Influence of Structure on Chemical and Thermal Stability of Aliphatic


Oct 30, 2013 - Trent Centre for Biomaterials Research, Departments of Physics and Astronomy, and Chemistry, Trent University, 1600 West Bank. Drive ...
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Influence of Structure on Chemical and Thermal Stability of Aliphatic Diesters Latchmi Raghunanan and Suresh S. Narine* Trent Centre for Biomaterials Research, Departments of Physics and Astronomy, and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada ABSTRACT: Ester group interactions with each other and with the atoms between them were investigated in order to determine dependence of chemical and thermal stabilities of aliphatic diesters on structure. Novel glycol-derived diesters with chemical formula (C17H33COO)2CnH2n were used as model systems. Chemical stability was determined using 1H NMR and FTIR, and thermal stability and weight-loss kinetics were examined using nonisothermal TGA. Chemical stability increased with the number of methylene units (n, carbon) between the ester groups until n = 6, and no significant improvement was observed past n > 6. It is argued that other ester-dense materials, including polyesters, would behave similarly. Evidence of a strong dependence of thermal stability on chemical stability is also provided. This work shows that the chemical and thermal stabilities of ester-dense functional materials such as diesters, oligoesters, and polyesters can be manipulated by varying the distance between the ester groups, and hence the interactions of the electron-withdrawing ester groups with its neighbors. branching; and (iii) Hojabri et al.4 showed that copolymers such as poly(ester-urethanes) can be tuned to possess polyamide-like or polyethylene-like properties based on the lengths of the ester-chain copolymer segments. However, comprehensive and systematic structure−performance relationships for complex ester materialsnecessary to make informed decisions with respect to the type and position of the structural modification(s) required in order to achieve optimal physical propertiesare still lacking. Specifically, the influence of the electron-withdrawing effect of ester groups within oligo-ester- and polyester-containing materials on the chemical and thermal stabilities has not yet been established. The present study investigates the effect(s) of structure on the chemical and thermal stabilities of pure linear unsaturated diesters. The acid chain segments were fixed at 18 carbon units and the diol chain moieties varied from 2 to 10 carbon units (n) to give diesters with the chemical formula (C17H33COO)2CnH2n and structure as shown in Scheme 1. Table 1 lists the nomenclature of the diesters as used in this report.

1. INTRODUCTION Global research efforts to find renewable replacement feedstock for producing commodity materials such as plastics, fuels, lubricants, and cosmetics are mainly driven by an increasing demand for environmentally responsible materials1 and prospects of the end of cheap fossil oil.2 In this regard, vegetable oils are an attractive alternative since they are renewable and biodegradable, and possess high molecular diversities with very low toxicities. Esters can be readily derived from vegetable oils.3 They find uses in diverse applications such as polymer chemistry,4−6 antifreeze and lubricant formulations,7 cosmetics,8 pharmaceuticals,9 and foods.10 Because biodegradability increases with the density of ester groups per molecule,11,12 complex esters, i.e., with more than one ester group per molecule, are favored over monoesters where increased biodegradability is beneficial. However, because the ester group is susceptible to both nucleophilic attack at its carbonyl carbon center13 and thermolysis at the ester bonds,14 increasing the number of ester groups per molecule will adversely affect bulk physical properties, and hence material performance. For example, polyglycolic acid, a very hydrophilic polyester on account of its high density of ester groups, suffers from low hydrolytic stability and poor tensile strength properties compared to polyesters with lower ester group densities such as poly(3hydroxyalkanoate)s.15 It has been acknowledged that such limitations may be addressed with structural modifications to the cores of the molecules. (i) Patel et al.15 reported on the influence of total carbon length, unsaturation, double bond position, and branching at the alcohol moiety on the melting point of wax monoesters; (ii) Bouzidi et al.16−18 reported that crystallization, melt, and flow behaviors of linear aliphatic monoesters depend in a predictive manner on chain length, symmetry, and © 2013 American Chemical Society

Scheme 1. General Structure of the Model Linear Unsaturated Diesters

Received: September 10, 2013 Revised: October 22, 2013 Published: October 30, 2013 14754

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Table 1. Nomenclature and Structure of the Linear Diestersa compound

IUPAC name

nT

n

C18-2-18

2-[(Z)-octadec-9-enoyl]oxyethyl-(Z)-octadec-9enoate 3-[(Z)-octadec-9-enoyl]oxypropyl-(Z)-octadec-9enoate 4-[(Z)-octadec-9-enoyl]oxybutyl-(Z)-octadec-9enoate 6-[(Z)-octadec-9-enoyl]oxyhexyl-(Z)-octadec-9enoate 9-[(Z)-octadec-9-enoyl]oxynonyl-(Z)-octadec-9enoate 10-[(Z)-octadec-9-enoyl]oxidecyl-(Z)-octadec-9enoate

38

2

39

3

40

4

42

6

45

9

46

10

C18-3-18 C18-4-18 C18-6-18 C18-9-18 C18-10-18

the conversion m is the mass loss. The temperature dependence of the rate constant is typically described by the Arrhenius equation18

k(T ) = A e−Ea / RT

(2)

where A is the pre-exponential factor, Ea is the activation energy, and R is the gas constant. Using eqs 1 and 2, an experimentally useful form for determining the kinetic triplet A, Ea, f(m)may be derived:

dm = A e−Ea / RT f (m) dt

(3)

Nonisothermal isoconversional methods, which are recommended by ICTAC for kinetic investigations, assume that for a given conversion m, f(m) is the same over all heating (or cooling) rates, β.19 Nonisothermal isoconversional methods, therefore, require the use of several β to calculate Ea. From eq 3, an expression for the determination of a conversion-specific apparent activation energy Ea,m can be derived20

a

nT = total length (carbon atoms); n = diol chain length (carbon atoms).

2. MATERIALS AND METHODS 2.1. Materials. The C18-n-18 diesters were prepared by the reaction of oleoyl chloride with the respective diols in the presence of pyridine as described in Raghunanan et al.16 Materials with >99% purities were used in the subsequent investigations described herein. 2.2. 1H-Nuclear Magnetic Resonance. 1H spectra were acquired on a Bruker Advance III 400 spectrometer (ν(1H) = 400.22 MHz; Bruker BioSpin MRI GmbH, Karlsruhe, Germany) equipped with a 5 mm Broadband Observe (BBO) probe. Spectra were acquired at 25 °C over a 16 ppm spectral window with a 1 s recycle delay, 32 transients. Spectra were Fourier transformed, phase corrected, and baseline corrected. Spectra were apodized through multiplication with an exponential decay corresponding to 0.3 Hz line broadening. Chemical shifts were referenced relative to the residual solvent peak (CDCl3, δ(1H) = 7.24 ppm). 2.3. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FTIR) spectra were obtained using a Thermo Scientific Nicolet 380 FT-IR spectrometer (Thermo Electron Scientific Instruments, LLC, USA) equipped with a PIKE MIRacle attenuated total reflectance (ATR) system (PIKE Technologies, Madison, WI). Liquid samples were loaded onto the ATR crystal area, and sample spectra were acquired over a scanning range of 400−4000 cm−1 for 64 repeated scans at a spectral resolution of 1 cm−1. 2.4. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) measurements were carried out on a Q500 model TGA (TA Instruments, New Castle, DE) under nitrogen gas (sample flow 60 mL/min; balance flow 40 mL/min). Thermal stability tests were performed on fully melted samples (10.5−12.5 mg) loaded onto a platinum pan and heated to 600 °C at a rate of 10 K/min. Measurements were performed in triplicate. Kinetic experiments were performed similarly using six different heating rates: 0.1, 1.0, 3.0, 5.0, 10, and 20 K/min. TGA data were analyzed using the TA Universal Analysis software. 2.5. The Isoconversional Model. The overall rate of reactions is commonly modeled by the solid-state kinetic equation (eq 1) under the assumption that the rate constant (k(T)) and the reaction model (f(m)) can be separated.17 dm = k(T )f (m) (1) dt In eq 1, t is time, m is the extent of the reaction, or conversion, and T is temperature in kelvin. In the case of TGA experiments,

Ea, m ⎛ dm ⎞ ⎟ ln⎜ = ln[A mf (m)] − ⎝ dt ⎠ m , i RTm , i

(4)

where the subscript i denotes the ordinal number of a nonisothermal experiment conducted at the heating rate β, and the subscript m the quantities evaluated at conversion m. Using the different β described in section 2.4, values of Ea,m were obtained from plots of the left side of eq 4 versus 1/Tm,i at constant values of m (0.05 ≤ m ≤ 0.95 in 0.05 increments). The conversion rates dm/dt were determined from the first derivative (DTG) curves of the TGA experiments.

3. RESULTS AND DISCUSSION 3.1. Structure Analysis. 3.1.1. Effect of n on Ester Chain Segments: FTIR Analysis. A typical FTIR spectrum for the C18-n-18 diesters is given in Figure 1. The peaks at 1737−1742

Figure 1. FTIR spectrum of C18-10-18, typical of the C18-n-18 diesters.

and 1160−1170 cm−1 are characteristic of the OC(O) and (O)C−O−C ester bond stretching vibrations, respectively, and the peak at 3003−3006 cm−1 is characteristic of the cis C−H bond stretching vibration.21−23 Interestingly, only the vibrations associated with the CO and C−O−C bonds of the ester groups varied with n, indicating a measurable variation of their electronic environment (full FTIR characterization presented in Raghunanan et al.16). The peak wavenumbers of 14755

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Figure 2. Stretching frequency as a function of diol chain length (n) of (a) CO bond and (b) C−O−C bonds on C18-n-18 diesters. Dashed lines are exponential fits.

the CO and C−O−C bonds versus n are presented in Figure 2, a and b, respectively. As can be seen, the peak wavenumbers of the CO bonds decreased exponentially with increasing n (Figure 2a, r2 = 0.9967; characteristic diol chain length, n0 = 1.17), whereas the peak wavenumbers of C−O−C bonds increased exponentially with n (Figure 2b, r2 = 0.9582; n0 = 1.29). The exponential trends observed in the vibrational wavenumbers of the C−O−C and CO bonds (Figure 2) resulted from a combination of the competing effects of mass and bond strength. According to Hooke’s law, the absorption wavenumber for a stretching vibration is dependent on both the force constant between the two vibrating atoms and the mass of the two atoms.24,25 By assuming that the force constant is proportional to the bond strength,26 the peak wavenumber would increase with increasing bond strength and decrease with increasing mass of the atoms that make up the bond.24,25 Because the vibration of a bond is not isolated from the rest of the molecule,26 the “effective mass” of the ester group (CO, C−O−C) of the C18-n-18 diesters do in fact increase with the increasing n, i.e., the mass of the diol chain moiety. Furthermore, because ester groups are electron-withdrawing,13,27 the atoms between them will feel their electronwithdrawing effects (EWE) to varying extents depending on the esters’ separation distance, given as the length of the diol chain moiety, n (in carbons). The bond strength will, therefore, be influenced accordingly. At small n (n = 2), when the electron-withdrawing influence is at its maximum, the electrons on each carbonyl oxygen atom (CO) are inductively attracted toward the other ester group; the CO bonds become shorter and stronger and the vibrational wavenumber is at its highest (Figure 2a). As the diol chain length is increased, the CO bond strength decreases and its vibrational wavenumber decreases. An increase in the effective mass of the vibrating atoms may have also contributed to the decreasing wavenumber. Conversely, the movement of the electrons on the C−O−C bonds toward the other electron-withdrawing ester group results in bond lengthening. Hence, the C−O−C bonds are at their weakest when the electron-withdrawing effect is at its greatest (n = 2), and the bond stretching wavenumbers are at their lowest (Figure 2b). As n increases, the C−O−C bonds strengthen, increasing the vibrational wavenumbers. The obvious contribution of bond strength to the vibrational frequencies of the C−O−C bonds masked any influence of increasing effective mass which accompanies increasing n. These observations are all consistent with previous reports on

the influence of other electron-withdrawing substituents on ester groups.13 As is clearly shown in Figure 2a,b, the C−O−C and CO bond vibrations were unchanged at n ≥ 6. This indicated that when the ester groups are sufficiently far apart, the electronwithdrawing effect no longer affected the bond strengths, and both the C−O−C and CO bonds were influenced only by the electronegativity of the atoms which made up the bond. These results highlight the significant role of the size of the diol chain moieties separating the electron-withdrawing groups in stabilizing the ester centers from this electron-withdrawing effect. The chemical reactivity of the atoms at the ester groups is directly related to the electron-withdrawing effect whose magnitude is determined by the size of the diol. In fact, as the diol chain length is increased, the diol moiety gradually shields the ester group atoms from the electron-withdrawing effect, effectively rendering it inoperative at n ≥ 6. The chemical stability of the C18-n-18 diesters of the present study as inferred from the FTIR data is, therefore, least at n = 2 and increases to reach its maximum at n ≥ 6. This finding implies that the chemical susceptibility can be customized predictably in materials containing multiple electron-withdrawing groups by structural modifications which would control the degree of shielding between the electron-withdrawing groups. 3.1.2. Effect of n on Ester Chain Segments: 1H NMR Analyses. A typical 1H NMR spectrum for the C18-n-18 diesters is given in Figure 3. The proton chemical shifts were assigned based on tabulated values.28 From these results (full 1 H NMR characterization presented in Raghunanan et al.16), three features, in particular, stood out: (i) C18-2-18 did not show a β-proton (O−CCH) chemical shift, (ii) only the chemical shifts of the α (O−CH) and β-protons varied with n, and (iii) the chemical shifts of both the α- and β-protons decreased with n. Scheme 2 shows the positions of the α- and β-protons on the C18-n-18 diesters, and Figure 4, a and b, shows the chemical shifts of the α- and β-protons as a function of n, respectively. The chemical shift versus n curves of both the α- and the β-protons presented exponential decays (r2 > 0.9921) with characteristic values n0 = 1.48 and 0.80, respectively. The absence of a defined β-proton chemical shift on the C18-2-18 diester occurs because, unlike the other diesters, this molecule is symmetric about an ethyl bond. This means that the β-protons of one ester group become the α-protons of the neighboring ester group. Hence, the β-characteristics of these protons will be masked by the stronger (more deshielded) α14756

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moiety is the largest in the diester with n = 2, resulting in the greatest deshielding of its α-protons compared to the other diesters (Figure 4a). As n increases, both the α- and the βprotons become progressively more shielded from the electronwithdrawing effect, and their δ move upfield (Figure 4b). That is, as n increases, the lability of these protons decreases. The plateau in chemical shift which occurred at n ≥ 6 for both the α- and the β-protons indicates that at this point the ester groups are sufficiently distant from each other that only a single ester group effectively acts on these protons. This is in accordance with the shielding effect on the C−O−C bond strengths reported in section 3.1.1. These results indicate that, at n = 2, the diester is the most susceptible to deprotonation and hence unwanted chemical reactions and that the resistance of the C18-n-18 diesters to deprotonation at the α- and the β-positions increases with increasing n up to n = 6. At n ≥ 6, the resistance to deprotonation does not improve any further. Combined with the results of section 3.1.1, the destabilizing influence of electron-withdrawing groups on the chemical stability of functionalized materials has been clearly established. These results also indicate that chemical stability can be manipulated by controlling the amount of shielding allowed between successive electron-withdrawing substituents. 3.2. Thermal Stability. The TGA and corresponding DTG curves of the C18-n-18 diesters are shown in Figure 5, a and b, respectively. Figure 6 shows the temperatures at which 5% weight loss (T5%) and 50% weight loss (T50%) occurred, along with the center maxima of the DTG peaks (TPDTG) of the C18n-18 diesters. As can be seen from Figure 6, the T5%, T50%, and TPDTG versus n curves demonstrate linear trends (r2 > 0.96) which indicate a continuous increase of the thermal stability of the C18-n-18 diesters of this study. Note that similar trends were observed for these temperatures regardless of heating rate, as demonstrated by the experiments conducted using a wide range of heating rates (0.1, 1, 3, 5, 20 K/min). The TGA profiles of the C18-n-18 diesters consisted of an apparent single-stage weight loss (Figure 5a). The asymmetry in the DTG plots (Figure 5b), however, indicates that these profiles may include (i) several unresolved processes whose individual degradation profiles occurred at temperatures very close to each other 31 and/or (ii) phase change via evaporation.32 Ester groups generally undergo inert-atmosphere thermolysis by way of a cis-β-elimination mechanism.14,33 The reaction proceeds via the formation of a low-energy cyclic 6-membered

Figure 3. 1H NMR spectrum of C18-10-18, typical of the C18-n-18 diesters. Left and right arrows point to the α- and β-protons chemical shifts, respectively.

Scheme 2. α- and β-Positions on Typical C18-n-18 Diesters: from Top to Bottom: n = 2, 3, 10

characteristics. As can be seen from Scheme 2, this occurs only for n = 2. The significance of the electron-withdrawing effect and its dependence on n as discussed in section 3.1.1 also account for the trends observed in the variation of the chemical shifts of the α- and β-protons versus n. The chemical shift of a proton is influenced by its electronic environment.29,30 It is not surprising, therefore, that the chemical shifts of the protons directly associated with the electron-withdrawing effects of the ester groups, i.e., the α- and β-protons, would vary with n. The electron-withdrawing effect on the protons of the diol chain

Figure 4. 1H NMR chemical shifts of (a) α-protons, and (b) β-protons as functions of diol chain length on C18-n-18 diesters. Dashed lines are exponential fits, and the error bars are the spread of the chemical shifts for a single experiment. 14757

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Figure 5. (a) TGA and (b) TGA derivative (DTG) curves of diesters obtained using a heating rate of 10 K/min.

also contained products with both higher and lower masses than the diesters themselves (smaller retention times are typical of lower mass fractions and vice versa; results not shown). It is likely that these higher-mass materials were the result of thermal modification of the diesters by way of catalyst-free cross-linking of the reactive olefinic centers at temperatures above 300 °C.36 Subsequent decomposition of these and unmodified diesters at higher temperatures may in turn account for the small amount of lower-mass products observed in the evolved gas stream. 3.3. Kinetics of Mass Loss of the C18-n-18 Diesters. The Friedman isoconversional method20 has been used as outlined in section 2.5 to determine the apparent activation energies which define the thermal stability of the C18-n-18 diesters. The TGA profiles obtained over the six different heating rates are given in Figure 7a for C18-3-18. These plots are typical of the other C18-n-18 diesters, and they show that the same apparent single-step mass loss and asymmetric DTG profiles (not presented) described in section 3.2 also apply over all the heating rates investigated. Figure 7b shows the Friedman plot obtained for C18-3-18 at 40% conversion (m = 0.40). This plot is also typical of all the C18-n-18 diesters. It shows that a single Ea may be obtained for each conversion which holds for all the heating rates investigated, indicating that, for any given conversion, the kinetic mechanism underlying mass loss is independent of the heating rate.37 The calculated standard errors on the Ea values were less than 6 kJ/mol (m < 0.90; r2 > 0.999). The Ea versus m curve for C18-3-18, again typical of the other C18-n-18 diesters, is given in Figure 7c. Three distinct mdependent regions can be observed: a first (I) and last (III) region wherein Ea decreased with increasing m, and a main central region (II) in which Ea was constant over a large range of m. A list of the obtained Ea for each diester is given in Table 2. Figure 7d shows the changes in Ea with increasing n which occur at the boundaries of regions I and II, and II and III, as well as the average Ea over region II. It can be seen from this plot that, at the delimiting boundaries of both regions I−II and II−III, Ea increases linearly with n over all n by approximately 3.0 ± 0.4 kJ/mol per carbon unit (r2 > 0.92), whereas for the main region over which mass loss occurs (region II), Ea increases linearly only for n ≥ 3 (r2 = 0.9998; slope 3.6 ± 0.03 kJ/mol per carbon unit).

Figure 6. Characteristic temperatures, T, of C18-n-18 diesters at 5% (●) and 50% (▽) mass loss, and center maxima of the DTG peaks (TPDTG, ◇) as functions of diol chain length (n).

intermediate,31 and is driven by the presence of an easily extractable proton at the β-position34 and the thermal susceptibility of the (O)CO-C bond compared to the OCO or OCO ester bonds (bond energy 86, 110, and 179 kcal, respectively).35 In section 3.1.2, it was shown that the atoms at the α- and β-positions of the diol chain moiety of the C18-n-18 diesters (Scheme 2) with n < 6 are chemically more reactive than the atoms at the same positions of diesters with n ≥ 6. Accordingly, the increasing ease with which the β-proton can be extracted is inversely correlated to the thermal stability of these materials. That this n-dependence of T5%, T50%, or TPDTG with βproton reactivity was not observed indicates that either this degradation process is masked by another more dominant mechanism (such as evaporation), or that it occurs well above the evaporation temperatures (T5%, T50%) of the diesters. To verify that evaporation was in fact occurring, the evolved gas stream was condensed at −78 °C using a dry ice−acetone bath, and subsequently characterized against the starting materials using the HPLC calibration curve method as described in Raghunanan et al.16 Indeed, it was found that the condensed gas stream contained 90% diesters by mass, confirming that evaporation was the primary means of mass loss. The retention times of the remaining 10% of the materials from the HPLC column indicated that the evolved gas stream 14758

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Figure 7. (a) Typical TGA curves for C18-n-C18 (shown herein, C18-3-18) obtained at different heating rates (from left to right 0.1, 1.0, 3.0, 5.0, 10, 20 K/min). (b) Typical Friedman plot of C18-n-18 (shown herein, C18-3-18 at 40% conversion). (c) Apparent activation energy (Ea), obtained from the slopes of the Friedman plots, of C18-3-18 as a function of conversion (m). (d) Increase in Ea with increasing n which occur at the boundaries of regions I and II (○) and regions II and III (▽), as well as the average Ea over region II (△).

Table 2. Apparent Activation Energies (± Standard Errors) for C18-n-18 Diesters, Obtained Using the Friedman Method Ea (kJ/mol) m 0.05 0.10 0.15 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

n=2

n=3

121 ± 1 114 ± 3

110 110 110 109 109 109 109 111 110 109 108 107

± ± ± ± ± ± ± ± ± ± ± ±

2 2 2 2 2 1 1 1 0 1 1 1

105 104 102 99 80

± ± ± ± ±

1 1 1 2 9

122 ± 1 117 ± 1 112 ± 3

109 106 106 106 106 106 106 106 105 105 104 103 102

± ± ± ± ± ± ± ± ± ± ± ± ±

3 2 2 2 2 3 3 3 3 3 2 2 3

n=4 Region I 127 ± 2 123 ± 3 119 ± 4 Region II

117 115 114 113 112 110 109 109 108 109 108 108

± ± ± ± ± ± ± ± ± ± ± ±

n=6

n=9

n = 10

128 ± 2

131 ± 6

136 ± 3

122 120 120 119 118 118 118 117 117 116 116 115 115 114 113

5 5 5 4 4 4 4 3 3 3 2 2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4 4 3 3 2 2 2 3 3 3 2 2 2 3 3

129 129 129 129 129 129 128 128 127 127 126 125 123 122

± ± ± ± ± ± ± ± ± ± ± ± ± ±

5 5 4 4 3 3 3 3 3 3 3 3 3 3

133 132 131 131 132 131 132 132 131 131 130 129 128 127

± ± ± ± ± ± ± ± ± ± ± ± ± ±

4 3 3 2 2 2 1 2 2 2 2 2 2 2

120 117 115 108

± ± ± ±

4 4 7 23

125 122 117 111

± ± ± ±

2 2 3 9

Region III 0.75 0.80 0.85 0.90 0.95

99 ± 4 90 ± 8 50 ± 27

107 105 100 79

± ± ± ±

2 3 3 12

The constant Ea observed in region II (Figure 7c) indicates that the same thermal mechanism is occurring, whereas the

112 ± 3 112 ± 4 119 ± 10

decreasing Ea with increasing m observed in regions I and III suggests the influence of increasing free volume.38 Within 14759

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5-membered cyclic conformer (Scheme 4) rather than the 6membered analogue favored by the other diesters.

region I, free volume may be generated as a result of hightemperature bond rotation and reorganization of the diesters about their ester groups into cyclic conformers, as shown in Scheme 3. The formation of these cyclic conformers is

Scheme 4. Rearrangement of Linear C18-2-18 Diester into a 5-Membered Cyclic Conformer, Facilitated by Intramolecular Hydrogen-Like Bonds at the α-Position

Scheme 3. Rearrangement of Linear C18-n-18 Diesters (Shown Herein, C18-6-18) into Cyclic Conformers, Facilitated by Intramolecular Hydrogen-Like Bonds at the βPosition

facilitated by the formation of hydrogen-like bonds between the labile β-protons (section 3.1.2) and the carbonyl oxygen atoms of the ester groups. As the ester-group-mediated intermolecular attractions give way to the hydrogen-like intramolecular bonds, the net influence of intermolecular bonding lessens, thus allowing the diesters to volatilize at temperatures lower than their linear counterparts. That it is evaporation occurring within this region is further supported by the linear increase in Ea with increasing n, and hence mass, observed at the boundary of regions I and II (Figure 7d). Within the liquid phase of each material, decreasing intermolecular attractions allow increased thermal expansion with increasing temperature, thereby accounting for the increasing free volume of region I and hence the observed decreasing activation energies (Figure 7c, Table 2). Region II, which accounts for most of the weight loss occurring under these thermal conditions, very likely represents the boiling range of the diesters. This assumption is corroborated by the experimental results presented in section 3.2 which described the presence of significant amounts of the diester in the evolved gas stream relative to other “reaction” products. Free volume, and hence Ea, stays relatively constant over this range since temperature is unchanged during boiling. The apparent activation energies for the C18-n-18 diesters obtained within this region span from 105 to 130 kJ/mol (Figure 7d). That these values are less than those reported in the literature (also using the Friedman isoconversional method) for the thermal degradation of linear aliphatic polyesters such as poly(propylene glutarate) and poly(propylene suberate) (190−237 kJ/mol39) support the argument that the bond cleavage activation energy is higher than the energy required to overcome the weakened intermolecular forces of attraction in the C18-n-18 diesters. Figure 7d also shows that, when C18-2-18 is excluded, the Ea values of region II can be well fitted by a linear regression (r2 = 0.9998) with a slope of 3.6 ± 0.03 kJ/mol per carbon. This indicates that the same phase-change mechanism is operative for the C18-n-18 diesters at n ≥ 3, and that the predictable increase in Ea with n is due to their increasing mass. For C18-2-18, however, Ea lies outside of this linear trend (Figure 7d), alluding to a difference in its phase-change mechanism compared to the other diesters. In section 3.1.2, it was shown that, unlike the other diesters, C18-2-18 does not possess any well-defined β-protons. Instead, all of the protons attached to its diol chain moiety display αcharacteristics, indicating that C18-2-18 is more likely to form a

Finally, the activation energy within region III decreased due to increasing free volume which may have been the result of an increase in the density of the vapor phase just above the liquid surface. As for region I, the role of the vapor phase on the activation energies of this region is supported by the linear dependence of the delimiting values of Ea at region II−III over all n (Figure 7d). Furthermore, this phase can increase in temperatures well above the boiling point of the liquid molecules. This introduces the opportunity for further reactions to occur, such as polymerization across the carbon−carbon double bonds and subsequent decomposition of these or vaporphase diester molecules. Such vapor-phase reactions may, therefore, account for the small amounts of reaction products observed in the evolved gas stream discussed in section 3.2, and thus supports the assumption that ester-mediated decomposition of the diesters may occur at higher temperatures.

4. CONCLUSIONS The influence of the electron-withdrawing ester groups in esterdense materials such as oligo-esters and polyesters on chemical and thermal stabilities has been investigated using as model systems linear aliphatic unsaturated diesters. Both FTIR and 1H NMR data have been used to show that chemical stability is in fact influenced by the electronwithdrawing effect of the ester groups on each other and on their neighboring atoms, and that these effects can be minimized by increasing the length of the spacer groups and hence the levels of shieldingbetween successive ester groups up to a maximum of six methylene groups. Thus, a polyester whose ester groups are separated by two methylene units may be chemically less stable than an analogous polyester whose ester groups are separated by four or five methylene units, while a polyester whose ester groups are separated by 18 methylene units may not be significantly more stable than one separated by a minimum of six methylene units. Both TGA and isoconversional kinetic experiments revealed that the diesters of this study tended to undergo phase change well before ester-group-mediated thermal degradation could occur. Thermal stability of these materials, therefore, was found to be dependent upon increasing molecular mass, and independent of the levels of shielding between the electronwithdrawing ester groups. TGA experiments showed that the earliest mass loss (5%) occurred at 280 °C for the smallest 14760

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diester (C18-2-18), and increased by 5 °C for every methylene group added to the diol chain moiety. Apparent activation energies, determined using the Friedman isoconversional method, revealed that evaporation generally occurred at 85% conversion. Activation energies for the main phase-change process (boiling) were in the order of 105−130 kJ/mol, and were dependent upon both molecular mass and the availability of β-protons at the alcohol moieties of the ester groups. Overall, these results are of fundamental significance in that they allow for the design and syntheses of a variety of complex ester molecules with chemical and thermal properties optimized as per their applications.



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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-705-748-1011, ext 6105. E-mail: [email protected] trentu.ca. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors wish to acknowledge the technical support of Dr. Shaojun Li and Dr. Laziz Bouzidi, and the financial support of Elevance Renewable Sciences, NSERC, Grain Farmers of Ontario, GPA-EDC, Industry Canada, and Trent University.



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