The Influence of pH on the Melamine-Dimethylurea-Formaldehyde Co-Condensations: A Quantitative 13C-NMR Study

1,3-dimethylurea (DMU) was used to mimic urea and to model melamine-urea-formaldehyde (MUF) co-condensation reactions. The products of 1,3-dimethylurea-formaldehyde (DMUF), melamine-formaldehyde (MF), and melamine-1,3-dimethylurea-formaldehyde (MDMUF) reactions under alkaline and weak acidic conditions were compared by performing quantitative carbon-13 nuclear magnetic resonance (13C-NMR) analysis. The effect of pH on the co-condensation reactions was clarified. With the presence of the methyl groups in DMU, the appearance or absence of the featured signal at 54–55 ppm can be used to identify the co-condensed methylene linkage –N(–CH3) –CH2–NH–. Under alkaline condition, MDMUF reactions produced primarily MF polymers and the featured signal at 54–55 ppm was absent. Even though the co-condensations concurrently occurred, undistinguishable and very minor condensed structures with ether linkage were formed. Differently, under weak acidic condition, the relative content of co-condensed methylene carbons accounts for over 40%, indicating the MDMUF co-condensation reactions were much more competitive than the self-condensations. The formation of reactive carbocation intermediate was proposed to rationalize the results.