Ind. Eng. Chem. Res. 2003, 42, 4147-4155
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APPLIED CHEMISTRY Density Functional Investigation of Melamine-Formaldehyde Cross-Linking Agents. 1. Partially Substituted Melamine Michael T. Benson† Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415-2208
Computational modeling has been performed on the cross-linking mechanism of partially substituted melamine reacting with poly(vinyl alcohol). Ab initio calculations were performed with density functional theory employing the BLYP functional and double numerical DND basis set. The mechanism, general acid catalysis, has been modeled with respect to structures, protonation, and reaction with a polymer. Protonation of the oxygen is required initially, followed by liberation of methanol. The reaction can take one of two pathways after methanol liberation. The conjugate base of the weak acid can abstract a proton, generating a Schiff-base intermediate. The O-H on the polymer then adds to the C-N double bond, producing the polymer-melamine bond. The alternative is that the polymer adds to the charged melamine, using the specific acid catalysis mechanism. After polymer addition, the proton is abstracted, producing the product. 1. Introduction The reaction mechanism for cross-linking with melamine-formaldehyde (MF) resins has been understood for over 20 years and is known to be dependent on the imine (-NH) content.1,2 Specific acid catalysis is required for highly substituted melamines (no imine groups), while melamines with imine groups (referred to as MF-3, Figure 1) can use general acid catalysis (Figure 2). The first step is protonation of the crosslinking agent, followed by liberation of methanol. In step 2, the conjugate base of the weak acid required for general acid catalysis abstracts the proton, leaving a Schiff-base intermediate behind, which then reacts with the hydroxyl group on the polymer. Buckley and Oppenheimer3 have used semiempirical methods to investigate general versus specific acid catalysis for aldehyde hydrates, hemiacetals, acetals, methylribosides, and glucosides with respect to protonation, the leaving group, and the remaining cation. The mechanism was shown to depend on the stability of both the leaving group and carbocation. The leaving group and cation are not an issue in this investigation. Methanol is the leaving group, and the cation is stabilized through resonance structures with the triazine ring. This is apparently the first paper on MF cross-linking agents employing first principle calculations, discussing structures, protonation (activation and deactivation), and reaction with a polymer. Previous computational modeling has been reported for melamine.4-6 In one paper, calculated structures for ammeline, melamine, and 2,4-diamino-1,3,5-triazine, as well as the protonated structure of each compound, are presented.4 Larkin et al.7 calculated the IR and Raman frequencies for trisand hexa(methoxymethyl)melamine, though they did not discuss the optimized structures. A decomposition † Tel.: (208) 526-1316. Fax: (208) 526-8541. E-mail: bensmt@ inel.gov.
Figure 1. Three-arm melamine (MF-3). In all figures, the square lines filled pattern represents nitrogen, the dotted filled pattern represents oxygen, the solid filled pattern represents carbon, and the open circles represent hydrogen.
study of N-(hydroxymethyl)pentamethylmelamine, an antitumor drug, has been performed using semiempirical MNDO and PM3 techniques.8 Computational studies have been performed on acid catalysis, specifically Brønsted acids.3,9,10 Ozment et al.9 investigated protonation of 3-acetyltriazene to look at the decomposition of triazenes, with predicted bond cleavage based on the protonation site. Where appropriate, comparisons of the present calculations with previous calculations will be made. This is the first paper in a computational investigation of MF cross-linking agents. The second paper11 will cover fully alkylated, fully substituted melamines, while this paper presents an investigation of a fully alkylated, partially substituted melamine (Figure 1), where alky-
10.1021/ie020638e CCC: $25.00 © 2003 American Chemical Society Published on Web 07/22/2003
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Figure 2. General acid catalysis, with R ) CH3.
Figure 3. BLYP/DND optimized structure of melamine. Mulliken atomic charges are as follows: Nring, -0.46; Namino, -0.68; C, 0.42. In all figures, distances are in angstroms.
lated refers to a -CH3 terminal group as opposed to -H, and substitution refers to an arm on the amino nitrogen as opposed to hydrogen. In this study, MF-3 is substituted symmetrically (one arm and one hydrogen on each amino nitrogen) to simplify modeling. Although a highly symmetrical, partially substituted melamine is not synthetically feasible,1 a high degree of symmetry is possible.12 While an understanding of the cross-linking mechanism is known, detailed knowledge of the structures, both stationary and transition states, and energetics is not available. Computational modeling can simplify an otherwise very complex reaction, due to a multitude of competing reactions, and fill in the gaps. 2. Methodology 2.1. Computational Methods. All calculations were performed with DMOL3,13 contained in the Materials Studio Suite.14 Structures were optimized by employing density functional theory (DFT),15 with the Becke exchange functional16 and the Lee-Yang-Parr correlation functional (BLYP).17 BLYP incorporates gradient corrections into the electron density. The basis set used was the double numerical DND basis set.15 Vibrational analysis was used to determine minima (no imaginary frequencies) or transition states (one imaginary fre-
quency). No symmetry was used in the calculations. Atomic charges were calculated with a Mulliken population analysis.18 Although there are deficiencies in calculating charge distributions in this way, the calculated atomic charges are sufficient for a qualitative discussion. Energies were corrected using the zero-point energies and from absolute zero to 298.15 K using vibrational frequencies. Proton affinities were calculated by subtracting the enthalpy of the neutral species from the enthalpy of the cation, as performed previously.9 All calculations are gas phase and, as such, do not include solvent effects. The low-energy conformations studied here may not be the lowest energy in the solution. Also, many of the melamine systems modeled in this paper are charged. Solvation effects, such as H bonding, can stabilize some conformations better than others. Despite this, gas-phase calculations have provided useful information for related systems, such as the acid-catalyzed decomposition of 3-acetyltriazene,9 and thus can also provide an understanding of substituted melamine cross-linking agents. 2.2. Melamine Optimized Structure. Melamine (3A in Figure 3) was calculated to compare geometric data to literature values, both computational4,6 and experimental.19,20 The BLYP structure compares very
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well with the previously calculated structures. Bond lengths are all within 0.03 Å, angles are identical, and the out-of-plane NH2 angles (3B) are also close, calculated to be 8.4° and 8.6° for the two NH2 groups down below the triazine ring plane and 9.7° for the up NH2 group. These values correspond closer to the X-ray structure discussed in ref 5 (8.43°, 4.03°, and 3.18°) than the calculated structure (12.2°, 9.9°, and 9.9°), also from ref 5. X-ray bond distances for the ring C-N bonds are between 1.340 and 1.355 Å, while the Cring-Namino distances are 1.343, 1.362, and 1.337 Å.19 Calculated ring angles (126° for N-C-N and 113° for C-N-C) are in excellent agreement with the X-ray structure (125.5°, 125.9°, and 124.9°; 114.3°, 114.7°, and 114.6°). All BLYP/DND calculated distances, angles, and out-ofplane angles correspond well to both previously calculated structures and both X-ray structures. On the basis of these findings, BLYP/DND was considered a suitable level of theory for the present calculations. 3. Results and Discussion The discussion is broken into four subsections, beginning with the optimized structure for MF-3. The second, third, and fourth subsections are organized to follow the reaction mechanism shown in Figure 2, namely, protonation, methanol liberation, and polymer addition. 3.1. MF-3 Optimized Structure. The optimized structure for the fully alkylated, partially substituted melamine cross-linking agent is shown in Figure 1. Table 1 lists geometric data, as well as selected Mulliken atomic charges. Two of the arms in MF-3 come “up”, out of the triazine ring plane, and the third points “down”. Geometrically, MF-3 is very similar to melamine (Figure 3), with bond lengths differing by no more than 0.02 Å, while the only difference in angles is in the amine angle. The H-N-H angle in melamine is 122°, while H-N-Carm in MF-3 is 117°. Atomic charge is an indicator of where an electrophile, such as a proton, will preferentially attack. The oxygen and ring nitrogen in MF-3 carry similar charges, at -0.45 and -0.48, but the nonring nitrogen has more electron density, -0.53, making it more susceptible to electrophilic attack. This is in stark contrast to the fully
Table 1. Bond Lengths, Angles, and Mulliken Atomic Charges for MF-3a bond
MF-3 bond lengths (Å)
N1-C2 C2-N3 C2-N5 N5-C6
1.35 1.35 1.38 1.44
bond
MF-3 bond lengths (Å)
C6-O7 O7-C8 N5-H9
1.45 1.43 1.02
angle
angles (deg)
angle
angles (deg)
N1-C2-N3
126.1
C2-N3-C4
113.9
atom
atomic charge
atom
atomic charge
N1 C2 N5
-0.48 0.49 -0.53
C6 O7 C8
-0.03 -0.45 -0.35
a See Figure 1 for atom numbering. In all figures, unless otherwise stated, symmetrical bonds or atoms have identical lengths or atomic charges, respectively.
substituted melamine (referred to as MF-6), where the ring nitrogen and oxygen are again roughly the same (-0.47 and -0.45, respectively), yet the nonring nitrogen is -0.36.11 The nonring nitrogen in MF-3 is more susceptible to electrophilic attack than that in MF-6. This is in agreement with experimental data,21 which shows that basicity increases as substitution decreases. Melamine (MF-0) also fits into this trend, with an atomic charge of -0.46 for the ring nitrogen and -0.68 for the nonring nitrogen. 3.2. Protonated MF-3 Cross-Linking Agent. The general acid catalysis mechanism is shown in Figure 2. For the reaction mechanism to proceed, protonation of the oxygen is required, with the subsequent liberation of methanol. The placement of the proton has been discussed in the literature. Hill and Lee21 and Berge et al.2 have both discussed the location of protonation, although they discussed ring protonation versus oxygen protonation and neglected the third possibility, protonation of the imine nitrogen. Protonated structures are discussed first, followed by energetics. Figures 4 and 5 show the protonated structures for MF-3: protonated oxygen (MF-3+-O) (4A), protonated nonring nitrogen (MF-3+-Namino) (4B), and protonated
Figure 4. MF-3+-O (4A) and MF-3+-Namino (4B). In all figures, the black sphere is the added proton.
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Figure 5. MF-3+-Nring. All three Cring-Namino distances (in 5A-C) are 1.36 Å: (5A) not stabilized; (5B) stabilized by one arm; (5C) stabilized by two arms.
ring nitrogen (MF-3+-Nring) (5). In 4A, the C-O bond is elongated to 2.34 Å, Namino-C is shortened to 1.30 Å, and Cring-Namino is lengthened to 1.46 Å. The long C-O bond places the protonated intermediate closer to the product and liberated methanol on the reaction path. When optimizing this protonated oxygen structure, if the arm was rotated slightly toward the triazine ring, the structure optimized to 5B, MF-3+-Nring with one arm stabilizing the proton. After oxygen protonation, methanol is very close to breaking free in 4A and could, at this point, be considered an adduct. Structural evidence for this is found in the C-N double bond, 1.30 Å (4A) vs 1.29 Å in Figure 6 (vide infra). Further, Mulliken atomic charges also show the structures to be very near the product. The charge on the protonated oxygen is -0.65 (4A), very close to the calculated charge for oxygen in methanol, -0.63. Also, the nitrogen on the protonated arm has a charge of -0.36 (4A), compared to -0.32 in the product (6). On the basis of these data, the charge is formally placed on the nitrogen, as shown in 4A, and the C-N multiple bond is almost identical to that of the product. The second possible protonation site is the nonring nitrogen, MF-3+-Namino (4B). The bonds around the protonated nitrogen are longer than those of MF-3, with Cring-Namino ) 1.51 Å and Namino-Carm ) 1.53 Å.
Sterically, the nonring nitrogen is easily accessible to an incoming proton. The triazine ring and amino nitrogens are all planar, allowing access from either side. After attack, the nitrogen adopts a tetrahedral geometry. The Cring-N-Carm angle decreases by 10.6° (114.9° vs 125.5° in the nonprotonated arm), and the C-N-H angles decrease by 5° (Cring-N-H) or 9° (N-H-Carm). The third potential protonation site is the ring nitrogen, MF-3+-Nring, with several structures possible. 5A shows MF-3+-Nring with no stabilization from an adjacent arm, 5B has one arm stabilizing the proton, and 5C has two arms stabilizing the proton. Rotation of the Cring-Namino bond (from the minimum-energy structure, Figure 1) is required for 5A and 5C; however, the energy is raised by only 0.3 kcal/mol as a result of this rotation. Because rotation is not an issue sterically, 5C is the most likely conformation because it was found to have the lowest energy (vide infra). In all three MF-3+-Nring structures, the charge and bond lengths suggest that the charge is delocalized between the nitrogen and both adjacent carbons. The adjacent ring C-N bond lengths are either 1.39 or 1.40 Å, depending on whether the arm is in a stabilizing position (1.39 Å) or not (1.40 Å). Clearly, there is less double-bond character in the adjacent bonds. The atomic charges on the ring atoms also show the charge to be
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Figure 6. Transition state (6A) and product (6B) from methanol liberation. Table 2. Atomic Charges on MF-3+-Nring protonated nonprotonated N N MF-3+-Nring, 5A MF-3+-Nring, 5B MF-3+-Nring, 5C
-0.64 -0.67 -0.70
-0.42, -0.43 -0.42, -0.42 -0.40, -0.41
o-C
p-C
0.61, 0.62 0.51 0.60, 0.57 0.51 0.56, 0.57 0.50
Table 3. Calculated Energies, Enthalpies, and Proton Affinities for Protonated MF-3 Structures
MF-3+-O, 4A MF-3+-Namino, 4B MF-3+-Nring, 5A MF-3+-Nring, 5B MF-3+-Nring, 5C
energy (hartrees)
enthalpy correction (kcal/mol)
proton affinity (kcal/mol)
-908.366 396 1 -908.371 347 1 -908.391 711 0 -908.398 043 5 -908.400 695 9
201.909 205.118 204.266 204.346 204.346
176.2 176.1 189.7 193.6 195.3
delocalized. Table 2 lists the atomic charges for 5A-C. The ortho carbons have a higher atomic charge in all three structures than in MF-3 (0.49). As more stabilizing arms are added, the atomic charge on the ortho carbons decreases, from 0.61 and 0.62 in 5A to 0.60 and 0.57 in 5B and 0.56 and 0.57 in 5C. Enthalpies for protonated MF-3 are listed in Table 3. MF-3+-Nring (5C) is the lowest in energy, while MF3+-O (4A) is the highest, by 19.1 kcal/mol. For the mechanism (Figure 2) to proceed forward, the oxygen must be protonated, facilitating the liberation of methanol. Energetically, MF-3+-Namino (4B) is roughly the same as MF-3+-O. This is an energetically feasible dead end for the mechanism though. Not only is there no elongation of the C-O bond, the bond length actually decreases (1.40 vs 1.45 Å in MF-3). MF-3+-Nring, Figure 5, is the lowest in energy, with three possible conformations, as previously stated. A structure with one arm rotated around the Cring-Namino bond, with respect to MF-3, is 0.3 kcal/mol higher in energy as a result of sterics, making the stabilization energy from the first arm 3.6 kcal/mol and 1.3 kcal/mol for the second arm. As with MF-6, this too is a dead end for the mechanism, regardless of conformation. Proton affinities for protonated MF-3 are listed in Table 3. The largest difference is between 4A and 5C. The higher proton affinities are due to hydrogen bonding stabilizing the proton. 4A does not have any stabilizing arms, while 5C has two stabilizing arms. To our knowledge, the only similar system with calculated proton affinities is 3-acetyltriazene.9 3-Acetyltriazene
has proton affinities between 180.4 and 234.6 kcal/mol, depending on the protonation site and hydrogen bonding, while calculated proton affinities for MF-3 are between 176.2 and 195.3 kcal/mol. If the protonation site was based only on atomic charges, MF-3+-Namino would be the lowest, with MF3+-O and MF-3+-Nring roughly the same. When the ring nitrogen is protonated, the bond lengths and angles undergo very little change from MF-3, with the largest change being an increase in adjacent triazine C-N bond lengths by 0.04 Å. MF-3+-Namino and MF-3+-O undergo large structural changes due to the proton. In the case of MF-3+-O, the two large changes are the C-O bond increase by 0.93 Å and the Namino-C bond decrease by 0.14 Å. In the case of MF-3+-Namino, the Carm-Namino and Cring-Namino bonds increase by 0.09 and 0.13 Å, respectively, and the nitrogen becomes tetrahedral, from trigonal planar. These large structural changes cause MF-3+-O and MF-3+-Namino to be much higher in energy than MF-3+-Nring. These data indicate that one possible mechanism is ring protonation followed by stabilization through one arm. MF-3+-Nring with one stabilizing arm (5B) has the shortest O-H interaction distance (1.99 vs 2.19 Å and 2.24 Å in 5C, MF-3+-Nring with two stabilizing arms). Proton transfer to the oxygen, followed by methanol liberation (vide infra) is likely because of the elevated cure temperatures, removing methanol from the reaction medium and driving the reaction forward. 3.3. Methanol Liberation. The reactant for general acid catalysis (Figure 2) is the protonated oxygen melamine (4A). The C-O bond length is elongated, at 2.34 Å. Heat from the curing process will drive the methanol off, leaving behind a protonated intermediate (6B). In Figure 2, the reaction is shown as being reversible. It is assumed that the methanol will be volatilized and removed from the reaction medium. If methanol remains longer, it could also add to the Schiffbase intermediate, in a manner similar to poly(vinyl alcohol) (PVA) addition (section 3.4.1, vide infra), reforming MF-3. The transition state and product for methanol liberation are shown in Figure 6 (6A and 6B, respectively). The transition state (6A) has one imaginary frequency, at 132i cm-1. The only significant change from MF3+-O (4A) is the elongation of the C-O bond to 2.39 Å, from 2.34 Å, further separating methanol from the MF-3 fragment. The product (6B) after methanol liberation
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Figure 7. Energy scheme (in kcal/mol) for methanol liberation from MF-3.
is a protonated Schiff base, with the proton on the amino nitrogen. The Schiff-base C-N bond is 1.29 Å, only 0.01 Å longer than the transition state and the neutral Schiff-base product (9A, vide infra). The Cring-Namino bond is lengthened, from 1.36 Å in MF-3 to 1.47 Å, only slightly longer than that of 9A, 1.43 Å. Figure 7 shows the energy scheme for methanol liberation. The product is the separated melamine and methanol and not an intermolecular complex. An intermolecular complex was not located, most likely because it is very weak. The transition state is higher than MF-3+-O by 0.15 kcal/mol, making the reaction essentially barrierless. The reaction is endothermic by 9.35 kcal/mol. The reverse reaction, addition of methanol, is exothermic with a downhill transition state. Although liberating methanol is endothermic, the elevated cure temperatures required for cross-linking with substituted melamines are required more to volatilize methanol to remove it from the reaction medium, thereby preventing the reverse reaction, than to provide the energy input to drive methanol liberation. 3.4. Polymer Addition. To keep the calculation size reasonable, the polymer fragment studied contained only 3 repeating units. PVA calculated with 3, 9, and 20 repeating units has identical bond lengths for the inner OH groups (C-C ) 1.54 Å; C-O ) 1.46 Å) and identical atomic charges (C, 0.01; O, -0.64). The center OH group in the three-unit PVA was used as the reactive site for polymer addition. On the basis of the bond lengths and atomic charges, the three repeating unit polymer should accurately model addition to a full polymer. There are two pathways a polymer can add to MF-3 after liberation of methanol, as shown in Figure 8. The first pathway is a noncatalyzed addition (mechanism 1) and involves addition of the O-H across the C-N double bond with the nitrogen unprotonated. The second pathway (mechanism 2) involves addition of the oxygen to the Schiff-base carbon, with the adjacent nitrogen protonated. In both cases, abstraction of a proton is required to produce the product. This could happen after
methanol liberation, producing a Schiff-base intermediate, or after polymer addition, as occurs for polymer addition to the fully substituted melamine.11 The strength of the acid catalyst present determines the mechanism. The pKb of MF-3 has been measured to be 10.4,20 indicating that the pKa of an acid catalyst would have to be greater than 3.6 in order for mechanism 1 to dominate. A stronger acid catalyst (pKa < 3.6) would favor the reaction proceeding primarily through mechanism 2. 3.4.1. Noncatalyzed Polymer Addition. The reactant and transition state for the noncatalyzed mechanism are shown in Figure 9 (9A and 9B, respectively). The protonated Schiff base (6B) is produced after methanol liberation. A weak acid abstracts the proton, producing the neutral Schiff-base intermediate (9A). Except for the Schiff-base (SB) arm, the bond lengths are roughly the same as those in MF-3, varying by no more than 0.01 Å. The Cring-NSB bond is slightly longer than MF-3, at 1.43 Å, and the C-N bond is 1.28 Å, a typical length for a carbon-nitrogen double bond. The transition state is shown in 9B. The optimized structure has one imaginary frequency at 624i cm-1. It is a four-centered transition state, with the O-H adding across the C-N double bond. The C-N double bond lengthens substantially to 1.40 Å, an increase of 0.12 Å, while the Cring-Namino bond is only different from the product by 0.01 Å. The O-H bond is lengthened to 1.18 Å, and the N-H bond is 1.47 Å. The C-O bond is at 1.67 Å, 0.2 Å longer than that in the product. Except for the hydrogen transfer, the transition state is close to the product, indicating a late transition state. The MF-3-PVA product is shown in 9C. The structure is very close to MF-3, with identical triazine ring and Cring-Namino bond lengths. Sterically, the polymer was found not to interact with the cross-linking agent aside from the formation of the ether linkage. The polymer is positioned at an angle with respect to the triazine ring plane. If the polymer was extended beyond three repeating units, each unit would be farther from the cross-linking agent. This is a simplistic approach to keep the model feasible because in a real system the cross-linking agent could interact with or bond to the same polymer chain more than once. 3.4.2. Acid-Catalyzed Polymer Addition. The reactant for the acid-catalyzed mechanism is the product after methanol liberation (6B). Figure 10 shows the transition state (10A) and protonated MF-3-PVA product (10B). The reaction proceeds in the same manner as the reverse of the methanol liberation reaction. The transition state is very close to the protonated oxygen structure, in this case the product. The C-O bond is
Figure 8. Noncatalyzed (mechanism 1) and acid-catalyzed (mechanism 2) polymer addition.
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Figure 9. Schiff-base reactant (9A), transition state (9B), and product (9C) for noncatalyzed polymer addition.
Figure 10. Transition state (10A) and protonated MF-3-PVA (10B).
longer than the product by only 0.04 Å. 10A is a true transition state, though with one imaginary frequency of 178i cm-1. Because the product is the protonated structure, the transition state in this addition reaction is very late. The protonated product, 10B, is structurally
very similar to MF-3+-O (4A). The C-O bond is slightly shorter in 10B, 2.32 vs 2.34 Å in 4B. To form the neutral product (9C), abstraction of the proton is required. 3.4.3. Energetics for Polymer Addition. Figure 11 shows the energy scheme for the addition of PVA to MF-
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Figure 11. Energy scheme (in kcal/mol) for (11A) noncatalyzed polymer addition and (11B) acid-catalyzed polymer addition.
3, through the noncatalyzed (11A) and acid-catalyzed (11B) mechanisms. As in the case of methanol liberation, intermolecular complexes were not located. Given the very long C-O bonds in the transition states, an intermolecular complex is expected to be very weak. The noncatalyzed mechanism is exothermic by 15.4 kcal/mol, with a 36.4 kcal/mol barrier (11A). The acid-catalyzed reaction is exothermic by 11.5 kcal/mol and has a downhill transition state. The proton affinity calculated for the protonated product, 10B, is 216.9 kcal/mol as compared to 220.7 kcal/mol for the protonated Schiff base (6B). Based solely on energetics, the acid-catalyzed reaction should dominate the addition reaction. The proton should be easier to abstract after the addition of the polymer, and the reaction is exothermic with a downhill transition state, similar to the reverse reaction for methanol liberation (7). However, this is not in agreement with experiment.1 It has been shown that partially substituted melamines, such as MF-3, have a better cure response with a weak acid than a strong acid, indicating that the general acid catalysis mechanism is operating. This discrepancy is best explained by noting that the reverse reaction is facile for one mechanism yet much more difficult for the other. In general acid catalysis, the product is a neutral polymermelamine. To reverse the reaction, protonation of the polymer oxygen has to occur, followed by breaking of the C-O bond. Protonation is unlikely due to competing sites, the ring and nonring nitrogens, and the oxygens on the other arms, whereas in the specific acid-catalyzed reaction, the product is the protonated polymermelamine. Reversing this reaction is simply breaking the C-O bond. 4. Conclusions A partially substituted melamine cross-linking agent has been investigated, with respect to the general acid catalysis mechanism. The initial steps in the mechanism are protonation of the melamine followed by liberation of methanol. Although it is required for the mechanism to proceed, the oxygen is the least likely to be protonated (by 19.1 kcal/mol), with the ring nitrogen the most likely. Mulliken atomic charges indicate that the nonring nitrogen is the most likely to be protonated, while the oxygen and ring nitrogen are roughly equivalent. This is not what is observed, though, because of the large structural changes that occur when the oxygen and nonring nitrogen are protonated. Protonation of the ring nitrogen (stabilized with only one arm) is likely the first step, followed by proton transfer to the oxygen.
Liberation of methanol is an endothermic reaction (by 9.35 kcal/mol) and the transition state barrier is thermoneutral. The reverse, addition of methanol, is exothermic with a downhill transition state, although the elevated cure temperatures are required to remove methanol from the reaction medium, driving the reaction to completion. After liberation of methanol, the addition of the polymer can follow an acid-catalyzed or a noncatalyzed mechanism, depending on the acid strength. Energetically, following the acid-catalyzed mechanism appears to be an easier mechanism, although the proton has to be abstracted to obtain the neutral product. In the noncatalyzed mechanism, a barrier of 36.4 kcal/mol has to be overcome to add the polymer. The protonated polymer product has a lower gas-phase acidity (216.9 kcal/mol) than the protonated Schiff base (220.7 kcal/mol), making proton abstraction easier after polymer addition. This suggests that polymer addition will proceed through the acid-catalyzed mechanism, the same as the fully substituted melamine, MF-6.11 This is not the case experimentally, though, where the reverse reactions dominate the cure response behavior.1 To reverse the general acid catalysis reaction, protonation of the polymer oxygen has to occur, followed by breaking of the C-O bond. Protonation is unlikely due to competing sites, the ring and nonring nitrogens, and the oxygens on the other arms, whereas in the specific acid-catalyzed reaction, the product is the protonated polymer-melamine. Reversing this reaction is simply breaking the C-O bond. Acknowledgment This research was supported by the United States Department of Energy, under Contract DE-AC0799ID13727. Literature Cited (1) Blank, W. J. Reaction-Mechanism of Melamine Resins. J. Coat. Technol. 1979, 51, 61. (2) Berge, A.; Kvæven, B.; Ugelstad, J. Melamine-Formaldehyde Compounds. 2. Acid Decomposition of Methylol Melamines and Methoxymethyl Melamines. Eur. Polym. J. 1970, 6, 981. (3) Buckley, N.; Oppenheimer, N. J. Reactions of Charged Substrates. 7. The Methoxymethyl Carbenium Ion Problem. 2. A Semiempirical Study of the Kinetic and Thermodynamic Stabilities of Linear and Cyclic Oxo- and Thiocarbenium Ions Generated from Aldehyde Hydrates, Hemiacetals, Acetals, and Methyl Ribosides and Glucosides. J. Org. Chem. 1996, 61, 8048. (4) Wang, Y.; Pittman, C. U., Jr.; Saebo, S. Investigation of the Structure and the Properties of Ammeline, Melamine, and 2,4-
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Received for review August 16, 2002 Revised manuscript received June 15, 2003 Accepted June 15, 2003 IE020638E
