Investigation of the reaction of 1,3‐dimethylurea with formaldehyde by quantitative on‐line NMR spectroscopy: a model for the urea–formaldehyde system

Abstract

Quantitative on-line NMR spectroscopy is applied to study equilibria and reaction kinetics of the reaction of formaldehyde with 1,3-dimethylurea. This reaction system serves as a model system for the much more complex but industrially relevant urea-formaldehyde system. The aim is to study individual reactions and intermediates. The 1,3-dimethylurea-formaldehyde system undergoes only four reactions and, unlike urea-formaldehyde, does not form polymers. The following reactions are studied in detail: (1) the hydroxymethylation, (2) the formation of hemiformals of the hydroxymethylated intermediate, and (3) two condensation reactions of which the first leads to methylene bridges, the other to ether bridges. NMR spectroscopic chemical shift data of the reacting species are provided for the (1) H, (13) C, and (15) N domains. Equilibrium data of reactions (1), (2), and (3) are determined by quantitative (1) H and (13) C NMR spectroscopy at molar ratios of formaldehyde to 1,3-dimethylurea between 1:2 and 16:1 at a pH value of 8.5. Reaction kinetic experiments using an NMR spectrometer coupled to a batch reactor led to a reaction kinetic model parametrized with true species concentrations. The model takes into account reactions (1), (2), and (3). It describes the reaction system well for molar ratios of 1:1, 2:1, and 4:1, temperatures of 303 to 333K, and pH values from 5.0 to 9.5. Dilution experiments with a micro mixer coupled to the NMR spectrometer are conducted to estimate the time to equilibrium of reaction (2) of which the time constant is significantly lower than those of reactions (1) and (3). Copyright © 2015 John Wiley & Sons, Ltd.

NMR reaction monitoring in flow synthesis

“…Similarly, Steinhof et al [ 47 ] studied the equilibria and kinetics of the reaction of 1,3-dimethylurea with formaldehyde, which is a model for the industrially relevant urea–formaldehyde system. The reaction was performed in a batch reactor and the analysis was carried out using a commercial NMR flow probe ( Fig.…”

Copolymerization of UF Resins with Dimethylurea for Improving Storage Stability without Impairing Adhesive Performance

“…The results are shown in Table 1 . The assignment of the chemical shifts were made based in articles of UF resins [ 18 , 19 ] and articles with 13 C NMR analysis of DMeU with formaldehyde [ 8 , 20 ]. The peaks present a little shift to the right due to the high DMSO- d 6 content [ 18 ].…”

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

“…Figure 2 shows the 13 C-NMR spectrum of the sample under alkaline condition (A1). By referencing the literature [21], the chemical shifts were assigned to different types of methylene carbons and the quantitative results (relative Polymers 2017, 9, 109 4 of 12 contents) were list in Table 1. Due to the methyl groups on DMU are not directly involved in the reactions, only the chemical shifts were given in Figure 1, and quantitative calculations were not carried out.…”

“…Therefore, DMUF and UF reactions should share common mechanisms. The peak at 68-70 belongs to methylene carbons in the triazine ring [21], suggesting that a slight hydrolysis of DMU and recombination of the hydrolysis products occurred.…”

“…It has been proved that 1,3-dimethylurea (DMU) is an ideal compound and its reactions with formaldehyde can be used to mimic the urea-formaldehyde reactions [21]. With the presence of two methyl groups, the co-condensed structures may exhibit different spectroscopic signals and allow us to identify them.…”