The polymerization of styrene oxide was carried out at 20 degrees C in chloroform with an acid-exchanged montmorillonite as acid solid ecocatalyst (Mag-H+). The effect of the amount of catalyst, solvent, and concentration of monomer on yield and molecular weight of polymer was studied. A typical reaction product (PSTO) was analyzed by infrared and nuclear magnetic resonance spectroscopy, as well as by gel-permeation chromatography and MALDI-TOFMS. The mechanism of the polymerization appears to be cationic. (c) 2006 Wiley Periodicals, Inc.