The application of catalytic chain transfer in the emulsion homopolymerization of methyl methacrylate and copolymerization of methyl methacrylate and styrene has been examined. [Bis[mu-[(2,3-butanedione dioximato)(2-)-O:O’]] tetrafluorodiborato(2-)-N,N’,N ",N"’]cobalt (COBF, 1) has been used as a low-spin cobalt(II) catalytic chain transfer agent. The number-average molecular weight of poly(methyl methacrylate) is decreased from over 120 000 in the absence of catalytic chain transfer agent to less than 3000 on addition of 36 ppm of 1 remaining constant throughout the reaction while the polydispersity index was maintained at approximately equal to 2. The mechanism is essentially the same as in solution or bulk with very small amounts of primary radical initiation, undetectable by matrix-assisted laser desorption ionization time of flight mass spectrometry, and vinyl unsaturation at the terminal end of each polymer molecule. Copolymerization of methyl methacrylate with styrene has also been demonstrated. Catalytic chain transfer polymerization has been shown to be an extremely effective method of controlling molecular weight under emulsion conditions. The effects of catalytic chain transfer agent level, feed conditions, and styrene level are presented. The behavior of catalytic chain transfer in emulsion proves to be more complicated than in solution, due to partitioning of the catalytic chain transfer agent in the compartmentalized system. COBF partitions almost equally between the organic and aqueous phases. The feed profile of each reaction ingredient is important, with the best results being obtained when catalyst is fed continually as a solution in the monomer feed. A critical level of catalyst is apparent which is shown to be related to the number of catalyst molecules per particle.