The investigation of polymer brushes grafted from layered clay nanoparticles adsorbed on flat surfaces is reported. The protocol involves adsorption of clay nanoparticle layers on self-assembled monolayer-modified flat surfaces of Si wafers (SiOx) and Au-coated glass. Organic cation free radical initiators were then adsorbed electrostatically onto the nanoparticle layer providing functionality for free radical surface-initiated polymerization (SIP). In this manner, grafting of the polymer from clay nanoparticle surfaces was observed in situ as compared to SIP procedures using particle dispersion in solution or in bulk. Surface sensitive spectroscopic and microscopic analytical techniques were used to characterize these polymer brushes. A comparison is made on similar free radical SIP protocols where the polymer was grafted directly from flat SiOx, and Au surfaces. Important issues on initiator density, substrate effects, and initiator stability are discussed with respect to polymer brush molecular weight, conformation, and grafting density. The protocol provides a general procedure for preparing model substrate surfaces to investigate SIP mechanism on particle and nanoparticle surfaces.