Hydrolytic stability of oligoesters: comparison of steric with anchimeric effects

EUROPEAN POLYMER JOURNAL

European Polymer Journal 40 (2004) 2773–2781

www.elsevier.com/locate/europolj

Hydrolytic stability of oligoesters: comparison of steric with anchimeric effects A.H. Johnson, J. Wegner, M.D. Soucek

*

Department of Polymer Engineering, University of Akron, 250 South Forge Street, Akron, OH 44325, USA Received 10 November 2003; received in revised form 14 July 2004; accepted 23 July 2004 Available online 18 September 2004

Abstract A comparison of steric and anchimeric effects on the hydrolytic stability of polyesters was studied. Twelve monomers were selected based on their propensity toward steric and anchimeric interactions: adipic acid, isophthalic acid, phthalic anhydride, hexahydrophthalic anhydride, 1,4-cyclohexanedicarboxylic acid, maleic anhydride, ethylene glycol, 1,2-propanediol, 1,3-propanediol 1,4-butanediol, 1,5-pentanediol, and neopentyl glycol. Hydroxyl terminated oligoesters consisting of one diacid and one diol and one hydroxyl terminated oligoester consisting of two diacids and one diol were prepared. The hydrolytic stability was evaluated in an acetone/water solution. The acid number was monitored as a function of time. It was found that telechelic groups favor anchimeric interactions, while steric groups determine the rate of hydrolysis for the main chain.
2004 Elsevier Ltd. All rights reserved. Keywords: Polyesters; Hydrolysis; Kinetics; Degradation; Coatings

1. Introduction Polyesters are used as components in elastomers [1,2] coatings [15,19] lubricants [3] fibers [4,5] and thermosetting molded plastics [6]. The most important aspect of polyesters in coatings is hydrolytic stability as shown in Fig. 1 [7,8]. The major concern is caused by exposure to aggressive environments that can promote main chain polymer scission, and in turn, the failure of physical properties [9,10]. Natural pollutants can create an environment suitable for promotion of hydrolysis by altering the pH of rainwater in the range of 5–7. In addition, fossil fuel pollution concentrated near cities can further re* Corresponding author. Tel: +1 330 972 5638; fax: +1 330 258 2339. E-mail address: [email protected] (M.D. Soucek).

duce the pH [11]. As water droplets evaporate or due to Marangoni type flows toward droplet edge [12] local hydronium and hydroxide ion concentrations readily increase, resulting in surface erosion, etching, pitting and gloss reduction. Humidity alone has dramatic effects on the dimensional stability of polyester and polyurethane elastomers [13]. This makes formulation criteria based on hydrolytically stable monomer selection of great interest. An effort to improve lifetime deficiencies of polyesters requires knowledge of hydrolysis rates relevant to the formulation and environmental criteria. Model compound studies have been performed based on steric [14] anchimeric [8] hydrophobic [15] inductive [16] and resonance [17] effects coupled to the ester moiety. This has provided insight to approximate lifetime prediction. However, when both steric hinderance and anchimeric

0014-3057/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.07.028

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A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781 H O R' R

+ H2O

R

O

Ester

H

O

O

O

+ O

R

R

R'

OH

Carboxylic acid

Water

HO

Alcohol

Fig. 1. Equilibrium reaction of esters and the tetrahedral transition state of an ester during hydrolysis.

pathways are possible, there are discrepancies in the literature as to which is the dominate hydrolysis pathway. This makes further investigation into the influence of steric and anchimeric effects germane [15,18–20]. In some cases, cycloaliphatic esters outperformed sterically equivalent acyclic analogues, and esters formed from anhydrides had better resistance to hydrolysis than less anchimericly implicated oligoesters [21]. An approximation of steric influences on the rate of ester hydrolysis, known as NewmanÕs rule, has been used to define the effects of the carboxylic acid component for model compound studies of esters [22]. In general, NewmanÕs rule states that increasing the number of atoms in the six position, counting away from the carbonyl oxygen, decreases the rate of hydrolysis. Atoms in the six position improve hydrolytic stability by crowding the carbonyl group when the molecule is in a coiled state. Mechanistically, this physically impedes the approach of a Lewis acid catalyst and nucleophiles from the carbonyl oxygen and carbon, respectively. Crowding of the carbonyl also interferes with the spatial requirements of the tetrahedral intermediate as depicted in Fig. 1. Since its development, NewmanÕs rule has been improved by including atoms in the seven position; however, the predictive ability is at best qualitative [15]. In addition, hetero atoms in the steric positions may have a contradictory effect on the rate. Intramolecular catalysis, otherwise known as the anchimeric effect or synartetic assistance, is the ability of an attached hetero atom or group to participate in the hydrolysis of an ester. Proximity of the assisting group relative to the ester is essential. It has been established by Turpin [15] using linear aliphatic polyesters that anchimeric effects escalate with less than three carbon spacers and rates of hydrolysis decrease to a plateau beyond four carbon spacers. Consequently, this is the reason anchimeric effects are commonly known as ‘‘neighboring group effects.’’ However, an enhancement of the hydrolysis rate has not been shown to increase proportionately for two carbon spacers in polyesters containing cyclic diacids [20,23]. In past studies [15,19] of polyesters, it has been concluded that using monomers comprised of carboxylic acids and glycols containing two carbon spacers is detrimental to the longevity of the polymer. However, research on phthalic anhydride

[20] and 1,2-cyclohexanedicarboxylic anhydride [23] have challenged the concept that anchimeric influences dominate main chain polymer scission. Since a comparison of steric and anchimeric effects on the rate of oligoester hydrolysis has not been previously reported, it was appropriate to revisit the area of polyester hydrolytic stability with this comparison in mind. In this paper, rates of hydrolysis in an acetone medium were evaluated as a function of oligoester monomer composition. In total, 11 linear oligoesters were used in this study. The dibasic acids were chosen to illustrate differences between aromatic, cycloaliphatic, and acyclic aliphatic polyesters, including anchimeric effects. The diols were chosen to elucidate the difference in both steric and anchimeric effects. Hydrolysis of the polyesters was assessed by monitoring the formation of carboxylic acid by titration and gel permeation chromatography in an accelerated test as a function of time (0– 145 days).

2. Experimental 2.1. Materials Hexahydrophthalic anhydride (HHPA) (95%), 1,2propanediol (1,2-PD) (99.5%), 1,4-butanediol (1,4-BD) (99%), 1,5-pentanediol (1,5-PeD) (96%), neopentyl glycol (NPG) (99%), maleic anhydride (MA) (99%), 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 purchased from Aldrich. The 1,4-cyclohexanedicarboxylic acid (1,4-CHDA) (99%), and isophthalic acid (IPA) (99%), were supplied by Eastman. Stepan provided phthalic anhydride (PA) (99%). Dow supplied ethylene glycol (EG) (99%). Adipic acid (AA) (99%) was received from Dupont. Shell supplied 1,3-propanediol (1,3-PD) (98%). All materials were used as received, without further purification. For identification purposes, oligoester acronyms start with the acid followed by the alcohol. For example, PA. NPG represents an oligoester synthesized from phthalic anhydride and neopentyl glycol.

A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781

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Table 1 Mass of monomers used for polyester synthesis Resin

Diacid (g)

Diol (g)

Diacid (mol)

Diol (mol)

MA.NPG PA.NPG HHPA.NPG IPA.1,4-CHDA.NPG AA.NPG 1,4-CHDA.EG 1,4-CHDA.1,2-PD 1,4-CHDA.1,3-PD 1,4-CHDA.1,4-BD 1,4-CHDA.1,5-PeD 1,4-CHDA.NPG

82.78 103.22 105.22 53.92a,55.90b 102.55 135.10 125.84 125.84 117.79 110.71 110.71

117.22 96.78 94.78 90.18 97.45 64.90 74.16 74.16 82.21 89.29 89.29

0.844 0.697 0.682 0.325a,0.325b 0.702 0.785 0.731 0.731 0.684 0.643 0.643

1.125 0.929 0.910 0.866 0.936 1.046 0.976 0.976 0.912 0.875 0.875

a

IPA,

b

1,4-CHDA.

2.2. Preparation of oligoesters The oligoesters in this study were designed to contain an average of six ester units per hydroxyl-functional molecule, using a 3–4 molar ratio of diacid to diol, as shown in Table 1. The ester monomers were reacted in a 1-l breakaway reaction flask equipped with a mechanical stirrer and a modified Dean–Stark trap, and a heat mantle. The reactions proceeded under nitrogen to minimize oxidative degradation. To reduce the reaction time, 0.4 wt% of DBTO, a transesterification catalyst, was used. In an effort to minimize evaporative glycol loss, the reaction temperature was carefully controlled using a Love Controls Series 2600 auto tuning PID (proportion integral derivative) temperature controller (±0.1 C) and a J-type thermocouple. The following temperature schedule was used, unless otherwise noted: 20– 150 C at a rate of 4.3 C/min, 150–160 C at a rate of 0.17 C/min, 160–195 C at a rate of 0.29 C/min, and 195–210 C at a rate of 0.50 C/min. When the temperature reached 195 C, xylene was added to the Dean– Stark trap up to the reflux connection to azeotrope water away from the resin. The final temperature was held until the resin had an acid number less than or equal to 11.8 mgKOH/gresin. Deviations from the temperature schedule occurred with the IPA.1,4-CHDA. NPG, 1,4-CHDA.1,2-PD, MA.NPG and 1,4-CHDA.EG oligoesters. To dissolve crystalline IPA, the copolymer resin containing 50% IPA was reacted with diol and IPA until clear before addition of the 1,4-CHDA. In addition, 1,4-CHDA.EG was reacted with half the 1,4-CHDA monomer until clear before adding the balance of the monomers (Scheme 1). For reactions that contained a diol with a low vapor pressure, such as EG and 1,2-PD, the reactor was cooled, and the contents of the trap were reintroduced

to the reaction vessel to maintain the carboxyl to hydroxyl ratios. The initial stage of the reaction with MA required removal of the heating mantle to maintain a constant temperature due to the exothermic nature of the ring-opening reaction. Oligoesters were purified by drying at 110 C under vacuum (1 mm Hg) to remove the residual xylene and low molecular weight byproducts. Final acid concentration and hydroxyl concentration were measured by ASTM standards D 1639-89 and D 4274-94, respectively. Resin color was compared using a Gardner color standard ASTM D 1544-98. The physical and chemical properties of the neat oligomers are described in Tables 2 and 3. 2.3. Evaluation of hydrolytic stability Equivalent amounts (30 g) of oligoester, acetone, and water were used to maintain a large molar excess of water relative to the formation of carboxylic acid of the partially hydrolyzed oligoester. Oligoesters were dissolved in acetone. This was followed by addition of distilled water and vigorous agitation. To accelerate the experiment, the solution was sealed and placed in a 40 C constant temperature water bath. Periodically, an aliquot of the oligoester solution was removed and dried in a convection oven at 110 C for 3 h. After drying, 1 g of the dried resin was dissolved in 25 g of an equal mixture of acetone and ethanol for titration with a 0.1 N solution of potassium hydroxide in methanol to a pink phenolphthalein endpoint (two drops of 0.5 wt.% phenolphthalein in methanol). Four titrations were performed per sample and a standard deviation was taken. Each sample had less than ±0.15 error. Error is represented by the standard error of fit of the line to the acid number points. In addition, the reduction in molecular weight was also monitored by gel permeation chromatography. The kinetic data were treated as a

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A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781 O O

O

O

H

H

O

O O

O

n n

MA.NPG

PA.NPG

O

O

O

H

H

O

O

O

O

n

O

HHPA.NPG

n

AA.NPG

O

O

O

O

O

O H

H

O

O n

n

1,4-CHDA.EG

1,4-CHDA.1,2-PD

O

O

O

O

O

H O

H

O

O n

n

1,4-CHDA.1,3-PD

1,4-CHDA.1,4-BD O

O

O

H

O

O

O

H O

O

n

n

1,4-CHDA.1,5-PeD

O

1,4-CHDA.NPG

O

O

O

H O

O

O

O

n

IPA.1,4-CHDA.NPG

Scheme 1. Oligoester resin structures.

composite and as three discrete intervals: (1) initial 0–30 days; (2) 30–60 days; (3) 60–140 days. A Waters 515 pump connected to a 2410 refractive index detector, data module, and three columns in succession (HR 0.5, HR 1, and HT 2) were used. Samples were prepared at 0.5 wt% in HPLC grade tetrahydrofuran. Flow rate was 1 ml/min with a constant temperature of 35 C. Molecular weight curves were calculated using monodisperse polystyrene standards ranging from 106 to 50,000 Da (ASTM D 697500).

3. Results The selection of monomers is based on potential for steric and/or anchimeric effects. For comparison of the diols the 1,4-CHDA, was chosen for its previously reported hydrolytic stability [19]. Furthermore, the ring structure of the 1,4-CHDA precludes anchimeric interactions. Glycols with 2–5 carbon spacers were chosen for their anchimeric interactions [15] and widespread commercial usage [24,25]. The selection of glycols

A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781

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Table 2 Physical properties of neat oligoester resins Diacid.diol

Gardner color

Number average molecular weight (g/mol)

Polydisperity index

Physical state

MA.NPG PA.NPG HHPA.NPG IPA.1,4-CHDA.NPG AA.NPG 1,4-CHDA.EG 1,4-CHDA.1,2-PD 1,4-CHDA.1,3-PD 1,4-CHDA.1,4-BD 1,4-CHDA.1,5-PeD 1,4-CHDA.NPG

3 1 4 1 1 5 1 1 1 3 1

1128 954 1001 1446 1438 1872 1246 1508 1368 1300 1418

1.68 1.46 1.46 1.54 1.78 1.79 1.47 1.94 1.76 1.64 1.56

CV* CW* CV CV CV CV CV TW* TW CV TW

*CV

is a clear viscous resin, CW is a clear wax, and TW is a turbid wax.

Table 3 Chemical properties of neat oligoester resins Diacid.diol

Reaction time (h)

Acid number (mgKOH/gresin)

Hydroxyl number (mgKOH/gresin)

MA.NPG PA.NPG HHPA.NPG IPA.1,4-CHDA.NPG AA.NPG 1,4-CHDA.EG 1,4-CHDA.1,2-PD 1,4-CHDA.1,3-PD 1,4-CHDA.1,4-BD 1,4-CHDA.1,5-PeD 1,4-CHDA.NPG

9.5 8.5 10.5 7.5 6.0 18 7.2 6.5 8.5 7.5 7.2

11.8 9.3 5.3 3.8 5.8 8.9 3.9 5.1 4.8 7.5 3.6

107 123 81.7 88.5 95.6 81.8 99.6 86.4 89.4 101 115

includes a gradation of steric hindrance. The widely reported steric stability and common use of NPG made an obvious standard for the diacid or anhydride comparison [15,17,22]. The MA, PA, and IPA were selected for their intrinsic lack of steric hindrance, and a comparison of anchimeric proclivity MA, HHPA, and PA. Both cyclic 1,4-CHDA and acyclic AA aliphatic monomers were selected for steric interactions. The quantity of base necessary to neutralize latent carboxylic acid in the oligoester may be converted into molar concentrations of ester hydrolysis. At a given moment of time (t) in the duration of the hydrolysis, the acid number, Aac (mg KOH/g of resin), can be calculated from Eq. (1). AacðtÞ ¼ ðV
N
FWKOH Þ=m

ð1Þ

The variables, V and N, are the volume (ml) and normality (mol/l) of potassium hydroxide solution, and FWKOH is the formula weight of potassium hydroxide (56.11 g/mol). The initial weight of the oligomer sample, m, is measured in grams. If the density of the esters is considered relatively constant, ponderal effects can be excluded. Therefore, Aac is amenable to a kinetic simulation.

The generalized kinetic equation for ester hydrolysis in neutral conditions is expressed in Eq. (2). Ester and water concentrations are subject to partial order coefficients, a and b. For esters, these coefficients are generally accepted to equal unity, except under the influence of autocatalysis. d½ester=dt ¼ k½RCO2 Ra ½H2 Ob :

ð2Þ

If the concentration of water is much greater than the hydrolyzed ester concentration, as in Eq. (3), then the hydrolyzed ester concentration, [H2O]b, can be considered constant, and k 0 can be defined as depicted in Eq. (4). The combination of Eqs. (2)–(4) and if a is set to unity results in the rate being solely dependent on ester concentration, as expressed in Eq. (5). ½H2 O P ½RCO2 R=10

ð3Þ

k 0 ¼ k½RCO2 Ra

ð4Þ

d½RCO2 R=dt ¼ k 0 ½RCO2 R

ð5Þ

For first order reactions, the plot of ln[RCO2R] versus t is linear. However, for the initial stages of the ester hydrolysis, this can be inconsistent. Inspection of the

Acid Number (mg/g)

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A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781

30 25 20 15 10 5 0

0

20

40

60

80

100

120

140

Time (days)

Fig. 2. Sample plot of Aac verses t of oligoesters: AA.NPG (·), 1,4-CHDA.1,2-PD (+).

linear nature of the sample plots for Aac verses t, as depicted in Fig. 2, reveals hydrolysis to be only timedependent (Eq. (6)). This yields the zero-order hydrolysis rate equation in Eq. (7). In has been previously reported that under the experimental conditions where the both the ester and water are in excess with respect to the rate of hydrolysis that ester polyester hydrolysis is independent ester concentration [20,26–29]. d½RCO2 R=dt ¼ k 0 t

ð6Þ

½RCO2 R ¼ k 0 t þ ½RCO2 R0

ð7Þ

The relative rates of oligoester hydrolysis and standard error of fit for the Aac versus t plots are depicted in Table 4. In oligoesters comprised of NPG and the dibasic acid or anhydrides, the hydrolysis rates (mgKOH/ (gresinday)) decreases in the following order: MA (773 · 103)  AA(183 · 103) > PA (89.1 · 103) > IPA.1,4-CHDA (37.0 · 103) = 1,4-CHDA (35.4 · 103)  HHPA (1.87 · 103). In comparison to the old adage, ‘‘hard to make, hard to break,’’ for polyesters [22] the reaction times (h) in increasing order from Table 2 are: AA (6) < 1,4-CHDA (7.2) < IPA.1,4-CHDA (7.5) < PA (8.5) < MA (9.5) < HHPA (10.5). Although

Table 4 Relative rates of oligoester hydrolysis for dibasic acids reacted with NPG and 1,4-CHDA reacted with select diols Oligoester

103k 0 (mgKOH/(gresinday))

MA.NPG PA.NPG HHPA.NPG IPA.1,4-CHDA.NPG AA.NPG 1,4-CHDA.EG 1,4-CHDA.1,2-PD 1,4-CHDA.1,3-PD 1,4-CHDA.1,4-BD 1,4-CHDA.1,5-PeD 1,4-CHDA.NPG

773 ± 36 89.1 ± 9.2 1.9 ± 1.2 37.0 ± 2.3 183 ± 5.7 54.8 ± 7.2 79.4 ± 2.1 95.6 ± 2.1 61.3 ± 2.1 65.9 ± 2.5 35.4 ± 2.8

the oligoesters were not reacted to identical Aac (Aac=11.8) corresponds to 99.995% conversion of acids to esters in MA.NPG numbers to avoid discoloration [7] there is no clear correlation of hydrolysis of diacids and ease of ester formation. However, solvent effects must also be considered as the media around the transitions state in a neat resin during synthesis in dissimilar to the polar acetone/water solution [30]. For 1,4-CHDA based oligoesters, the following rates (mgKOH/(gresinday)) are observed in decreasing order: 1,3-PD (95.6 · 103) > 1,2-PD (79.4 · 103) > 1,5-PeD (65.9 · 103) = 1,4-BD (61.3 · 103) > EG (54.8 · 103) > NPG (35.4 · 103). Similar to the diacids, there is not a direct correlation between the hydrolysis rate and reaction time (h), as shown in increasing order: 1,3-PD (6.5) < NPG (7.2) = 1,2-PD (7.2) < 1,5-PeD (7.5) < 1,4-BD (8.5) < EG (18). It was interesting to observe that the oligoester based on 1,3-PD had higher hydrolysis rates than 1,2-glycols oligoesters, and for the 1,2-PD oligoester to have had a higher rate of hydrolysis than EG due to the steric nature of 1,2glycols. Anchimeric effects of a hydroxyl moiety appear to be less severe relative to anchimeric effects of an acid moiety. This is described by the comparison to the range of hydrolysis rates based on diacids (1.8 · 103 to 773 · 103 (mgKOH/(gresin day))), and the hydrolysis rates based on the diols being contained to a smaller relative range (35.4 · 103 to 95.6 · 103 (mgKOH/(gresinday))). The controlled diacid (1,4-CHDA) or diol (NPG) are considered to have good hydrolytic stability based on steric hindrance [19]. It is postulated that this disparity in rates is due to acidity of the carboxylic acid verses the hydrogen bonding strength of the hydroxyl and the ability to form cyclic structures for intramolecular catalysis in the telechelic groups depicted in Fig. 3. A comparison of rates at different stages throughout the measurement period is depicted in Fig. 4 for acyclic non-branched glycols, and in Fig. 5 for the diacid and anhydride comparisons. For the 1,4-CHDA.EG oligoester, it is evident that the initial rate is substantially larger than the rates in later time periods. This trend is also observed in the oligoester containing 1,2-PD. Oligoesters containing 1,3-PD, 1,4-BD, and 1,5-PeD were consistent in rate throughout the reaction within standard

OH

HO O

a O

O

O

b Polyester

O

Fig. 3. Intra-molecular catalysis of telechelic groups by hydrogen bonding: (a) two carbon spacer diol and (b) two carbon spacer carboxylic acid.

A.H. Johnson et al. / European Polymer Journal 40 (2004) 2773–2781 Initial

Middle

2779

End

0.25

Rate k'

0.2

0.15 0.1

0.05 0 1,4-CHDA.EG 1,4-CHDA.1,2-PD 1,4-CHDA.1,3-PD 1,4-CHDA.1,4-BD 1,4-CHDA.1,5PD

Fig. 4. Comparison of diols reacted with 1,4-CHDA with respect to the hydrolysis rate (k 0 ) at periods (initial, middle, and end) during the duration of the experiment.

Initial

Middle

End

1 0.9 0.8

Rate k'

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 MA.NPG

AA.NPG

PA.NPG

IPA.1,4CHDA.NPG

1,4CHDA.NPG

HHPA.NPG

Fig. 5. Comparison of dibasic acids and anhydrides reacted with NPG with respect to the hydrolysis rate (k 0 ) at periods (initial, middle, and end) during the duration of the experiment.

error. However, a slight trend towards auto-acceleration was observed. The effects of auto-acceleration are most noticeable for the MA.NPG resin in Fig. 5. The oligoester comprised of AA, IPA.1,4-CHDA, 1,4-CHDA, and HHPA have consistent rates of hydrolysis. If a trend were to be concluded, it would be that there is a slight decrease in hydrolytic velocity as hydrolysis progressed. The PA.NPG oligoester has a large decrease in velocity from the initial to the middle of the experiment. In comparing acyclic non-branched oligoesters ranging from two carbon spacers (EG 0–5 carbon spacers (1,5-PeD), trends can be observed (Table 4) that EG has a lower overall velocity than 1,3-PD. Regression of the initial velocity, as depicted in Fig. 4, for EG has a

significantly higher velocity, 215 · 103 (mgKOH/(gresinday)), relative to the long-range average of 54.8 · 103 (mgKOH/(gresinday)). In addition, 1,2-PD showed a slightly higher rate in the initial stage with a velocity of 97.1 · 103 (mgKOH/(gresinday)) versus the long-range average of 79.4 · 103 (mgKOH/(gresinday)). The 1,3-PD had the highest overall rate with 95.6 · 103 (mgKOH/ (gresinday)). The 1,4-BD and 1,5-PeD have similar hydrolytic velocities with 61.3 · 103 and 65.9 · 103 (mgKOH/(gresinday)), respectively. The hydrolytic proximity of 1,4-BD relative to 1,5-PeD is concurrent with the results of Turpin [15] for anchimeric effects above four carbon spacers having velocity-independent of anchimeric effects.

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4. Discussion It has long been considered that the hydroxyl functional monomer has a more substantial role in the steric effect on rates of hydrolysis than the acid-functional monomer [15]. This is conceivably due to the accumulation of inductive studies performed to develop Hammett sigma constants [22]. These studies closely resemble the substituted benzene derivatives commonly used as a hard segment in ester-based polymers such as IPA, PA, and terephthalic acid. However, the benzene group in the main chain has little steric effect except when a substituent is placed in the ortho position and then steric effects prevail over inductive effects [31]. This places emphasis on the hydroxyl functional component for steric stabilization. It was further observed by Jones and Thomas [14] that increasing the number of a-methyl substitutions on the hydroxyl functional group systematically reduced the rate of hydrolysis to a greater extent than a corresponding increase of the acid-functional group. However, examination of hydrolysis velocities in Table 4 provides evidence that the acid component of the oligoester has an effect on the rate of hydrolysis. The MA.NPG oligoester hydrolyzes nearly twenty-three times faster than the corresponding 1,4-CHDA based resin. The easiest conclusion would be that this is a ponderal effect [15] since the ester to carbon mass ratio is higher for MA than CHDA. In comparison, the resin comprised of AA.NPG has a relatively close ester to carbon mass ratio and maintains a rate of hydrolysis five times higher than the cycloaliphatic based resins. This implies that cycloaliphatics have a greater ability to physically impede the nucleophilic approach of water, but it does not fully justify the high rate of the MA.NPG oligomers. Rate constants comparing MA to IPA was previously reported by Verdu and coworkers [27]. They showed that MA to hydrolyzed appreciably faster than IPA. The rates were attributed to a difference in hydrophilicity resulting from an increased polarity generated by the MA. It is reasonable to postulate that the instability of MA is due to intramolecular catalysis. However, the rate can also be affected by the spontaneous isomerization reaction of maleic acid to undergo cis– trans conversion to fumaric acid [32,33]. The trans structure of fumaric acid leaves the carbonyl exposed to nucleophilic approach, whereas the cis structure of maleic acid places a c-carbon substituted with oxygen in a sterically-shielding position. Hence, fumaric acids would be expected to hydrolyze at an appreciably faster rate than maleic acid. Jones [19] has also shown that rates of hydrolysis correlated to hydrophobicity with IPA being more stable than AA. However, this pertains only to acidic conditions. Under basic conditions, saponification of IPA was shown to be extremely facile [19]. This is conceivably due to the ability of IPA to resonance stabilize the

catalytic addition of hydrogen ion to the carbonyl, but IPA has no ability to stabilize the nucleophilic addition of hydroxide. The surprising stability of HHPA compared with the instability of MA seems to belie explanation of an anchimeric effect. The classic example of an anchimeric is PA where the ring would provide significantly less steric obstruction than HHPA or MA due to the rigid planar orientation of the ester groups. Nevertheless, PA is somewhat stable, with a k 0 of 89.1 · 103 (mgKOH/(gresinday)). This is roughly twice the rate of the cycloaliphatic and cycloaromatic based oligoester. However, it has about half the rate of the linear aliphatic (AA) resin with a k 0 of 183 · 103 (mgKOH/(gresinday)). It would be expected that PA would have steric shielding on anterior face of each ester from the opposing ortho ester and its a- and b-carbons. This in itself would not justify the intermediary rate. It is probable that resonance effects reduce the rate in acid environments. Upon protonation, the carbonyl carbon can draw electron density away form the conjugated ring; hence, the positive charge of the carbonyl is reduced. It has also been pointed-out by OÕBrien et al. [20] that the hydrogen ion can be shared between the carbonyls. Effectually, this would reduce the nucleophilic attraction to the carbonyl carbon, thereby lowering the rate of hydrolysis in acid catalyzed media. Conversely, conjugated mechanisms of stabilization would not be applicable for saponification, and this is what is observed. The initial rates of hydrolysis for the alkyl moiety with two carbons suggest intramolecular nucleophilic catalysis or an anchimeric affect is specific to the hydroxyl functional end-group. Anchimeric structures of the telechelic groups are illustrated for the glycols and carboxylic acids in Fig. 3. End-group effects have previously been implicated in formulating polyester melamine [18] and other studies [8,9]. It is also possible that the overall rate reduction of EG involves steric shielding of the adjacent CHDA, allowing it to afford lower velocity than 1,3-PD. However, in this case the end-groups would not be affected by the same steric hindrance. The hydroxyl proton in EG is six atoms away from the carbonyl, allowing for alignment of the oxygen lone pair for hydrogen bonding with the end hydroxyl. Hence, the electron withdrawing effect as a result of hydrogen bonding increasing the positive charge on the carbonyl carbon, increasing susceptibility to nucleophilic attack. The 1,3-PD places the hydroxyl hydrogen seven atoms away from the carbonyl oxygen resulting in a lack of overlap of the oxygen lone pairs and the hydroxyl hydrogen results in no anchimeric effect. Therefore, it is postulated that the higher velocity of 1,3-PD relative to 1,2-glycols was not due to an anchimeric effect but to a reduction in steric shielding. Steric effects appear to have the principal role in polyester hydrolysis when end-group concentration is lim-

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ited. As the concentration of protic end-groups increase, as in the case of half-esters, [15] anchimeric effects become more significant. Therefore, elimination of protic end-groups by cross-linking or using end-group scavengers should significantly increase the durability of esters. As end-group concentration increases due to hydrolysis, anchimeric implicated esters would result in exponential loss of properties, unless the intramolecular catalysis is sterically impeded. It is apparent that cycloaliphatic moieties have an ability to interfere with the anchimeric-assisted autocatalysis; in turn, this results in durability against polymer chain scission. In addition, the ability of the sterically hindered glycol to coil and act as an intramolecular catalyst has an effect on the overall rate of polymer degradation. 5. Conclusions After an initial period, hydrolysis of 1,2-diols occurred at lower velocities than diols with more carbon spacers due to the steric stabilization by the cycloaliphatic diacid. Anchimeric effects associated with 1,2-glycols were postulated to be end-group dependent, which facilitated intra-molecular hydrogen bonding. Similar to the 1,2-diols, 1,2-diacid rates of hydrolysis were curtailed relative to their analogs with a greater number of carbon spacers with the exception of maleic anhydride based esters. It was proposed that isomerization of maleic to fumaric during the 1/2 ester formation led to a ester structure with little steric shielding. Steric considerations were predominant for adipic and phthalic based esters as the rates of hydrolysis were greater than that of the cycloaliphatic diacid based esters. The oligomers were formulated with excess glycol. For this reason, anchimeric effects associated with the acid end groups were insignificant relative to steric interactions. It can be concluded that steric interactions are the dominant factor relative to anchimeric effects for main chain scission, and anchimeric interactions are predominate for end groups. Acknowledgment Part of this work was performed at the Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105. References [1] Schollenberger CS, Stewart FD. J Elastoplast 1971;3:28. [2] Athey RA. Rubber Age 1965;96:705.

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[3] Echin AI, Novosartov GT, KondratÕeva TB. Chemistry and technology of fuels and oils. New York: Plenum; 1986. [4] Tinker RB. ChemSA 1989(June):186. [5] Ravens DA, Ward IM. Trans Faraday Soc 1961;57:150. [6] Mortaigne B, Bellenger V, Verdu J. Polym Networks Blends 1992;2:187. [7] Jerry M. Advanced organic chemistry: reactions mechanisms, and structure. fourth ed.. New York: Wiley; 1992. [8] Bender ML. Chem Rev 1960;60:53. [9] Bellenger V, Ganem M, Mortaigue B, Verdu J. Polym Degrad Stab 1995;49:91. [10] Ossefort ZT, Testroet FB. Rubber Chem Technol 1966;39:1308. [11] Northwestern Society for Coatings Technology and Montreal Society for Coatings Technology. J Coat Technol 1995;67:19. [12] Deegan RD, Bakajin O, Dupont TF, Huber G, Nagel SR, Witten TA. Nature 1997;389:837. [13] Magnus G, Dunleavy RA, Critchfield FE. Rubber Chem Tech (Rubber Division, American Chemical Society) 1965;39:1328. [14] Jones RWA, Thomas JDR. J Chem Soc 1966;B:661. [15] Turpin ET. J Paint Technol 1975;47:40. [16] Brown TL. J Am Chem Soc 1959;81:3229. [17] Eastman Chemical Company, Publication N-342, 1994. [18] Misˇkovi-Stankovi V, Natasˇa D. Serb Chem Soc 1998; 63:53. [19] Jones TE, McCarthy JM. J Coat Technol 1995;67:57. [20] OÕBrien, ME, Faunce JA, Hillshafer DK, Adhesive Sealant Council. Spring Convention. J. Adhes. Seal. Pittsburgh, PA; 1997. p. 89. [21] Ni H, Daum JL, Thiltgen PR, Soucek MD, Simonsick W, Zhong W, et al. Prog Org Coat 2002;45(1):49. [22] Newman MS. Steric effects in organic chemistry. New York: Wiley; 1956. [23] Chapman NB, Shorter J, Toyne KJ. J Chem Soc 1961: 2543. [24] Lawrence JR. Polyester resins. New York: Reinhold; 1960. [25] Bjorksten Research Laboratories. Polyesters and their applications. Reinhold: New York; 1956. [26] Payne KL, Jones FN, Brandenburger LW. J Coat Technol 1985;57:35. [27] Be´lan F, Bellenger V, Mortaigne B, Verdu J. Polym Degrad Stab 1997;56:310. [28] Soucek MD, Johnson AJ, Meeken LE. Effect of additional hydroxyl functionalities on the hydroylic stability of oligoesters. Macromol Chem Phys 2004;205(1):35. [29] Soucek MD, Johnson AJ. New intramolecular effect observed for polyesters: an anomeric effect. J Coat Technol Res 2004;1(2):111. [30] Venkoba Rao G, Venkatasbramanian N. Aust J Chem 1971;24:201. [31] Taft Jr RW. J Am Chem Soc 1952;74:3120. [32] Kisker CT, Crandall DI. Tetrahedron 1963;19:701. [33] Taube J. J Am Chem Soc 1943;65:527.