Nitroxide-mediated controlled/living radical precipitation polymerization of styrene in supercritical carbon dioxide has been performed using SG1 and AIBN as mediator and initiator, respectively. The equivalent bulk polymerization proceeded at a rate ≈1.58 times faster than the precipitation polymerization. The number of chains increased with conversion to a similar extent in precipitation and bulk systems. A chain transfer mechanism to the Diels-Alder dimer is used to explain the increase in numbers of chains. High conversions were also achieved for a conventional precipitation polymerization of styrene in scCO2 at 110 °C. The polymer was obtained as a powder, which contained cavities when observed under an SEM.