The oligomerisation reaction of the lactic acid is a polycondensation, also named a polyesterification reaction.

In the case of lactic acid, because the molecule is bifunctional (it is a hydroxy acid), it will be the sole reagent of the polycondensation.

Lactic acid monomers will react together to form longer chains of oligomers by eliminating the water molecules.

The polymerisation of lactic acid leads to linear chains of oligomers with an average molecular weight that depends on process conditions.

The general lactic acid polycondensation process looks like this:

To obtain oligomers of lactic acid of sufficient molecular weight it is important to apply the process conditions that facilitate the elimination of water molecules. However, in the oligomerisation reaction of the lactic acid, the cyclic dimer of the acid – lactide – may also be formed. There is therefore competition between the oligomerisation reaction and the lactide formation reaction.

As for all polymerisations, polycondensation leads to highly viscous mixtures but through rigorous parameterisation of the polycondensation it is possible to obtain oligomers with a wide range of molecular weight (size of chains) . This characteristic consequently induces different product viscosities, a particularly useful property in the applications of such product.

PLA of high molecular weight is generally produced by lactide ring opening polymerisation (ROP). The average molecular weight of the PLA obtained is directly related to the molar ratio of the initiator (introduced into the reaction environment). It is therefore possible to synthesise oligomers and/or lactide polymers of variable molecular weight by controlling the initiator content.

Depending on the initiator used it is possible to obtain linear or branched oligomers.

This synthesis method is used to obtain lactide oligomers:

• Specified by a molecular weight varying from 1,000 to 5,000 daltons (PS eq.)
• With a more finely controlled molecular weight
• Able to offer a linear or branched structure
• With one or more functionalisation options