Composite latex particles based on homopolymers and graft-copolymers composed of polynorbornene (PNB) and poly(tert-butyl acrylate) (PtBA) were synthesized in microemulsion conditions by simultaneous combination of two distinct methods of polymerization: Ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP). Only one commercial compound (first generation Grubbs catalyst) was Lied to initiate the ROMP of norbornene (NB) and activate the ATRP of tert-butyl acrylate (tBA). Well-defined nanoparticles with hydrodynamic diameters smaller than 50 nm were prepared with original morphologies depending on the monomer compositions, the type of combination (polymer blend or graft-copolymer), and the conditions of microemulsion polymerizations.