New intramolecular effect observed for polyesters: an anomeric effect.
Keywords: Gel permeation, NMR, polyesters, reaction kinetics, service life prediction, weatherability, polyurethane, water-based
Recent research within the Soucek group (1,2) has shown that viscosity can be reduced by the > 20% addition of 1,3-cyclohexanedicarboxylic acid (1,3-CHDA) to a resin comprised of 1,4-cyclohexanedicarboxylic acid (1,4-CHDA) and a glycol. Due to the lower resin viscosity, the incorporation of 1,3-CHDA can be used to reduce the organic solvent concentration necessary to adjust the viscosity for coatings applications. (3) In addition, the resulting polyester decreases the viscosity of the neat resins, making it viable for high-solids applications without significant differences in the ultimate mechanical properties after application. This unique physical quality drives interest in the durability of resins developed with cyclohexyl dibasic acids, especially in the area of hydrolytic resistance.
In a preliminary study (4) using a variety of dibasic acids, including hexahydrophthalic anhydride, anchimeric effects and steric effects were investigated for oligoesters. The overall conclusion was that steric effects dominated for chain scission, and anchimeric effects were only important for end group scission. It was observed that the 1,2-cyclohexyl diester did not appear to have an anchimeric effect. When compared to maleic, or phthalic diesters, the 1,2-cyclohexyl diesters appeared to be remarkably stable. No explanation was given for this enhanced stability.
Previously, comparison of the hydrolytic stability of cycloaliphatic diacids had been reported using methyl ester of dicarboxylic acids (5) and CHDA-based polyols crosslinked with melamine. (6) Conformation (cis or trans) was found to be a predominate factor in controlling the rate of hydrolysis for the 1,3- and 1,4-CHDA methyl esters. The conformations that favored esters with an axial position (trans-1,3-CHDA and cis-1,4-CHDA) had lower hydrolytic velocity than conformations that favored an equatorial ester position. Isomer conformations are depicted for 1,3-CHDA in Figure 1 and for 1,4-CHDA in Figure 2. Configurations of both cis- and trans-CHDA were three to four times more stable in acidic conditions than alkaline. The analogs of 1,3- and 1,4-CHDA with equivalent axial and equatorial ratios had roughly the same hydrolysis rates. Moreover, the cis to trans ratio is important to resins, since it directly affects the solubility of the resins. (5)
However, polymers often behave very differently than model compounds. The cis to trans isomerization is dependent on conversion during synthesis of high molecular weight polymers. (7) For the melamine-formaledehyde (M-F) crosslinked CHDA-based polyols, there was a large discrepancy found between the 1,3- and 1,4-CHDA coatings. After a 16-hr exposure to 30% [H.sub.2]S[O.sub.4], coatings composed of 1,3-CHDA [and neopentylglycol (NPG)] crosslinked with M-F resins maintained approximately 90% of their film thickness and up to 80% of their gloss. However, when the analogs derived from 1,4-CHDA were exposed to the same conditions, ~100% decrease in film thickness and gloss was observed. (7) It was surprising that 1,4-CHDA-based polymers had such poor resistance considering their known hydrolytic stability (8) and Cleveland humidity resistance. (9)
[FIGURE 1 OMITTED]
The unique properties of cyclohexyl-based esters are ideal for the reduction of VOCs via a high-solids or a waterborne approach. For either approach, hydrolytic stability of the oligoester within a thermosetting coating is important. In addition, the shelf life of polyesters in an aqueous environment is of principle interest. Accordingly, it is of great interest to investigate the difference in hydrolytic stability between the diacids in an accelerated aqueous acetone environment. For comparison of the diacids, two glycols, NPG and cyclohexanedimethanol (CHDM), were chosen for this study. Hydrolysis was monitored by the formation of carboxylic acid and the oligoesters conformation was analyzed using NMR.
[FIGURE 2 OMITTED]
Neopentyl glycol (NPG) (99%), hexahydrophthalic anhydride (HHPA) (95%), mixed xylenes (98.5%), reagent grade acetone (99.5%), reagent grade ethanol (99.5%), standardized potassium hydroxide in methanol (0.1026 N), reagent grade phenolphthalein, inhibitor-free HPLC grade tetrahydrofuran (99.9%), and dibutyltin oxide (DBTO) (98%) were all purchased from Aldrich. The 1,4-CHDA (99%), 1,3-CHDA (99%), and CHDM (99%) were supplied by Eastman Chemical Co. All reagents were used as received. The naming system for the oligoesters uses acronyms that start with the acid followed by the alcohol and separated with a period. For example, 1,4-CHDA.NPG corresponds to an oligoester synthesized from 1,4-cyclohexanedicarboxylic acid and neopentyl glycol.
Preparation of Oligoesters
The designed neat oligoesters were to have a molar ratio such that there was an equivalent average of six ester units per hydroxyl-functional molecule, as listed with regards to the monomer mass and moles in Table 1. The monomers were reacted in a 1-L two-piece reaction kettle equipped with a stirrer, modified Dean-Stark trap, nitrogen purge valve, and a jacketed heat supply. The reaction temperature was controlled using a Love Controls Series 2600 auto-tuning proportion integral derivative temperature controller ([+ or -] 0.1[degrees]C) and a J-type stainless steel jacketed thermocouple. The following temperature schedule was used: 20-150[degrees]C at a rate of 4.3[degrees]C/min, 150-160[degrees]C at a rate of 0.17[degrees]C/min, 160-195[degrees]C at a rate of 0.29[degrees]C/min, and 195-210[degrees]C at a rate of 0.50[degrees]C/min. When the temperature reached 195[degrees]C, the xylene (3-5% by mass) was added to form an azeotrope with water. The final temperature was held constant until the resin had an acid number < 10 [mg.sub.KOH]/[g.sub.resin]. An inert atmosphere (nitrogen purge) was maintained throughout the reaction. A catalyst, DBTO (0.4 wt%) was used to accelerate the synthesis. Products were distilled under vacuum (to 1 mm Hg) for five hours at 110[degrees]C in order to remove residuals. Final acid concentration and hydroxyl concentration were measured by ASTM standards D 1639-89 and D 4274-94, respectively. Resin color was compared using the Gardner color standard ASTM D 1544-98. Characterization of the neat oligoesters is depicted in Tables 2 and 3.
Hydrolysis Conditions and Analysis Method
For hydrolysis, 30 g equivalent masses of oligoester, acetone, and water were combined by solvating the resin in acetone and followed by the addition of water. The solution was sealed in glass jars and stored in a 40[degrees]C (104[degrees]F) water bath. Aliquots of the oligoester solutions were extracted and then dried in a convection oven (110[degrees]C for 3 hr). The dried resin (1 g) was dissolved in an equal mixture of neutralized acetone and ethanol (25 g) for titration with a solution of potassium hydroxide (0.1 N in methanol) to phenolphthalein endpoint. A Varian Mercury 300 MHz spectrometer was used to record [.sup.1]H NMR spectra with TMS (tetramethylsilane) as the standard reference peak. The NMR samples of the oligoester resins were solvated in CD[Cl.sub.3] and taken under ambient conditions.
The isomeric diacids were used to explore the effect of the monomer conformation on the hydrolytic stability of the resultant oligoester. For these cycloaliphatic monomers, the 1,4-isomer spatially separated the ester or carboxylic acid moiety. The 1,2-isomer and the 1,3-isomer have the potential to have neighboring group interactions. The glycols used in this study are traditionally used in polyester resins to increase steric hindrance by methyl substitution of the [beta]-carbon for NPG, and methylol substitution of the cycloaliphatic ring in CHDM. The CHDM is often substituted for NPG to raise the glass transition temperature of the resin. (10) The choice of diols also adds an acyclic versus cyclic dimension to the study.
The relationship of acid number versus time is shown in Figure 3. It reveals that the rate is independent of carboxylic acid concentration over time. A linear rate relationship, as depicted in Figure 3, has been previously reported for macromolecules. (4,11-13) It is postulated that this is the result of the low relative concentration of acid formed. Linear regression of the data resulted in a line where the slope was proportional to the hydrolysis rate ([DELTA] acid number/[DELTA] days). The slope (relative rate of hydrolysis) and standard error of fit for the oligoesters are listed in Table 4. The 1,4-CHDA.NPG oligoester [(35 [mg.sub.KOH]/([g.sub.resin] *day)] had a 25% lower hydrolytic velocity than the 1,3-CHDA.NPG oligoester [47 [mg.sub.KOH]/([g.sub.resin] *day)]. A comparison of the hydrolytic velocity of the HHPA.NPG oligoester [1.3 [mg.sub.KOH]/([g.sub.resin] *day)] to the 1,3-CHDA.NPG oligoester revealed that the HHPA-based oligoester was 2500% lower, and the HHPA.NPG oligoester was 1800% lower when compared to the 1,4-CHDA.NPG oligoester.
A similar reduction occured with the cycloaliphatic glycol-based resins where 1,4-CHDA.CHDM [(24 [mg.sub.KOH]/([g.sub.resin] *day)] had a 45% lower rate than 1,3-CHDA.CHDM [43 [mg.sub.KOH]/([g.sub.resin] *day)]. When comparing the HHPA.CHDM oligoester [3.0 [mg.sub.KOH]/([g.sub.resin] *day)] to the 1,3-CHDA. CHDM and 1,4-CHDA. CHDM oligoesters, a reduction in the hydrolytic velocity of 1400% and 790% was observed, respectively. The rate reduction for the cycloaliphatic-based 1,4-CHDA was significantly larger than that for the acyclic. In the 1,3-CHDA, there was nearly a 9% reduction in velocity when the glycol was changed from NPG to CHDM. However, in the 1,4-CHDA-based resin, there was a 33% rate reduction. This signifies that the cycloaliphatic glycol had a greater influence on the 1,4-CHDA isomer than the 1,3-CHDA isomer. Overall, 1,4-CHDA- and CHDM-based resins resisted hydrolysis to a greater degree than that of the 1,3-CHDA- and NPG-based oligoesters. Surprisingly, the 1,2-based cyclic oligoester HHPA.CHDM had a higher hydrolytic velocity than the acyclic HHPA.NPG oligoesters, which is the opposite of the trend of reduction in hydrolytic velocity observed for the oligoesters, based on the 1,3 and 1,4 acid isomers.
For conformational analysis, the NMR specta of 1,3-CHDA.NPG and 1,4-CHDA.NPG are depicted in Figures 4 and 5, respectively. The NMR integration indicates the relative concentration of cis and trans isomers within the oligoesters. Under ambient conditions, the facile ring flip resulted in distinct peaks for each of the isomers. For 1,4-CHDA.NPG, the [alpha]-proton on the cycloaliphatic ring produced a peak at [delta] 2.71 ppm for the cis configuration, and a peak at [delta] 2.36 ppm for the trans structure. This coincides with the unpublished results of Eastman, (14) where the peaks for cis and trans were [delta] 2.67 and [delta] 2.46 ppm, respectively. Normalization of cis to trans integrals of 1,4-CHDA.NPG results in a cis to trans ratio of 1.00 to 1.38, respectively. Hence, the oligoester is 42% cis and 58% trans. In the case of 1,3-CHDA.NPG, the [alpha]-proton of the cis isomer is found at 2.32 ppm, and the trans at 2.51 ppm. This also corresponds to the Eastman (15) results for 1,3-CHDA for cis and trans at 2.52 and 2.83 ppm, respectively. If the integral for 1,3-CHDA.NPG is normalized to the trans peak, the ratio for cis to trans is 1.00 to 2.38, respectively. This corresponds to an average molecule being 30% cis and 70% trans. The ratio of the isomers is important for hydrolysis, as the rate-determining step corresponds to differences in the concentration of equatorial and axial moieties.
[FIGURE 3 OMITTED]
Hydrolysis is influenced by steric factors found within the monomeric units of the oligoesters. Monomers containing cycloaliphatic moieties are known to be hydrolytically resistant in both acidic and alkaline conditions, as well as stable toward UV-induced Norrish degradation due to lack of strong chromophores. (15) The cycloaliphatic structure is favorable for steric hindrance. However, comparison of the 1,2-, 1,3-, or 1,4-placement of the ester on the ring affects the steric interaction to different extents. This has been shown to have an impact on the velocity of hydrolysis.
The 1,2-cyclohexyl-based oligoesters are different from the 1,3- or 1,4-based oligoesters, since the starting material is an anhydride and not a diacid, as shown in Figures 1 and 2. As a consequence, the system begins in a cis position, and after the first ester (1/2 ester) is formed the ester and carboxylic acid are in a trans position. This is a simplistic model; however, the possible conformations are a little more complex, as shown in Figure 6. There were two trans conformations where the esters are either axial or equatorial, as shown in Figure 6. The equatorial conformation arranged in the Cram model shows that the nucleophilic approach is sterically hindered. Similar to the cis model, the alkyl oxygen and alpha carbon are sterically blocking one side of each of the symmetrical carbonyls.
The oligoesters based on HHPA are unique relative to 1,4-CHDA since the acyl groups are fixed in close proximity. In the cis configuration, ring inversion was equivalent. Thus, it formed a constant axial and equatorial acyl moiety. In the trans configuration there was potential for both esters to be either equatorial or axial. Modeling the nucleophilic approach after Cram's rule shows that the carbonyl carbon on both esters was not equal in the cis conformation. An alternative model was forwarded by Burgi-Dunitz, as shown in Figure 7. (16) They proposed that the nucleophile would approach at 107[degrees] away from the carbonyl oxygen, where the HHPA axial carbonyl carbon was sterically shielded by the 3 and 5 protons of the ring. To approach the axial carbonyl carbon from the opposite direction, the alkyl oxygen and an alpha carbon sterically shielded it. In a similar manner, the equatorial carbonyl carbon was shielded by the alkyl oxygen and an alpha carbon. However, the equatorial carbonyl was open to a 107[degrees] approach from the backside.
In the case of cis or trans 1,4-CHDA configurations, the cycloaliphatic ring had the equivalent of four carbon spacers physically separating the esters from each other. There was one possibility for neighboring group interaction that would accelerate hydrolysis in the boat conformation of cis-1,4-CHDA (Figure 2); however, torsional strain and flagpole interactions would destabilize this conformer. (17) Hence, the steric effects were limited by the conformation and configuration of the cycloaliphatic ester. In the trans chair configuration, the placement of the ester groups in the equatorial position formed the most stable conformation, and ring flip that would place the esters into an axial position was sterically limited by 1,3-diaxial interactions resulting from van der Waals repulsion with the hydrogens in the 3 and 5 positions. However, cis/trans isomerization that can occur during the synthetic process of 1,4-CHDA, which is originally 70 to 79% cis isomer, (18) had been found to lead to a thermodynamic equilibrium mixture of 35% cis and 65% trans, (19) or a mixture of 55% cis and 45% trans isomers, depending upon the method of synthesis. (3)
In addition, CHDM contains 31 to 32% of the cis conformation. (3,20) The cis configuration was known to intensify the rate of hydrolysis of the overall oligoester because the rates of hydrolysis differ between axial and equatorial esters of CHDA. For example, equatorial methoxycarbonyl had been found to react 4.8 times faster than an axial methoxycarbonyl in cis- and trans-4-t-butylcyclohexanecarboxylate. (21) In oligoesters containing 1,4-CHDA, the equilibrium is relevant to conformations commonly used in industrial applications as an average of the rates corresponding to cis and trans configurations formed during the polymerization reaction. In this synthesis, isomerization was driven toward the trans configuration, resulting in a 58% trans 1,4-CHDA.NPG resin. A trans oligoester has an equilibrium which favors esters in the equatorial positions. In turn, the oligoester should be more susceptible to hydrolysis due to the higher concentration of equatorial esters.
However, for 1,3-CHDA there is a potential for anomeric interaction, (22) as there are three carbon spacers in a cyclic structure between the esters. The anomeric effect had been previously used to describe 1,3-diaxial interactions in cyclic (5 and 6 member rings) carbohydrates, which lead to instability. In this case, it refers to a cyclohexane ring that has substitution on the 1 and 3 positions. The esters derived from the trans-1,3-CHDA are physically separated in any ring conformation. However, in the cis-1,3-CHDA configuration, the carbonyl oxygen of the ester is in proximity of the other ester, provided that the ester groups are in the axial position. The esters interact to form a six-member ring as a transition state. If, however, the ester groups are in the equatorial position, then no anomeric effect can be generated due to physical separation.
The cis configuration can show anomeric activity provided that it undergoes a facile conformational inversion from the esters or acids that are in equatorial conformation to esters in an axial conformation. It has been noted that substitution on a cyclohexane ring does not significantly affect the rate of ring inversion (10.8 kcal/mol); however, it does shift the equilibrium to favor the equatorial conformation of the substituted group. Substitution of the ethyl ester or carboxylic acid gives conformational free energies of 1.1 to 1.35 kcal/mol. (18) The NMR data indicate that 30% of the esters of 1,3-CHDA.NPG oligoester were in the cis configuration. Thus, both the cis and trans conformer can be subject to accelerated hydrolysis via inter-conversion. On the backside of the 1,3-CHDA oligoesters, it appears that the protons of the 2 and 6 position may inhibit the 107[degrees] approach. If the ring is inverted, the ester is moved into axial positions, leaving the carbonyl carbon sterically exposed on one side and sterically shielded by the ring on the other. However, the increased size of the esters hydrolytic transition state would induce 1,3-diaxial interactions by the active ester and the opposing ester on the opposite side. (17) This relayed steric effect would account for increased activation energy of the transition state; hence, it may justify the observed hydrolysis rate.
The difference between 1,3-CHDA.NPG and 1,4-CHDA.NPG is a pseudo rate of 12.1 X [10.sup.-3] [[mg.sub.KOH]/([g.sub.resin] *day)] or 25% more conformationally labile for 1,4-CHDA, which gives it stability. In the CHDM analog, 1,4-CHDA is nearly 45% more stable than the 1,3-CHDA version. Based on equatorial-axial contributions to hydrolysis, it would be expected that 1,3-CHDA.NPG would have a lower hydrolytic velocity due to a greater percentage of the trans (axial) configuration. The 1,4-CHDA.NPG, being 58% trans (equatorial), would favor a faster rate of hydrolysis. Therefore, another factor must predominate over equatorial-axial effects in cycloaliphatic ester hydrolysis. It is clear that the steric effects of an individual ester moiety among the monomeric units are similar in most conformations. Therefore, the difference in rate must be attributed to an anomeric effect established by the cis-1,3-CHDA configuration containing the ester groups in the axial conformation.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Since cyclohexane-based monomers are used in the coating industry and will experience increased usage in the future, it is necessary to have insight into the inherent interactions of difunctional substitution upon the resultant coating properties. It is clear that the expected 1,2 anchimeric behavior does not operate, and the 1,3- and 1,4-isomers have a more complex behavior due to interconversion of the cyclohexyl ring. This article also introduces a new effect known as an anomeric effect, not previously used to describe the hydrolytic stability of oligoesters. This effect was invoked to explain the lack of hydrolytic stability inherent in 1,3-cyclohexanediacid-based oligoesters.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The 1,3- and 1,4-cyclohexane-based oligoesters exhibited better hydrolytic stability when CHDM was used instead of NPG. However, the 1,2-cyclohexane diacid/CHDM-based oligoesters had slightly less hydrolytic stability than the 1,2-diacid/NPG-based oligoesters. The hydrolytic stability of the dibasic acid 1,2-cyclohexane based oligoesters was far superior (greater) than either the 1,3- or 1,4-cyclohexane diacid-based oligoesters. Between the 1,3- and 1,4-cyclohexane diacid-based oligoesters, the 1,4 had better hydrolytic stability. Under the conditions used for polymerization, it was found that the trans configuration was favored for both 1,3- and 1,4-CHDA, resulting in 70% and 58% percent trans moieties, respectively. It was also found that hydrolysis was dependent upon ring configuration and the interchangeability of conformers. Finally, an anomeric effect was used to explain the hydrolytic instability of 1,3-cyclohexane diacid oligomers.
Table 1 -- Mass and Moles of Monomers Used for Polyester Synthesis Oligoester Diacid Diol Diacid Diol (Diacid.Diol) (g) (g) (mol) (mol) HHPA.NPG 105.22 94.78 0.682 0.910 1,3-CHDA.NPG 110.71 89.29 0.643 0.875 1,4-CHDA.NPG 110.71 89.29 0.643 0.875 HHPA.CHDM 89.00 111.00 0.577 0.770 1,3-CHDA.CHDM 94.48 105.51 0.549 0.731 1,4-CHDA.CHDM 94.48 105.51 0.549 0.731 Table 2 -- Physical Properties of Neat Oligoester Resins Oligoester Gardner Number Average Polydisperity Physical (Diacid.Diol) Color Molecular Weight Index State (g/mol) HHPA.NPG 4 1001 1.46 CV 1,3-CHDA.NPG 2 1404 1.54 TW (a) 1,4-CHDA.NPG 1 1418 1.56 TW HHPA.CHDM 2 1078 1.70 CW (a) 1,3-CHDA.CHDM 1 1164 1.97 CV 1,4-CHDA.CHDM 1 1265 1.84 CV (a) CW = clear wax; TW = turbid wax. Table 3 -- Chemical Properties of Neat Oligoester Resins Oligoester Reaction Time Acid Number Hydroxyl Number (Diacid.Diol) (hr) ([mg.sub.KOH]/ ([mg.sub.KOH]/ [g.sub.resin]) [g.sub.resin]) HHPA.NPG 10.5 5.3 82 1,3-CHDA.NPG 7.0 4.4 83 1,4-CHDA.NPG 7.2 3.6 115 HHPA.CHDM 9.2 2.6 101 1,3-CHDA.CHDM 5.7 3.0 87 1,4-CHDA.CHDM 6.5 6.4 105 Table 4 -- Relative Rates of Oligoester Hydrolysis Oligoester [10.sup.3] k' [+ or -] Standard Error (Diacid.Diol) ([mg.sub.KOH]/([g.sub.resin] *day)) HHPA.NPG 1.9 [+ or -] 1.2 1,3-CHDA.NPG 47.5 [+ or -] 2.2 1,4-CHDA.NPG 35.4 [+ or -] 2.8 HHPA.CHDM 3.0 [+ or -] 1.1 1,3-CHDA.CHDM 43.4 [+ or -] 2.6 1,4-CHDA.CHDM 23.8 [+ or -] 4.9
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Mark D. Soucek and Aaron H. Johnson -- University of Akron*
Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12-14, 2003, in Philadelphia, PA.
*Dept. of Polymer Engineering, Akron, OH 44325.
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|Author:||Johnson, Aaron H.|
|Date:||Apr 1, 2004|
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