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Surface-initiated ethylene “l(fā)iving” polymerization with covalently tethered Pd–diimine catalysts represents a novel technique for covalent surface functionalization of silica nanoparticles with polyethylene (PE) brushes. In this paper, we report on the successful tuning of various structural parameters of PE brushes in this surface-initiated polymerization technique, including brush length, density, and topology. To control/reduce the brush density, the density of the surface-tethered acryloyl groups for catalyst immobilization is adjusted by using mixed silane agents comprised of effective 3-acryloxypropyltrichlorosilane and inert ethyltrichlorosilane at different compositions in the surface functionalization step, which in turn adjusts the density of immobilized catalysts for rendering PE brushes. This approach gives rise to low-polydispersity PE brushes of controllable densities at the polymerization condition of 27 atm/5 °C: 0.022–0.055 chains per nm2 on a precipitated silica (Silica-I) and 0.07–0.17 chains per nm2 on a fumed silica (Silica-II). The length of PE brushes is controlled by adjusting the polymerization time, with the highest brush length of about 45 kg mol?1 achieved at 6 h of polymerization at 27 atm/5 °C. Unlike the linear brushes with short branch structures obtained at 27 atm/5 °C, hyperbranched PE brushes with compact topology are obtained at 1 atm/25 °C, benefiting from the chain walking mechanism of the Pd–diimine catalyst. The PE-grafted silicas of varying brush density and length are subsequently used as nanofillers to construct polymer nanocomposites with an ethylene–α-olefin copolymer as the matrix polymer. The effects of brush length and density on the nanofiller dispersion and physical properties of the composites are examined. This represents the first study on polyolefin composites containing silica nanoparticles grafted with polyolefin brushes as nanofillers.