A polymer chain transfer agent was synthesized by the reaction between poly (vinylbenzyl chloride) latex particles and 2-aminoethanethiol in a basic environment. In subsequent emulsion polymerization reactions, low molecular weight species were formed when water-born oligomeric radicals diffused to the surface of these seed particles. These low molecular weight oligomers were separated by membrane filtration and their composition and molecular weight were analyzed by FTIR and mass spectroscopy. The measured composition results were compared with those that were calculated from the copolymerization equation. The molecular weights were compared with earlier experimental results that were obtained by isolation of oligomers formed when a water-soluble inhibitor was added to a reacting emulsion. Three seeded emulsion copolymerization systems, that is, styrene-acrylic acid, styrene-methacrylic acid, and styrene-methyl methacrylate, were investigated. The distribution of monomer in the water phase and in the copolymer particles was analyzed. The results show that the oligomer compositions for different copolymerization systems can be approximated reasonable well by the copolymerization equation, using the reactivity ratios obtained from bulk copolymerization. The length of the oligomer radicals formed depends on their composition and the properties of the polymer particles, such as surface charge, composition, size, and concentration.