Effects of Pressure and Temperature on the Kinetics of<scp>l</scp>-Lactide Polymerization in Supercritical Chlorodifluoromethane

Abstract

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 °C. 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.