Conventional free-radical copolymerization of acrylonitrile (AN) and styrene (St) was realized in room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) under mild conditions. The copolymerization in RTILs was more rapid than that in traditional solvent DMF. Poly(styrene-co-acrylonitrile) (SAN) prepared in RTILs had higher molecular weight than that prepared in DMF or by bulk copolymerization. SAN with bimodal molecular weight distribution (MWD) were obtained in most of the reaction conditions in [Bmim][BF4] and some conditions in [Bmim][PF6]. By the analysis of reaction phenomena and fluorescence behavior, the reason of the difference in MWD could be attributed to the difference of reaction system compatibility mainly caused by the immiscibility of macromolecule with RTIL. (c) 2006 Wiley Periodicals, Inc.