There is a controversial debate about the mechanism of the Cu(0)-catalyzed radical polymerization. Herein, a comparative analysis of a series of reactions catalyzed by different valent copper shows that the induction period and the subsequent autoaccelerated polymerization of a Cu(0)/Me6TREN-catalyzed system originate from the accumulation of soluble copper species, and Cu(I) is still a powerful activator under its disproportionation favored conditions.