The synthesis and characterization of thermosensitive poly(D,L-lactic-co-glycolic acid) (PLGA)-polyethylene glycol (PEG)-PLGA triblock copolymers as in situ gelling matrices were investigated. Ring-opening polymerization using supercritical carbon dioxide (scCO(2)) was utilized, and the results were compared with microwave irradiation method. The effect of parameters such as temperature, pressure, and time of synthesis was studied and optimized by central composite design. The structure of the resulting PLGA-PEG-PLGA and lactide (LA)-to-glycolide (GA) molar ratio was determined by H-1 NMR. The number average molecular weight (M-n), molecular weight (M-w) and their size distributions were both measured by gel permeation chromatography (GPC). The sol-gel transition temperature were determined by refrigerated bath circulator instrument. Maximum Mw was obtained for temperature of 144 degrees C, pressure of 367 bar, and time of 13 h. The LA/GA molar ratio matched very well with the initial ratios of the monomers that were used in the polymerization process using scCO(2).