Abstract: The enthalpies of formation of lactide,lactic acid oligomer and lactic acid high polymer were measured indirectly by measuring combustion heat.The standard entropies of three compounds were estimated.Based on the data of enthalpies of formation and standard entropies,the Gibbs free energies and equilibrium constant of three reactions of the direct polymerization of lactic acid,the formation of lactide and the ring opening polymerization of lactic,were calculated.It was concluded that the first and the secon...