Homopolymerization of styrene and methyl methacrylate was carried out at 60-130 degrees C in the presence of a mono-captodatively (cd) substituted ethane bearing nitrile and ethylsulfenyl substituents on the same carbon atom. It was found that the cd-ethane accelerated both styrene and methyl methacrylate polymerizations with no induction period, but the polymerization mode of methyl methacrylate was different from that of styrene. The polymerization rate of styrene was proportional to the 0.46th power of the cd-ethane concentration. However, the cd-ethane produced a reversible radical termination in the case of methyl methacrylate. The mechanism of both polymerizations is discussed in terms of the kinetic and ESR data.