Polyethylene-graft-poly(methyl methacrylate) (PE-g-PMMA) copolymers were prepared using a combination of ring-opening metathesis polymerization (ROMP), hydrogenation, and atom transfer radical polymerization (ATRP). Approximately 20 PMMA side chains per molecule with average degrees of polymerization 6, 12, and 24 were grown via a "grafting from" approach from a Br-substituted linear PE backbone with M-w = 56 000. The resulting graft copolymers were evaluated as compatibilizing agents for binary PE/PMMA homopolymer blends. The roles of PMMA side chain length and different compatibilizer concentration in the polymer blends were investigated, and the mechanical and morphological properties of blends containing 70% PE and 30% PMMA by weight were examined. The presence of the compatibilizer reduced the average PMMA droplet size substantially, even at compatibilizer loadings as low as 1%. Furthermore, the compatibilized blends exhibited significant improvements in elastic modulus, yield strength, and scratch resistance as compared to the binary blends. Adhesion testing confirmed the ability of PE-g-PMMA to act as an effective PE/PMMA adhesion promoter. Remarkably, the graft copolymer with the shortest side chains was the most effective compatibilizer. This counterintuitive result is tentatively attributed to kinetic limitations in partitioning of the graft copolymers to the interface.