Lubricating and Waxy Esters, V: Synthesis, Crystallization, and Melt

Article pubs.acs.org/IECR

Lubricating and Waxy Esters, V: Synthesis, Crystallization, and Melt and Flow Behaviors of Branched Monoesters Incorporating 9‑Decenol and 9‑Decenoic Acid Shaojun Li,† Laziz Bouzidi,† and Suresh S. Narine* Trent Centre for Biomaterials Research, Departments of Physics & Astronomy and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada S Supporting Information *

ABSTRACT: Branched derivatives of waxy monoesters incorporating 9-decenol and 9-decenoic acid were synthesized using epoxidation and ring-opening esterification. The reactions were conducted at two different temperatures and monitored over time. The crystallization, melting, and viscosity of the compounds were all controlled strongly as a function of incremental branching. Isomerism was shown to be critically important: an OH group at the end of the hydrocarbon chain completely suppressed crystallization, whereas its isomer with a terminal acyl chain did not. The structure of the linear monoesters were shown to provide the templates for crystallization, melting, and flow behavior, whereas the branching effect extended but could not erase the effect of the base molecular architecture. These compounds present a large range of properties that are suitable for a variety of applications ranging from waxes to lubricants.

1. INTRODUCTION Plant oils represent an important resource primarily used for food and feed purposes. They are increasingly explored as potential feedstocks for industrial materials and fuels,1 because of their biodegradability and renewability,2 low toxicity,3 low volatility, high viscosity indices,4 and excellent lubrication performance in many practical applications.5 However, low oxidative and thermal stabilities, poor low-temperature properties, and narrow ranges of available viscosities limit the scope of their application.6,7 Generally, their properties can be improved by structural modification at the fatty-acid-chain level. Such changes include decreasing the chain length, decreasing the internal symmetry of the molecule, and/or introducing branches into the fatty acid chain and changing the positions of the branches.8,9 The chain lengths of fatty acids and their derivatives can be decreased by cross-metathesis reactions using ethylene,10−13 butene, hexene,14 or substitute alkenes.15−17 Cross-metathesis with ethylene, so-called ethenolysis, is an important example, because ethylene is a favorable substrate with a low cost and abundant supply. The resultant terminal double bond can be further functionalized by carboxylation, amination, or epoxidation.11,18 Branching has been shown to alter the low-temperature performance and oxidation stability and lubricity behavior of ester and ether products.19,20 Several methods have been developed to synthesize branched fatty acid esters.21−27 Epoxidation followed by ring-opening esterification has received special attention because it can be carried out under moderate reaction conditions and also because of the high reactivity of the resultant epoxide ring.28 The epoxide ring can react with different nucleophiles to provide multifunctional products,29 such as polyols,30 hydroxyl esters,31 hydroxyl ethers,32 and amino alcohols.29 © 2014 American Chemical Society

The Trent Centre for Biomaterials Research (TCBR) is conducting comprehensive fundamental studies of the synthesis, crystallization, melt, polymorphism, solid fat content, viscosity, and modeling of the structure and function of jojoba-like wax ester (JLE) analogues: linear mono-, di-, and triesters and their branched derivatives. The oil from the jojoba plant is very interesting because it is mainly composed of linear wax esters rather than triacylglycerols (TAGs) as is the case for all other known seed oils.33−35 Because of its molecular structure, jojoba oil displays quite distinct properties suitable for a variety of highend applications such as pharmaceuticals, cosmetics, and highgrade lubricant oil formulations.36 Unfortunately, its cost and availability impede its large-scale use.37 The production of JLEs from commonly available and economical feedstocks, such as oleic acid, is an alternative that has attracted significant interest.37−41 TCBR is particularly interested in elucidating the effects of chain length, branching, symmetry, and functional groups on the physical properties. We recently reported on the physical properties of four JLEs synthesized using a Lewis base as the catalyst under mild reaction conditions.42 Following this work, we conducted a comprehensive study on a series of multibranched polyol esters prepared from these JLEs using an ecofriendly and green method, incorporating a solvent-free and catalyst-free epoxide ring-opening reaction, followed by a onepot condensation esterification.20 These studies revealed that properties scale predictively with mass and number and position of branches and are affected greatly by the number of hydroxyl groups. More recently, we reported on the calorimetry, solid-fatcontent evolution, and flow behavior of three shortened JLEs Received: Revised: Accepted: Published: 12339

April 10, 2014 July 4, 2014 July 14, 2014 July 14, 2014 dx.doi.org/10.1021/ie5014844 | Ind. Eng. Chem. Res. 2014, 53, 12339−12354

Industrial & Engineering Chemistry Research

Article

2. MATERIALS AND METHODS 2.1. Materials. Octadec-9-enyl dec-9-enoate (JLE-281), dec9-enyl oleate (JLE-282), and dec-9-enyl dec-9-enoate (JLE-20) were prepared in our laboratory.43 Chloroform, dichloromethane, calcium hydride (CaH2), N,N′-dicyclohexylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP) were purchased from Sigma-Aldrich. Hexane and ethyl acetate, from ACP Chemicals Inc. (Montreal, Quebec, Canada), were used without further treatment. Chloroform was distilled over calcium hydride. 2.2. Analytical Methods. 2.2.1. 1H NMR Spectroscopy. One-dimensional 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker BioSpin MRI GmbH, Karlsruhe, Germany) at frequencies of 400 and 100 MHz, respectively, using a 5-mm Broadband Observe (BBO) probe. One-dimensional 1H NMR spectra were acquired at 25 °C over a 16 ppm spectral window with a 1-s recycle delay and 32 transients. One-dimensional 13C NMR spectra were acquired at 25 °C over a 240 ppm spectral window with a 0.2-s recycle delay and 2048 transients. The NMR spectra were Fourier transformed, phase corrected, and baseline corrected. Window functions were not applied prior to Fourier transformation. 1H NMR spectroscopy was performed in deuterated chloroform (CDCl3) and deuterated dimethyl sulfoxide (DMSO-d6), and 13 C NMR spectroscopy was performed in CDCl3. Chemical shifts were referenced to residual solvent peaks. 2.2.2. HPLC. Purities of compounds were measured on a Waters e2695 HPLC instrument with a Waters 2424 ELS detector (Waters Corporation, Milford, MA) and an XBridge column (C18, 150 mm × 4.6 mm, 5.0 μm). The mobile phase was a mixture of 40% chloroform/60% acetonitrile (ACN) with a flow rate of 1 mL/min. To study the migration of the acyl group in the terminal location to its adjacent hydroxyl group, samples were measured with a diol column (BETASIL Diol-100, 250 × 4 mm, particle size 5 μm, part no. 72605-254030, Thermo Scientific). The mobile phase was a 90:10 (v/v) hexane/ isopropanol mixture with a flow rate of 0.5 mL/min 2.2.3. Electrospray Ionization Mass Spectrometry. Electrospray ionization mass spectrometry (ESI-MS) analysis was performed using a QStar XL quadrupole time-of-flight mass spectrometer (AB Sciex, Concord, ON, Canada) equipped with an ion-spray source and modified hot-source-induced desolvation (HSID) interface (Ionics, Bolton, ON, Canada). The ion source and interface conditions were adjusted as follows: ionspray voltage (IS) = 4500 V, nebulizing gas (GS1) = 45, curtain gas (GS2) = 45, declustering potential (DP) = 60 V, and HSID temperature (T) = 200 °C. Multiply charged ion signals were reconstructed using the BioTools 1.1.5 software package (AB Sciex, Concord, ON, Canada). 2.2.4. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was carried out in triplicate on a TGA Q500 apparatus (TA Instruments, New Castle, DE). Approximately 8.0−15.0 mg of fully melted and homogeneously mixed sample was loaded into the open TGA platinum pan. The sample was equilibrated at 25 °C and heated to 600 °C at a constant rate of 10 °C/min. The TGA measurements were performed under a dry nitrogen flow of 40 mL/min for balance purge and 60 mL/ min for sample purge. The first derivatives of the TGA (differential thermogravimetry, DTG) curves were obtained and analyzed with TA Universal Analysis software. 2.2.5. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) analysis was carried out on a Q200

that had not previously been investigated in monoester formulations.43 These molecules, namely, octadec-9-enyl dec-9enoate (JLE-281), dec-9-enyl oleate (JLE-282), and dec-9-enyl dec-9-enoate (JLE-20), were synthesized from fatty acids and fatty alcohols. The present work uses the peculiar double-bond structures of JLE-281, JLE-282, and JLE-20 to prepare branched polyol esters with branches at both the middle and end of the aliphatic carbon chain. Fifteen branched derivatives were prepared from these JLEs by reactions including epoxidation, ring-opening esterification of epoxides by propionic acid, and normal condensation. Their generalized structures are shown in Scheme 1, and related Scheme 1. Syntheses of Branched Derivatives of JLEsa,b

a

n and m values and R groups are specified in Table 1. bR(OH), the substituted group is R, and the substituted group in the neighboring position is OH; OH(R), the substituted group is OH, and the substituted group in the neighboring position is R.

information, including the nomenclature and abbreviations used, is provided in Table 1. The individual structures of the branched derivatives of the JLEs are provided in Scheme S1 of the Supporting Information. The compounds were fully characterized by 1H nuclear magnetic resonance (NMR) spectroscopy, and their purities were measured by high-performance liquid chromatography (HPLC). Mass spectroscopy was used to assist in the identification of selected compounds (see the list in provided in Table S1, Supporting Information). 12340

dx.doi.org/10.1021/ie5014844 | Ind. Eng. Chem. Res. 2014, 53, 12339−12354


Article

Table 1. Jojoba-Like Monoesters (JLEs) with Terminal Double Bonds Synthesized in This Work compound

structurea

IUPAC name Base Esters

n1 = 0, n2 = 8 m1 = m2 = 5 n1 = 8, n2 = 0 m1 = m2 = 5 n1 = n2 = 0 m1 = m2 = 5

JLE-281

octadec-9-enyl dec-9-enoate

JLE-282

dec-9-enyl oleate

JLE-20

dec-9-enyl dec-9-enoate

JLE-281-E

8-(3-octyloxiran-2-yl)octyl 8-(oxiran-2-yl)octanoate

JLE-282-E

8-(oxiran-2-yl)octyl 8-(3-octyloxiran-2-yl)octanoate

JLE-20-E

8-(oxiran-2-yl)octyl 8-(oxiran-2-yl)octanoate

JLE-281-2-1

Branched Derivatives 9-hydroxy-10-(propionyloxy)octadecyl 9-hydroxy-10-(propionyloxy)decanoate or/and 10-hydroxy-9-(propionyloxy)octadecyl 9hydroxy-10-(propionyloxy)decanoate

JLE-281-2-2

10-hydroxy-9-(propionyloxy)octadecyl 10-hydroxy-9-(propionyloxy)decanoate or/and 9-hydroxy-10-(propionyloxy)octadecyl 10hydroxy-9-(propionyloxy)decanoate

JLE-281-3-1

10-((9-hydroxy-10-(propionyloxy)octadecyl)oxy)-10-oxodecane-1,2-diyl dipropionate or/and 10-((10-hydroxy-9-(propionyloxy) octadecyl)oxy)-10-oxodecane-1,2-diyl dipropionate

JLE-281-3-2

1-((9-hydroxy-10-(propionyloxy)decanoyl)oxy)octadecane-9,10-diyl dipropionate (I) or/and 1-((10-hydroxy-9-(propionyloxy) decanoyl)oxy)octadecane-9,10-diyl dipropionate (II)

JLE-281-4

1-((9,10-bis(propionyloxy)decanoyl)oxy)octadecane-9,10-diyl dipropionate

JLE-282-2-1

9-hydroxy-10-(propionyloxy)decyl 9-hydroxy-10-(propionyloxy)octadecanoate or/and 9-hydroxy-10-(propionyloxy)decyl 10hydroxy-9-(propionyloxy)octadecanoate

JLE-282-2-2

10-hydroxy-9-(propionyloxy)decyl 10-hydroxy-9-(propionyloxy)octadecanoate or/and 10-hydroxy-9-(propionyloxy)decyl 9hydroxy-10-(propionyloxy)octadecanoate

JLE-282-3-1

10-((9-hydroxy-10-(propionyloxy)octadecanoyl)oxy)decane-1,2-diyl dipropionate or/and 10-((10-hydroxy-9-(propionyloxy) octadecanoyl)oxy)decane-1,2-diyl dipropionate

JLE-282-3-2

1-((9-hydroxy-10-(propionyloxy)decyl)oxy)-1-oxooctadecane-9,10-diyl dipropionate (I) or/and 1-((10-hydroxy-9-(propionyloxy) decyl)oxy)-1-oxooctadecane-9,10-diyl dipropionate (II)

JLE-282-4

1-((9,10-bis(propionyloxy)decyl)oxy)-1-oxooctadecane-9,10-diyl dipropionate

JLE-20-2-1

9-hydroxy-10-(propionyloxy)decyl 9-hydroxy-10-(propionyloxy)decanoate

JLE-20-2-2

10-hydroxy-9-(propionyloxy)decyl 9-hydroxy-10-(propionyloxy)decanoate or/and 9-hydroxy-10-(propionyloxy)decyl 10-hydroxy9-(propionyloxy)decanoate

JLE-20-2-3

10-hydroxy-9-(propionyloxy)decyl 10-hydroxy-9-(propionyloxy)decanoate

JLE-20-3-1

10-((9-hydroxy-10-(propionyloxy)decanoyl)oxy)decane-1,2-diyl dipropionate or/and 10-((9-hydroxy-10-(propionyloxy)decyl) oxy)-10-oxodecane-1,2-diyl dipropionate

JLE-20-3-2

10-((10-hydroxy-9-(propionyloxy)decanoyl)oxy)decane-1,2-diyl dipropionate or/and 10-((10-hydroxy-9-(propionyloxy)decyl) oxy)-10-oxodecane-1,2-diyl dipropionate

JLE-20-4

10-((9,10-bis(propionyloxy)decanoyl)oxy)decane-1,2-diyl dipropionate

Epoxides n1 = 0, n2 = 8 m1 = m2 = 5 n1 = 8, n2 = 0 m1 = m2 = 5 n1 = n2 = 0 m1 = m2 = 5

12341

n1 = 0, n2 = 8 m1 = m2 = 5 R = C2H5COO n1 = 0, n2 = 8 m1 = m2 = 5 R = C2H5COO n1 = 0, n2 = 8 m1 = m2 = 5 R = C2H5COO n1 = 0, n2 = 8 m1 = m2 = 5 R = C2H5COO n1 = 0, n2 = 8 m1 = m2 = 5 R = C2H5COO n1 = 8, n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = 8, n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = 8, n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = 8, n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = 8, n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0 m1 = m2 = 5 R = C2H5COO n1 = n2 = 0



Article

Table 1. continued compound

IUPAC name

structurea

Branched Derivatives m1 = m2 = 5 R = C2H5COO a

Generalized base ester structures and branched polyol esters shown in Scheme 1.

to give compounds with three or four branches (so-called threeor four-branched derivatives, respectively). The ring-opening esterification of the epoxides to prepare the two-branched compounds was carried out without the need of either a catalyst or a solvent. In this reaction, carboxylic acid (propoinic acid) served as both a reactant and a solvent. 2.3.1. Epoxidation of the Jojoba-Like Esters (Scheme 1). To a stirred solution of ester (10 mmol) and formic acid (60 mmol) in 10 mL of CH2Cl2 at 4 °C, H2O2 (44 mmol) was slowly added. The reaction proceeded at room temperature with vigorous stirring for 4−36 h. After removal of the aqueous phase, additional CH2Cl2 (30 mL) was added to the organic phase, which was washed sequentially with water (2 × 20 mL), saturated aqueous NaHCO3 (2 × 10 mL), and brine (2 × 20 mL) and then dried with Na2SO4. After filtration, the filtrate was concentrated, and the residue was purified by column chromatography with 1:10 (v/v) ethyl acetate/hexanes. The pure epoxides 8-(3-octyloxiran-2-yl)octyl 8-(oxiran-2yl)octanoate (JLE-28S-E), 8-(oxiran-2-yl)octyl 8-(3-octyloxiran-2-yl)octanoate (JLE-28A-E), and 8-(oxiran-2-yl)octyl 8(oxiran-2-yl)octanoate (JLE-20-E) prepared from JLE-281, JLE-282, and JLE-20, respectively, were obtained as colorless oils with purities of >98%. The yields and purities of the epoxidation products are listed in Table 2. 2.3.2. Synthesis of the Branched Compounds (Scheme 1). To a solution of each epoxidation product (10 mmol) was added 220 mmol of propionic acid (or nonanoic acid). The reaction was carried out under a N2 atmosphere at a temperature of 95 °C. The reaction mixtures were stirred at 95 °C for 16−24 h. The reaction temperature was raised to 120 °C for 24−36 h to achieve three- or four-branched compounds. The resulting products were poured into 200 mL of water and extracted with ethyl acetate (2 × 50 mL). The organic phase was washed sequentially with water (2 × 100 mL), saturated aqueous NaHCO3 (2 × 100 mL), and brine (2 × 200 mL) and dried on Na2SO4. After filtration, the filtrate was concentrated, and the residue was purified by column chromatography with ethyl acetate and hexanes. The derivatives of JLEs with two and three branches were prepared at 95 °C for 16 h, and those with four branches were prepared at 120 °C for 32 h. Pure derivatives of JLE-281 and JLE282 were obtained as colorless oils by column chromatography, and pure derivatives of JLE-20 were obtained as white solids. The yields, ethyl acetate/hexanes ratios in column chromatography, and purities of the branched derivatives of the JLEs are listed in Table 2.

model instrument (TA Instruments, New Castle, DE) equipped with a refrigerated cooling system (RCS 90, TA Instruments) under a nitrogen flow of 50 mL/min. The sample [5.0−6.0 (±0.1) mg] in a hermetically sealed aluminum pan was cooled from the melt (50 °C) to −90 °C and subsequently reheated to 70 °C at the same constant rate of 3.0 °C/min to obtain the crystallization and melting profiles, respectively. TA Universal Analysis software together with a method developed by our group44 was used to analyze the data and extract the main characteristics of the peaks. The measurement temperatures are reported to a certainty of ±0.5 °C. 2.2.6. Viscosity Measurements. Sample viscosities were measured on a computer-controlled AR2000ex rheometer (TA Instruments, New Castle, DE) using a standard-size recessed-end concentric cylinder (stator inner radius = 15 mm, rotor outer radius = 14 mm, part no. 545023.001). Temperature control was achieved by the Peltier effect with an accuracy of 0.1 °C. Viscosities of samples were measured from temperatures above each sample’s melting point to 110 °C. The measurements were performed using shear rate/share stress curves and a constant heating rate (0.1 and 3.0 K/min) with a constant shear rate (200 s−1). The two methods yielded measured viscosities in good agreement within the experimental uncertainty. Note that, in this investigation, the shear-rate range was optimized for torque (lowest possible value of 10 μN m) and velocity (maximum supplier suggested value of 40 rad/s). The shear rate−shear stress curves were fitted with the Herschel−Bulkley equation (eq 1), a model commonly used to describe the general behavior of materials characterized by a yield stress τ = τ0 + Kγ ṅ

(1)

where γ̇ denotes the shear stress, τ0 is the yield stress below which there is no flow, K is the consistency index, and n is the power index. n depends on the constitutive properties of the material. For Newtonian fluids, n is equal to 1. The flow is shear-thickening for n > 1 and shear-thinning for n < 1. 2.3. Synthesis of Branched Derivatives of the JojobaLike Esters. The base JLEs (structures are shown in Scheme 1) were previously prepared with high yield from oleic acid, 9decenic acid, and their related alcohols by Steglich esterification with 4-dimethylaminopyridine as the catalyst or from their related chlorides and alcohols in the presence of pyridine.43 The JLE epoxides (Scheme 1) were synthesized according to a published procedure.31 The catalyst, peroxyacid, was formed from formic acid and hydrogen peroxide in situ with CH2Cl2 as the solvent. The epoxidation reaction took about 36 h, longer than the 8−16 h required for the JLEs without terminal carbon double bonds in our previous work,20 because of the lower electron density of the terminal carbon double bonds, which decreased the rate of epoxidation.45 The branched compounds were prepared in a two-step reaction in one pot using a ring-opening esterification to give the compounds with two branches (so-called two-branched derivatives) and an in situ normal condensation esterification

3. RESULTS AND DISCUSSION 3.1. Chemical Analysis and Identification of the Branched Compounds. 3.1.1. Identification of the Branched Compounds. All of the purified compounds were identified by 1 H NMR spectroscopy. Mass spectroscopy was used to assist in the identification of the compounds. 1H NMR data (δ, ppm) of the epoxidation products in CDCl3, 1H NMR data (δ, ppm) of the branched derivatives of the JLEs in CDCl3 and DMSO, and 12342



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Table 2. Yield, 1H NMR, and MS Data of Epoxidation Products and Branched Derivatives of Jojoba-Like Monoesters (JLEs). EA/HE= ethyl acetate/hexanes

ratio between nonterminal and terminal hydroxyl derivatives based on 1H NMR spectroscopy was about 70:30, similar to what was determined by HPLC. In the 1H NMR spectra of JLE-281-3 and JLE-282-3, the peak of −CH(R)CH(R)CH2−, appearing at 4.98−4.95 ppm, was very weak, indicating that JLE-28-3-1 was the main product. Although, theoretically, as shown in Scheme 1, two regioisomers of the three-branched derivatives of JLE-281 and JLE-282 can be afforded, only trace amounts of JLE-281,2-3-2 were detected by 1 H NMR spectroscopy. This is because the OH groups of the two-branched derivatives at the end of the chain are less sterically hindered than the OH groups in the middle of the chain, so it was easier to produce JLE-28-3-1 than JLE-28-3-2. Two derivatives with three branches (JLE-20-3) were achieved from JLE-20. The 1 H NMR spectrum of JLE-20-3 indicated the presence of both the terminal hydroxyl derivative (JLE-20-3-2) and the nonterminal hydroxyl derivative (JLE-20-3-1) with a ratio of 25:75. 3.1.2. Migration of the Acyl Group and Its Adjacent Hydroxyl Group in JLE-281-2. Migration of the terminal acyl group of JLE-281-2-1 to its adjacent hydroxyl group was detected by HPLC in samples that were stored at room temperature for 1 month (curve “JLE-281-2-1 at RT” in Figure 1) and in samples

Epoxidation Products of JLEs yield (%) JLE-281-E JLE-282-E JLE-20-E Branched Derivatives of JLEs EA/ HE

purity (%)

JLE-281-2-1 JLE-281-2-2 JLE-281-3

1:4 1:4 1:6

97 94 98

JLE-281-4

1:6

98

JLE-282-2-1 JLE-282-2-2 JLE-282-3

1:4 1:4 1:6

97 97 98

JLE-282-4

1:6

98

JLE-20-2-1

1:4

97

JLE-20-2-2

1:4

97

JLE-20-3-1

1:6

98

JLE-20-3-2

1:6

98

JLE-20-4

1:5

99

76 71 72

yield 26% JLE-28S-3 + 58% JLE-28S-2-M at 95 °C 26% JLE-28S-3 + 58% JLE-28S-2-M at 95 °C 26% JLE-28S-3 + 58% JLE-28S-2-M at 95 °C 32.5% JLE-28S-4 + 21.5% JLE-28S-3 + 33.7% JLE-28S-2 at 120 °C 32.5% JLE-28S-4 + 21.5% JLE-28S-3 + 33.7% JLE-28S-2 at 120 °C 19.8% JLE-28A-3 + 64.3% JLE-28A-2-M 19.8% JLE-28A-3 + 64.3% JLE-28A-2-M 19.8% JLE-28A-3 + 64.3% JLE-28A-2-M at 95 °C 51.3% JLE-28A-4 + 30% JLE-28A-3 at 120 °C 51.3% JLE-28A-4 + 30% JLE-28A-3 at 120 °C 47.7% JLE-20-3 + 51.2% JLE-20-2-M at 95 °C 47.7% JLE-20-3 + 51.2% JLE-20-2-M at 95 °C 47.7% JLE-20-3 + 51.2% JLE-20-2-M at 95 °C 57.2% JLE-20-4 + 21.5% JLE-20-3 at 120 °C 47.7% JLE-20-3 + 51.2% JLE-20-2-M at 95 °C 57.2% JLE-20-4 + 21.5% JLE-20-3 at 120 °C 57.2% JLE-20-4+ 21.5% JLE-20-3 at 120 °C

MS data of selected compounds are detailed in Table S1 (Supporting Information). Two potential alcohols can be formed on one epoxide ring during the preparation of the two-branched compounds. The resultant polyol esters have regiochemistry (for example, in JLE281-2, 9-alkanonate-10-hydroxyoctadecanoate versus the equally likely alkyl 10-alkanoate-9-hydroxyoctadecanoate regioisomer) and stereochemistry [for example, left- (S) or right- (R) handed enantiomers at C9 and C10 in JLE-281-2 products]. The regiochemistry (but not stereochemistry) products of the twobranched polyol esters were separated and characterized. The differences in polarity due to the amount and position of OH groups made it easier to separate them by column chromatography. Generally, a compound with a terminal hydroxyl group (such as JLE-281-2-2) has more polarity than its regioisomer with a nonterminal hydroxyl group (such as JLE-281-2-1). The nonterminal hydroxyl two-branched derivatives (e.g., JLE-28-21) were easily identified by the chemical shift of −CH2[OCOC2H5(R)]CH(OH)CH2−, −CH2(R)CH(OH)CH2−, −CH(R)CH(OH)CH2−, and −CH(R)CH(OH)CH2− in CDCl3 (at δ 4.12−4.10 and 3.94−3.90, 3.80, 4.80, and 3.55 ppm, respectively; see Table S1, Supporting Information). The terminal hydroxyl two-branched derivatives (e.g., JLE-28-2-2) were identified by the chemical shifts of −CH2(OH)CH(R)CH2− and −CH2(OH)CH(R)CH2− (at δ 3.70−3.60 and 4.91 ppm, respectively; see Table S1, Supporting Information). The

Figure 1. HPLC of the two-branched derivatives of JLE-281 showing the migration of the acyl group and its adjacent hydroxyl group. JLE-281-2M, mixture of JLE-281-2-1 and JLE-281-2-2. The nomenclature used is provided in Table 1, and the structures are shown in Scheme 1. JLE-2812-1 at RT, compound stored at room temperature for 30 days; JLE-2812-1 cycled, compound subjected to eight cooling and heating cycles from 50 to −90 °C during a 1-month period.

subjected to cooling and heating cycles (eight) from 50 to −90 °C during a 1-month period (curve “JLE-281-2-1 cycled” in Figure 1). As estimated from HPLC, 3% of JLE-281-2-1 in the sample stored at room temperature and 13% of the cycled sample were transformed into the JLE-281-2-2 isomer. The higher rate of migration observed for the sample subjected to cycling was probably due to the higher temperature to which JLE-281-2-1 was subjected (50 °C compared to ∼22 °C). Acyl migration to an adjacent hydroxyl group is not specific to the compounds of the present study. It has been reported to occur in acylated 12343



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Scheme 3. Progress of the Ring-Opening Reactiona−c

polyhydroxylic compounds, such as glucopyanose46 and glycerols47 and has been demonstrated to be intramolecular and not intermolecular in nature.47 Fischer48 suggested that the mechanism of acyl migration involves a 1,2-ortho acid or a cyclic ortho ester intermediate (Scheme 2). Scheme 2. Mechanism of Acyl Migration Involving a Cyclic Ortho Ester Intermediate

When observed by 1H NMR spectroscopy in CDCl3, acyl migration at room temperature proceeded relatively rapidly, reaching equilibrium in less than 2 days, but was not observed in DMSO-d6 even after 30 days. The Fisher mechanism48 suggests that the formation of intermediates was prevented in the presence of DMSO-d6. Apparently, the strong hydrogen bonds formed by DMSO-d6 with the hydroxyl groups prevents the formation of the cyclic ortho ester intermediate, slowing or averting acyl migration. Acyl migration in solvent was not studied further in the present work. 3.2. Effects of Temperature on the Progress of the Ring-Opening Esterification Reaction and on the Composition of the Reaction Products. The progress of ring-opening esterification in the one-pot reaction is suggested in Scheme 3. The yields and rates of production of the different branched compounds in the one-pot ring-opening esterification reaction was monitored for the derivatives of JLE-281 and JLE20 at 95 and 120 °C. The reaction products were analyzed every hour by HPLC for 25 h, and the composition was determined using calibration curves obtained for the pure compounds. The results at 95 and 120 °C are presented in panels a and b, respectively, of Figure 2 for JLE-281 (JLE-281-E) and in panels c and d, respectively, of Figure 2 for JLE-20 (JLE-20-E). As expected, the time for obtaining any given derivative was shorter and the production of the three- and four-branched derivatives increased dramatically when the reaction was conducted at the higher temperature. For example, JLE-20-E, the starting material for the derivatives of JLE-20, was completely consumed in ∼1 h at 120 °C and in ∼6 h at 95 °C. The rate of consumption of JLE-20-E starting material was higher than that of JLE-28S-E at 95 °C (solid circles in Figure 2a,c) but lower at 120 °C (solid circles in Figure 2b,d). This is understandable, as the reaction rate depends not only on the reaction temperature but also on other competing factors, particularly the size of the starting materials and the activity of their functional groups. Although JLE-20-E is a smaller molecule than JLE-281-E, it has only two terminal functional groups compared to JLE-281-E, which has one internal and one terminal functional groups. At the lower reaction temperature (95 °C), the effect of the smaller size of JLE-20-E on the reaction rate predominated over the functional groups, whereas the effect of the functional groups predominated over size at the higher temperature (120 °C), both of which can be explained by the combined relative effects of the mobility and reactivity of the molecules. One of the epoxide rings of the starting material is opened first to give the intermediates. JLE-20-E gave one intermediate (JLE-

a

JLE-281: n1 = 0, m1 = m2 = 5, n2 = 8. JLE-20: n1 = n2 = 0, m1 = m2 = 8. R: C2H5COO. bR(OH): the substituted group is R and the substituted group in the neighboring position is OH. OH(R): the substituted group is OH, and the substituted group in the neighboring position is R. cJLE-E, epoxide of JLE; JLE-R1 and JLE-R2, types of intermediates resulting from the opening of one of the two rings of the epoxide; JLE2, -3, and -4, two-, three-, and four-branched derivatives, respectively, of the JLE.

20-R1) because of its symmetrical structure about the ester headgroup, whereas JLE-281-E gave two types of intermediates, JLE-281-R1 and JLE-281-R2, resulting from the opening of one of the two rings on its asymmetric structure. To better understand the progress of the reaction, the intermediates (JLE-281-R1 and JLE-20-R1) were separated and characterized by 1H NMR spectroscopy. The 1H NMR spectrum of JLE-281R1 is shown in Figure 3. The JLE-281-R1 intermediate presented three chemical shifts at δ 2.8, 2.7, and 2.4 ppm indicative of a terminal epoxide ring and chemical shifts at δ 4.8 and 3.5 ppm characteristic of protons at positions 9 and 10 of an opened internal epoxide ring. JLE-281-R1 presents an unopened terminal epoxide ring and an open internal epoxide ring, and JLE-281-R2 presents an unopened internal epoxide and an open terminal epoxide ring; see Scheme 3. Note that, because of their small quantity and the laborious chromatographic work that it would entail, the intermediates with one epoxide ring were not separated. 12344



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Figure 2. Relative content versus time for the products of the ring-opening esterification reaction: (a) JLE-281 at 95 °C, (b) JLE-281 at 120 °C, (c) JLE20 at 95 °C, and (d) JLE-20 at 120 °C. Solid circles, starting material (SM); open downward-pointing triangles, intermediates R1 (see Scheme 3); thick ×, intermediates R2 (see Scheme 3); open upward-pointing triangles, two-branched derivatives; open circles, three-branched derivatives; open stars, four-branched derivatives.

Figure 3. 1H NMR spectrum of JLE-281-R1. The structure of JLE-281-R1 is shown in Scheme 3.

Scheme 4. SN1 Mechanism for the Ring-Opening Esterification of Epoxides

20-E at both 95 and 120 °C. Consistent with expectations, the maximum conversion was reached much faster in the reaction conducted at 120 °C (3 h) than in that conducted at 95 °C (8 h).

The intermediates were further reacted with carboxylic acid to give two-branched compounds (JLE-281-2 and JLE-20-2). The conversion was as high as 90% for JLE-281-E and 80% for JEL12345



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suggests that SN1 rather than SN2 was the dominant mechanism of the ring opening of the internal epoxide rings. If the SN2 mechanism were responsible, the nucleophiles (carboxylic acid) would more easily attack the sterically hindered terminal epoxide ring than the internal ring and the reaction would yield more JLE-281-R2 than JLE-281-R1. The ratio of JLE-281-2-1, the two-branched derivative with a terminal acyl group, to JLE-281-2-2, the two-branched derivative with a terminal OH group, was 70:30, indicating a more facile opening of the terminal epoxide ring, which suggests SN2 as the dominant mechanism for ring opening of the epoxide. When nucleophiles attack the terminal epoxide ring based on an SN2 mechanism, the reaction affords two asymmetric compounds based on the position attacked by the nucleophile. In the case of JLE-281-E, attack of the nucleophile on the less sterically hindered carbon gives JLE-281-2-1, and attack on the other carbon gives JLE-281-2-2. If the reaction were based on the SN1 mechanism, the nucleophile would attack the carbocation, which is stable in the more sterically hindered carbon, and would give more JLE-281-2-2 than JLE-281-2-1. 3.3. Thermal Stability. TGA indicated that the branched derivatives of the JLEs follow almost similar degradation patterns. The start and completion temperatures of decompositions of these compounds (∼200 and ∼480 °C, respectively) are typical of ester compounds. The temperature for 10% initial weight loss, T10, considered to be the compound decomposition temperature, was 313 ± 3, 307 ± 3, and 300 ± 3 °C for the derivatives of JLE-281, JLE-282, and JLE-20, respectively. Subtle differences in decomposition behavior due to the level of branching and type of base ester were observed. As shown in Figure 4 representing the DTG curves of the compounds, the decomposition of the branched derivatives occurred in one main step followed by three or two steps depending on the base JLE and number of branches. The branched derivatives of JLE-281,2 lost 70%−80% of the mass in the first step, whereas JLE-20 lost more than 90%. One can also note that the degradation rate ≈ 364 ± 3 °C in the two- and three-branched peaked at TDIG p derivatives of JLE-281,2 and at TDIG p ≈ 350 ± 4 °C in those of JLE20. This temperature was ∼10 °C higher for the four-branched

The resultant branched compounds were further reacted with carboxylic acid in a normal condensation reaction, where water was released as a byproduct. At 95 °C, JLE-20-2 was easily converted into JLE-20-3 and JLE-20-4, and JLE-281-2 was easily converted into JLE-281-3, but not into JLE-281-4 because its dangling chain prevented an easy condensation reaction between the hydroxyl groups and the carboxylic acid. The higher temperature (120 °C) was required to achieve JLE-281-4 in good yield. 3.2.1. Mechanisms of the Ring-Opening Esterification of the Epoxides. Depending on the nature of the epoxide and the reaction conditions, ring-opening reactions of epoxides with carboxylic acid can proceed by either an S N1 or SN2 mechanism.49 In the SN1 mechanism, an intermediate, a carbocation, is formed first, and then the substrate becomes susceptible to attack by the nucleophile (Scheme 4). In the SN2 mechanism, the ring opening is based on a nucleophilic attack by a carboxylate ion (Scheme 5). Scheme 5. SN2 Mechanism for the Ring-Opening Esterification of Epoxides

Although the kinetics of the reaction were not determined in the present work, the time-dependence data of the intermediates obtained from JLE-281-E (JLE-281-R1 and JLE-281-R2) and of the two-branched derivatives (JLE-281-2-1 and JLE-281-2-2) strongly suggest that the internal epoxide rings were opened mainly by the SN1 mechanism and the terminal epoxide rings were opened mainly by the SN2 mechanism. As shown in Figure 2a,b, JLE-281-R1 (with a terminal acyl group) rather than JLE281-R2 (with a terminal OH group) was detected as the main component of the reaction, indicating that the internal epoxide ring was protonated first before the nucleophilic attack. This

Figure 4. TGA first-derivative (DTG) curves of the branched derivatives of (a) JLE-281, (b) JLE-282, and (c) JLE-20. 12346



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Figure 5. DSC cooling thermograms of the base monoesters and their branched derivatives: (a) JLE-281, (b) JLE-282, and (c) JLE-20. (d) Crystallization onset (Ton, downward-pointing solid triangles) and peak (TP, solid circles) temperatures of JLE-20 and its branched derivatives.

crystallization and melting behaviors similarly. Crystallization was completely suppressed in the three- and four-branched derivatives, as well as in the two-branched derivatives having OH groups at the end of the chain (derivatives 2-2 in Figure 5a,b). Concurrently, no recrystallization or melting peaks were detected for these derivatives upon subsequent heating. In these cases, only a glass transition at very low temperature was detected. The addition of two terminal acyl branches decreased the onset of crystallization by 8.9 °C (from −5.8 ± 0.1 to −14.7 ± 0.1 °C) in JLE-282 and by 12.3 °C (from −13.3 ± 0.1 to −25.6 ± 0.1 °C) in JLE-281. The enthalpy of crystallization also decreased dramatically in both cases (from 137 J/g in JLE-282 to 10 J/g and from 133 J/g in JLE-281 to 35 J/g). The effect on thermal properties of the position of the “dangling” chain relative to the ester headgroup, which was very well established for the base JLE-281 and JLE-282,43 although weaker, still operates in their branched derivatives. The chain that is attached to the singly bonded oxygen in the case of JLE-281 and its two-branched derivatives can more easily rotate to conform for easier packing

derivatives. Note that no significant differences were observed in the decomposition patterns between the 2-1 and 2-2 or 3-1 and 3-2 positional isomers, probably because of similar reactivities. Obviously, the degradation mechanisms are initiated at the weakest bond in the ester link. The differences observed between the degradation patterns of JLE-20 and JLE-281,2 derivatives can be attributed to the dangling chain of JLE-281,2 and its position relative to the weakest carbon on the ester linkage. 3.4. Crystallization, Melting, and Glass Transition Behaviors. 3.4.1. Effect of Number of Branches. The thermal behavior of the JLEs was evaluated using DSC. The cooling and heating thermograms of the branched derivatives of the JLEs are shown in Figures 5 and 6, respectively. The related crystallization, melting, and glass transition data are provided in Tables 3−5, respectively. The thermal transitions were found to depend on the number of branches in a complex manner. The hydroxyl groups and their positions (terminal or not) had a significant effect on the magnitude of the variations in crystallization temperature and transformation behavior. One can notice that branching of JLE-281 and JLE-282 affected their 12347



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Figure 6. DSC heating thermograms of the base monoesters and their branched derivatives: (a) JLE-281, (b) JLE-282, and (c) JLE-20.

Table 3. Crystallization (3.0 °C/min) Data of the Presented Compoundsa main exotherm compound

b

JLE-281-2-1 JLE-281-2-M JLE-281-2-2 JLE-281-3 JLE-281-4 JLE-282-2-1 JLE-282-2-M JLE-282-2-2 JLE-282-3 JLE-282-4 JLE-20-2-1 JLE-20-2-2 JLE-20-2-M JLE-20-3-1 JLE-20-3-M JLE-20-4

Tcon

(°C)

−25.6 ± 4.0 −42.3 ± 1.0 glass transition glass transition glass transition −14.7 ± 0.1 −28.9 ± 0.5 glass transition glass transition glass transition 37.7 ± 0.1 −8.1 ± 0.1 19.3 ± 0.2 −21.8 ± 0.2 −33.9 ± 0.2 glass transition

Tcoff

(°C)

Tc1

(°C)

ΔHc1

second exotherm (J/g)

−72.7 ± 0.5 −68.7 ± 0.3

−35.2 ± 0.1 −57.3 ± 0.3

27.6 ± 0.6 10.3 ± 2.5

−23.2 ± 0.2 −54.6 ± 1.5

−17.5 ± 0.1 −43.6 ± 1

43.1 ± 1.4 10.8 ± 0.4

34.5 ± 0.5 −18.3 ± 0.7 −35.5 ± 1.3 −26.8 ± 0.4 −52.0 ± 0.2

36.7 ± 0.1 −9.3 ± 0.2 18.1 ± 0.4 −22.6 ± 0.3 −40.5 ± 0.1

106.4 ± 5.1 62.5 ± 5.5 41.0 ± 2.3 40.1 ± 0.1 12.5 ± 0.9

Tc2

(°C)

−25.6 ± 0.1

ΔHc2 (J/g) 0.2 ± 0.2

−16.8 ± 0.3

Tcoff, and Tc1,2: Onset, offset, and peak (1, 2) temperatures of crystallization, respectively. ΔHc1,2: Enthalpies of crystallization. bM in the compound name refers to a mixture of 1- and 2-isomers. a c Ton,

JLE-20-3-2, which did not record any exotherm upon cooling (JLE-20-3-2 curves in Figures 5c and 6c). The difference in the effects of branching on the thermal behavior between JLE-28 and JLE-20 is attributable to the presence of the dangling chain in the former. The addition of two branches in the case of JLE-20 is, in fact, a concatenation of acyl chains and an increase of the mass of the molecule, which understandably led to the increase of the crystallization temperature. The large value with which the onset of crystallization was increased when the two acyl groups were added at the end of the molecule is attributable to the combined effects of eliminating the terminal double bonds and increasing the van der Waals forces subsequent to the lengthening of the hydrocarbon chain. Branching was effective in depressing the onset and scope of crystallization only when the molecule was no longer linear. This clearly shows that, to be effective in disturbing regular packing, a “branch” must be protuberant.

than in JLE-282, where the chain is attached to the doubly bonded oxygen. The addition of two branches to JLE-20 increased the onset of crystallization by as much as 57.2 °C when the acyl chains were terminal and 11.4 °C when an OH group was terminal. As can be seen in Figure 5d, displaying the onset and peak temperature of crystallization of the branched derivatives of JLE-20, the crystallization was lowered only when three branches were added to the base JLE. Crystallization was completely suppressed when the OH group was at the end of the chain (JLE-20-3-2 curve in Figure 5c) and was depressed by only 2.3 °C for the three-branched derivatives with the OH group in the middle of the chain (JLE-20-3-1 curve in Figure 5c). Except for JLE-20-4, which presented only a glass transition, all of the heating traces of the branched derivatives of JLE-20 showed relatively strong recrystallizations and subsequent melting endotherms, including 12348



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Table 4. Melting (3.0 °C/min) Data of the Presented Compoundsa main endotherm compoundb JLE-281-2-1 JLE-281-2-M JLE-281-2-2 JLE-281-3 JLE-281-4 JLE-282-2-1 JLE-282-2-M JLE-282-2-2 JLE-282-3 JLE-282-4 JLE-20-2-1 JLE-20-2-2 JLE-20-2-M JLE-20-3-1 JLE-20-3-2 JLE-20-3-M JLE-20-4

second endotherm

R1

R2

TM off (°C)

TM 1 (°C)

ΔHM 1 (J/g)

TM 2 (°C)

TR1 (°C)

TR2 (°C)

27.3 ± 0.1 −25.7 ± 0.1

−13.9 ± 0.2 −28.9 ± 0.3

38.2 ± 0.7 8.0 ± 0.7

18.6 ± 0.3 −

2.1 ± 0.2 −38.9 ± 0.1

−48.3 ± 0.1 −

41.6 ± 0.1 32.9 ± 0.3

34.4 ± 0.1 28.5 ± 0.4

62.3 ± 1.0

−8.5 ± 0.1 −14.7 ± 0.7

−47.3 ± 0.1 −44.9 ± 0.5

−5.5 ± 0.1 −10.4 ± 0.6

54.6 ± 0.1 32.7 ± 5.8 43.1 ± 0.7 −15.6 ± 0.1 −37.1 ± 0.4 −25.5 ± 0.2

53.3 ± 0.1 −8.39 ± 0.2 39.2 ± 0.2 −17.7 ± 0.1 −43.0 ± 0.1 −28.5 ± 0.2

79.5 ± 4 60.9 ± 3.0 39.6 ± 1.7 49.8 ± 2.0 7.3 ± 0.3 21.8 ± 0.9

43.6 ± 0.1 24.2 ± 0.3 −12.9 ± 0.3

−53.8 ± 0.7

8.2 ± 0.1

TM on (°C) −55.9 ± 0.1 −46.7 ± 0.5 glass transition only glass transition only glass transition only −55.7 ± 0.4 −52.8 ± 0.6 glass transition only glass transition only glass transition only −49.0 ± 1.0 −59.5 ± 0.5 −24.4 ± 0.7 −63.5 ± 0.6 −57.8 ± 0.2 −60.7 ± 0.3 glass transition only

−41.5 ± 0.5

−58.1 ± 0.4 −51.9 ± 0.3 −55.3 ± 0.2

a M M M Ton, TM off, and T1,2: Onset, offset, and peak (1, 2) temperatures of melting, respectively. ΔH1 : Enthalpy of melting of the main endotherm. R1, R2, and TR1,2: recrystallization peaks and corresponding peak temperatures. bM in the compound name refers to a mixture of 1- and 2-isomers.

for its three-branched derivatives. The Tg value of JLE-20-4 was −87 ± 3 °C, a temperature close to the experimental limit of our DSC equipment (−90 °C). Note that the position of the branches (terminal or not) did not show any significant effect on the glass transition temperature. 3.4.2. Effects of the Positions of the Branches and Isomerization and Colligative Effects. The position of the branches and hence the OH groups was shown to be critical in determining the thermal properties. The effect was evident in the two-branched derivatives of the three JLEs and in the threebranched derivatives of JLE-20. Upon cooling, the isomers having at least one terminal OH group (isomers 2-2 in Scheme 1, i.e., JLE-28-2-2) demonstrated only a glass transition, whereas the isomer with terminal ester groups (isomer 2-1 in Scheme 1, i.e., JLE-28-2-1) showed strong exothermic events although much lower than the base JLE (see Figure 5a,b). The mixtures predominantly made of the 2-1 isomer (2-1/2-2 ratio of 75:25) presented much lower crystallization points and only very weak exotherms, indicating very strong colligative effects. Similarly, isomers 2-1 of JLE-281 and JLE-282 showed both recrystallization and melting events in their heating thermograms, whereas their 2-2 isomers did not (see Figure 6a,b). As can be seen in Figure 6a,b, both the recrystallization peaks and the subsequent melting endotherms were dramatically reduced for the 2-1/2-2 mixtures, indicating that colligative effects were at play during heating similar to what was observed during cooling. This “isomerization effect” was much stronger in JLE-281 than in JLE282. The second melting event observed at ∼22 °C in JLE-281-21 was suppressed in JLE-281-2-M, whereas it was only reduced in JLE-282-2-M, underscoring the fundamental role of the position of the “dangling chain” not only in the pure compounds but also in their mixtures. This indicates again that the effect of the structural details of the base ester on the physical properties can linger with not only branching but also isomerization. JLE-20-2-M, the mixture containing ∼75% of JLE-20-2-1 and 25% of JLE-20-2-2, also showed similar mitigating effects. The crystallization and melting point as well the polymorphic activity were dramatically reduced by presence of JLE-20-2-2 (curves JLE-20-2-M in Figures 5c and 6c). Note that JLE-20-2-1 is

Table 5. Glass Transition (Tg) Data Obtained from the DSC Cooling and Heating Thermograms of the Presented Compoundsa Tg (°C) compound

cooling

heating

JLE-281-2-1 JLE-281-2-2 JLE-281-2-M JLE-281-3 JLE-281-4 JLE-282-2-1 JLE-282-2-2 JLE-282-2-M JLE-282-3 JLE-282-4 JLE-20-2-1 JLE-20-2-2 JLE-20-2-M JLE-20-3-1 JLE-20-3-2 JLE-20-3-M JLE-20-4

−66.5 ± 0.4 −64.2 ± 0.2 −64.6 ± 0.2 −75.8 ± 0.7 −77.1 ± 0.2 −67.7 ± 1.0 −62.8 ± 0.2 −63.6 ± 0.9 −77.3 ± 0.9 −85.2 ± 0.7 −73.1 ± 1.2 −73.9 ± 0.7 −78.4 ± 1.0 −78.7 ± 0.6 −75.1 ± 0.1 −75.9 ± 0.2 >−90

−65.1 ± 0.1 −61.4 ± 0.2 −61.7 ± 0.1 −73.4 ± 0.6 −75.0 ± 0.1 −64.9 ± 0.3 −60.1 ± 0.1 −61.4 ± 0.1 −75.1 ± 1.0 −85.2 ± 0.6 −70.3 ± 0.3 −70.6 ± 1.0 −74.9 ± 1.4 −76.7 ± 0.3 −72.5 ± 0.1 −73.2 ± 0.2 >−90

Samples were cooled from the melt to −90 °C and subsequently heated at the same constant rate of 3.0 °C/min.

a

The glass transition (Tg) of each reported compounds was detected during both the cooling and heating cycles. It was affected by the number of branches, but not by their position. The effect of branching on Tg was stronger in JLE-282 than in JLE-281. The two-branched derivatives of JLE-282 and JLE-281 presented similar Tg values (−65 ± 2 °C). Their three-branched derivatives also presented similar Tg values (−75 ± 1 °C), but the Tg value of JLE-281-4 (−77 ± 1 °C) was 8 °C higher than that of JLE-282-4. The peculiarity of JLE-20 also manifested in the glass transition of its branched derivatives. The glass transition as observed by DSC was extended (from −65 to −72 °C) for the two-branched derivatives of JLE-20 (JLE-20-2-1 and JLE-20-22 curves in Figure 6c) and well-defined ( approximately −72 °C) 12349



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Figure 7. Shear stress−shear rate curves obtained at 30 °C for the branched derivatives of (a) JLE-281 and (b) JLE-20. Dashed lines are fits of the data to the Herschel−Bulkley equation (eq 1).

Figure 8. Viscosity versus temperature curves of the base monoesters and their branched derivatives: (a) JLE-281, (b) JLE-282, and (c) JLE-20. (d) Viscosities of the two-branched derivatives of the JLEs measured at 45 °C.

JLE-20-2-2. One can conjuncture that, to completely suppress crystallization with two branches, such monoesters should be provided with terminal OH groups at both ends of the molecule. This, of course, has to be confirmed by appropriate experimental

actually a triester with two nonterminal hydroxyl groups and that JLE-20-2-2 is a diester with a terminal ester branch and one terminal hydroxyl group (see Scheme 1). This might explain why the crystallization was suppressed in JLE-281,2-2-2 but not in 12350



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Figure 9. Parameters of the fit of the viscosity−temperature data to the generalized van Velzen equation (eq 2): (a) parameter A and (b) exponent m.

study. The residuals determined for the region of temperatures where the compounds were liquid were less than 1% and, in some cases, less than 0.2%. The fit parameters (m and A) are are presented in Figure 9. The two parameters of the GvVE satisfactorily reflected the trend observed in the rheology of the different compounds and elucidated some aspects of the contribution to the viscosity of structural features such as the number of branches and the relative positions of hydroxyl groups. The parameter A captured very well the variation of the magnitude of the viscosity with the structure of the JLE derivatives. It also clarified and allowed a more global insight into the effect of the main drivers of viscosity such as molecular mass and nature (acyl or OH groups) and position of the branches. One can notice first that the magnitude of A reflects the hierarchy observed in viscosity for any given temperature. The A value was found, from the highest to the lowest, for the two-, three-, and four-branched derivatives and the base JLE (Figure 9a) to scale with the number of hydroxyl groups (no OH groups in the base JLEs and in their four-branched derivatives, and one and two OH groups in the three- and twobranched derivatives, respectively). The position of the dangling chain in the JLE compounds with 28 carbons affected the A value of the base JLEs and the two-branched derivatives with terminal OH groups only, highlighting the role played by the hydroxyl groups and their positions in the rheology of these materials. For any given number and position of branches, the A values of JLE281 and JLE-282 (solid circles and triangles, respectively, in Figure 9a) were higher than that of JLE-20 (solid squares in Figure 9a), as expected from the magnitudes of their respective viscosity versus temperature curves. The values obtained for the exponent m increased from ∼3 for the two-branched compounds to ∼4 for the base JLEs, similarly to the trend observed for the parameter A. Obviously, the same structural features are driving the differences observed in the two parameters. Exponent m of JLE-281 and JLE-28 2 was significantly different for only the base JLEs and their twobranched derivatives, indicating that the differences in activation energy due to the position of the dangling chain relative to the singly bonded oxygen are more relevant to the rheology of the fluid base JLE-28 and its two-branched derivatives than to its three- and four-branched derivatives, where the effect of molecular mass is dominant. Note that the difference in m is larger between the two-branched derivatives with terminal OH groups than those with terminal acyl groups, indicating a more important and effective contribution to opposing the flow. The m values of JLE-281 and JLE-282 are lower than that of JLE-20 for similarly branched compounds. Although the trend observed for parameter m can be related to particular structural features, such

evidence. On a practical level, isomers having at least one OH group at the end of the hydrocarbon chain (isomers 2-2) are efficient in mitigating the onset of crystallization and reducing the polymorphism at relatively low levels and can therefore be used as cold-flow improvers of the more crystal tending and polymorphism-prone 2-1 isomers. 3.5. Flow Behavior and Viscosity versus Temperature. The branched derivatives of JLE-20, JLE-282, and JLE-281 exhibited Newtonian behavior over the examined range of shear rates for all examined temperatures. Shear stress−shear rate curves obtained at 30 °C for the derivatives of JLE-281 and JLE20, typical of the flow behavior of all samples, are presented in panels a and b, respectively, of Figure 7. The application of the Herschel−Bulkley equation (eq 1) to the share rate−shear stress data of the JLEs (R2 > 0.9999) yielded power index (n) values all practically equal to unity and yield stress values below the sensitivity limits of our equipment. Fits to the Herschel−Bulkley (eq 1) model are included in Figure 7. Viscosity versus temperature curves of the branched derivatives of JLE-281, JLE-282, and JLE-20 obtained using the ramp procedure are presented in Figure 8a−c. As exemplified in Figure 8d, showing the viscosity measured at 45 °C of the twobranched derivatives of the JLEs, the derivatives with a terminal OH group (samples 2-2) have higher viscosities than those in which acyl group is terminal (samples 2-1). At any given temperature, the viscosity of the two-branched derivatives of JLE-282 was consistently higher than that of JLE-281, which, in turn, was higher than that of JLE-20. Note that the viscosity differences are larger when the acyl branches are terminal compared to when one or two OH groups are terminal. The experimental viscosity−temperature data were analyzed using a generalized form of the van Velzen expression (GvVE) ⎛ 1 ⎞ ln(η) = A⎜ −1 + m ⎟ ⎝ T ⎠

(2)

The GvVE is especially interesting as its parameters are physically meaningful: Its parameter A relates directly to the magnitude of the viscosity of the liquid, and its exponent m is related, although not explicitly, to the energy barrier to motion, similarly to the parameters of the Andrade and generalized Andrade models.50 Similarly to the Andrade approach, the “activation energy” that is at the origin of m increases as the size of the molecule increases (adding branches) and is sensitive to the position of the OH groups (terminal versus internal position) and, to a lesser extent, to the position of the dangling chains relative to the ester. The application of eq 2 to the viscosity versus temperature data yielded exceptionally good fits for all of the compounds in this 12351



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as the position of the “dangling” hydrocarbon chains relative to the C−O bond, the overall symmetry about the bent ester group, the presence of hydroxyl groups, and the relative positions of the branches, its relations to an actual activation energy and their interactions are not clear. 3.6. Comparison of the Branched Derivatives of the JLEs with Similar Compounds. The development of vegetable-oil-based materials has been the subject of increasing interest. Polyesters derived from the transesterification/esterification of plant oils and fatty acids, branched polyols, and estolides have been extensively investigated for various applications, particularly as biolubricants.51 Thermal and oxidative stability, appropriate viscosity index (VI, i.e., viscosity change as a function of temperature), and low-temperature properties are the most commonly optimized properties. Pour point, in place of or along with melting temperature, is reported because it provides a good estimate of low-temperature fluidity. The materials are usually designed to have combined improved performance. The polyol esters are prepared completely esterified or with free hydroxyl groups. The polyol esters from jojoba-like esters are prepared from starting materials not only with double bonds in the middle of the chain but also with double bonds at the end of the chain.20,42,43 To the best of our knowledge, none of the compounds reported so far have been synthesized from starting materials with terminal double bonds. Polyol esters having free hydroxyl group present lower melting points but exhibit more change in viscosity with temperature than the corresponding completely esterified polyols. The challenge is therefore to combine low-temperature performance with low VI. Excellent low-temperature properties usually perform well in low-viscosity applications, and a compromise is needed for general biolubricant formulations. The present work shows that appending short-chain ester branches at the end of the chain combined with the possibility of an OH group at the outer position (terminal position) widens dramatically the range of melting/crystallization points, as well as viscosity versus temperature behavior. As a result, a new window of opportunity is open for the design of much more targeted materials suitable for biolubricant formulations as well as waxes. Very recently, acyl derivatives from jatropha oil52 and karandja oil53 have been synthesized and evaluated as potential lubricant basestocks. The jatropha-oil-based acylated derivatives were reported to have viscosity indices (kinematic viscosity in the ranges of 19−30 cSt at 40 °C and 4−6 cSt at 100 °C) and pour points (between 9 and −15 °C) that might be suitable for use as biolubricants,52 whereas those of karanja oil were reported to be high.53 Nevertheless, the derivatives from these “exotic” oils performed far worse than the present derivatives and the derivatives of the jojoba-like esters in our previous work.20 Note that, for both jatropha and karanja, pour point and viscosity improved with the increase in acyl chain length, similarly to what was reported for vegetable oil derivatives.54 Melting points as low as −37.7 °C and viscosities of 8.6 cP at 40 °C and 4.29 cP at 70 °C, deemed suitable for biolubricants or biofuels, were reported for esters branched with acid or alcohol moieties.55 Melting point in the range from −2 to −49 °C and viscosities of 8.4−65 cP at 40 °C and 2.2−10.8 cP at 100 °C were also reported for oleate and decanoate esters.56 Such performances are not surprising given the short hydrocarbon branches and absence of OH groups involved. However, the compounds in these studies, although excellent, performed worse than the compounds of the present study, which, with similar structural

characteristics, do present only a glass transition at very low temperature (less than −60 °C). Diesters that might prove useful as lubricants, surfactants, and fuel additives are actively being investigated. Miller et al.57 reported pour points as low as −20 °C and kinematic viscosities of 3 cSt at 100 °C for diesters branched with hydrocarbons of different chain lengths (C5−C15). Moser et al.58 prepared singly branched diesters from oleates and evaluated their lowtemperature operability and oxidation stability as a function of position and chain length of the branch. They recorded pour points as low as −13 °C. Both studies showed that increasing chain length in the midchain ester and branching in the end-chain ester had a positive influence on the low-temperature properties of diesters. Triester derivatives have also been shown to be good biolubricant base stock candidates. For example, Salih et al.59 showed that an increase in midchain substituent length oleic acid triester derivatives improved the pour point and antiwear properties. None of the above was able to match the performances of our JLE derivatives, again because of the inherent versatility of the JLE structures. The low-temperature performance and flow behavior of estolides as reported in the literature are the closest to those of our JLEs. The superior properties of the estolide esters are believed to be dictated mainly by the combination of the degree of oligomerization; the decrease in the level of unsaturation; the absence of hydroxyl functionalities on the estolide backbone; and, to a lesser extent, the nature of the specific ester moiety. It is the distribution of secondary linkage positions in the estolide that generally affects the low-temperature and flow behavior very favorably and not so much their position.60 The short-capped estolide esters are reported to present better low-temperature properties than the long-capped ones and to potentially perform well in low-viscosity applications.61 Because of their promise, estolides have been sourced from various vegetable oils such as oleic and different saturated fatty acids,62 oleins from high-oleic sunflower oil,63 and ricinoleic acid.64 It is of note that, for all of these compounds, pour point and viscosity were found to increase as the saturated capping chain length was increased. Disregarding the economics of making them, estolides have properties similar to those of branched derivatives of JLEs and are very suitable for extreme applications. Acetic- and butyric-capped estolide esters, for example, displayed excellent pour points (from −45 to −27 °C) and viscosity indexes (VI) of 161 and 163,61 the estolides sourced from oleic acid presented pour points as low as −33 °C,62 and the estolides sourced from ricinoic acid showed melting points as low as approximately −51 °C and viscosities of 182.6 cP at −5 °C,64 all deemed particularly promising for biolubricants.

4. CONCLUSIONS Ring-opening esterification over time at two temperatures demonstrated that product composition can be easily and effectively controlled, providing a powerful tool for the custom design and optimization of the production of controlled mixtures of branched derivatives of the JLEs investigated in this work. The thermal stability of the branched JLEs was not affected significantly by branching. However, the introduction of branches considerably influenced the thermal transition behavior. The crystallization and melting were very dramatically impacted and even suppressed in the highly branched derivatives. The physical properties were also markedly influenced by the positions of the OH groups. Crystallization was only slightly depressed in branched derivatives with internal OH groups, but it 12352



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(6) Gryglewicz, S.; Piechocki, W.; Gryglewicz, G. Preparation of polyol esters based on vegetable and animal fats. Bioresour. Technol. 2003, 87, 35−39. (7) Bhatia, V.; Chaudhry, A.; Sivasankaran, G.; Bisht, R.; Kashyap, M. Modification of jojoba oil for lubricant formulations. J. Am. Oil Chem. Soc. 1990, 67, 1−7. (8) Brown, M.; Fotheringham, J. D.; Hoyes, T. J.; Mortier, R. M.; Orszulik, S. T.; Randles, S. J.; Stroud, P. M. Synthetic Base Fluids. In Chemistry and Technology of Lubricants; Springer-Verlag: Berlin, 2010; Chapter 2, pp 35−74. (9) Salimon, J.; Salih, N. Modification of Epoxidized Ricinoleic Acid for Biolubricant Base Oil With Improved Flash and Pour Points. Asian J. Chem. 2010, 22, 5468−5476. (10) Burdett, K. A.; Harris, L. D.; Margl, P.; Maughon, B. R.; MokhtarZadeh, T.; Saucier, P. C.; Wasserman, E. P. Renewable Monomer Feedstocks via Olefin Metathesis: Fundamental Mechanistic Studies of Methyl Oleate Ethenolysis with the First-Generation Grubbs Catalyst. Organometallics 2004, 23, 2027−2047. (11) Forman, G. S.; Bellabarba, R. M.; Tooze, R. P.; Slawin, A. M. Z.; Karch, R.; Winde, R. Metathesis of renewable unsaturated fatty acid esters catalysed by a phoban-indenylidene ruthenium catalyst. J. Organomet. Chem. 2006, 691, 5513−5516. (12) Schrodi, Y.; Ung, T.; Vargas, A.; Mkrtumyan, G.; Lee, C. W.; Champagne, T. M.; Pederson, R. L.; Hong, S. H. Ruthenium olefin metathesis catalysts for the ethenolysis of renewable feedstocks. Clean: Soil, Air, Water 2008, 36, 669−673. (13) Zlatanic, A.; Petrovic, Z. S.; Dusek, K. Structure and properties of triolein-based polyurethane networks. Biomacromolecules 2002, 3, 1048−1056. (14) Patel, J.; Mujcinovic, S.; Jackson, W. R.; Robinson, A. J.; Serelis, A. K.; Such, C. High conversion and productive catalyst turnovers in crossmetathesis reactions of natural oils with 2-butene. Green Chem. 2006, 8, 450−454. (15) Miao, X.; Malacea, R.; Fischmeister, C.; Bruneau, C.; Dixneuf, P. H. Ruthenium-alkylidene catalysed cross-metathesis of fatty acid derivatives with acrylonitrile and methyl acrylate: A key step toward long-chain bifunctional and amino acid compounds. Green Chem. 2011, 13, 2911−2919. (16) Jacobs, T.; Rybak, A.; Meier, M. A. R. Cross-metathesis reactions of allyl chloride with fatty acid methyl esters: Efficient synthesis of α,ωdifunctional chemical intermediates from renewable raw materials. Appl. Catal. A: Gen. 2009, 353, 32−35. (17) Miao, X. W.; Dixneuf, P. H.; Fischmeister, C.; Bruneau, C. A green route to nitrogen-containing groups: The acrylonitrile cross-metathesis and applications to plant oil derivatives. Green Chem. 2011, 13, 2258− 2271. (18) Behr, A.; Gomes, J. P. The refinement of renewable resources: New important derivatives of fatty acids and glycerol. Eur. J. Lipid Sci. Technol. 2010, 112, 31−50. (19) Moser, B. R.; Erhan, S. Z. Branched chain derivatives of alkyl oleates: Tribological, rheological, oxidation, and low temperature properties. Fuel 2008, 87, 2253−2257. (20) Bouzidi, L.; Li, S. J.; Di Biase, S.; Rizvi, S. Q.; Dawson, P.; Narine, S. S. Lubricating and Waxy Esters II: Synthesis, Crystallization, and Melt Behavior of Branched Monoesters. Ind. Eng. Chem. Res. 2012, 51, 14892−14902. (21) Cermak, S. C.; Brandon, K. B.; Isbell, T. A. Synthesis and physical properties of estolides from lesquerella and castor fatty acid esters. Ind. Crop. Prod. 2006, 23, 54−64. (22) Dailey, O. D., Jr.; Prevost, N. T.; Strahan, G. D. Synthesis and characterization of branched-chain derivatives of methyll olleate. Clean: Soil, Air, Water 2008, 36, 687−693. (23) Erhan, S. Z.; Adhvaryu, A.; Liu, Z. Chemically modified vegetable oil-based industrial fluid. Patent PCT/US2003/002102, 2003. (24) Sharma, B. K.; Adhvaryu, A.; Erhan, S. Z. Synthesis of hydroxy thio-ether derivatives of vegetable oil. J. Agric. Food Chem. 2006, 54, 9866−9872.

was completely suppressed in the isomers with terminal OH groups. Moreover, the thermal transition points and polymorphic activity were dramatically reduced when the isomers were mixed, even at low ratios, indicating strong colligative effects. The influence of the structural elements of the base JLEs, such as symmetry about the ester headgroup and position of the “dangling” chain relative to the singly bonded oxygen, although being reduced as branching was increased, remained noticeable, enduring not only with branching but also with isomerization. The flow behaviors of branched derivatives of the JLEs were all Newtonian, presenting viscosity versus temperature data profiles similar to those of liquid hydrocarbons. A simple two-parameter model, the generalized van Velzen equation (GvVE, eq 2) described very well the viscosity data as a function of the structural parameters (number and position of branches and OH groups). The large ranges of melting and crystallization points and viscosities recorded for these compounds make them suitable for a very wide variety of applications that could range from waxes to lubricants. Combined with temperature−time controlled reactions, isomerization might be used as a powerful and efficient tool to prepare ester materials with custom-designed crystallization and melting and flow behaviors.

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ASSOCIATED CONTENT

* Supporting Information S

Individual structures of the branched derivatives of the jojobalike esters (Scheme S1). 1H NMR shifts in CDCl3 of the epoxidation products, 1H NMR shifts in CDCl3 and DMSO of the branched derivatives of the JLEs, and MS data for selected compounds (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-705-748-1011 Ext. 6105. Fax: 1-705-748-1652. E-mail: [email protected]. Notes

The authors declare no competing financial interest. † These authors contributed equally to this work.

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ACKNOWLEDGMENTS The financial support of Elevance Renewable Sciences, NSERC, Grain Farmers of Ontario, GPA-EDC, Industry Canada, and Trent University is gratefully acknowledged.

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REFERENCES

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