Lactide is the cyclic di-ester of lactic acid, i.e., 2-hydroxypropionic acid. Lactic acid can not form a lactone as other hydroxy acids do because the hydroxy group is too close to the carboxylic group. Instead, lactic acid first forms a dimer, which is similar to a 5-hydroxyacid. The dimer contains a hydroxy group at a convenient distance from the carboxylic group for the formation of a lactone. Indeed, the dimer readily forms a six-membered cyclic diester known as lactide.
In general, a lactide is the cyclic diester, i.e., the di-lactone of two molecules of any 2-hydroxycarboxylic acid. It can be polymerized to polylactide using suitable catalysts, with either syndiotactic or a heterotactic stereocontrol, to give materials with many useful properties.
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