The combined effects of sonochemical and conventional chemical initiation on the emulsion polymerization of methyl methacrylate (MMA) and MMA-CaCO3 systems have been studied. Combining ultrasound (US) and conventional initiation by potassium persulfate (KPS) for the MMA and MMA-CaCO3 systems, helped increase the final conversion. An increase of 15% for the MMA only system (from 72% to 87%) and of 10% for the MMA-CaCO3 system (from 76% to 86%) was observed as compared to initiation by KPS alone. Also, an increase of 18% (from 69% to 87%) and 20% (from 66% to 86%) for the MMA only and MMA-CaCO3 systems, respectively was observed for the combined initiation as compared to initiation by US alone. Although all particles synthesized were in the size range of 60-130 nm, the excellent dispersion ability of ultrasound helped to obtain narrow size distribution and smaller average sizes in both the PMMA and PMMA-CaCO3 systems. Possible mechanisms have been proposed for both the polymerization and the formation of poly-MMA-CaCO3 composite and core-shell nanoparticles taking into account the results obtained by analyzing the synthesized materials. (C) 2013 Elsevier B.V. All rights reserved.