Ring-opening polymerization of epoxidized soybean oil (ESO) catalyzed by boron trifluoride diethyl etherate (BF3·OEt2), in liquid carbon dioxide, was conducted in an effort to develop useful biobased biodegradable polymers. The resulting polymers (RPESO) were characterized by FTIR spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), 1H NMR, 13C NMR, solid state 13C NMR spectroscopies and gel permeation chromatography (GPC). The results indicated that ring-opening polymerization of ESO occurred at mild conditions, such as at room temperature, and a subcritical CO2 pressure of 65.5 bar. The formed RPESO materials were highly crosslinked polymers. The glass transition temperatures of these polymers ranged from −11.9 °C to −24.1 °C. TGA results showed that the RPESO polymers were thermally stable at temperatures lower than 200 °C and significant decomposition mainly occurred above 340 °C.