Binary and ternary acrylonitrile (AN) copolymers with methyl acrylate (MA) and either itaconic acid (IA, 1) or IA derivatives (monomethyl itaconate (2), monoethyl itaconate (3), IA monoamide (4), IA mono-n-octylamide (5)) were prepared in CO₂. The obtained copolymers have good morphological and molecular weight characteristics. They demonstrate broad heat evolution during thermal cyclisation according to DSC measurements; therefore, they may be used to produce carbon fibres. The polymer yield per reaction volume is 2 to 3 times higher relative to that produced in solution (in DMSO or DMF) or an aqueous suspension.