Condensation of Methylolmelamine

The condensation of methylolmelamine in the range of ca. pH 8–1 was investigated. An aqueous-dimethylsulfoxide (DMSO) solution (ca. 1 : 2, vol. ratio) was mainly used as the solvent, because it was revealed that the condensate in the latter stage of the condensation is soluble in DMSO solvent, and also that in this solvent methylolmelamine scarcely decomposes into a lower methylolmelamine liberating formaldehyde; then, the condensation mechanism of that methylolmelamine whose composition corresponds approximately to di-, tri-, and hexa-methylolmelamine, was investigated, not only in the early stage but also in the latter stage. It was found that the condensation rates of di- and tri-methylolmelamines (low methylolmelamines) in the early stage show maxima at ca. [HGl]/[methylolmelamine]=0.5 (molar ratio), i.e., at ca. pH= logK, where K is the equilibrium constant between the molecular form of the methylolmelamine and its conjugate acid, and that in hexa-methylolmelamine (high methylolmelamine) the above maximum appears at ca. [HCl]/[methylolmelamine]=0.5–1.0 (molar ratio). Also, the above maxima were found to shift toward smaller [HGl]/[methylolmelamine] (molar ratio) values as the condensation proceeded (the latter stage). From these results, a condensation mechanism was proposed for the early stage of the condensation.