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Polymers 2017, 9(9), 426; https://doi.org/10.3390/polym9090426

Mechanism of Base-Catalyzed Resorcinol-Formaldehyde and Phenol-Resorcinol-Formaldehyde Condensation Reactions: A Theoretical Study

1
The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China
2
Key Lab for Forest Resources Conservation and Utilisation in the Southwest Mountains of China, Southwest Forestry University, Ministry of Education, Kunming 650224, China
3
School of Chemical Science and Technology, Yunnan University, Kunming 650091, China
*
Authors to whom correspondence should be addressed.
Received: 13 August 2017 / Revised: 1 September 2017 / Accepted: 5 September 2017 / Published: 7 September 2017
(This article belongs to the Collection Polymeric Adhesives)
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Abstract

The base-catalyzed resorcinol-formaldehyde condensation reactions were theoretically investigated in this study by employing a quantum chemistry method. The condensation reaction includes two steps: (1) formation of the quinonemethide (QM) intermediate from hydroxymethylresorcinol; (2) Michael addition between the quinonemethide and resorcinol anion. The first step is the rate-determining step. Two mechanisms, unimolecular elimination of the conjugate base (E1cb) and water-aided elimination (WAE), were identified for the formation of QM. The hydroxymethylresorcinol anion produces neutral QM while the dianion produces a quinonemethide anion (QMA). The calculated potential energy barriers suggested that the QMA formation is much more favorable. Although resorcinol-formaldehyde and phenol-formaldehyde condensations share a common mechanism, the former would be faster if the QMA participates in condensations. The potential energy barriers for formation of 2-QM, 4-QM, 6-QM, 2-QMA, and 4-QMA were calculated. The results show that the formations of 6-QM and 4-QMA have relatively lower energy barriers. This rationalized previous experimental observations that the 2,4-(2,6-) and 6,6′-(4,4′-) methylene linkages were dominant, whereas the 2,2′-linkage was almost absent. The resorcinol-phenol-formaldehyde co-condensations were also calculated. The cold-setting characteristic of phenol-resorcinol-formaldehyde co-condensed resin can be attributed to participation of resorcinol quinonemethides in condensations. View Full-Text
Keywords: resorcinol-formaldehyde; base-catalyzed; condensation; quinonemethide resorcinol-formaldehyde; base-catalyzed; condensation; quinonemethide
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Li, T.; Cao, M.; Liang, J.; Xie, X.; Du, G. Mechanism of Base-Catalyzed Resorcinol-Formaldehyde and Phenol-Resorcinol-Formaldehyde Condensation Reactions: A Theoretical Study. Polymers 2017, 9, 426.

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