Abstract: A closed-loop flow system was developed and used to study semicontinuous emulsion polymn. reactions by ESR . Latex was continuously circulated during a polymn. from a custom reactor through the ESR cavity equipped with a customized flat cell and then back to the reactor. The utility of this system was illustrated by following the propagating free radical concentration. in a semicontinuous emulsion polymn. of Me methacrylate at 50°C. The experimental results showed that the average number of free radicals per particle increased during the reaction due to a steady accumulation of free radicals as the particle size increased, which indicated the presence of 'trapped' radicals. The relatively small increase in radical concentrations. based on polymer volume suggested that a radical required a min. polymer volume to remain unterminated.