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

polymers

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

Taohong Li 1,2,* ID , Ming Cao 1,2, Jiankun Liang 1,2, Xiaoguang Xie 3 and Guanben Du 1,2,*

1 The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China; [email protected] (M.C.); [email protected] (J.L.); 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; [email protected] * Correspondence: [email protected] (T.L.); [email protected] (G.D.); Tel.: +86-871-6386-2581 (T.L.); +86-871-6386-3472 (G.D.)

Received: 13 August 2017; Accepted: 5 September 2017; Published: 7 September 2017

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 6,60-(4,40-) methylene linkages were dominant, whereas the 2,20-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.

Keywords: resorcinol-formaldehyde; base-catalyzed; condensation; quinonemethide

1. Introduction 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 [1–7], forms [8], and organic gels [9–12]. Depending on the F/R molar ratio, RF reactions can result in novlac or resol type resin [2]. RF resin is generally not used separately as wood adhesive due to its instability and high cost. Alternatively, in order to accelerate the cure of PF resin, resorcinol is generally mixed with PF resol resin, obtaining the so-called phenol-resorcinol-formaldehyde (PRF) co-condensed resins [2,3,5–7]. Due to the similarity of chemical structure to phenol, it is natural to believe the RF polymers are also similar to phenol-formaldehyde (PF) polymers in which methylene bridges can be formed though reactions between phenol and hydroxymethylphenol or between hydroxymethylphenols. However, a signiﬁcant difference between PF and RF resin is that curing of PF resin generally requires a hot-press temperature above 160 ◦C, whereas RF and PRF are cold-setting resins. This indicates that

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RF condensations encounter a much lower energy barrier and occur much faster. Superficially, higher reactivity may be attributed to the stronger nucleophilicity of resorcinol since it has two phenolic hydroxyl groups which lead to higher electron density on the aromatic ring. 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. Therefore, comparing the kinetics of the PF and RF intermediates formation can help us better understand the different behaviors of the two resins. But, the experimental kinetics study seems to be incapable of doing so because capture and quantitative description of such short-lived intermediates is currently almost impossible, especially for solution reactions. Fortunately, theoretical methods have become powerful tools that allow us to explore the microscopic world of chemical reactions in silico. In the field of wood adhesive resins, pioneering works that employed molecular mechanics methods are from the group of Professor A. Pizzi [13–17]. These studies have investigated the interactions between resin components and wood cellulose, aiming to establish theoretical models for resin–wood surface interaction and giving insight into the relationship between resin structure and performance. The obtained results can help us design novel resins, but the prerequisite is that we can control the reactions based on our knowledge about mechanisms. Therefore, understanding the reaction mechanisms and resin chemistry is another task. Different from molecular mechanics, which is based on force filed theory and mainly deals with the hydrogen bonding interactions, Van der Waals’ force and their influences on molecule energy and conformations, quantum chemistry methods that is based on quantum mechanics can deal with formation and breakage of chemical bonds, kinetics and thermodynamics of chemical reactions through rigorous calculations on the electronic structures of the involved species. To explain the different condensation reactivity of PF and RF, quantum chemistry calculations appear to be necessary. By employing a quantum chemistry method, we recently studied the base-catalyzed phenol- formaldehyde reactions and the quinonemethide (QM) was confirmed to be the key intermediate that initiates condensations [18]. RF condensation reactions may share the same mechanism. But, the chemistry of resorcinol is obviously more complex than phenol. Resorcinol has two hydroxyl groups and therefore dissociation of the protons can produce a singly charged or doubly charged anion (dianion) with the presence of a base. As a result, different quinonemethide intermediates may be formed. As can be seen in Figure1, five possible intermediates may be formed according to the proposed mechanism. 2-QM, 4-QM, and 6-QM are neutral species that are similar to phenol quinonemethides. But, 2-QMA and 4-QMA are anions which may exhibit different reactivity. According to the results for PF reactions [18], formation of quinonemethide is the rate-determining step of the overall condensation reaction. Thus, it can be speculated that formations of RF quinonemethides encounter lower energy barriers. But which of the neutral QMs and QM anions are more favorable? 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. Theoretically, resorcinol has three reactive sites on the ring, namely, the three ortho positions of the two hydroxyl groups. Thus, three types of methylene linkages through 2,20-, 2,4-, and 4,40-condensations should be formed. But, it is interesting that some 13C NMR studies indicated that the 2,40- and 4,40-linkages were found to be dominant, whereas 2,20-linkage was observed to be minor or absent [2–4]. Further, 4,40-linkage appeared to be more favorable than 2,4-linkage. These observations imply that the 4-position is more reactive than 2-position. A plausible explanation is the 4-position is more reactive toward formaldehyde, leading to higher concentration of 4-hydroxymethylresorcinol. But the hydroxymethylation reaction should be faster than condensations, considering that resorcinol is highly reactive toward formaldehyde in the presence of catalysts. Therefore, different reaction rates of rate-determining steps may be another reason. That is to say, 2-hydroxymethylresorcinol and 4-hydroxymethylresorcinol may have different rates of forming reactive intermediates. Which is the more important factor? Or do both of them work? This issue has never been addressed in detail, according to our survey of the literature. Polymers 2017, 9, 426 3 of 14 of forming reactive intermediates. Which is the more important factor? Or do both of them work? ThisPolymers issue2017 has, 9, 426 never been addressed in detail, according to our survey of the literature. 3 of 14

Figure 1. TheThe proposed proposed mechanisms for format formationion of resorcinol quinonemethides.

Finally, inin the synthesisthe synthesis of phenol-resorcinol-formaldehyde of phenol-resorcinol-formaldehyde resins, PF andresins, RF self-condensations PF and RF self-condensationsand PRF co-condensations and PRF should co-condensations occur simultaneously. should The occur competitive simultaneously. relationship The of thecompetitive reactions relationshipis also an important of the reactions issue that is deserves also an attentionimportant because issue that such deserves a relationship attention determines because whether such a relationshipco-condensed determines structures whether can be formed co-condensed efﬁciently. structures can be formed efficiently.

2. Theoretical Calculations By employingemploying the the density density functional functional theory theory [19, 20[19,20]] (DFT) (DFT) method method B3LYP B3LYP [21–23] with[21–23] the with standard the standardbasis set 6-31++G**basis set [6-31++G**24–27], full [24–27], structural full optimizations structural optimizations were performed were on allperformed the stationary on all points the stationaryon the reaction points potential on the reaction energy surfacespotential (PES), energy including surfaces reactants,(PES), including intermediates, reactants, transition intermediates, states, transitionand products. states, The and harmonic products. vibrational The harmonic frequencies vibrational were calculatedfrequencies at were the same calculated theoretical at the level same to theoreticalcharacterize level the natureto characterize of the stationary the nature point of as the a localstationary minimum point or as first-order a local minimum saddle point or first-order (transition saddlestate) on point the PES.(transition The zero-point state) on vibrational the PES. The energies zero-point (ZPVE) vibrational were used energies to correct (ZPVE) the relative were energies.used to correctEach transition the relative state energies. (TS) was Each identified transition as it state has a(TS) unique was imaginaryidentified as frequency. it has a unique Intrinsic imaginary reaction frequency.coordinate (IRC)Intrinsic analysis reaction was alsocoordinate performed (IRC to) confirmanalysis the was connectivity also performed between to a TSconfirm and a localthe connectivityminimum. To between compare a theTS relativeand a nucleophilicitylocal minimum. of To different compare carbons the ofrelative the resorcinol nucleophilicity anion and of differentdianion, thecarbons APT (atomicof the resorcinol polar tensor) anion charges and dian wereion, also the calculated. APT (atomic To simulate polar tensor) the implicit charges solvent were alsoeffects, calculated. for all the To calculations simulate the the implicit self-consistent solvent reactioneffects, for field all (SCRF) the calculations method was the employed self-consistent with reactionthe polarizable field (SCRF) continuum method model was (PCM) employed [28–30 with] by the defining polarizable water ascontinuum the solvent model (Water: (PCM)ε = 78.3553).[28–30] Allby defining the calculations water as were the carriedsolvent out(Water: with ε the = 78.3553). GAUSSIAN All 03the program calculations package were (Gaussian, carried out Pittsburgh, with the GAUSSIANPA, USA) [31 03]. program package (Gaussian, Pittsburgh, PA, USA) [31]. The potential energy barrier (theoretical activation energy) of each calculated reaction step was described byby the the relative relative energy energy (energy (energy gap) betweengap) between a transition a transition state and state a reactant and ora reactant-likereactant or reactant-likeintermediate. intermediate. The relative energiesThe relative were energi obtainedes were by theobtained following by the formulas: following formulas:

∆ΔEE(TS)(TS) = =[( [(EE(TS)(TS) ++ ZPVE(TS)]ZPVE(TS)] −− [[EE(R)(R) + + ZPVE(R)] ZPVE(R)]

∆E(P) = [(E(P) + ZPVE(P)] − [E(R) + ZPVE(R)] Polymers 2017, 9, 426 4 of 14

Here, E(TS) is the total electron energy of a TS, while E(R) represents the total electron energy of a local minimum which can be an initial reactant or a reactant-like intermediate. Thus, ∆E(TS) represents the relative energy between TS and reactant or reactant-like intermediate, namely the energy barrier of a reaction step. Similarly, ∆E(P) corresponds to the relative energy between a product or product-like intermediate. In this context, the potential energy proﬁle shown in this paper was demonstrated by choosing the energy of reactant or a reactant-like intermediate as reference.

3. Results and Discussion

3.1. Formations of Quinine Methide Intermediates Figure1 shows two mechanisms for base-catalyzed formation of quinonemethide which was derived from that identified for phenol-formaldehyde reactions [18]. Pathway (A) demonstrates the E1cb (unimolecular elimination of conjugate base) mechanism. Pathway (B) is another one in which intra-molecular water elimination is involved. According to the proposed mechanisms, three neutral quinonemethide intermediates, 2-QM, 4-QM, and 6-QM, can be produced from singly charged hydroxymethylresorcinol. If a resorcinol dianion participates in the reaction, a quininemethide anion will be formed. Concentrations of the two types of intermediate are dependent on the concentration of the alkaline catalyst (or pH). In the experiment of Christiansen [4], 76% formaldehyde was converted to 4- or 6-hydroxymethyl groups (4- and 6-position are equal due to symmetry of resorcinol), whereas only 12% formaldehyde took the form of 2-hydroxymethyl groups. Three factors may be attributed to this result. First, the 4- and 6-positions are more reactive toward formaldehyde due to their stronger nucleophilicity. Another one is a statistical factor. Namely, two positions versus one. Finally, Durairaj pointed out that the steric hindrance on the 2-position caused by the two adjacent hydroxyl groups is a key factor [32]. To confirm the first factor, the APT atomic charges were calculated and are given in Figure2. In the resorcinol anion (RA), the carbons numbered as 2, 4, and 6 are negatively charged as −0.574, −0.479, and −0.642, while carbon 5 is positively charged as 0.378, clearly indicating the three ortho positions are strong nulceophilic centers. But, according to the charge distribution, the three positions are different in reactivity as the nucleophilic order appears to be 6 > 2 > 4. Position 4 is less reactive than position 2, suggesting that, for RA, high reactivity of 6-position is responsible for dominant formation of corresponding hydroxymethyl groups. In contrast with the statistical factor, nucleophilicity plays a more important role. For the resorcinol dianion (RDA), the negative charges of the three ortho positions were almost equal. But, the statistical factor determines 4- and 6-positions have higher probability in reacting with formaldehyde. Steric hindrance may be partially responsible for the slower hydroxymethylation in 2-postion, but it cannot rationalize the results that the 2-hydroxymethyl groups and 2,40-methylene linkages were produced while 2,20-methylene linkages were absent. These results also cannot be explained by nucleophilicity and statistical effects. Slower hydroxymethylation is not definitely in accordance with slower condensation reactivity. 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]. This explained the experimental results that ortho-ortho-methylene linkage was always observed to be minor in comparison with ortho-para- and para-para-methylene linkages. So, it is also necessary to compare the reactivity of 2-, 4-, and 6-hydroxymethylresorcinol anion (HMRA) in formation of quinonemethide intermediates. Polymers 2017, 9, 426 5 of 14 Polymers 2017, 9, 426 5 of 14

FigureFigure 2. 2. TheThe calculated calculated atomic atomic charges charges for for reso resorcinolrcinol anion anion (RA) (RA) and dianion (RDA).

FigureFigure 33 shows the calculated calculated structures structures of of the the reactant reactant intermediates intermediates and and transition transition states states for thefor theE1cb E1cb formation formation of ofdifferent different neutral neutral quinonemethides quinonemethides from from hydroxymethylresorcinol hydroxymethylresorcinol anions. anions. The structuresstructures of producedof produced quinonemethides quinonemethides are collected inare Figure collected4. In 2-hydroxymethylresorcinol in Figure 4. In 2-hydroxymethylresorcinolanion (2-HMRA), a strong intra-molecular anion (2-HMRA), hydrogen a strong bond (1.747intra-molecu Å) was formedlar hydrogen between bo hydroxymethylnd (1.747 Å) wasgroup formed and the between carbonyl hydroxymethyl oxygen. In contrast, group the and hydrogen the carbonyl bond inoxygen. 4-hydroxymethylresorcinol In contrast, the hydrogen anion bond(4-HMRA) in 4-hydroxymethylresorcinol is weaker since the bond length anion is longer(4-HMRA) (1.814 Å).is weaker In 6-HMRA, sincethe the hydrogen bond length bond is formedlonger (1.814between Å). two In 6-HMRA, hydroxyl groupsthe hydrogen and the bond very is long formed bond between length of two 2.660 hydroxyl Å indicates groups the and interaction the very is longalmost bond ignorable. length of The 2.660 hydrogen Å indicates bonding the effectsinteraction seems is toalmost be a ignorable. factor that The stabilizes hydrogen these bonding species. effectsBut, the seems bond to length be a orderfactor is that not consistentstabilizes these with thespecies. stability But, order, the bond as it canlength be seen order that is thenot 4-HMRAconsistent is withthe most the stablestability one, order, therefore as it itcan was be chosen seen that as reference the 4-HMRA in the is potential the most energy stable profiles. one, therefore In fact, besidesit was chosenthe hydrogen as reference bonding in interaction, the potential the electronicenergy profiles. structures In offact, the besides isomers the also hydrogen play an importantbonding interaction,role, especially the theelectronic delocalization structures extent of the of electronsisomers also in the play conjugate an important system. role, However, especially as canthe delocalizationbe seen from theextent potential of electrons energy in profiles the conjug in Figureate 5system., the E1cb However, transition as statecan be (2-E1cb-TS) seen from has the a potentialhigher energy energy barrier profiles (113.7 in kJ/mol) Figure than5, the 4-E1cb-TS E1cb transition (107.3 kJ/mol) state (2-E1cb-TS) by 6.7 kJ/mol. has Further, a higher 6-E1cb-TS energy barrierhas the (113.7 lowest kJ/mol) energy than barrier 4-E1cb-TS of 93.5 (107.3 kJ/mol. kJ/mol) This by order 6.7 kJ/mol. is consistent Further, with 6-E1cb-TS the order has of the hydrogen lowest energybonding barrier strength. of 93.5 Obviously, kJ/mol. This the hydrogen order is consistent bonding effectwith the suppresses order of thehydrogen leaving bonding of an OH strength. ion and Obviously,the relative the bonding hydrogen strength bonding is the effect key factorsuppresse thats determines the leaving the of relative an OH reactivityion and forthe HMRAsrelative bondingto form quinonemethidestrength is the key intermediates. factor that Indetermines our calculations the relative for PF reactivity reactions for [18 ],HMRAs such effect to form was quinonemethidealso found. In addition,intermediates. the quinonemethides, In our calculations 2-QM, for PF 4-QM, reactions and [18], 6-QM such are effect different was also in stability. found. InSpecifically, addition, 6-QMthe quinonemethides, is the most stable 2-QM, species 4-QM, and isand more 6-QM stable are thandifferent 2-QM in and stability. 4-QM Specifically, by 20.8 and 6-QM7.4 kJ/mol, is the respectively.most stable Thus,species considering and is more both stable the energythan 2-QM barriers and and 4-QM relative by 20.8 stability and of7.4 different kJ/mol, respectively.quinonemethides, Thus, formation considering of 6-QM both is energeticallythe energy thebarriers most favorableand relative and condensations stability of involvingdifferent quinonemethides,6-QM are the fastest, formation whereas of condensations 6-QM is energe relatedtically to 2-QMthe most are lessfavorable competitive. and condensations The reaction involvingof 6-QM with6-QM another are the monomer,fastest, whereas resorcinol condensa or hydroxymethylresorcinol,tions related to 2-QM are will lesslead competitive. to 2-6 or The 4-6 reactionmethylene of linkage.6-QM with Due another to the symmetric monomer, structure resorcinol of resorcinol,or hydroxymethylresorcinol, they are equal to 2,4 will or 4,4 lead0-methylene to 2-6 or 4-6linkage. methylene Therefore, linkage. in addition Due to to the highersymmetric concentration structure ofof 4- resorcinol, or 6-hydroxymethyl they are group,equal to the 2,4 lower or 4,4energy′-methylene barrier linkage. for 6-QM Therefore, formation inalso addition plays to an th importante higher concentration role. Correspondingly, of 4- or 6-hydroxymethyl because of the group,lower concentrationthe lower energy of the barrier 2-hydroxymethyl for 6-QM formatio groupn and also higher plays energyan important barrier role. for formation Correspondingly, of 2-QM, because2,20-methylene of the linkagelower concentration was almost prohibited, of the 2-hydroxymethyl or only very minorly group formed. and higher energy barrier for formation of 2-QM, 2,2′-methylene linkage was almost prohibited, or only very minorly formed.

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Figure 3. The calculated structures of the intermediates and transition states for E1cb formation of FigureFigure 3. 3. TheThe calculated calculated structures structures of of the the intermediate intermediatess and and transition transition states states for for E1cb E1cb formation formation of of quinonemethide from hydroxymethyresorcinol anion. quinonemethidequinonemethide from from hydroxymethyresorcinol hydroxymethyresorcinol anion.

Figure 4.4. TheThe calculated structures for the neutralneutral quinonemethides (QM) and quinonemethidequinonemethide Figure 4. The calculated structures for the neutral quinonemethides (QM) and quinonemethide anions (QMA). anions (QMA).

Polymers 2017, 9, 426 7 of 14 Polymers 2017, 9, 426 7 of 14

Polymers 2017, 9, 426 7 of 14

Figure 5. The potential energy profiles proﬁles for E1cb formations of quinonemethides (QM).

In our recent calculations on PF condensation reactions [[18],18], a newnew mechanismmechanism which involves Figure 5. The potential energy profiles for E1cb formations of quinonemethides (QM). intra-molecular waterwater eliminationelimination waswas identified. identified. For For RF, RF, such such a a mechanism mechanism was was shown shown in in Figure Figure1 as 1 as pathway (B). Here, only the reaction on carbon 6 was considered as an example. The calculated pathway (B).In our Here, recent only calculations the reaction on PF on condensation carbon 6 was reactions considered [18], as a new an example. mechanism The which calculated involves results results are given in Figures 6 and 7. Note that the 6-HMRA (Figure 3) is an anion, while the are givenintra-molecular in Figures 6water and7 elimination. Note that was the identified. 6-HMRA (FigureFor RF, 3such) is ana mechanism anion, while was the shown intermediate in Figure 1 (IM) intermediate (IM) 6-HMR-IM is a neutral species. Starting from this reactant-like intermediate, a 6-HMR-IMas pathway is a neutral(B). Here, species. only the Starting reaction fromon carbon this reactant-like6 was considered intermediate, as an example. a water The calculated molecule can water molecule can be eliminated (WE) via the four-member ring transition state WE-TS, leading to be eliminatedresults are (WE) given via in theFigures four-member 6 and 7. ringNote transitionthat the 6-HMRA state WE-TS, (Figure leading 3) is an to anion, 6-QM. while This reactionthe 6-QM. This reaction encounters a notable barrier of 160.4 kJ/mol, indicating that such a mechanism encountersintermediate a notable (IM) barrier 6-HMR-IM of 160.4 is a kJ/mol,neutral species. indicating Starting that from such this a mechanism reactant-like cannot intermediate, compete a with cannotwater compete molecule with can E1cb be eliminated at all. However, (WE) via thefor four-memberPF, it was found ring transition that solvent state WE-TS,water moleculesleading to can E1cb at all. However, for PF, it was found that solvent water molecules can catalyze the proton catalyze6-QM. the This proton reaction transfer encounters from carbon a notable to barrieroxygen of and 160.4 the kJ/mol, energy indicating barrier wasthat suchsignificantly a mechanism lowered transfer from carbon to oxygen and the energy barrier was significantly lowered [13]. We call such [13]. cannotWe call compete such witha mechanism E1cb at all. aHowever, water-aided for PF, elimination it was found (WAE). that solvent It was water found molecules that cansuch a a mechanism a water-aided elimination (WAE). It was found that such a mechanism can also be mechanismcatalyze thecan proton also transferbe applied from carbon to RF.to oxygen 6-W-HMR-IM1 and the energy is barrier the wascomplex significantly formed lowered through applied[13]. to We RF. call 6-W-HMR-IM1 such a mechanism is the a complex water-aided formed elimination through (WAE). inter-molecular It was found hydrogen that such bonding a inter-molecular hydrogen bonding between 6-HMR and a water molecule. Via the six-member ring betweenmechanism 6-HMR andcan aalso water be molecule.applied to Via RF. the six-member6-W-HMR-IM1 ring is transitionthe complex state formed WAE-TS, through the proton transition state WAE-TS, the proton on carbon 6 migrates to water molecule. Meanwhile, a proton on carboninter-molecular 6 migrates hydrogen to water bonding molecule. between Meanwhile, 6-HMR and a a proton water molecule. on water Via shifts the tosix-member the oxygen ring atom on water shifts to the oxygen atom of the hydroxymethyl group. Consequently, the product-like of thetransition hydroxymethyl state WAE-TS, group. the Consequently, proton on carbon the product-like6 migrates to water intermediate molecule. 6-W-HMR-IM2 Meanwhile, a proton is formed. intermediate 6-W-HMR-IM2 is formed. WAE-TS has energy barrier of 98.2 kJ/mol which is WAE-TSon water has energy shifts to barrier the oxygen of 98.2 atom kJ/mol of the which hydroxymethyl is significantly group. lower Consequently, than WE-TS the product-like by 62.2 kJ/mol, significantly lower than WE-TS by 62.2 kJ/mol, indicating that solvent water has a very strong indicatingintermediate that solvent 6-W-HMR-IM2 water has ais veryformed. strong WAE-TS catalytic ha effect.s energy Elimination barrier of of 98.2 a two-water kJ/mol which complex is via catalyticsignificantly effect. Elimination lower than ofWE-TS a two-water by 62.2 kJ/mol,complex in dicatingvia 6-Disso-TS that solvent finally water leads has to a 6-QM.very strong This step 6-Disso-TS finally leads to 6-QM. This step is almost barrierless. The barrier of WAE is slightly higher is almostcatalytic barrierless. effect. Elimination The barrier of a two-waterof WAE iscomplex slightly via higher 6-Disso-TS than finally that ofleads E1cb to 6-QM.(6-E1cb-TS) This step by 4.7 than that of E1cb (6-E1cb-TS) by 4.7 kJ/mol, implying that such a mechanism may also contribute to kJ/mol,is almostimplying barrierless. that such The a mechanismbarrier of WAE may is alsoslightly contribute higher thanto the that formation of E1cb (6-E1cb-TS)of 6-QM. Differently, by 4.7 the formation of 6-QM. Differently, for PF, the WAE mechanism was found to be energetically more for PF,kJ/mol, the implyingWAE mechanism that such a mechanism was found may to also be contribute energetically to the formationmore favorable of 6-QM. thanDifferently, the E1cb favorablefor PF, than the the WAE E1cb mechanism mechanism was [18 found]. Specifically, to be energetically the barrier more of the favorable WAE reaction than the on E1cb the para mechanism [18]. Specifically, the barrier of the WAE reaction on the para position was found to be positionmechanism was found [18]. to Specifically, be 8.0 kJ/mol the barrier lower thanof the that WAE of reaction the E1cb on reaction. the para Thisposition difference was found suggests to be that 8.0 kJ/mol lower than that of the E1cb reaction. This difference suggests that the WAE and E1cb the WAE8.0 kJ/mol and E1cb lower mechanisms than that of playthe E1cb different reaction roles. This for difference PF and RF suggests systems. that the WAE and E1cb mechanismsmechanisms play play different different roles roles for for PF PF and and RF RF systems.systems.

FigureFigure 6. The 6. The calculated calculated structures structures of of the the intermediates intermediates and and transition transition states states for forthe theformation formation of 6-QM of 6-QM (quinonemethide)(quinonemethide) via via the the WE WE (water (water elimination) elimination) andand WAE (water-aided (water-aided elimination) elimination) mechanisms. mechanisms. Figure 6. The calculated structures of the intermediates and transition states for the formation of 6-QM

(quinonemethide) via the WE (water elimination) and WAE (water-aided elimination) mechanisms.

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Figure 7. The potential energy profiles for formation of 6-QM via the WE and WAE mechanisms. FigureFigure 7. 7. TheTheThe potentialpotential energyenergy profilesprofiles proﬁles forfor formationformation of of 6-QM 6-QM via via the the WE WE and and WAE WAE mechanisms.mechanisms.

Dissociation of the two phenolic hydroxyl groups of resorcinol leads to a dianion which should Dissociation of the two phenolic hydroxyl groups of resorcinol leads to a dianion which be a stronger nucleophile. Therefore, hydroxymethylation of such a dianion can produce should be a stronger nucleophile. Therefore, hydroxymethylation of such a dianion can produce hydroxymethylresorcinol dianion (HMRDA), from which a quinonemethide intermediate can also hydroxymethylresorcinol dianion (HMRDA), from which a quinonemethide intermediate can also be be formed. According to the proposed mechanism in Figure 1, the formed quinonemethides should formed. According to the proposed mechanism in Figure1, the formed quinonemethides should be be anions like 2-QMA and 4-QMA. The calculated results for the formation of them are shown in anions like 2-QMA and 4-QMA. The calculated results for the formation of them are shown in Figures8 Figures 8 and 9. The transition states 2-E1cb-TSA and 4-E1cb-TSA have energy barriers of 76.7 and and9. The transition states 2-E1cb-TSA and 4-E1cb-TSA have energy barriers of 76.7 and 69.1 kJ/mol, 69.1 kJ/mol, which are significantly lower than 2-E1cb-TS and 4-E1cb-TS by 37.0 and 24.4 kJ/mol, which are signiﬁcantly lower than 2-E1cb-TS and 4-E1cb-TS by 37.0 and 24.4 kJ/mol, respectively. respectively.respectively. Therefore,Therefore, condensationscondensations relatedrelated toto 2-QMA2-QMA andand 4-QMA4-QMA shouldshould bebe muchmuch faster.faster. Therefore, condensations related to 2-QMA and 4-QMA should be much faster. Similarly, formation Similarly, formation of 4-QMA is energetically more favorable than 2-QMA. However, as resorcinol of 4-QMA is energetically more favorable than 2-QMA. However, as resorcinol is a weak acid, isis aa weakweak acid,acid, thethe concentrationsconcentrations ofof resorcinolresorcinol dianiondianion (RDA)(RDA) andand hydroxymethylresorcinolhydroxymethylresorcinol the concentrations of resorcinol dianion (RDA) and hydroxymethylresorcinol dianion (HMRDA) dianion (HMRDA) are dependent on the concentrationion ofof thethe basebase oror thethe base/resorcinolbase/resorcinol molarmolar are dependent on the concentration of the base or the base/resorcinol molar ratio. ratio.ratio.

Figure 8. TheThe calculated structuresstructuresstructuresfor forforthe thetheintermediates intermediateintermediatess and andand transition transitiontransition states statesstates for forfor the thethe E1cb E1cbE1cb formation formationformation of ofquinonemethide quinonemethide anions. anions.

Figure 9. The potential energy proﬁles for the E1cb formation of quinonemethide anions (QMAa). Figure 9. The potential energy profiles for the E1cb formationrmation ofof quinonemethidequinonemethide anionsanions (QMAa).(QMAa).

3.2. RF and PRF Polyaddtion Reactions As it it was was found found for for PF PFreactions reactions [18], [once18], oncethe quinonemethide the quinonemethide is formed, is formed, a Michael a addition Michael reactionadditionreaction reactionwillwill taketake will placeplace take betweenbetween place between aa neutralneutral a neutral quinonemethidequinonemethide quinonemethide andand and aa phenol aphenol phenol anionanion anion oror or a a hydroxymethylphenol anion. anion. Ther Therefore,efore, “polyaddition” “polyaddition” may may be be a a more more appropriate appropriate word than “polycondensation”“polycondensation” toto describedescribe thethe reactionreaction fromfrom aa mechanisticmechanistic viewpoint.viewpoint. Such Such a reaction was identifiedidentiﬁedidentified to to be be muchmuch fasterfaster thanthanthan quinonemethidequinonemethidequinonemethide formation.formation. ThisThis This featurefeature feature shouldshould stillstill bebe kept kept for for neutral RF quinonemethides. But does it still hold truetrue forfor aa quinonemethidequinonemethide anion,anion, likelike 4-QMA?4-QMA? ItIt isis expectedexpected thatthat thethe reactionreaction betweenbetween twotwo anionsanions (or(or electron-richelectron-rich species)species) wouldwould encounterencounter aa

Polymers 2017, 9, 426 9 of 14

Polymers 2017, 9, 426 9 of 14 neutralPolymers RF 2017 quinonemethides., 9, 426 But does it still hold true for a quinonemethide anion, like 4-QMA?9 of 14 It is expected that the reaction between two anions (or electron-rich species) would encounter a repulsive repulsive effect. Then, what if the condensation occurs between a quinonemethide anion and a effect.repulsive Then, effect. what ifThen, the condensation what if the occurscondensation between occurs a quinonemethide between a quinonemethide anion and a neutral anion monomer? and a neutral monomer? To answer these questions, the condensation reactions shown in Figure 10 were neutral monomer? To answer these questions, the condensation reactions shown in Figure 10 were Tocalculated answer these and questions,the results are the shown condensation in Figures reactions 11 and 12. shown in Figure 10 were calculated and the resultscalculated are shown and the in results Figures are 11 shown and 12 in. Figures 11 and 12.

Figure 10. The selected Michael addition reactions of resorcinol anion (RA) and neutral resorcinol FigureFigure 10. 10.The The selected selected Michael Michael addition addition reactions reactions of of resorcinol resorcinol anion anion (RA) (RA) and and neutral neutral resorcinol resorcinol(R) (R) with quinonemethides. with(R)quinonemethides. with quinonemethides.

FigureFigure 11. 11.The The calculated calculated structures structures of transition of transition states st andates product-likeand product-like intermediates intermediates for the for Michael the Figure 11. The calculated structures of transition states and product-like intermediates for the additionMichael reactions addition in reac Figuretions 10in. Figure 10. Michael addition reactions in Figure 10.

Polymers 2017, 9, 426 10 of 14 Polymers 2017, 9, 426 10 of 14

FigureFigure 12. 12. TheThe potential potential energy energy profiles proﬁles for the Mich Michaelael addition reacti reactionsons in Figure 1010..

Reactions (1) and (2) are the reactions of neutral 2-QM and 6-QM with a resorcinol anion. Reactions (1) and (2) are the reactions of neutral 2-QM and 6-QM with a resorcinol anion. Theoretical calculations predicted that they have energy barriers of 38.5 and 31.9 kJ/mol, Theoretical calculations predicted that they have energy barriers of 38.5 and 31.9 kJ/mol, represented represented by 2,6-Add-TS and 6,6′-Add-TS, respectively, which are much lower than those for by 2,6-Add-TS and 6,60-Add-TS, respectively, which are much lower than those for formations of the formations of the quinonemethides. Therefore, they are indeed faster reactions. As the formation of quinonemethides. Therefore, they are indeed faster reactions. As the formation of quinonemethide is quinonemethide is the rate-determining step, the overall condensation reaction can be identified as the rate-determining step, the overall condensation reaction can be identified as having a unimolecular having a unimolecular pattern. The formed intermediate products 6,6′-IM and 2,6-IM can evolve to pattern. The formed intermediate products 6,60-IM and 2,6-IM can evolve to 6-60-dimer (4,40-dimer) 6-6′-dimer (4,4′-dimer) and 2,6-dimer (2,4-dimer), respectively. Several steps of proton transfers or and 2,6-dimer (2,4-dimer), respectively. Several steps of proton transfers or migrations may occur in migrations may occur in this process, but they are generally fast reactions. Another reaction leading this process, but they are generally fast reactions. Another reaction leading to the 2,4-dimer is the one to the 2,4-dimer is the one between 4-QM or 6-QM and the 2-position of the resorcinol anion. But, between 4-QM or 6-QM and the 2-position of the resorcinol anion. But, the situation should be similar the situation should be similar to reaction (2). to reaction (2). Differently, reaction (3), the condensation of 4-QMA with neutral resorcinol encounters a Differently, reaction (3), the condensation of 4-QMA with neutral resorcinol encounters a notable notable barrier of 103.4 kJ/mol which significantly exceeded the barrier of 4-QMA formation (69.1 barrier of 103.4 kJ/mol which significantly exceeded the barrier of 4-QMA formation (69.1 kJ/mol) and kJ/mol) and became the rate-determining step. This must be caused by the much lower nucleophilic became the rate-determining step. This must be caused by the much lower nucleophilic reactivity of reactivity of neutral resorcinol. It was not our expectation that reaction (4) would encounter a much neutral resorcinol. It was not our expectation that reaction (4) would encounter a much lower barrier lower barrier of 63.1 kJ/mol. Although the repulsive effect makes the barrier higher than that of of 63.1 kJ/mol. Although the repulsive effect makes the barrier higher than that of reactions (1) and reactions (1) and (2), the barrier is still lower than that of the formation of 4-QMA (69.1 kJ/mol). (2), the barrier is still lower than that of the formation of 4-QMA (69.1 kJ/mol). Therefore, such a Therefore, such a condensation pattern is feasible. But, the overall reaction may not be a typical condensation pattern is feasible. But, the overall reaction may not be a typical unimolecular mechanism unimolecular mechanism because the 4-QMA formation and following Michael addition have because the 4-QMA formation and following Michael addition have similar energy barriers. In fact, similar energy barriers. In fact, 4-QMA may not directly participate in the condensation because it can 4-QMA may not directly participate in the condensation because it can convert to 4-QM or 6-QM convert to 4-QM or 6-QM through fast protonation of one of the carbonyl groups. Thus, the pathway through fast protonation of one of the carbonyl groups. Thus, the pathway with the lowest energy with the lowest energy barrier should be 4-HMRDA→4-QMA→protonation→4-QM (or 6-QM). barrier should be 4-HMRDA→4-QMA→protonation→4-QM (or 6-QM). So far, there is no experiment that compares PF and RF condensation reaction rates under the So far, there is no experiment that compares PF and RF condensation reaction rates under the same conditions (the same F/P or F/R molar ratio, catalyst concentration, and temperature) that has same conditions (the same F/P or F/R molar ratio, catalyst concentration, and temperature) that been reported. But, the calculated potential energy barriers for PF and RF quinonemethides allow us has been reported. But, the calculated potential energy barriers for PF and RF quinonemethides to make comparisons since the formation of these intermediates is the rate-determining step. allow us to make comparisons since the formation of these intermediates is the rate-determining step. According to our theoretical calculations of PF [18], the lowest energy barrier for E1cb formation of According to our theoretical calculations of PF [18], the lowest energy barrier for E1cb formation of neutral para-quinonemethide was calculated to be 99.7 kJ/mol, which is higher than that of 6-QM neutral para-quinonemethide was calculated to be 99.7 kJ/mol, which is higher than that of 6-QM (93.5 kJ/mol) by 6.2 kJ/mol, but the WAE mechanism for PF has a lower barrier of 87.8 kJ/mol. In (93.5 kJ/mol) by 6.2 kJ/mol, but the WAE mechanism for PF has a lower barrier of 87.8 kJ/mol. this sense, RF condensations are not faster. But, taking into account the participation of the In this sense, RF condensations are not faster. But, taking into account the participation of the quinonemethidesanions like 4-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. Such a big for its formation is significantly lower than the neutral PF QMs by 20–30 kJ/mol. Such a big difference difference is sufficient to explain the fact that the RF resin cure is much faster than PF resin, and that RF resin can be used as PF cure accelerator.

Polymers 2017, 9, 426 11 of 14 is sufﬁcient to explain the fact that the RF resin cure is much faster than PF resin, and that RF resin can be used as PF cure accelerator. Polymers 2017, 9, 426 11 of 14 SynthesisPolymers 2017, 9 of, 426 phenol-resorcinol-formaldehyde (PRF) co-condensed resin has been11 of 14 a hot topic [1–3Synthesis,5,6,8]. In of preparation phenol-resorcinol-formaldehyde of traditional phenol-resorcinol-formaldehyde (PRF) co-condensed resin has been resin, a hot resorcinol topic is generally[1–3,5,6,8]. added In into preparation the PF resol of resin. traditional It is believed phenol-resorcinol-formaldehyde that resorcinol can react withresin, the resorcinol hydroxymethyl is groupsgenerally of PF resin,added forming into the P–CH PF 2resol–R methylene resin. It is linkages believed [3 ].that With resorcinol such a pattern, can react resorcinol with the can be connectedhydroxymethyl to the end groups of PF of chains.PF resin, Conversely,forming P–CH can2–R phenolmethylene react linkages with [3]. the With hydroxymethyl such a pattern, group on resorcinol?resorcinol can What be connected will happen to the if phenol,end of PF resorcinol, chains. Conversely, and formaldehyde can phenol are react mixed with together? the Howhydroxymethyl do co-condensations group on compete resorcinol? with What self-condensations? will happen if phenol, These resorcinol, issues have and neverformaldehyde been addressed are mixed together? How do co-condensations compete with self-condensations? These issues have mechanisticallymixed together? in detail. How Accordingdo co-condensations to our understanding, compete with theself-condensations? co-condensation These efﬁciency issues is have not only never been addressed mechanistically in detail. According to our understanding, the dependent on the rates of PF and RF quinonemethides formation, but also on the selectivity of these co-condensation efficiency is not only dependent on the rates of PF and RF quinonemethides intermediates toward PF and RF monomers. Namely, both PF and RF quinonemethide intermediates formation, but also on the selectivity of these intermediates toward PF and RF monomers. Namely, can beboth formed PF and under RF quinonemethide alkaline conditions intermediates and their can be relative formed formation under alkaline rates conditions and reactivity and their toward phenol,relative resorcinol, formation and rates their and hydroxymethyl reactivity toward compounds phenol,phenol, resorcinol, determinesresorcinol, andand the theirtheir competitive hydroxymethylhydroxymethyl formations of self-compounds and co-condensed determines the polymers. competitive To formations address this of self- issue, and we co-condensed carried out polymers. further calculationsTo address on the representativethis issue, we carried PRF co-polyaddition out further calculations reactions on whichthethe representativerepresentative are shown PRFPRF in Figure co-polyadditionco-polyaddition 13 as reactions reactionsreactions (5)–(7). The calculatedwhich are shown results in areFigure given 13 as in reactions Figures (5)–(7). 14 and Th 15e calculated. results are given in Figures 14 and 15.

Figure 13. The Michael addition reactions of phenol anion (PA) with resorcinol quinonemethides Figure 13. TheThe MichaelMichael additionaddition reactionsreactions ofof phenolphenol anion (PA) with resorcinol quinonemethides (6-QM(6-QM and and 4-QMA) 4-QMA) and and the the reaction reaction of of resorcinolresorcinol anion anion (RA) (RA) with with phenol phenol quinonemethide quinonemethide (para-QM). (para -QM).

Figure 14. TheThe calculatedcalculated structuresstructures ofof thethe transitiontransition statesstates andand product-likeproduct-like intermediatesintermediates forfor thethe Figure 14. The calculated structures of the transition states and product-like intermediates for the Michael addition reactions in Figure 13. Michael addition reactions in Figure 13.

Polymers 2017, 9, 426 12 of 14 Polymers 2017, 9, 426 12 of 14

FigureFigure 15. 15. TheThe potential potential energy energy profiles proﬁles for for the the Mi Michaelchael addition addition reacti reactionsons in in Figure Figure 13.13.

ReactionReaction (5) (5) is is the the addition addition between between the the phenol phenol anion anion and and resorcinol resorcinol quinonemethide quinonemethide (6-QM). (6-QM). TheThe calculated calculated energy energy barrier barrier for for this this reaction reaction is 39.8 is 39.8 kJ/mol, kJ/mol, which which is higher is higher than reaction than reaction (1) by 7.9 (1) kJ/mol,by 7.9 kJ/mol,and higher and than higher the thanaddition the additionof the phen of theol anion phenol with anion phenol with quinonemethide phenol quinonemethide by 13.2 kJ/molby 13.2 [18]. kJ/mol This [18 ].result This resultindicates indicates that thatthe theco-condensation co-condensation reaction reaction between between phenol phenol and and hydroxymethylresorcinolhydroxymethylresorcinol isis lessless competitivecompetitive thanthan PF PF and and RF RF self-condensations. self-condensations. Similarly, Similarly, reaction reaction (6) (6)encounters encounters a higher a higher barrier barrier than self-condensations.than self-condensations. In contrast, In contrast, reaction reaction (7) has the(7) lowesthas the barrier lowest of barrier27.0 kJ/mol, of 27.0 which kJ/mol, is lowerwhich than is lower the barrier than the of reactionbarrier of (1) reaction by 12.8 (1) kJ/mol by 12.8 and kJ/mol very closeand very to the close barrier to theof PFbarrier self-condensation of PF self-condensation (26.6 kJ/mol), (26.6 suggesting kJ/mol), suggesting that the co-condensation that the co-condensation between the between resorcinol the resorcinolanion and anion phenol and quinonemethide phenol quinonemethide is energetically is ener moregetically competitive. more competitive. In this context, In this if context, resorcinol, if resorcinol,phenol, and phenol, formaldehyde and formaldehyde are mixed together, are mixed hydroxymethylation together, hydroxymethylation of resorcinol shouldof resorcinol be faster should than behydroxymethylation faster than hydroxymethylation of phenol due of to higherphenol reactivitydue to higher of resorcinol. reactivity In of addition resorcinol. to faster In addition formation to fasterof resorcinol formation quinonemethides, of resorcinol RFquinonemethides, self-condensation RF occurs self-condensation prior to PRF co-condensation, occurs prior to but PRF it is co-condensation,unlikely that the PRFbut reactionsit is unlikely can bethat excluded the PRF since reactions the gap can between be excluded the energy since barriersthe gap arebetween moderate. the energyAs barriers mentioned are inmoderate. the literature [3], a general strategy in the synthesis of PRF is to let resorcinol react withAs hydroxymethylphenol mentioned in the literature or with [3], PF resina general in excess strategy of un-reacted in the synthesis hydroxymethyl of PRF is groups. to let resorcinol In such a reacttype ofwith resin, hydroxymethylphenol the polymers are resorcinol or with PF terminated. resin in excess In the of curing un-reacted process, hydroxymethyl condensations groups. can occur In suchbetween a type terminal of resin, resorcinol the polymers moieties, are or resorcinol between phenolterminated. and hydroxymethylatedresorcinol.In the curing process, condensations In both cancases, fastoccur formation between of the resorcinolterminal quinonemethidesresorcinol moieties, can contribute or tobetween the curing phenol condensations. and hydroxymethylatedresorcinol.This is why PRF resin bears the In cold-setting both cases, characteristic. fast formation of the resorcinol quinonemethides can contribute to the curing condensations. This is why PRF resin bears the cold-setting characteristic. 4. Conclusions 4. Conclusions Base-catalyzed resorcinol-formaldehyde condensation reactions were theoretically investigated in thisBase-catalyzed study by employing resorcinol-formaldehyde a quantum chemistry condensati method.on reactions The experimentally were theoretically observed investigated polymer instructures this study were by rationalizedemploying a according quantum tochemistry the identiﬁed method. mechanisms The experimentally and the calculated observed energetics. polymer structuresThe results were were rationalized discussed and according compared to the with iden thosetified obtained mechanisms previously and th fore phenol-formaldehydecalculated energetics. Thereactions. results The were main discussed conclusions and compared have been with drawn those as follows:obtained previously for phenol-formaldehyde reactions. The main conclusions have been drawn as follows: (1) The condensation reaction includes two steps: Formation of a quinonemethide (QM) intermediate (1) Thefrom condensation hydroxymethylresorcinol, reaction includes followed two by Michaelsteps: Formation addition between of a thequinonemethide quinonemethide (QM) and intermediatethe resorcinol from anion. hydroxymethylresorcinol, followed by Michael addition between the (2) quinonemethideTwo mechanisms, and unimolecular the resorcinol elimination anion. of conjugate base (E1cb) and water-aided elimination (2) Two(WAE), mechanisms, were identiﬁed unimolecular for the formation elimination of quinonemethide. of conjugate Hydroxymethylresorcinolbase (E1cb) and water-aided anion eliminationproduces a neutral(WAE), quinonemethide were identified while thefor dianionthe producesformation a quinonemethideof quinonemethide. anion. HydroxymethylresorcinolThe calculated potential energy anion barriers produces suggested a neutral that quinonemethide quinonemethide anion while formation the dianion is much producesmore favorable. a quinonemethide Although resorcinol-formaldehyde anion. The calculated potential and phenol-formaldehyde energy barriers suggested share common that quinonemethidecondensation mechanisms, anion formation the former is much would more be favorable. faster if the Although quinonemethide resorcinol-formaldehyde anion is involved. (3) andThe phenol-formaldehyde potential energy barriers share for common formation condensation of 2-QM, 4-QM, mechanisms, 6-QM, 2-QMA, the former and 4-QMA would were be fastercalculated. if the quinonemethide The results show anion that is the involved. formation of 6-QM and 4-QMA have relatively lower

Polymers 2017, 9, 426 13 of 14

energy barriers. In addition to higher reactivity of the position-6 hydroxymethylation reaction, the previous experimental observation that the 2,4-(2,6-) and 6,60-(4,40-) methylene linkages were dominant polycondensed structures has been rationalized. (4) The cold-setting characteristic of phenol-resorcinol-formaldehyde co-condensed resin can be attributed to participation of resorcinol quinonemethides in condensations.

Acknowledgments: This work was supported by the Programs of the National Natural Science Foundation of China (NSFC) (31360159 and 51273163). Author Contributions: Taohong Li and Guanben Du contributed to the theoretical calculations and editing of the paper. Ming Cao and Jiankun Liang contributed to the collection of references and editing of the paper. Xiaoguang Xie contributed to the analysis of the calculated results. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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