We report the successful precipitation polymerization of acrylic acid in supercritical carbon dioxide at pressures ranging from 125 to 345 bar utilizing AIBN as a free radical initiator. Analyses by gel permeation chromatograpy (GPC) and scanning electron microscopy (SEM) indicate that for the pressure range studied there is no appreciable effect on product molecular weight, molecular weight distribution, or particle size or morphology. In addition, effective molecular weight control was demonstrated for precipitation polymerizations of acrylic acid in CO2 through the use of ethyl mercaptan as a chain transfer agent.