Dicarboxylic esters, acidolysis

Liquid crystal polyesters are made by a different route. Because they are phenolic esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenolic hydroxyl groups are acylated with a lower aliphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxylic acid, sometimes in the presence of a catalyst. The phenolic polyester forms readily as the volatile lower acid distills from the reaction mixture. Many liquid crystal polymers are derived formally from hydroxyacids (97,98) and their acetates readily undeigo self-condensation in the melt, stoichiometric balance being automatically obtained. 

The most important method used in the preparation of polyamides is direct amidation, usually through the intermediate formation of a salt of the diamine and dicarboxylic acid, but without it in the case of aminoacids or for pairs of monomers that do not readily form a salt. Esters can react with diamines to form polyamides with liberation of alcohol or phenol. Diamines can be reacted with diamides yielding polyamides and freeing ammonia. Polyamides have been prepared by acidolysis of acyl derivatives of diamines (compare Section 5.4 for acidolysis in polyester preparation). Bis-anhydrides react with diamines to form polyamides and, if reacted further, polyimides. The low-temperature reaction of acid chlorides with diamines has been used, interfacially or as a solution technique, to prepare certain polyamides (compare Section 5.7 for related reactions in polyester synthesis). 

Polyesterification by acidolysis ester exchange is also a useful synthetic method, for example, reaction of the diacetate ester of a dihydric phenol with a dicarboxylic acid in the presence of a catalyst to eliminate acetic acid. 

Ester interchange reactions in the melt at high temperatures of either of two different types (a) reaction of a diphenyl ester of the aromatic dicarboxylic acid monomer with the difunctional phenol monomer to liberate phenol, which is removed under vacuum, or (b) reaction of a diacetate ester of the difunctional phenol monomer with the dicarboxylic acid monomer by an acidolysis reaction to liberate acetic acid, which is readily removed under vacuum... 

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