The reversible kinetics of L-lactide bulk polymerization with tin(II) ethylhexanoate was determined over a wide range of temperatures, 130-220 °C, and monomer to initiator molar ratios, 1000-80 000. Both polymerization and depolymerization are accurately described by a reversible model with a propagation term that is first order in monomer and catalyst. The activation energy of propagation is 70.9 ( 1.5 kJ mol -1 . The enthalpy, entropy, and ceiling temperature of polymerization are -23.3 ( 1.5 kJ mol -1 , -22.0 ( 3.2 J mol -1 K -1 , and 786 ( 87 °C, respectively. Crystallization increases the propagation rate and decreases the apparent monomer equilibrium in proportion to the degree of crystallinity. Natural hydroxyl impurities stoichiometrically control the polymer molecular weight but do not significantly affect the propagation rate.