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If water is the initiator, R equals H and hydrolysis of lactide produces lactoyl lactic acid (HL₂). Propagation with lactide in the presence of a polymerization catalyst produces PLA with a hydroxyl and one carboxylic acid end group, as if the PLA was obtained by polycondensation of lactic acid.

If the hydroxyl group of lactic acid acts as an initiator, PLA with one hydroxyl end group and a lactic acid end group (HOOC─CH(CH₃)─O─C(O)─) is obtained.

If the initiator itself is polymeric in nature, for example, polyethylene glycol (PEG), lactide can polymerize from the hydroxyl end group(s) of PEG resulting in PEG–PLLA diblock or triblock copolymers.

The molar ratio of monomer to initiator (M/I)—where initiator can also be read as total hydroxyl content—basically controls the final, average molecular weight (M _n) of the PLA. A high amount of initiator produces short polymer chains, and a low amount of initiator produces high‐molecular‐weight polymer. The lower the amount of potentially initiating hydroxyls in the lactide monomer, the higher the maximum attainable degree of polymerization [69]. Since water and lactic acid can both cause ring scission of the lactide and initiate polymerization, their amounts in the lactide must be low and should be specified.