Recent advances in surface-initiated polymerization have given rise to a range of brush nanocomposites and hybrid functional materials. However, the synthesis of pure polyolefin-grafted nanocomposites by surface-initiated ring-opening metathesis polymerization (SI-ROMP) is a significant challenge due to the particle aggregation and irreversible particle coupling. This study presents a synthetic approach toward well-defined poly(cyclooctene)- and polyethylene-grafted nanoparticles by tethering Grubbs third generation catalyst on the particle surface and initiating the polymerization in a rapid manner. This work also serves as a template to prepare other hairy nanoparticles and functions as a basis toward understanding their thermomechanical behaviors.