The effect of itaconic acid (IA) as functionalizing agent on the static mechanical properties (Young's modulus, tensile stress, ultimate properties, and hardness) of core/shell polymers made of styrene and butyl acrylate by a two-stage microemulsion polymerization process is reported. High-polymer content (>35 wt%) was obtained in the first stage (seed) by adding more monomer semicontinuously to the parent microemulsion-latex; however, the characteristics of microemulsion-made latexes were preserved, that is, nanometer particles (<50 nm) and high molar masses (>2 × 106 g/mol). The IA content in the shell was varied from 0 to 20 wt% with respect to that of the polymer in the shell, either polystyrene or poly(butyl acrylate) (PBA). High conversions in both stages (>84 %), final core-shell latex with relatively high-polymer content (ca. 20 wt%), and particle with diameters smaller than 78 nm were obtained. The fact that particle size grew from the first (ca. 50 nm) to the second stages (ca. 75 nm) and that the polymers showed two glass transitions suggests that a core shell structure was obtained. The presence of the functionalizing agent (IA) modified the mechanical properties, especially when PBA was in the shell. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.