Polydimethylsiloxane-block-polystyrene-block-polydimethylsiloxane (PDMS-b-PS-b-PDMS)was synthesized by the radical polymerization of styrene using a polydimethylsiloxane-based macroazoinitiator (PDMS MAI) in supercritical CO 2 . PDMS MAI was synthesized by reacting hydroxyterminated PDMS and 4,4 -azobis(4-cyanopentanoyl chloride) (ACPC) having a thermodegradable azo-linkage at room temperature. The polymerization of styrene initiated by PDMS MAI was investigated in a batch system using supercritical CO 2 as the reaction medium. PDMS MAI was found to behave as a polyazoinitiator for radical block copolymerization of styrene, but not as a surfactant. The response surface methodology was used to design the experiments. The parameters used were pressure, temperature, PDMS MAI concentration and reaction time. These parameters were investigated at three levels (−1, 0 and 1). The dependent variable was taken as the polymerization yield of styrene. PDMS MAI and PDMS-b-PSb-PDMS copolymers obtained were characterized by proton nuclear magnetic resonance and infrared spectroscopy. The number-and weight-average molecular weights of block copolymers were determined by gel permeation chromatography.