Quantitative and qualitative 1H, 13C, and 15N NMR spectroscopic investigation of the urea-formaldehyde resin synthesis

Article abstract of DOI:10.1002/mrc.4044

Urea-formaldehyde resins are bulk products of the chemical industry. Their synthesis involves a complex reaction network. The present work contributes to its elucidation by presenting results from detailed NMR spectroscopic studies with different methods. Besides¹H NMR and¹³C NMR, ¹⁵N NMR spectroscopy is also applied.¹⁵N-enriched urea was used for the investigations. A detailed NMR signal assignment and a model of the reaction network of the hydroxymethylation step of the synthesis are presented. Because of its higher spectral dispersion and the fact that all key reactions directly involve the nitrogen centers,¹⁵N NMR provides a much larger amount of detail than do¹H and¹³C NMR spectroscopy. Symmetric and asymmetric dimethylol urea can be clearly distinguished and separated from monomethylol urea, trimethylol urea, and methylene-bridged urea. The existence of hemiformals of methylol urea is confirmed. 1,3,5-Oxadiazinan-4-on (uron) and its derivatives were not found in the reaction mixtures investigated here but were prepared via alternative routes. The molar ratios of formaldehyde to urea were 1, 2, and 4, the pH values 7.5 and 8.5, and the reaction temperature 60 °C. Copyright 2014 John Wiley & Sons, Ltd. ¹⁵N-enriched urea is used in combination with quantitative¹⁵N and¹³C NMR spectroscopy and a Virtual Reference. This allows for a detailed peak assignment and absolute quantification of the early steps of this industrial process, which involves a complex reaction network. A detailed peak assignment for all three nuclei, a full quantitative description of the reaction mixture's composition and a model describing changes in shift depending on formaldehyde substitution are given. Copyright