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Core-shell polymers of organic/organic pair that has two different properties within a particle were prepared by a sequential emulsion polymerization of methyl methacrylate (MMA), styrene (ST), and ethyl acrylate (EA) by using an water-soluble initiator in the presence of an anionic surfactant. The characteristics and structures of these core-shell polymers were evaluated. In organic/organic core-shell polymerization, the pre-emulsion method, which minimized required quantity of surfactant, has been used to increase the conversion rate and the stability of core-shell polymer particle as well as to reduce the formation of secondary particle that cause problems of soap-free emulsion during shell polymerization. Therefore, PMMA/PS, PS/PMMA and PS/PEA core-shell latex particles were prepared by a monomer pre-emulsion using sodium dodecyl benzene sulfonate (SDBS), which proceeded by a two-stage emulsion polymerization with ammonium persulfate as an initiator after the seed emulsion polymerization. We used several methods to observe the core-shell structure. The core-shell structure was studied by measuring pH changes during hydrolysis by NaOH; glass transition temperature by Differential Scanning Calorimeter (DSC); morphology of latex by Transmission Electron Microscope (TEM); and change of particle size and distribution by a particle analyzer.