Hybrid nanocomposites of polystyrene (PS) and methacryl phenyl polyhedral oligomeric silsesquioxane (POSS) were synthesized by reactive melt blending in the mixing chamber of a torque rheometer using dicumyl peroxide (DCP) as a free radical initiator and styrene monomer as a chain transfer agent. The effects of mixing intensity and composition on the molecular structure and morphology of the PS-POSS hybrid nanocomposites were investigated. The degree of POSS hybridization (alpha(POSS)) was found to increase with the POSS content, DCP/POSS ratio, and rotor speed. For the PS-POSS materials processed in the absence of styrene monomer, an increase in the alpha(POSS) led to a reduction in the molecular weight by PS chain scission, as a consequence of the free radical initiation. On the other hand, the use of styrene monomer as a chain transfer agent reduces the steric hindrance in the hybridization reaction between POSS and PS, enhancing the degree of POSS hybridization and avoiding PS degradation. The PS-POSS morphology consists of nanoscale POSS clusters and particles and microscale crystalline POSS aggregates. PS-POSS with higher alpha(POSS) values and lower amounts of nonbound POSS showed improved POSS dispersion, characterized by smaller interfacial thickness (t) and greater Porod inhomogeneity lengths (l(p)). The processing-molecular structure-morphology correlations analyzed in this study allow the POSS dispersion level in the PS-POSS materials to be tuned by controlling the reactive melt blending through the choice of the processing conditions. These insights are very useful for the development of PS-POSS materials with optimized performance. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 811-827, 2013