In this research, thermoresponsive copolymer latex particles with an average diameter of about 200-500 nm were prepared via surfactant-free emulsion polymerization. The thermoresponsive properties of these particles were designed by the addition of hydrophilic monomers [acrylic acid (A-A) and sodium acrylate (SA)] to copolymerize with N-isopropylacrylamide (NIPAAm). The effects of the comonomers and composition on the synthesis mechanism, kinetics, particle size, morphology, and thermoresponsive properties of the copolymer latex were also studied to determine the relationships between the synthesis conditions, the particle morphology, and the thermoresponsive properties. The results showed that the addition of hydrophilic AA or SA affected the mechanism and kinetics of polymerization. The lower critical solution temperature (LCST) of the latex copolymerized with AA rose to a higher temperature. However, because the strong hydrophilic and ionic properties of SA caused a core-shell structure, where NIPAAm was in the inner core and SA was in the outer shell, the LCST of the latex copolymerized with SA was still the same as that of pure poly(N-isopropylacrylamide) latex. It was concluded that these submicrometer copolymer latex particles with different thermoresponsive properties could be applied in many fields. (c) 2005 Wiley Periodicals, Inc.