Hydrogen bonding effects on aspartate ester reactions.
Keywords Hydrogen bonding, Aspartate ester, Polyurea coatings, Isocyanates
The high abrasion resistance, gloss, hardness, and corrosion resistance of polyurea coatings are used in industrial maintenance coatings applications as well as in truck bed and secondary containment liners. Two component formulations are required due to the rapid reaction between the NH and NCO components. The components are normally mixed at the spray head, with the polymer's molecular weight and viscosity building rapidly after application. The rapid curing rates for primary NHs prevent sufficient flow and leveling to occur. Thus, reducing reactivity via proper diamine selection is one of the main challenges in polyurea coating formulation. Aliphatic NHs have enhanced reaction rates as compared to their aromatic counterparts due to increased nucleophilicity, and impart superior mechanical and weathering properties to the coating. In addition, cure rates can be lowered by utilizing blocked NHs (i.e., aldimines and ketimines) that become active upon reacting with atmospheric moisture and/or formulating with ASP-based secondary diamines. (1) Polyurea coatings formulated with ASP-based diamines display enhanced resistance to weathering, superior stain resistance, and an acceptable pot life for flow and leveling needed to yield high gloss films. These modified diamines exhibit reduced reaction rates with NCOs primarily because their NH groups are secondary and therefore can H-bond with the closest carbonyl to form a five-membered ring (Fig. 1). (2)
[FIGURE 1 OMITTED]
HB polymers are reported to have lower viscosity and higher end group functionalities over linear polymers with comparable molecular weights. NMR spectroscopic analysis is the primary method for characterizing the type of architecture formed and the degree of branching. HB polymers are typically formed from [AB.sub.2] type monomers having equally reactive B groups (Fig. 2). When one of the reactive B groups displays reduced reactivity, a less branched architecture or even linear chains are formed depending on the reactivity discrepancy.
[FIGURE 2 OMITTED]
HB polymers have received significant attention due to their three-dimensional structure, which provides unique advantages over linear polymers in coatings applications. Commercially feasible HB polymers have "tree-like" architectures and are prepared in a one-step reaction to make them more economical than their linear counterparts. Nevertheless, manufacturing HB polymers on a commercial scale is limited by poor reproducibility and gelation concerns arising from an elevated concentration of functional groups and a large number of side reactions. Tomalia (3) was the first to utilize Michael addition reactions to synthesize HB polymers.
A highly branched polymer architecture is advantageous for coatings applications due to their unique rheological behavior, increased functional group concentration, and enhanced solubility in organic solvents. HB polymers are ideal film formers since their three-dimensional architecture reduces viscosity (fewer chain entanglements) and promotes crosslinking reactions (increased density of reactive sites). Moreover, solvent demands required for high gloss films are reduced due to improved solvation of polar reactive groups, reduction in chain entanglements, and high molar mass buildup via thermosetting reactions after application. (4)
Aspartate ester (ASPE) functional reactants have received considerable attention in patent literature. (5-7) Commercial ASPE-based diamines are derived from the Michael addition reaction between aliphatic diamines and diethyl maleate (DEM). These low molecular weight reactants bear two secondary NHs to be crosslinked with multifunctional NCOs. Synthesis of polyaspartate esters (PASPEs) with functionalities greater than two is accomplished by reacting an aminoalcohol (e.g., AP) with a maleate (e.g., DEM), followed by subsequent polyesterification (Fig. 3). This synthesis is described in patent literature and discloses the resulting polymers as having a "hyperbranched" structure. (5-7) The aim of this work was to prepare hyperbranched polyesters bearing amine groups of low nucleophilicity due to H-bonding. It was believed that these products would make interesting candidates for hyperbranched polymer studies related to interaction of polar surface polymers and as novel reactants for polymer network formation. The first part of this research focuses on H-bonding effects on the chemoselectivity of the esterification reaction. The second section investigates the influence of H-bonding on the reaction between the ASPE and an aliphatic NCO.
[FIGURE 3 OMITTED]
DEM, 3-amino-1-propanol (AP), cyclohexyl isocyanate (cyclohexyl NCO), d-chloroform (containing 0.03 vol.% tetramethylsilane) ([CDCl.sub.3]), dimethyl sulfoxide-[d.sub.6] (DMSO-[d.sub.6]), and titanium butoxide were used as received from Sigma-Aldrich. All reactions were performed neat in a 250 mL, one-neck glass flask equipped with an egg-shaped magnetic stir bar and heated using a mineral oil bath.
The preparation techniques for both the monomer and polymer were published in patent examples. The Michael addition reaction between AP (37.5 g; 0.5 mole) and DEM (86.1 g; 0.5 mole) was performed by adding the maleate dropwise under a nitrogen environment to the aminoalcohol at 60[degrees]C over 7 h.
After the Michael addition reaction was complete as demonstrated by NMR, polyesterification was performed by increasing the reaction temperature to 120[degrees]C, adding 0.3 wt% titanium butoxide catalyst and applying reduced pressure (ca. 5-6 Torr) to facilitate ethanol removal. Reaction conversion reached 88% and was monitored gravimetrically by condensing the ethanol into a dry ice/acetone bath and weighing the recovered ethanol.
ASPE reaction with isocyanate
This reaction was characterized using NMR and Fourier transform infrared (FTIR) spectroscopy. ASPE in Fig. 2 (61.7 g; 0.25 mole) was reacted with cyclohexyl NCO (31.3 g; 0.25 mole) at room temperature in equimolar quantities. Upon blending ASPE with cyclohexyl NCO, an exothermic reaction was observed immediately and the product solidified.
NMR spectral data were acquired using two instruments. Routine [.sup.1]H and [.sup.13]C spectra along with homonuclear correlation spectroscopy (COSY), heteronuclear single-quantum correlation (HSQC), and gradient-assisted heteronuclear multiple bond correlation (gHMBC) data were obtained using a Varian Mercury 300 NMR spectrometer operating at a frequency of 300.13 MHz for proton. Proton spectral acquisition parameters were as follows: recycle delay was 1 s, 45[degrees] pulse width was 7.1 [micro]s, and acquisition time was 2 s. The number of transients acquired was 128, with FIDs composed of 12,000 data points. No zero-filling or digital filtering was used prior to Fourier transformation. Carbon and attached proton test (APT) spectra were obtained using a 1 s recycle delay, a 7.8 [micro]s, 45[degrees] pulse width, and an acquisition time of 1.8 s. The FID was composed of 57,436 data points, with 1500 scans accumulated for each spectrum. Acquisition parameters for gradient-assisted COSY spectra were as follows: recycle delay was 1 s, 90[degrees] pulse width was 14.2 [micro]s, sweepwidth was 10.0 ppm, and acquisition time was 150 ms. The number of t1 increments was 256 with 4 scans per increment. N-type data were acquired, and the data were processed with sinebell apodization in both dimensions. An additional 768 points were added to the F1 dimension via linear prediction, and both t1 and t2 were zero-filled to 4096 data points prior to Fourier transformation. HSCQ spectra were obtained using a recycle delay of 1 s, a [.sup.1]H 90[degrees] pulse width of 14.2 [micro]s, and an acquisition time of 150 ms. The [.sup.1]H and [.sup.13]C sweepwidths were 10.0 and 140 ppm, respectively. The number of t1 increments was 400 with 8 scans per increment. States-Haberkorn phase cycling was used to obtain phase-sensitive data, and an additional 600 points were added to the F1 dimension via linear prediction. Both t1 and t2 were zero-filled to 4096 and 1024 data points and apodized using a Gaussian function prior to Fourier transformation. The acquisition parameters for the gHMBC spectra were as follows: recycle delay was 1 s, 90[degrees] pulse width was 14.2 [micro]s, [.sup.1.H] sweepwidth was 16.0 ppm, [.sup.13]C sweepwidth was 200 ppm, and acquisition time was 150 ms. The number of t1 increments was 400 with 8 scans per increment. N-type data were acquired, and the data were processed with sinebell apodization in both dimensions. An additional 1200 points were added to the F1 dimension via linear prediction, and both t1 and t2 were zero-filled to 4096 and 2048 data points, respectively, prior to Fourier transformation. Adequate sensitivity DoublE QUAnTum spEctroscopy (ADEQUATE) spectra were obtained using a [.sup.UNITY]INOVA 500 NMR spectrometer operating at a frequency of 499.8 MHz for proton. The acquisition parameters were as follows: recycle delay was 1.2 s, [.sup.1]H 90[degrees] pulse width of 14.0 [micro]s, and acquisition time was 114 ms. The [.sup.1]H and [.sup.13]C sweepwidths were 9.0 and 310 ppm, respectively. The number of t1 increments was 574 with 64 scans per increment. States-Haberkorn phase cycling was used to obtain phase sensitive data, and an additional 861 points were added to the F1 dimension via linear prediction. Both t1 and t2 were zero-filled to 8192 and 2048 data points and apodized using a shifted-sinebell function prior to Fourier transformation. Samples were 15% weight solutions in [CDCl.sub.3], and the deuterated solvent served as spectral reference ([[delta].sup.1]H = 7.20 ppm and [[delta].sup.13]C = 77.0 ppm, respectively).
FTIR spectroscopy characterization was performed on a Bruker Tensor 37 spectrometer in a nitrogen atmosphere. A background spectrum of a clean salt plate was obtained by acquiring 32 scans, and the reaction products were characterized by averaging 32 scans of a thin film applied to a clean salt plate.
Results and discussion
ASPE preparation and characterization
The Michael addition reaction reached complete conversion at 60[degrees]C and was found to approach stoichiometric conversions at 30[degrees]C. NMR analysis revealed that the maleate rapidly isomerized to the fumarate in the presence of a primary amine acting as a catalyst for the rearrangement (Fig. 4). (8), (9) Upon fumarate formation, reaction with the primary NH occurred more rapidly than expected. Typically, Michael addition type reactions proceed to stoichiometric conversions at elevated temperatures and long reaction times. The reaction rate of this Michael addition reaction was enhanced by the presence of two electron withdrawing groups stabilizing the negatively charged transition state. The secondary amine addition product was not observed, indicating the secondary amine nucleophilicity is substantially reduced. The [.sup.13]C NMR spectrum of ASPE is shown in Fig. 6 (note the presence of residual fumarate).
[FIGURE 4 OMITTED]
FTIR analysis (Fig. 5) revealed the presence of both H-bonded (peaks near 3331 and 3435 [cm.sup.-1]) and non-H-bonded OH groups (peak near 3575 [cm.sup.-1]). (10) The broad carbonyl peak near 1730 [cm.sup.-1] is comprised of both H-bonded and non-H-bonded bands, while absorbance near 3100 [cm.sup.-1], associated with unsaturated sites, was not evident.
[FIGURE 5 OMITTED]
The Michael addition product (ASPE) was further characterized using a number of NMR techniques to determine the assignments given in Figs. 6 ([.sup.13]C) and 7 ([.sup.1]H). The [.sup.13]C NMR spectrum (Fig. 6) shows two carbonyl peaks at 170 and 173 ppm (carbons 11 and 10, respectively) as well as two resonances for the methylene groups of the ethyl esters (near 61 ppm), indicating each ester is different chemically. This effect is most likely due to H-bonding between the secondary NH proton and closest carbonyl site forming a five-membered ring.
[FIGURE 6 OMITTED]
In addition, the protons attached to the methylene adjacent to the secondary NH (protons 7a and 7b in Fig. 7) have different chemical shifts in [CDCl.sub.3]. When the lock solvent is changed to DMSO-[d.sub.6] (NMR not shown), the peak separation is reduced between protons 7a and 7b due to the more polar environment decreasing H-bonding. Protons 7a and 7b nonequivalence in the [.sup.1]H NMR is evidence that the aspartate is H-bonded into a ring structure. After examining all of the aforementioned 2D NMR spectra, the two carbonyls and methylenes of the ethyl esters could not be assigned and required further 2D NMR characterization.
[FIGURE 7 OMITTED]
Characterization of PAPSE
Polyesterification of ASPE was accomplished by moderate heating at reduced pressures in the presence of a titanate catalyst. The reaction was characterized by the generation of ethanol and a large increase in viscosity. A conversion of 88% (based on EtOH evolution) was typically achieved before an excessive viscosity was obtained. [.sup.13]C NMR spectral characterization of the PASPE showed the disappearance of the [CH.sub.2]OH carbon resonance at 62.5 ppm with a concurrent appearance of the corresponding ester resonance at 63.1 ppm (Fig. 8). These measurements also revealed that the esterification reaction proceeds selectively through one of the two ethyl esters (Fig. 8) with a reduction of only one of the [OCH.sub.2][CH.sub.3] signals at 60.9 ppm. This unequal reactivity forms a polymer with a linear or perhaps only slightly branched structure in contrast to the HB architecture originally hypothesized. Furthermore, the disappearance of only one ester group indicates that transesterification is not a predominant side reaction.
[FIGURE 8 OMITTED]
H-bonding involving the polymer's interior secondary NHs and those at the chain's end is supported by HSQC spectroscopic data. The 2D NMR spectrum of the polymer (Fig. 9) reveals that the carbon attached to protons 7a and 7b (see Fig. 7) of the chain's interior and on the chain ends are not equivalent and resonate at 37 and 44 ppm, respectively. Protons 7a and 7b (Fig. 7) attached in the chain's interior are more equivalent as compared to the analogous protons on the chain ends as evidenced by the protons chemical shifts being less differentiated. This is attributed to the polymer's conformation restricting the optimized H-bond geometry, especially in the chain's interior. Note the residual monomeric ASPE peak observed at 47 ppm along with the differing chemical shifts of protons 7a and 7b. Additional FTIR characterization of the polymer (not shown) shows an expected reduction in the OH peak and the presence of both H-bonded and non-H-bonded OH groups. While these experiments provided insight into the polymer and monomer's H-bonding environment, additional 2D NMR experiments were required to determine which ethyl ester was active in esterification.
[FIGURE 9 OMITTED]
In order to distinguish which ethyl ester group was participating in esterification, an ADEQUATE 2D NMR correlation experiment was implemented (Fig. 10). In this technique, the F1 axis shows correlations between a proton and the sum of the [.sup.13] C shifts of the attached carbon and its neighbor. The H-bonded carbonyl (carbon 10 in Fig. 6) has an adjacent methine (carbon 6) with a shift of 57.6 ppm, while the non-H-bonded carbonyl (carbon 11) has an adjacent methylene (carbon 5) with a chemical shift of 37.8 ppm. In the ADEQUATE spectrum there are correlations at 207 and 231 ppm. By comparing the sum of the two carbonyl and methylene shifts, the proper assignment can be made. Here the addition of carbon 6 shift (57.6) to that of carbon 10 (173.5) yields 231.1 ppm in agreement with the data. Similarly, adding the shift of carbon 5 (37.8) to carbon 11 (170) yields 207.8, once again in agreement with experiment. The result of this method makes the unambiguous assignment of the monomer and polymer structure possible.
[FIGURE 10 OMITTED]
The ADEQUATE experiment confirmed that the esterification occurs at the H-bonded carbonyl. Here the H-bonded site is preferred because the carbonyl carbon has increased positive character due to electrons being withdrawn by the H-bond (Fig. 11). The increased positive character of the H-bonded carbonyl promotes nucleophilic attack. The proposed bicyclic mechanism is also supported by the infrared spectrum indicating the presence of H-bonded hydroxyl groups and the non-equivalence of protons 7a and 7b in the NMR spectrum (Fig. 7) indicating restricted mobility.
[FIGURE 11 OMITTED]
NCO reactions with ASPEs
H-bonding lowers the reactivity of ASP-based diamines with polyisocyanates enabling many applications feasible by reducing the reaction rate and hence increasing pot life. (2) In order to further identify reactive ester site, the ASPE was reacted with a molar equivalent (NCO/NH 1/1) of cyclohexyl NCO at ambient conditions (Fig. 12). The reaction proceeded exothermically and the product was analyzed using [.sup.13]C NMR and FTIR spectroscopy. The [.sup.13]C NMR spectrum revealed that the NH was unreacted. Rather, the NCO group reacted with the primary OH to form urethane bonds. The result was confirmed by the disappearance of the methylene adjacent to the OH at 63 ppm as well as the appearance of urethane carbonyl at 157 ppm in the [.sub.13]C NMR spectrum (Fig. 13). Two carbonyl peaks are seen in Fig. 13a as compared to only one in Fig. 13b due to the polarity differences between DMSO-[d.sub.6] and [CDCl.sub.3] lock solvents causing the ester groups to become equivalent in a polar environment. In addition, the FTIR spectrum showed a reduction of the OH peak and no residual NCO (not shown). These results were not expected since the reaction of secondary NHs with NCOs occurs 200-500 times faster than the analogous reaction with primary OHs.(11) Furthermore, uncatalyzed urethane forming reactions typically proceed at relatively low rates at room temperature in contrast to the substantial exotherm that was observed here.
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
Tertiary NHs are widely known to be catalysts in urethane forming reactions; thus the observed increase in reactivity is attributed to the secondary NH internally catalyzing the OH/NCO reaction. To unambiguously prove that the OH group was consumed by the NCO, the product was heated overnight at 60[degrees]C in an open scintillation vial and monitored for the formation of hydantoin rings (Fig. 14). Hydantoins are five-membered rings formed when ASP isocyanates are heated, releasing ethanol and increasing the polymer's glass transition temperature. (12) After heating the reaction product overnight, both NMR and FTIR measurements indicated no change in the carbonyl regions. The lack of hydantoin ring formation reinforces the conclusion that urea formation was not taking place to a significant extent.
[FIGURE 14 OMITTED]
PASPEs were synthesized via Michael addition followed by esterification and the resulting polyester characterized by FTIR and NMR spectroscopy. The polyester formed linear chains in contrast to the expected HB structures. Esterification reactions predominated at the H-bonded carbonyl, while the non-H-bonded carbonyl did not undergo reaction. The chemoselectivity of the esterification reaction was attributed to the H-bond withdrawing electrons from the carbonyl, thereby promoting the nucleophilic attack by an OH group.
Chemical shift differences were noted on the methylene adjacent to the secondary NH in both monomeric and polymeric aspartic esters and were believed to originate from the geometrical effects of the five-membered ring. H-bonding in the PASPE displayed reduced intensity as compared to the ASP monomer and was attributed to polymer restricting the conformation corresponding to the optimum geometry for H-bonding.
H-bonding of the amine with the ester carbonyl increases the negative character of the amine nitrogen. This in turn allows the formation of a 6-member H-bonded ring involving the hydroxyl proton and the amine nitrogen. Such a structure is supported by the inequality of methylene protons [alpha] and [beta] to the amine. The axial and equatorial protons are in radically different environments causing their chemical shifts to be different.
Ambient reactions of cycloaliphatic NCO with the ASPE were investigated to determine the relative reactivity of the secondary NH and primary OHs. The secondary NHs were unexpectedly found to have less reactivity compared to the primary OH, with urethane formation being the major reaction product. The ASPE's primary OH groups displayed an unusually high reaction rate with the aliphatic NCO; here the H-bonded secondary NH is believed to function as an internal catalyst for the NCO/OH reaction. Urea-based ASPs were not detected as evidenced by the lack of hydantoin ring formation upon heating the NCO/AE product.
Acknowledgments The authors thank the Robert M. Hearin Support Foundation and Bayer Material Science, Pittsburgh, PA, for their support of this research. Partial support of this work from the National Science Foundation MRSEC (DMR 0213883) is gratefully acknowledged.
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C. T. Williams, D. A. Wicks (*). W. L. Jarrett
School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, MS 39401, USA