Radical copolymerization of styrene (St) and maleic anhydride (Man) was performed in supercritical carbon dioxide (SC CO2) at 50-80 degrees C with 2,2'-azoisobutyronitrile as the initiator. A novel, ultrahigh molecular mass St-Man alternative copolymers (M-w > 10(6)) has been synthesized. The NMR spectra demonstrated that the copolymers obtained possess strictly alternating structure. Aqueous solutions of the alternative copolymers exhibit lower critical solution temperatures (LCST). These LCST's are highly sensitive to changes in the molecular weight and pH. St/MAn copolymerization in SC CO2 as the solvent yielded higher molecular weight products and more cis configuration linkage of cyclic MAn units in the copolymer chains than those made in common organic solvents. These facts demonstrate that SC CO, as the solvent plays an important role in the copolymerization of styrene with maleic anhydride, originating from the special intermolecular interactions between SC CO2 and St-Man alternative copolymer.