Molecular weight Due to common side-reactions in a condensation polymerization (such as, incomplete reaction, the presence of water etc.), only low molecular weight polymers are obtained. So low molecular weight polyester-prepolymer is first formed from hydroxy acid monomers, and then the prepolymer is copolymerized with a monomer which reacts with its end groups in order to form a polymer having a high molecular weight . For example reaction with succinic anhydride. As a reactor, a 2 liter “Rotavapor”-type rotatory evaporator was employed. L-lactic acid, succinic can hydride and stannous octoate were introduced into the reactor. Dry nitrogen was fed below the liquid surface of the reaction mixture and the reactor was evacuated to an absolute pressure of 230 mbar. The reaction vessel was partly immersed into an oil bath having a temperature of 160°C, and the reaction mixture was mixed by starting the rotation at a speed of 100 rpm. The temperature of the oil bath was increased steadily at a rate of 20 °C/h up to a temperature of 200 °C., and further on at a rate of 5 °C /h to a temperature of 210 °C. Then polymerization was continued for a further 20 hours in the oil bath at a temperature of 210° C., the total polymerization time amounting to 24 h. The pressure was lowered after one hour of polymerization to a pressure of 170 mbar and then with one hour-intervals to 130 mbar, 100 mbar, 80 mbar, 60 mbar, 50 mbar and 40 mbar. The pressure was 40 mbar at the end of the polymerization. During the whole polymerization dry nitrogen was bubbled below the surface of the reaction mixture. The condensed water formed during polymerization was recovered as it was formed.