New Mechanism Proposed for the Base-Catalyzed Urea⁻Formaldehyde Condensation Reactions: A Theoretical Study.

Taohong Li,Ming Cao,Jiankun Liang,Xiaoguang Xie,Guanben Du

doi:10.3390/polym9060203

Taohong Li, Ming Cao + Show 3 more

Open Access

https://doi.org/10.3390/polym9060203

Copy DOI

Journal: Polymers	Publication Date: Jun 2, 2017
Citations: 13	License type: CC BY 4.0

Affiliation: Southwest Forestry University, Yunnan University

Abstract

Base-catalyzed urea–formaldehyde condensation reactions were investigated by using a quantum chemistry method. It was found that monomethylolurea or N,N’-dimethylolurea can produce the methyleneurea intermediate (–HN–CO–N=CH2) with the catalysis of base. The E1cb (unimolecular elimination of conjugate base) mechanism was identified for the formation of such an intermediate. The potential energy barrier was theoretically predicted to be 59.6 kJ/mol for the E1cb step, which is about half of that of previously proposed SN2 (bimolecular nucleophilic substitution) mechanism. In the subsequentcondensation reactions, Michael addition reactions that lead to different condensed structures can occur between the methyleneurea intermediate and the anions produced from methylolureas under alkaline conditions. Based on the theoretical calculations on the kinetics and thermodynamics of the selected reactions, the competitive formations of methylene linkages, ether linkages and uron were discussed in combination with our previous experimental observations.

Highlights

It is generally believed that base-catalyzed urea–formaldehyde (UF) reactions primarily form methylolureas, while condensations that form polymers can hardly occur [1,2]
According to the base-catalyzed quinonemethide intermediate formation from methylolphenol [12], we proposed the mechanism in Figure 1 where a methyleneurea intermediate (MU-IM) can be produced via E1cb elimination of OH−
An evidence that supports this mechanism is the condensation reactions of methylated dimethylureas were not observed under alkaline conditions [26,27] because methylol dimethylurea cannot produce methyleneurea intermediate according to this mechanism

Summary

A Theoretical Study

Taohong Li 1,2, *, Ming Cao 1,2 , Jiankun Liang 1,2 , Xiaoguang Xie 3 and Guanben Du 1,2, *. Key Lab for Forest Resources Conservation and Utilisation in the Southwest Mountains of China, Southwest Forestry University, Ministry of Education, Kunming 650224, China

Introduction

Quantum Chemistry Calculations

Results and Discussion

Conclusions