It is known that it is difficult to polymerize carboxylic acid-based monomer by atom transfer radical polymerization (ATRP) in polar solvents due to the protonation of ligand caused by acidic dissociation of the monomer. In this study, precipitation reverse ATRP of acrylic acid (AA) was carried out in supercritical carbon dioxide (scCO(2)), which is a nonpolar solvent to dissolve transition metal complexes, at 30 MPa and 45 degrees C. The polymerization proceeded smoothly and the conversion reached 86% for 3 h. After vending of scCO(2), a dry poly(acrylic acid) (PAA) powder was obtained. Weight-average molecular weight and polydispersity of the methylated PAA, which were measured by gel-permeation chromatography after methyl esterification, were 3.5 x 10(4) and 2.07, respectively, indicating that the precipitation reverse ATRP proceeded with a bad control manner. However, chain extension of the methylated PAA with styrene was possible by ATRP in a bulk system. Moreover, PAA-b-polystyrene was successfully prepared in scCO(2) directly by two-step ATRP, although its molecular weight distribution was broad. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2578-2584, 2012