Melt polycondensation of D,L-lactic acid: MALDI-TOF MS investigation of the ring-chain equilibrium

The melt polycondensation of D,L-lactic acid was studied by the MALDI-TOF M S method at temperatures ranging from 100 to 220 degreesC. At low temperatu re, i.e., up to 120 degreesC, linear, and at higher temperature both linear and cyclic oligomers were formed. The ring-chain equilibrium was investiga ted in detail and the concentration-based equilibrium constants (K,) and th eir dependence on the degree of polycondensation and on the temperature wer e determined. It was also found that the molecular size distribution of the linear chain oligomers obeys to "the most probable distribution". A detail ed investigation also showed that the ring size distribution of the cyclic oligomers follows the Jacobson-Stockmayer theory.