Acid-catalyzed Esterification and Alcoholysis

The two main reactions in these processes are esterification and alcoholysis, which share similar mechanisms, with a hypothetical tetrahedral intermediate. 

Reactions between anhydrides and alcohols are much faster than carboxyl + alcohol reaction, and full conversion of anhydrides (unless they are in excess) will occur in less than one minute while being melted and blended with the rest of the mixture. This stage is slightly exothermal and care is needed to avoid sudden boiling of the reacting mixture. Kinetic parameters of this reaction (apart from selectivities with respect to hydroxyls) are usually not needed. 

Acidolysis reactions have a different mechanism, involving mixed anhydrides [94]. Their rate is comparable to the esterification reactions only above 250°C. 

Esterification also occurs in high-temperature alcoholysis of aromatic esters, as carboxyl end groups are formed by side reactions, and should also be considered in the kinetic modeling of these processes. 

In spite of being the first reaction ever studied [95], esterification has been under investigation ever since, and much knowledge has accumulated, even if some points are stUl less clear. The basic kinetic model for polyesterification was established by Flory and is summarized in his classic book [5j. Esterification was shown to be acid-catalyzed, it is first-order with respect to hydroxyls and, with respect to carboxyls, its order is either one in the presence of foreign strong protic acids, or two in their absence [Eq. (52)]. 

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