A set of monomer miniemulsions are stabilized by a polyurethane made from polypropylene glycol and isophorone di-isocyanate and extended by butanediol, which was employed as hydrophobe, with sodium dodecyl-sulfate as surfactant. Upon polymerization they lead to polymer particles with about the same size than the initial droplets. Some homogeneous nucleation may take place when methylmethacrylate is used as monomer. The polymerization rate, and the level of monomer conversion are dependent on several parameters such as the nature and amount of the initiator, the temperature, the molecular weight and the amount of polyurethane, the amount of surfactant, and finally the composition of the monomer mixture.