The polymerization of vinyl monomers with a binary system composed of N-chlorosuccinimide (NCS) and some Lewis acids was investigated by means of kinetic and spectral determinations. It was found that the NCS-ZnCl2system was the most effective as an initiator of radical polymerization of methyl methacrylate, and this polymerization was initiated by a radical produced via the reaction of NCS and ZnCl2, and terminated bimolecularly. By applying a spin trapping technique to the reaction of NCS with ZnCl2, it was shown that the initiating radical was N-succinimidyl radical which was obtained through a homolysis at the N-Cl bond.