THERMODYNAMICS OF L-LACTIDE POLYMERIZATION - EQUILIBRIUM MONOMER CONCENTRATION

Thermodynamics of L-lactide [(3S,6S)-3,6-dimethyl-l,4-dioxacyclohexane-2,5-dione] polymerization in 1,4-dioxane solvent was studied. The equilibrium concentration of L-lactide, measured by gel permeation chromatography, decreased from 0.15 mol-LT1 at 406 K to 0.06 mol-LT1 at 353 K. The standard thermodynamic parameters, determined on the basis of the temperature dependence of the L-lactide — poly(L-lactide) equilibrium position, were as follows: enthalpy, Aff°88 = -22.9 kJ-πιοΓ1; entropy, AS°S8 = -41.1 J-mor’-K-1 (at 1 mol-LT1). Negative values of AH°as and AS°sa lead to the existence of ceiling transition temperature, Tc = 914 K (value computed for the bulk polymerization). The relatively high value of polymerization enthalpy compared with that of other six-membered monomers was attributed to the bond oppositions and angle distortions in the skew-boat conformation of the L-lactide molecule. © 1990, American Chemical Society. All rights reserved.