1 сентября, 2015 
Malyar A. Reaction Between Dicarboxylic Acids or Anhydrides and Diols
The synthesis of unsaturated polyesters usually involves a bulk reaction at elevated temperatures between dibasic acids or anhydrides and diols. A general reaction scheme for maleic anhydride and 1,2-ethanediol can be illustrated as follows:
During this reaction most of the maleate groups are isomerized into fumarate groups. Since esterification is a reversible process, reaction water must be removed efficiently, especially in the last stages of the reaction, where the decrease in carboxyl group concentration is slow and the increase in viscosity is fast. These last stages are usually carried out under vacuum. However, to avoid losses of volatile reactants, an azeotropic distillation of reaction water in the presence of added organic solvent such as toluene or xylene may be used [2]. The main drawbacks of this process are the longer reaction time and the difficulty in removing the last traces of solvent. Phthalic anhydride can be used to substitute maleic anhydride partially but extensively. The reaction scheme can be represented as follows:
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