4. Secondary reactions

Most polycondensations and polyadditions are accompanied by so-called secondary reactions, which can be very different from the main reaction. Advances in chemical analysis have revealed structural anomalies in systems long considered pure. In addition, the growing demands of applicators are forcing manufacturers to detect and eliminate these parasitic phenomena as far as possible. The case of unsaturated polyesters is typical of these phenomena.

The most conventional preparation of unsaturated polyesters involves polycondensing a mixture of diols and anhydrides, almost always containing maleic anhydride and, in some grades, phthalic anhydride. The main reaction is as follows: