Cross-linking polymerization of acrylic acid in supercritical carbon dioxide (scCO(2)) was studied in a batch reactor at 50 degrees C and 207 bar with either triallyl pentaerythritol ether or tetraallyl pentaerythritol ether as the cross-linker and with 2,2-azobis(2,4-dimethyl-valeronitrile) as the free radical initiator. All polymers were white, dry, fine powders. Scanning electron microscopy showed that the morphology of the polymer particles was not affected by cross-linking. As the cross-linker concentration was increased, the polymer glass transition temperature first decreased, then increased. Water-soluble and water-insoluble polymers were synthesized by adjusting the cross-linker concentration. Viscosity measurements showed that the polymer thickening effect strongly depended on the degree of cross-linking. Finally, cross-linking polymerization of acrylic acid in scCO(2) was carried out in a continuous stirred tank reactor. The use of cross-linker decreased the monomer conversion in this system. (c) 2006 Elsevier Ltd. All rights reserved.