Free-radical precipitation copolymerizations of acrylonitrile with methyl methaerylate and 2-chlorostyrene were carried out using AIBN as initiator at 70 degreesC and 220 bar in carbon dioxide at a total comonomer concentration of 13.6 wt%. Polymerizations were carried out without the use of stabilizers with different feed comonomer molar ratios of 100:0; 98:2; 96:4; 94:6; 90: 10; 50:50, and 0: 100 acrylonitrile: methyl methacrylate or acrylonitrile:2-chlorostyrene. The preliminary experiments conducted in a view cell showed that these systems that are initially homogeneous solutions undergo phase separation within about 3 h of polymerization time upon formation of polymer chains that are no longer completely miscible in the polymerization medium. Polymerizations that were carried out in a 50 mL high-pressure batch reactor for 20 h under rotational agitation of an impeller at 200 rpm resulted in end products that were either free-flowing powder, or soft solid, or gel (as observed upon depressurization and opening the reactor). The actual physical form was found to be dependent on the initial feed comonomer ratio. Homopolymerization of acrylonitrile led to powders, homopolymerization of 2-chlorostyrene resulted in a soft solid, and homopolymerization of methyl methaerylate resulted in a gel. In the copolymers, those with high acrylonitrile content resulted in powders and became gellike with increasing methyl methaerylate or soft solid with increasing 2-chlorostyrene content. Polymer yield which was about 50% in homopolymerizations was found to decrease in copolymerizations as the acrylonitrile content in the feed mixture was reduced. The number-average molecular weights of the polymers were in the range from 40 000 to 300 000, with polydispersities in the range from 1.4 to 4.8. The polymers were characterized by FTIR, DSC, TGA, H-1 NMR, and C-13 NMR, and the morphologies were evaluated with electron microscopic studies. The acrylonitrile:methyl methacrylate molar ratios in the copolymers as calculated from 1H NMR were lower than the respective feed comonomer ratios used in their polymerizations. These ratios were even lower in the acrylonitrile-2-chlorostyrene copolymers, suggesting relatively lower reactivity ratios in copolymerizations involving 2-chlorostyrene.