Polymerbrushesgrafted to the surface of silicananoparticles were fabricated by atom‐transfer radical polymerization (ATRP) and investigated as catalysts in the cleavage of phosphodiesters. The surfaces of silicananoparticles were functionalized with an ATRP initiator. Surface‐initiated ATRP reactions, in varying proportions, of a methacrylate moiety functionalized with a phenylguanidine moiety