The modification of the surface of nanoparticles with polymeric chains is ubiquitously used to engineer the physicochemical properties of nanoparticle fillers and to enable new material technologies based on polymer hybrid materials with controlled microstructure. The tethering of particles with polymeric chains of distinct (high and low) degree of polymerization (so-called "bimodal polymer grafts") has emerged as a particularly interesting strategy to combine the synergistic benefits of dense and sparse polymer grafts (i.e., good control of particle interactions facilitated by densely grafted polymer chains with the high inorganic content characteristic for sparsely grafted systems). In this contribution, surface-initiated atom transfer radical polymerization (SI-ATRP) is demonstrated to be a versatile tool that enables the synthesis of bimodal graft modifications with precise control of the degree of polymerization of the respective graft species. For the particular case of polystyrene-tethered silica particles, it was demonstrated that the presence of even small fractions of "long" chains provided an order-of magnitude increase of the mechanical toughness of particle films that is comparable to values found in densely tethered particle systems only in the limit of high degree of polymerization of tethered chains (and corresponding low inorganic content).