The synthesis of ultrathick concentrated poly(methyl methacrylate) (PMMA) brushes by atom transfer radical polymerization (ATRP) was investigated. The reactions were performed with a catalyst system of Cu(I)Br/dinonyl-2,2'-bipyridine (dN-bipy) and Cu(II)Br-2/dN-bipy at 60 degrees C under a high pressure of 500 MPa. The equilibrium constant for this catalyst system was determined to be 1.5 x 10(-6), which followed the kinetics study and indicated good polymerization rate control. Under the high pressure of 500 MPa, a micrometer scale thick PMMA brush was obtained. During chain growth under the high pressure, the concentration of the deactivator catalyst was demonstrated to significantly affect the graft density of PMMA brushes, which was correlated to the number of monomers added in activation-deactivation cycles. A novel "cutoff" experiment and gel permeation chromatography demonstrated similar propagation for free polymers and graft polymers even under high pressure.