DSC and ¹³C‐NMR studies of the imidazole‐accelerated reaction between epoxides and phenols

The imidazole‐accelerated reaction between epoxides and phenols has been studied through DSC and ¹³C‐NMR of model compounds. The selectivity of the accelerator has been found to be strongly dependent on its concentration. If a low amount of accelerator is used, the reaction takes place almost exclusively through the addition of phenols to epoxides even if the latter are used in excess. However, a larger amount of accelerator will cause the secondary hydroxyls (produced by the main reaction) to react with the epoxides. In any case, the lower reactivity of the secondary hydroxyls prevents them from reacting to any significant extent as long as there are any phenols present. When polyfunctional resins are used, the effect of the selectivity of the reaction on the properties of the crosslinked network is very clear. If a low amount of accelerator is used (to promote only the epoxy–phenol reaction), the maximum glass transition temperature (T_g) occurs when the epoxides and phenols are stoichiometrically balanced, reaching 178°C. However, when an excess of epoxides over phenols is used, along with a larger amount of accelarator, the maximum T_g increases its value to 199°C.