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.
Highlights
It is known that the base or acid catalyzed resorcinol-formaldehyde (RF) reactions can form polymeric resins which are currently used as wood adhesives and composites [1,2,3,4,5,6,7], forms [8], and organic gels [9,10,11,12]
Theoretical calculations on PF reactions revealed that quinonemethide formation is the rate-determining step in the overall condensation reaction, and formation of para quinonemethide is faster than the formation of ortho quinonemethide [18]
Taking into account the participation of the quinonemethidesanions like 4-quinonemethide anion (QMA), RF condensations should be much faster since the energy quinonemethidesanions like 4-QMA, RF condensations should be much faster since the energy barrier barrier for its formation is significantly lower than the neutral PF QMs by 20–30 kJ/mol
Summary
It is known that the base or acid catalyzed resorcinol-formaldehyde (RF) reactions can form polymeric resins which are currently used as wood adhesives and composites [1,2,3,4,5,6,7], forms [8], and organic gels [9,10,11,12]. In the viewpoint of a mechanism at the molecular level, formation of reactive intermediates is the key factor that determines the rate of the condensation reaction. By employing a quantum chemistry method, we recently studied the base-catalyzed phenolformaldehyde reactions and the quinonemethide (QM) was confirmed to be the key intermediate that initiates condensations [18]. This is the first issue to be addressed in this study Another task of this work is to rationalize the experimentally observed condensed structure of RF resin. To simulate the implicit solvent different carbons of the resorcinol anion and dianion, the APT (atomic polar tensor) charges were effects, for all theTocalculations theimplicit self-consistent field method was with calculated. ∆E(TS) represents the relative energy between TS and reactant or reactant-like intermediate, namely the energy barrier of a reaction step. The potential energy profile shown in this paper was demonstrated by choosing the energy of reactant or a reactant-like intermediate as reference
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