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Polymers 2017, 9(2), 45;

Theoretical Confirmation of the Quinone Methide Hypothesis for the Condensation Reactions in Phenol-Formaldehyde Resin Synthesis

The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China
Department of Chemistry, Yunnan University, Kunming 650091, China
Authors to whom correspondence should be addressed.
Academic Editor: Alexander Böker
Received: 20 December 2016 / Revised: 24 January 2017 / Accepted: 25 January 2017 / Published: 29 January 2017
(This article belongs to the Special Issue Renewable Polymeric Adhesives)
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The mechanisms for the base-catalyzed condensation reactions in phenol-formaldehyde resin synthesis were investigated by using the density functional theory method. The structures of the intermediates and transition states, as well as the potential energy barriers of the involved reactions, were obtained. The hypothesis of quinine methide (QM) formation was theoretically confirmed. Two mechanisms were identified for QM formation, namely E1cb (elimination unimolecular conjugate base) and water-aided intra-molecular water elimination. The latter is energetically more favorable and is proposed for the first time in this work. Based on the QM mechanism, the condensation should be a unimolecular reaction because the following condensation between an ionized species (dissociated phenol or hydroxymethylphenol) with QM is much faster. The previously proposed SN2 condensation mechanism was found to be not competitive over the QM mechanism due to a much higher energy barrier. The condensation reaction between neutral phenol or hydroxymethylphenol and QM was also found to be possible. The energy barrier of this reaction is close to or higher than that of QM formation. Therefore, the overall condensation reaction may appear to be bimolecular if such a reaction is incorporated. The theoretical calculations in this work rationalized the discrepant results reported in previous kinetics studies well. View Full-Text
Keywords: phenol-formaldehyde resin; condensation; quinone methide; mechanism phenol-formaldehyde resin; condensation; quinone methide; mechanism

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Li, T.; Cao, M.; Liang, J.; Xie, X.; Du, G. Theoretical Confirmation of the Quinone Methide Hypothesis for the Condensation Reactions in Phenol-Formaldehyde Resin Synthesis. Polymers 2017, 9, 45.

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