The feasibility of high-pressure atom transfer radical polymerization (ATRP) for synthesizing well-defined polymers of extraordinarily high molecular weights was demonstrated. ATRP of methyl methacrylate (MMA) under pressures up to 500 MPa was investigated at 60 degrees C. The addition of a small amount of a Cu(II)Cl-2/ligand complex along with the general benefits of high pressure of enhancing propagation and suppressing termination brought about an excellent control of polymerization even with an extremely low concentration of ATRP initiator. For example, there was produced PMMA with a number-average molecular weight M-n of 3.6 x 10(6) and a polydispersity index of 1.24, which had never been achieved by conventional ATRP. (c) 2008 Elsevier Ltd. All rights reserved.