Surface-initiated atom transfer radical polymerization (SI-ATRP) has emerged as a powerful tool to synthesize polymer-tethered particles (here called particle brushes) that can self-assemble into hybrid materials with well-defined microstructure, morphology, and enhanced mechanical properties or optical transparency, as compared to binary particle/polymer nanocomposite materials. However, side reactions such as the thermal self-initiation (TSI) of some monomers, such as styrene, during the polymerization can result in the formation of varying amounts of homopolystyrene, in addition to particle brushes. The presence of homopolystyrene impurity reduces the predictability of properties and impedes the interpretation of structure property relations in particle brush materials. This contribution presents a systematic evaluation of the formation of TSI homopolystyrene and its implications on the properties of polystyrene-tethered silica based particle brush materials. Kinetic and molecular weight studies reveal that the fraction of untethered chains generated by TSI depends on reaction conditions and increases with the degree of polymerization of surface-tethered chains. The presence of homopolystyrene results not only in a decrease of the softening temperature but also in an increase of ductility and toughness of particle brush solids. Structural analysis of particle brush assemblies admixed with small particle tracer inclusions suggests that the increase in ductility is related to the preferential segregation of the polystyrene impurity within the corner regions of the Wigner-Seitz cell of the particle brush array.