A supported and highly recyclable catalyst complex, CuBr/HMTETA physically adsorbed to silica gel, was used for the ATRP of MMA to elucidate the nature of the catalytic site. In some polymerizations, the reaction solutions were filtered and compared with their unfiltered references for catalytic activity. The filtered systems had high catalyst activity indicating the presence of active catalyst sites in solution. These sites are the primary catalytic contributors.