A series of fluorinated diblock copolymers, consisting of styrene (St)-acrylonitrile (AN) copolymer [poly(St-co-AN)] and poly-2-[(perfluorononenyl)oxy] ethyl methacrylate, with various compositions as well as with different molecular weights were synthesized by atom transfer radical polymerization and characterized. Dispersion polymerization of acrylonitrile in supercritical carbon dioxide (scCO(2)) at 30 MPa and at 65 degrees C with this kind of amphiphilic block copolymer as a stabilizer and 2,2'-azobisisobutyronitrile as an initiator was investigated. The experimental results indicated that, in the presence of a small amount of poly(St-co-AN) (5 wt% to AN), spherical particles of polyacrylonitrile (PAN) were prepared with small diameter and narrow polydispersity (d(n) = 153 nm, d(w)/d(n) = 1.12), resulting from the high stabilizing efficiency of this fluorinated block copolymer. Furthermore, the polymerization of AN in scCO(2) under different initial pressures especially under low pressure (< 14 MPa) was studied. When the polymerization was carried out around the critical pressure Of CO2 (7.7-7.8 MPa), the PANs with high molecular weight (M-v approximate to 130,000-194,000) were synthesized at high monomer conversion (> 90%) no matter whether the stabilizer was added, compared to those synthesized by dispersion polymerization at 30 MPa. It was also found that the crystallinity of PAN synthesized at 7.7-7.8 MPa was somewhat higher than that synthesized at 30 MPa, while its crystallite size did not change. (c) 2006 Elsevier Ltd. All rights reserved.