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A general procedure for batch prepolymerization is described in a patent by O'Brien et al. [75]. Typically, vacuum pressures of 70–250 mbar and temperatures up to 190°C are used to dewater lactic acid to a prepolymer with an average degree of polymerization (DP) of around 10 in a batch process time of 6 h. For lab‐scale equipment, it was also found that thin film and rotating flask vacuum equipment showed faster reaction times than a stirred tank, indicating the importance of mass transfer of water in the already viscous prepolymer.

Continuous prepolymerization has also been described in a number of patents, for example, in stirred tanks in series or in evaporator‐type equipment [68, 76, 77]. Usually patents describe prepolymers with a DP of 7–20 as feed to the lactide synthesis. Using modern HPLC methods, it has been shown that in oligomeric systems up to DP 10, an equilibrium is present with constant equilibrium constants between the oligomers [6, 72].