Effects of pressure and temperature on the kinetics of L-lactide polymerization in supercritical chlorodifluoromethane

The ring-opening polymerization of L-lactide initiated by 1-dodecanol/stannous 2-ethylhex-anoate (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 cm³/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.