Ring-Opening Metathesis Polymerization (ROMP) and Atom-Transfer Radical Polymerization (ATRP) were carried out simultaneously under microemulsion conditions with 1st generation Grubbs catalyst playing a double role: ROMP initiator and ATRP controlling agent. Starting from two distinct microemulsions-one containing the monomers and the other the catalyst-well-defined composite nanoparticles with z-average hydrodynamic diameter smaller than 50 nm were prepared. Depending on the monomers droplet composition, homopolymers blends or graft-copolymers were synthesized in a one-pot, one-step, one-catalyst approach. (C) 2009 Wiley Periodicals, Inc. J Polyin Sci Part A: Polyin Chem 47: 4014-4027, 2009