Organic/inorganic hybrid star-like nanocomposites from two different octafunctional cubic silsesquioxane (CSSQ) nano-cage cores and poly(methyl methacrylate) (PMMA) were synthesized using atom transfer radical polymerization (ATRP) at mild conditions, in which octafunctional octakis(3-hydroxypropyldimethylsiloxy)octasilsesquioxane, (OHPS) and octa(aminophenyl)silsesquioxane (OAPS) nano-cages were used as ATRP initiators. The polymerization was carried out at 50 degrees C in acetonitrile/water mixture. H-1-NMR and GPC were employed to characterize the obtained nanocomposites. GPC data revealed that the resulting nanocomposites exhibit unimodal and narrow molecular weight distributions indicating well-controlled synthesis and well-defined hybrid nanocomposites with star architecture. The influence of CSSQ nano-cages on the thermal property of nanocomposites was investigated using differential scanning calorimetry and thermal gravimetric analysis (TGA). It was observed that the nanocomposites exhibit significantly higher glass transition temperature compared with its linear counterpart because of slow relaxation caused by the star-like architecture. TGA study, however, did not reveal any significant improvement in thermal stability of nanocomposites as compared with linear PMMA. Finally, field emission scanning electron microscopy images of fractured surfaces of nanocomposite sample films showed well dispersed CSSQ nano-cages in PMMA matrix without phase separation. (C) 2007 Wiley Periodicals, Inc.