The reaction mechanism of living radical polymerization using organic catalysts, a reversible complexation mediated polymer experiments. The studied catalysts are tetramethylguanidine (TMG), ization (RCMP), was studied using both theoretical calculations and triethylamine (TEA), and thiophene. Methyl 2-iodoisobutyrate (MMA-I) was used as the low-molar-mass model of the dormant species (alkyl iodide) of poly(methyl methacrylate) iodide (PMMA-I). For the reaction of MMA-I with TEA to generate MMA(center dot) and I-center dot-TEA radicals (activation process), the Gibbs activation free energy for the inner-sphere electron transfer mechanism was calculated to be 39.7 kcal mol(-1), while the observed one was 25.1 kcal mol(-1). This difference of the energies suggests that the present RCMP proceeds via the outer-sphere electron transfer mechanism, i.e., single-electron transfer (SET) reaction from TEA to MMA-I to generate MMA(center dot) and I-center dot-TEA radicals. The mechanism of the deactivation process of MMA(center dot) to generate MMA-I was also theoretically studied. For the studied three catalysts, the theoretical results reasonably elucidated the experimentally observed polymerization behaviors.