Transesterification, carboxylic adds

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

Fatty acids. Fatty acids can be ethoxylated in what amounts to a two-step process where the first mole of EO adds slowly to the dry, precatalyzed acid to yield the hydroxyethyl ester. Because of the presence of carboxylic acid, the reaction runs slowly under general acid catalysis and does not produce substantial quantities of polyethoxylated product. Once the free carboxylic acids are completely capped, the pH of the system becomes alkaline and the reaction proceeds in a comparable manner to any primary alcohol ethoxylation. Some competing transesterification occurs at temperatures above 120°C, leading to a product distribution of PEG monoesters, diesters, and free PEG. 

---