The synthesis of core-shell polymers of styrene and butyl acrylate by a two-stage microemulsion polymerization process is reported. The microemulsion polymerization was optimized by adding more monomer in a semicontinuous fashion to the latex resulting from the polymerization of the parent microemulsion to produce a high-solid-content polystyrene latex (ca. 40% solids) with small particle size (< 30 nm) and high molar mass (> 2 x 10(6) g/mol). Core-shell polymers were characterized by transmission electron microscopy, quasielastic light scattering, IR spectroscopy, differential scanning calorimetry, and dynamic mechanical thermal analysis. The effect of adding a functional monomer (itaconic acid) on the mechanical properties (Young modulus, ultimate elongation, and hardness) of the microemulsion-made structured polymers is also reported. The core-shell polymers synthesized by microemulsion polymerization are more rigid and harder than core-shell polymers of similar composition made by emulsion polymerization. The presence of the functional monomer improves even more the mechanical properties of these materials.