Uncatalyzed Self-Catalyzed Polyesterification

The term uncatalyzed is to be understood here to mean that the reactions are carried out in the absence of externally added strong acid catalyst. In the absence of an added strong acid catalyst, the diacid monomer acts as its own catalyst for the esterification reaction. Assuming that [H ] is proportional to [COOH], Eq. (5.23) can be written as 

For equimolar initial concentrations of the diacid and diol, [COOH] = [OH] = C and Eq. (5.24) may then be simplified to 

It is convenient here and for many other purposes as well to introduce a parameter called the extent of reaction and designated by p, which represents the fraction of functional groups initially present that have undergone reaction at time t. Thus,  

The concentration C at time t of either hydroxyl or carboxyl groups is then given by 

A plot of 1/(1 — pY vs. t should thus be linear. However, when plots are made of experimental values of 1/(1 — pY versus time, it is found that Eq. (5.30) is not obeyed from p = 0 up to p = 0.80 (i.e., for the first 80% of the esterification of -COOH and -OH groups) [3]. Above 80% conversion the integrated rate expression (5.30) is obeyed very well. It may be noted that the deviations below 80% conversion are also observed for simple esterifications such as when the dicarboxylic acid is replaced by a monocarboxylic acid. 

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Equation (5.10) leads to two distinct kinetic situations depending on the source of H, that is, on whether or not a strong acid such as sulfuric acid or /7-toluene sulfonic acid is externally added as a catalyst. The former is known as catalyzed polyesterification and the latter as uncatalyzed or self-catalyzed polyesterification (Odian, 1991). 

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