The synthesis of nanosized poly(methyl methacrylate) initiated by 2,2 '-azoisobutyronitrile via differential microemulsion polymerization has been investigated. Poly(methyl methacrylate) with a molecular weight of around 1 X 10(6) and a particle size of about 20 nm was achieved under mild reaction conditions. A typical condition was that the surfactant amount required : could be as low as 1/130 of the monomer amount in weight, and the surfactant/water ratio could be as low as 1/600, which is much less than the corresponding amounts reported in the literature. "Molecular bricks", i.e., nanoparticles in which there are only one or two polymer chains, can be achieved using mild conditions by differential microemulsion polymerization, which may have potential applications for making molecular devices.