Competitive formation of the methylene and methylene ether bridges in the urea–formaldehyde reaction in alkaline solution: a combined experimental and theoretical study

Abstract

The competitive formation of the methylene and methylene ether bridges in the urea–formaldehyde reaction in alkaline solution was investigated by using 13C NMR and quantum chemistry calculations. Despite that the classical theory states that the condensations of methylolureas only form methylene ether bridges at alkaline pH, the competing formation of methylene bridges was observed in this study. The NMR spectra show that the methylene ether bridges are exclusively formed under conditions of 80 and 90 °C with F/U = 2/1. At 80 °C with F/U = 1/1, the linear methylene bridge (–NH–CH2–NH–) began to compete with ether bridges, but it was minor. At 90 °C with F/U = 1/1, the methylene bridges were found to be much more competitive. The theoretically calculated energy barriers for the formation of different types of methylene ether bridges are lower than those for methylene bridges by 12–26 kJ/mol. The steric hindrance was proposed to be another important effect that suppresses the condensations between di- and tri-methylolureas to form the branched methylene bridges [–(HOCH2)N–CH2–NH– or –(HOCH2)N–CH2–N(CH2OH)–]. However, the reaction between a free amino group –NH2 and a methylol group –CH2OH was proposed to encounter no steric hindrance. Hence, once the higher temperature and lower F/U ratio were guaranteed, the competitive formation of linear methylene bridges was observed.