Lactoyl lactic acid

Fig. 3 The relative amounts of lactide, lactic acid, lactoyl lactic acid and ethyl ester of lac- toyl lactic acid formed diming hydrolysis of PLLA in biotic and abiotic medium. Reprinted from [20] with permission of Elsevier. Elsevier (2000)...

The lactic acid molecule has a hydroxyl and an acid functional group, which may result in intermolecular and intramolecular esterification reactions. The first step is the formation of a linear dimer (lactoyl lactic acid). This condensation reaction can proceed to higher oligomers and is promoted by removal of water. Also a cyclic dimer, lactide, is formed in small amounts. Lactide can be formed by intramolecular esterification of lactoyl lactic acid or by breakdown of higher oligomers. All reactions are equilibrium reactions (Figure 1.2). 

Due to these reactions, a solution of lactic acid at equilibrium consists of monomeric lactic acid, dimeric lactic acid or lactoyl lactic acid, higher oligomers of lactic acid, and... 

Alcohols 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. 

Free acidity, for example, lactic acid or lactoyl lactic acid, slows down the rate of polymerization and limits the achievable degree of polymerization. According to... 

Migrants from PLA may include lactic acid, the linear dimer of lactic acid (lactoyl lactic acid), other oligomers of PLA ( = 3-13), and the cyclic dimer of lactic acid (lactide). Their chemical structures are shown in Figure 13.1. 

Carboxyl end group determination by the dye method was not successful with poly(caprolactone) due to the high pKg of the carboxyl groups which can be assumed to be similar to that of hexanoic acid (pK = 4.89). The pK of carboxyl groups in poly(lactide) will be con-siierably lower and very likely close to that of lactoyl lactic acid, pK 3.0 (24). 

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