The ring-opening polymerization of L-lactide initiated by 1-dodecanol/stannous 2-ethylhexanoate (DoOH/Sn(Oct)(2)) was carried out in supercritical chlorodifluoromethane at pressures ranging from 180 to 360 bar and at temperatures ranging from 100 to 130 degreesC. Increasing the pressure resulted in an accelerated polymerization rate. The activation volume for [DoOH](0)/[Sn](0) = 0 was determined to be -50 cm(3)/mol. The partial bond formation in the transition state contributed to the negative value of the activation volume and the pressure-induced acceleration in the rate of L-lactide polymerization, i.e., by producing a four-center transition state and a lower partial molar volume. A further stabilization of the transition state in terms of the activation volume of -61 cm(3)/mol by the use of DoOH ([DoOH](0)/[Sn](0) = 1.0) can be explained by sterically crowded transition state. A computational study using semiempirical methods showed the transition state is subject to weak electrostriction effects. The observed activation energy (88.7 kJ/mol), which is higher than that in bulk, reflects a partial blocking of the active coordinate site by chlorodifluoromethane.