created by the disproportionation of Cu(I)X/L inactive species, while in ATRP its concentration is mediated by the bimolecular termination. The combination of higher rate of activation with the creation of deactivator via disproportionation provides, via SET-LRP, an ultrafast synthesis of polymers with very narrow molecular weight distribution at room temperature. SET-LRP is mediated by a catalytic amount of Cu(O), and under suitable conditions, bimolecular termination is virtually absent. Kinetic and simulation experiments have also demonstrated that the amount of water available in commercial solvents and monomers is sufficient to induce the disproportionation of Cu(I)X/L into Cu(O) and Cu(II)X-2/L and, subsequently, to change the polymerization mechanism from ATRP to SET-LRP. (c) 2006 Wiley Periodicals, Inc.