Synthesis and Physical Properties of Triacylglycerol Oligomers

Article pubs.acs.org/IECR

Synthesis and Physical Properties of Triacylglycerol Oligomers: Examining the Physical Functionality Potential of Self-Metathesized Highly Unsaturated Vegetable Oils Shaojun Li, Laziz Bouzidi, and Suresh S. Narine* Trent Centre for Biomaterials Research, Departments of Physics and Astronomy and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, K9J 7B8 Ontario Canada S Supporting Information *

ABSTRACT: Seven model oligomers (from dimer to octamer) of the triacylglycerol (TAG) triolein were synthesized from oleic acid and fully characterized by 1H NMR, 13C NMR, mass spectroscopy, and gel permeation chromatography (GPC). The thermal stability of the oligomers as determined by TGA was excellent, with degradation beginning at 342 °C for the most thermally labile samples. The samples all presented glass transitions at low temperatures, with Tg continuously shifting to higher temperatures with increasing numbers of monomers. The crystallization and melting behavior scaled with molecular size and relative number of double bonds in the transconfiguration. Flow behavior was investigated over a large range of temperatures (−10 to 110 °C), and application of the Herschel−Bulkey model to shear stress versus shear rate data evidenced a flow behavior dependent on molecular size and temperature. The oligomers presented a thinning to Newtonian flow transition temperature proportional to molecular size. The viscosity versus temperature data, fitted with a generalized van Velzen equation, suggested that it is the competition between the trans- character and size of the molecules which determines the rheology of these molecules. Overall, all the investigated properties plateaued at the hexamer, suggesting that no further marginal utility can be obtained with larger oligomers.

1. INTRODUCTION Fats and oils are considered one of the most important renewable feedstocks for the chemical industry.1,2 They represent the largest part of the current consumption of renewable raw materials,3−5 with about 20% of the global production of fats and oils currently used for nonfood industrial applications.6 They are used for the manufacture of many products, such as polymers,7−9 biodiesel, detergents, waxes, and biodegradable lubricants,10−14 etc. Recently, modern synthetic methods have been applied extensively to fatty compounds for the selective functionalization of the alky chain, especially on the C−C double bond. Hydrogenation, oxidation, pericyclic reactions, radical addition, and olefin metathesis on the double bonds have led to a large number of novel fatty compounds with very interesting properties.1,15−19 Since olefin metathesis was first discovered in 1964 by Banks and Bailey,20 it has become an important industrial reaction for the preparation of monomers and specialty chemicals.21,22 Interest in the metathesis of unsaturated fatty acid derivatives has been steadily growing for decades.21,23−27 Metathesis of vegetable oils has been providing a large number of safer, less toxic fine chemicals1,9,28 and monomers for the polymer industry.9,29−34 Attempts35−37 have been made to produce and characterize self-metathesized vegetable oils after the promising results obtained with self-metathesis of soybean oil reported by Erhan et al.38 The composition of self-metathesized soybean oil was carefully studied by Larock et al.35,36 Metathesized soybean oil, as reported in these studies, is a complex mixture made of a number of linear oligomers (from dimer to pentamer) and © 2013 American Chemical Society

monocyclic structures, all of which contain both the trans and cis configurations.19 Because of the complex composition of metathesis products, isolation of individual components which would serve to facilitate the study of structure−function relationships of these materials is very difficult. It is therefore important to synthesize these compounds by conventional chemistry routes, as these molecules then serve not only as model systems for better characterization of the metathesis products but also in understanding the effect of the individual components on the physical properties of the composite materials. Such a pursuit significantly aids in the design of controlled metathesis reactions which would yield compositions enriched with specific components that deliver targeted functionality. In the present study, seven triacylglycerol (TAG) oligomers which may be found in metathesized vegetable oil products (deriving for example from soybean, canola, or olive oils) were synthesized from oleic acid and its derivatives using a multistep conventional reaction. The oligomers contain the positional isomers at sn-1 or sn-2, as well as cis- and trans- configurations. The general structure of the oligomers is presented in Scheme 1 and the individual structures are provided in the Supporting Information (SI) Scheme S1. The oligomers were fully characterized by 1H NMR, 13C NMR, mass spectroscopy, and gel permeation chromatography (GPC). Their thermal stability, crystallization, and melting Received: Revised: Accepted: Published: 2209

October 24, 2012 January 5, 2013 January 11, 2013 January 11, 2013 dx.doi.org/10.1021/ie302921h | Ind. Eng. Chem. Res. 2013, 52, 2209−2219

Industrial & Engineering Chemistry Research

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Scheme 1. Generalized Structure of the Oligomers: n = 2−8 represents dimer to octamer, n = 1 represents triolein

behaviors, as well as flow behavior, were investigated by TGA, DSC, and rheometry techniques and were examined in terms of molecular size (number of monomers) and trans- character (number of trans- double bonds).

loaded in the open TGA platinum pan. The sample was equilibrated at 25 °C and heated to 600 °C at a constant rate of 3 K/min. The TGA measurements were performed under dry nitrogen of 40 mL/min for balance purge flow and 60 mL/min for sample purge flow. 2.2.5. Differential Scanning Calorimetry (DSC). DSC analysis was carried out on a Q200 model (TA Instruments, New Castle, DE, USA) 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 K/min to obtain the crystallization and melting profiles, respectively. “TA Universal Analysis” software together with a method developed in our group39 was used to analyze the data and extract the main characteristics of the peaks. The measurement temperatures are reported to a certainty of better than ±0.5 °C. 2.2.6. Viscosities Measurement. Sample viscosities were measured on a computer-controlled AR2000ex using a 40-mm 2° steel cone geometry (SIN 511406.901). Temperature control was achieved by Peltier property with an accuracy of 0.1 °C. Shear rate/shear stress experiments with both increasing and decreasing shear rate were performed with the continuous ramp procedure. The shear rate range was optimized for each measurement temperature to take into account the lowest torque accessible with AR2000ex (10 μNm) and the maximum angular velocity suggested by the supplier (40 rad/s). Duration was 10 min in the log mode and sampling was 20 points per decade. The procedure was performed from 100 °C to −10 °C then back in 5 °C steps with 5 min equilibration between temperatures. Viscosity versus temperature data were obtained using the constant temperature rate procedure. In these experiments, the constant shear rate (200 s−1) was chosen in the common Newtonian region. The sample was quickly heated to 110 °C and equilibrated for 5 min then cooled to −10 °C at a constant rate (1.0 K/min). Sampling points were recorded every 1 °C. The shear rate/shear stress and constant temperature rate experiments yielded viscosities in very good agreement within the experimental uncertainty. Note that the viscosity of the octamer was not measured because of limited amount of synthesized material.

2. EXPERIMENTAL SECTION 2.1. Materials. Oleoyl chloride (80%), 1, 3-dihydroxyacetone (99%), solketal (99%), pyridine (99%), N,N′-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), and sodium borohydride were purchased from Sigma-Aldrich. Chloroform was purified by distillation over calcium hydride. 1,18-Octadec-9-enedioic acid was produced in our laboratory by the self-metathesis of oleic acid using Grubbs generation II catalyst. 2.2. Analytical Methods. The physical measurements were run at least in triplicate. The reported values and uncertainties attached are the mean and associated calculated standard deviations, respectively. 2.2.1. Nuclear Magnetic Resonance (NMR). 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker BioSpin MRI GmbH, Karlsruhe, Germany) at a frequency of 400 MHz and 100 MHz, respectively, using a 5-mm BBO probe. 1D 1H spectra were acquired at 25 °C over a 16-ppm spectral window with a 1-s recycle delay, 32 transients. 1D 13C spectra were acquired at 25 °C over a 240-ppm spectral window with a 0.2-s recycle delay, 2048 transients. NMR spectra were Fourier transformed, phase corrected, and baseline corrected. Window functions were not applied prior to Fourier transformation. Chemical shifts were referenced relative to residual solvent peaks. 2.2.2. 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) equipped with an ionspray source and a modified hot source-induced desolvation (HSID) interface (Ionics, Bolton, ON). 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). Multiple-charged ion signals were reconstructed using the BioTools 1.1.5 software package (AB Sciex, Concord, ON). 2.2.3. Gel Permeation Chromatography (GPC). GPC was carried out on a Waters e2695 HPLC (Waters Limited, Mississauga, Ontario) fitted with a Waters e2695 pump, Waters 2414 refractive index detector, and a Styragel HR5E column (5 μm). Chloroform was used as eluent with a flow rate of 1 mL/ min. The sample was made with a concentration of 4 mg/mL, and the injection volume was 30 μL for each sample. Polystyrene (PS) standards were used to calibrate the curve. 2.2.4. Thermogravimetric Analysis (TGA). The TGA measurements were carried out in triplicate on a TGA Q500 (TA Instruments, New Castle, DE, USA). Approximately 8.0− 15.0 mg of fully melted and homogenously mixed sample was

3. SYNTHESIS OF OLIGOMERS AND INTERMEDIARY PRODUCTS The synthesis procedure of the oligomers and the intermediary products is provided in SI Scheme S2. The yields of the different reactions, column chromatography method, and 1H NMR, 13C NMR, and MS characterization data are provided in SI Table S1. 3.1. Synthesis of 2-Hydroxypropane-1,3-diyl Dioleate (1 in Scheme S2). 3.1.1. 2-Oxopropane-1,3-diyl Dioleate. To a solution of 1,3-dihydroxylacetone (41.27 mmol) in 160 mL of 2210

dx.doi.org/10.1021/ie302921h | Ind. Eng. Chem. Res. 2013, 52, 2209−2219


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prepared from mono-ol 1 and 1, 18-octadec-9-enedioic acid. Monoacid 3 was prepared from mono-ol 2 and 1,18-octadec-9enedioic acid. 3.3.2. Synthesis of Mono-ols. To a solution of 1 mmol monoacid and 1.2 mmol diol (2, 3-dihydroxypropyl oleate) in 6 mL of CHCl3 under the protection of N2, 0.2 mmol DMAP was added, followed by 1.2 mmol DCC. The reaction was carried out at room temperature overnight. The precipitated dicyclohexylurea was removed by filtration. The organic phase was diluted with 10 mL of chloroform then washed sequentially with water (3 × 20 mL), 4% aqueous NaHCO3 (2 × 200 mL), and brine (3 × 200 mL), and then dried over Na2SO4. After filtration, the filtrate was concentrated with a rotary evaporator and the residue was purified by column chromatography with ethyl acetate and hexanes as the eluent. Mono-ol 1 was prepared from monoacid 1 and 2,3dihydroxypropyl oleate. Mono-ol 2 was prepared from mono acid 2 and 2,3-dihydroxypropyl oleate. Mono-ol 3 was prepared from mono acid 3 and 2,3-dihydroxypropyl oleate. 3.4. Synthesis of Oligomers. 3.4.1. Synthesis of Oligomers with Odd Numbers (Procedure A). To a solution of 2 mmol monoacids and 1 mmol diol (2,3-dihydroxypropyl oleate) in 10 mL of CHCl3 under the protection of N2, 0.4 mmol DMAP was added, followed by 2.2 mmol DCC. The reaction was carried out at room temperature overnight. The precipitated dicyclohexylurea was removed by filtration. The organic phase was diluted with 10 mL of chloroform, then washed sequentially with water (3 × 20 mL), 4% aqueous NaHCO3 (2 × 200 mL), and brine (3 × 200 mL), and then dried over Na2SO4. After filtration, the filtrate was concentrated with a rotary-evaporator and the residue was purified by column chromatography with ethyl acetate and hexanes as the eluent. 3.4.2. Syntheses of Oligomers with Even Numbers (Procedure B). To a solution of 2 mmol mono-ols and 1 mmol diacid (1,18-octadec-9-enedioic acid) in 10 mL of CHCl3 under the protection of N2, 0.4 mmol DMAP was added, followed by 2.2 mmol DCC. The reaction was carried out at room temperature overnight. The precipitated dicyclohexylurea was removed by filtration. The organic phase was diluted with 10 mL of chloroform, then washed sequentially with water (3 × 20 mL), 4% NaHCO3 (2 × 200 mL), and brine (3 × 200 mL), and then dried over Na2SO4. After filtration, the filtrate was concentrated with a rotary evaporator and the residue was purified by column chromatography with ethyl acetate and hexanes as the eluent. Dimer was prepared from monoacid 1 and 2-hydroxypropane-1,3-diyl dioleate following procedure B. Trimer was prepared from monoacid 1 and 2,3-dihydroxypropyl oleate following procedure A. Quatrimer was prepared from mono-ol 1 and 1,18-octadec-9-enedioic acid following procedure B. Pentamer was prepared from monoacid 2 and 2,3-dihydroxypropyl oleate following procedure A. Hexamer was prepared from mono-ol 2 and 1,18-octadec-9-enedioic acid following procedure B. Heptamer was prepared from mono acid 3 and 2,3-dihydroxypropyl oleate following procedure A. Octamer was prepared from mono-ol 3 and 1,18-octadec-9-enedioic acid following procedure B.

chloroform, oleoyl achloride (82.53 mmol) was added, followed by dropwise addition of pyridine (90.79 mmol). The reaction mixture was stirred at room temperature overnight, and then diluted with 160 mL of chloroform. The organic layer was washed sequentially with water (3 × 300 mL), followed by 5% aqueous HCl (2 × 300 mL), water (2 × 300 mL), 4% aqueous NaHCO3 (2 × 300 mL), and water (3 × 300 mL). The organic layer was dried over Na2SO4, which was subsequently removed by filtration. After the chloroform was removed with a rotary evaporator, the residue was recrystallized from 2-propanol (25 mL/g). 3.1.2. 2-Hydroxypropane-1,3-diyl Dioleate. To a solution of 1,3-diglyceroloxypropane-2-one (34.57 mmol) in 300 mL of THF at 5 °C, NaBH4 (51.86 mmol) in 5 mL of water was added slowly. The reaction mixture was stirred at 5 °C for 30 min then quenched by 5% aqueous HCl. Water (300 mL) was then added and the mixture was extracted with 400 mL of chloroform. The organic layer was washed with water (3 × 400 mL), 4% aqueous NaHCO3 (2 × 300 mL), and water again (3 × 400 mL). The organic layer was dried over Na2SO4, which was subsequently removed by filtration. After the chloroform was removed with a rotary evaporator, the residue was purified by column chromatography with ethyl acetate and hexanes as the eluent. 3.2. Synthesis of 2,3-Dihydroxypropyl Oleate (3 in Scheme S2). 3.2.1. 1,2-Isopropylidene-3-oleoyl Glycerol. To a solution of Solketal (100 mmol) in chloroform (200 mL) and pyridine (150 mmol), oleoyl chloride (100 mmol) in 100 mL of chloroform was added slowly. The reaction mixture was stirred overnight at room temperature. The chloroform solution was washed with water (2 × 150 mL), 5% aqueous HCl (150 mL), water (2 × 150 mL) again, 4% aqueous NaHCO3 (150 mL), and brine (2 × 150 mL) sequentially, and then dried over Na2SO4, which was removed by filtration. After the solvent was removed on a rotary evaporator, the yellow oil (44 g) was collected and used for the next step without separation. 3.2.2. 2,3-Dihydroxypropyl Oleate. To a solution of 1,2 -isopropylidene-3-oleoyl glycerol (34 g) in 500 mL of dioxane, 0.2 mol concentrated HCl (37%) was added. The reaction was stirred at room temperature for 5 h. The mixture was diluted by 500 mL of water and extracted with 500 mL of ethyl acetate. The ethyl acetate layer was washed by 4% aqueous NaHCO3 (300 mL), followed by water (3 × 300 mL), and then dried over Na2SO4, which was removed by filtration. The organic solvent was removed with a rotary evaporator and the residue was separated by column chromatography with ethyl acetate/ hexanes = 1:4 to 1:1. The product was given as colorless oil (a white crystal was given after overnight under vacuum). 3.3. Synthesis of Mono-ols and Monoacids. 3.3.1. Synthesis of Monoacids. To a solution of 1 mmol mono-ols and 1.2 mmol diacid (1,18-octadec-9-enedioic acid) in 10 mL of CHCl3 under the protection of N2, 0.4 mmol DMAP was added, followed by 1.1 mmol DCC. The reaction was carried out at room temperature overnight. The precipitated dicyclohexylurea was removed by filtration. The organic phase was diluted with 10 mL of chloroform then washed sequentially with water (3 × 20 mL), 4% aqueous NaHCO3 (2 × 200 mL), and brine (3 × 200 mL), and then dried over Na2SO4. After filtration, the filtrate was concentrated with a rotary evaporator and the residue was purified by column chromatography with ethyl acetate and hexanes as the eluent. Monoacid 1 was prepared from 2-hydroxypropane-1,3-diyl dioleate and 1,18-Octadec-9-enedioic acid. Monoacid 2 was

4. RESULTS AND DISCUSSION 4.1. Synthesis of Oligomers. The metathesis-like oligomers (dimer to octamer, Scheme 1) were synthesized from 2-hydroxypropane-1,3-diyl dioleate (1), 1, 18-octadec-92211



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Figure 1. (a) DSC cooling thermograms of olygomers. The number of monomer for each sample is indicated at the top right-hand-side of each curve. (b) Characteristic temperatures of crystallization versus number of monomers n: onset (▼, TOn) and offset (■, TOff) temperature, peak temperature of P1 (●, TP1) and P2 (⊕, TP2). (c) DSC heating thermograms of oligomers. The number of monomer for each sample is indicated at the top right-hand-side of each curve. (d) Characteristic temperatures of melting versus number of monomers n: onset (▼, TOn) and offset (■, TOff) temperature, peak temperature of P1 (○, TP1) and P2 (◊, TP2). (e) Span of crystallization (▲) and melting (○). (f) Enthalpy of crystallization (▲) and melting (○).

self-metathesis procedure of oleic acid using Grubbs second generation catalyst. The double bond on the alkyl chain of the diacid has a transgeometry similarly to what has been found earlier using the same procedure.30 2,3-Dihydroxypropyl oleate (3) was prepared by removing the protection group with concentrated HCl in dioxane from the protected compound, 1,2-isopropylidene-3-oleoyl glycerol that was synthesized by esterification of solketal and oleoyl chloride in the presence of pyridine. The reaction time was found to be critical at the deprotection step and greatly affects the yield. In fact,

enedioic acid (2), and 2,3-dihroxypropyl oleate (3) by Steglich esterification,40 where 4-dimethylaminopyridine (DMAP) was used as catalyst and N,N′-dicyclohexylcarbodiimide (DCC) was used as the condensing agent (see Scheme S2). 2-Hydroxypropane-1,3-diyl dioleate (1) was synthesized following a reported procedure.41 An intermediate, 2oxopropane-1,3 diyl dioleate was prepared from 1,3-dihydroxylacetone and oleoyl chloride in the presence of pyridine, which was subsequently reduced by NaBH4 in a solution of THF to give (1). 1,18-Octadec-9-enedioic acid (2) was produced by the 2212



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corresponding formula weights. The molecular weight to number average molecular weight (Mn) ratios determined by GPC for the oligomers of the present study are similar to reported values for similar structures.36 The discrepancy between the GPC results and the calculated molecular weight (FW) is due to the difference in the hydrodynamic volume between the oligomers samples and the polystyrene used as the GPC calibration standard. 4.3. Thermal Stability of the Oligomers. TGA derivative (DTG) curves of oligomers are provided in SI Figure S3, and the related TGA and DTG data are provided in SI Table S4. The TGA data indicated that the oligomers follow similar decomposition patterns. The onset temperature of degradation determined at 5% weight indicates that the oligomers are stable at temperatures higher than 340 ± 3 °C. Note that onset degradation temperatures higher than 370 ± 3 °C are obtained when the less restrictive definition of the inception of thermal degradation, as suggested by Racles et al.45 at 10% weight loss (T10, Table S4), is used. The DTG curves (SI Figure S3) clearly showed three temperature domains indicating that the oligomers decompose in three main steps. The first degradation step extended from ∼300 to ∼390 °C and involved ∼20% weight loss. The second step occurred from ∼390 to 440° and appeared to be the main decomposition process, as ∼60% of the weight was lost during this step. The third step, in which the remaining weight was lost (∼20%), ranged from ∼440 to 480 °C. The first degradation step is attributable to the decomposition of the ester groups initiated at the position of the βhydrogen (i.e., sn-2 of the glycerol backbone), as reported by Gryglewics et al.46 The second step is attributed to the decomposition of the sn-1(3) ester groups. The final steps are related to the decomposition of carbon chains and other fragments which may be produced at high temperatures under N2 and which have higher decomposition temperatures, as reported by Lin et al.47 4.4. Thermal Behavior of the Oligomers. The DSC profiles of the oligomers are shown in Figure 1a−f. There are clear first-order thermal transitions (crystallization and melting behaviors) as well as unmistakable prolonged second-order transitions (glass transitions). 4.4.1. Crystallization Behavior. The DSC cooling thermograms of the oligomers are shown in Figure 1a. They reveal an overall transformation path starting with low temperature crystallizations followed by prolonged glass transitions. The crystallization process of the oligomers is relatively simple and changes quite monotonously as the number of monomer (n) increases from dimer to octamer. The cooling thermograms of the dimer presented three exotherms (successive peaks are labeled PC1, PC2, and PC3 in Figure 1a) and the others displayed two exotherms. The crystallization offset temperature (TOff) as well as peak temperature of PC1 (TP1) and PC2 (TP2) versus n were readily fitted with an exponential rise to a maximum function (Figure 1b). Note that, obviously, the same trend is observed when the crystallization characteristics are plotted versus number of carbons (not shown). PC1 and PC2 are associated with the growth of two different crystal phases (phase 1 and phase 2, respectively). The relative amount of these phases depends strongly on the number of monomers. As estimated from the relative area of their respective exotherms (not presented here), phase 2 content increases with increasing n while phase 1 decreases and almost disappears in the heptamer and octamer samples. The onset temperature (TOn)

increasing deprotection time led to the hydroxylation of the ester group of oleyol glycerol. It was therefore optimized and controlled to around 5 h. The mono-ols and oligomers with even numbers were prepared separately from monoacids and 2,3-dihroxypropyl oleate by varying the input ratios of substrates, while the monoacids and oligomers with odd numbers were prepared separately from mono-ols and 1,18-octadec-9-enedioic acid, as presented in Sections 3.3 and 3.4. A high reaction temperature would lead to a conversion of cis- into transgeometry.42 To avoid this conversion, all of the reactions were carried out at room temperature. The steric hindrance effect of the large size substrates has led to relatively low reaction conversions, especially for the oligomers with large size (30% for octamer, 62% for dimer). Due to the requirement of further laborious chromatography, we did not distinguish and separate the regiochemistry of the intermediates (mono-ols and monoacids) and oligomers obtained. So the oligomers contain different positional isomers on position sn-1 and sn-2 of the glycerol skeleton. 4.2. Characterizations of Oligomers. All the compounds were characterized by 1H NMR. The oligomers were fully characterized by 1H NMR, 13C NMR, ESI-MS, and GPC. 4.2.1. 1H NMR. One of the typical 1H NMR spectra of oligomers is provided in SI Figure S1. The CHCH is presented at δ 5.38−5.32 ppm; CH2CH(O)CH2 and  OCH2CHCH2O are on glycerol skeleton at δ 5.27−5.25 ppm and 4.32−4.12 ppm, respectively; C(O)CH2 is at δ 2.33−2.28 ppm, α-H to CHCH at δ 2.03−1.98 ppm, C( O)CH2CH2 at δ 1.60 ppm, and CH3 at δ 0.88 ppm. The ratios of proton corresponding to CHCH,  OCH2CHCH2O, OCCH2, and CH3, in particular the ratios of OCH2CHCH2O and CH3, are important to identify the structure of the oligomers prepared in this work. The relevant 1H NMR data are provided in SI Table S2. As can be seen, the experimental values match the theoretical ones very well. Note that the ratio difference becomes smaller with increasing n, increasing the difficulty of confirming the larger size oligomers. Therefore, mass spectrometry was used as an important confirmation tool for the large size oligomers. The oligomers contain double bonds in both the trans- and cis- configurations as illustrated with the 1H NMR spectrum of the quatrimer in SI Figure S1. The chemical shifts at δ 5.38− 5.36 ppm in the 1H NMR spectra are assigned to the transgeometry, and those at δ 5.36−5.32 ppm are assigned to the cis- geometry based on 1H NMR of reference materials triolein (100% cis) and trielaidin (100% trans). The transgeometry was introduced by 1,18-octadec-9-enedioic acid (3), functioning as the bridge between the TAGs, and the cis- geometry was introduced by the cis- double bonds of oleic acid. 4.2.2. 13C NMR. TAG carbonyl carbons, alkenyl carbons, glyceryl carbons, and alkyl carbons of the oligomers are clearly identified in their 13C NMR (corresponding chemical shifts are listed in Experimental Section). The oligomers contain α- and β-TAG carbonyl carbons (SI Figure S2), which are presented at δ 173.45 and 173.04 ppm, respectively.43 The glyceryl carbon at position sn-2 on the glycerol skeleton and that at position sn1(3) are presented at δ 69.09 ppm and δ 62.31 ppm, respectively.44 Also, both cis (δ < 130.0 ppm) and trans (δ >130.0 ppm) of olefinic carbons36 are shown in 13C NMR spectra (SI Figure S2). 4.2.3. MS and GPC. ESI-MS and GPC results are provided in the SI Table S3. The results from ESI-MS match their 2213



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Figure 2. (a) Percentage of double bonds in the transconfiguration relative to the total number of double bonds (%trans-). Dashed line is a plot of the rational function ((Ntrans)/(Ntrans + Ncis)) = ((n − 1)/(2n + 1)) (eq 1). Upper horizontal line shows the limit of the function. (b) Peak temperatures of crystallization (▲) and melting (●) of P1 versus % trans- double bonds;.

versus n, on the other hand, presented a plateau at −22.14 ± 1.16 °C for n = 2−4 and then increased very slightly to −13.37 ± 0.04 °C for the octamer. This is simply an expression of the limited effect of molecular mass on the crystallization induction of the oligomers. Note that the difference in peak temperature (TP1 − TP2) between the two phases decreases as the size of the molecule increases (14.7 ± 0.3 °C for the dimer and 6.1 ± 0.5 °C for the octamer), indicating that both phases formed during cooling stabilize further as the number of monomers increases. 4.4.2. Melting Behavior. The phase development of the oligomers observed upon heating was consistent with that shown on cooling. The DSC heating thermograms displayed melting and recrystallization transitions (PM1, PM2, and R in Figure 1c) and similarly to the cooling profiles, showed unmistakable glass transitions (arrows in Figure 1c). The dimer and trimer displayed a polymorphic behavior with recrystallizations from the melt similar to that of triolein, whereas, the others (quatrimer to octamer) showed a two-endotherms melting (PM1 and PM2 in Figure 1c) without apparent recrystallization. PM1 and PM2 are associated with the melting of the two phases (phase 1 and phase 2) already formed during cooling. As estimated from the enthalpy of PM1 and PM2, the relative amount of Phase 1 increased with increasing n at the detriment of Phase 2; consistent with what was observed in the cooling experiments. The melting peak temperature of both PM1 and PM2 in the oligomers with n = 4−8 (TM1, ○, andTM2, ◊, in Figure 1d) versus n were readily fitted with exponential rise to a maximum functions, indicating increased stability of the associated phases similarly to what was inferred from their crystallization traces. Also, the difference between the two melting temperatures decreased in the same manner as was observed in their crystallization counterparts. The offset of melting (■ in Figure 1d) and the peak temperature of the last endotherm (○ in Figure 1d) decrease linearly with increasing n (slope∼ −5.9 ± 1.3 °C/monomer), up to n = 4, then increase exponentially afterward. This suggests that n = 4 is the tipping monomer size at which a qualitative change in the crystallization and melting behaviors, and in phase development of the oligomers occur. This can be understood in light of the competition between the extra attractive van der Waals contributions brought by the added monomers and the growing molecular complexity due to this addition. Therefore, more stable phases due to attractive forces overcoming the growing molecular complexity are possible only for molecules with n < 4. The sizable difference (∼19 °C) in melting offset temperature between triolein and the quatrimer

is also a manifestation of the strong competition between the interactions involved. Similar to the crystallization offset temperature (TCOff, ■ in Figure 1b), the melting onset temperature (TM On, ▲ in Figure 1d) versus n follows an exponential rise to a maximum function. The rather impressive range observed in TCOff (−35 to −65 °C, Figure 1b) and the exponential decay observed in span of crystallization versus n (▲ in Figure 1e) were perfectly mirrored by the melting onset (▲ in Figure 1d) and span of melt (○ in Figure 1e), respectively. This highlights the dramatic importance of the increasing complexity of the molecules in the phase development and indicates similarity of these effects on the crystallization and melting behavior of the oligomers. Interestingly, the crystallization onset (Figure 1b) and melting offset (Figure 1d) of the heptamer are below the respective trends observed, suggesting the possibility of symmetry related effects. Note, however, that no other evidence of symmetry related effects was observed. 4.4.3. Glass Transition. The unmistakable glass transitions observed in both the cooling and heating thermograms of oligomers are very prolonged and weak (arrows in Figure 1a and c, respectively). As can be seen, there are one- to possibly three-step glass transitions depending on the size of the oligomers. The first glass transition process (centered at ∼ −75 ± 3 °C) is identifiable in all the thermograms of the oligomers. A second glass transition event centered at ∼-60 ± 5 °C was visible in the heating and cooling traces of pentamer, heptamer, and hexamer, suggesting that the whole low temperature thermal event is a two-step glass transition in all oligomers. Another glass transition centered at ∼-35 ± 2 °C is unambiguous in the cooling thermogram of heptamer and octamer (right arrow in Figure 1a). The window of temperatures in which glass transition is recorded by the DSC is very wide due to the relatively low cooling and heating rates and also probably to very slow relaxation dynamics in these materials at such low temperatures. The DSC glass transition traces suggest that the oligomers of this study are complex and contain glassy liquid phases at low temperature. The apparent continuous shift of the glass transition to higher temperatures as n increases is a manifestation of the role of molecular size in the whole transition process. An increase of the glassy liquid phase content as n increases is suggested by the decrease of the amount of total crystallized material as evidenced by the exponential decrease of both the enthalpies of crystallization and melting of the oligomers (Figure 1f). 2214



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Figure 3. (a−c) Shear rate versus shear stress curves obtained at selected temperatures (indicated at the right-hand-side above each curve) of dimer, quatrimer, and hexamer, respectively. Dashed lines are fits to the Hershel−Bulkley equation, eq 2, R2 > 0.9999. (d) Power index, nHB, versus temperature curves of dimer, quatrimer, and hexamer.

4.4.5. Effect of trans-Configuration on the Thermal Properties of the Oligomers. As discussed before (Section 3.2.), the relative number of trans- configurations to the total number of double bonds is directly linked to the size of the molecules. It follows a simple rational function of number of monomers, n, (eq 1, Figure 2a). Ntrans n−1 = Ntrans + Ncis 2n + 1

number of monomers to significantly change the melting and crystallization characteristics of oligomers. This limit to what can be achieved is more clearly defined than what was inferred from the analysis of the exponential trends obtained using the number of monomers or equivalently molecular mass. The rapid rise of the relative number of trans double bonds to its limit can be related directly to the variation of the crystallization and melting characteristics and their plateauing. Note that the crystallization and melting characteristic values of heptamer are below the trend lines and again symmetry considerations can be invoked. There is obviously a limit to the extent to which molecular mass can be varied to impart significant changes to the properties of oligomers. The limit imposed by structural design to relative number of trans- double bonds is obviously a major factor. However, due to this link, one cannot fully distinguish between the pure effect of size (n or molecular mass) and transcontent from the data collected in this study. Such discrimination may be achieved if one parameter is fixed and the other is varied; this is the topic of another different study. Detailed insights onto the crystal structures and stacking conformations compatible with a given molecular structure and trans-/cis- configuration can be achieved by more specialized techniques such as XRD. Nevertheless, insights are already gained into the effectiveness of transconfiguration and into the delicate interplay between steric hindrances introduced by the widely different sizes and balanced interactions involved in the

(1)

The function of eq 1 rises to a maximum with increasing number of monomers and has a value of 0.44 for n = 8 (octamer) which is very close to its limit of 0.5 (Figure 2a). The number of trans- double bonds is a determining factor in the differences in the crystallization and melting characteristics observed between the oligomers. The extent of transconfiguration has a direct impact on the crystallization and melting temperatures of the oligomers, as more trans- will promote higher crystallization and melting temperatures. It is therefore not surprising to find linear relationships between the thermal (crystallization and melting) characteristics and transrelative amount. As can be seen in Figure 2b, P1 in both crystallization and melting increases linearly with increasing number of trans- double bonds. One can observe a “plateauing” or at least a sharp decrease in the rate of change of the peak temperature starting from the hexamer, a trend observed for all melting and crystallization characteristics (not shown). This indicates that there is a limit at which one can increase the 2215



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Figure 4. (a) Viscosity versus temperature curves of olygomers obtained in the Newtonian region and using the temperature rate method (1.0 K/ min, 200 s−1). The number of monomer for each sample is indicated at the top left-hand side of each curve. (b) Viscosity of the oligomers versus number of monomers, n, plots at selected temperatures. Dashed lines are exponential fits.

Figure 5. Variation as a function of number of monomer of the generalized van Velzen equation parameters (GvVE, eq 3) of (a) coefficient m and (b) parameter A.

Figures 3a−c), have been distinguished in the shear rate/shear stress curves both of which were very well fitted with eq 2. Note that no yield stress was observed at any temperatures, including those below 0 °C.

crystallization and melting as well as glass transition of the oligomers. 4.5. Rheology of the Oligomers. The flow behavior of oligomers was investigated using shear stress/shear rate experiments. This is illustrated in Figure 3 with shear stress/ shear rate curves obtained at selected temperatures. The viscosity versus temperature data of the oligomers are presented in Figure 4. The viscosities of the oligomers of this study are higher than those reported for jojoba-like monoesters (JLEs)48 and are similar to those of the branched derivatives of the JLEs,49 despite the relative higher molecular mass of the oligomers, suggesting the predominant effect of geometry on the viscosity. The viscosities of the oligomers of this study are higher than those reported for branched fatty acid esters50,51 and close to those of olein estolides,52 which possess similar molecular weights. 4.5.1. Flow Behavior of the Oligomers. Shear stress/shear rate curves obtained at selected temperatures of dimer, quatrimer, and hexamer, representative of the flow behavior of the oligomers, are shown in Figures 3 a−c, respectively. As can be seen, the low temperature rheology of the oligomers is much more complex than at high temperature. This has been evidenced clearly by the application of Herschel−Bulkley model (eq 2) to the shear stress/shear rate. The Herschel− Bulkley model (eq 2) fitted very well to the whole experimental shear rate/shear stress data of oligomers obtained at temperatures higher than 0 °C (dashed lines in Figure 3 a−c). At temperatures below 0 °C the model did not fit the whole experimental data set. In this case two distinct segments separated by a singularity, or critical shear rate (γ̇c, arrows in

τ = τ0 + Kγ ṅ HB

(2)

where γ̇ denotes the shear stress, τ0 is the yield stress below which there is no flow, K the consistency index, and nHB is the power index. n characterizes the flow behavior of the liquid. For Newtonian fluids, nHB = 1, and K = η is the fluid viscosity. For shear thickening fluids, nHB > 1 and for shear thinning fluids, nHB < 1. The power index versus temperature, nHB(T), obtained using the Herschel−Bulkley model for each oligomer varies continuously with temperature, rising exponentially to achieve a plateau. The temperature at which the nHB(T) curve plateaus is dependent on the size of the molecule, with the curves for larger molecules plateauing at higher temperatures. The variation of nHB(T), as illustrated in Figure 2d for dimer, quatrimer, and hexamer, is clear evidence that the oligomers are thinning at low temperature and becomes increasingly (exponentially) Newtonian at higher temperatures. It is worth noting that the standard errors obtained for nHB(T) are better than 0.001. Therefore, a pure Newtonian character of the flow has been strictly achieved only for the dimer, trimer, and quatrimer at the experimental temperatures (45, 50, and 60 °C, respectively) and all the others remained “weakly” thinning, even at the highest temperature used (100 °C). 2216



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The fit of nHB to an exponential rise to maximum function, eq 3, yielded characteristic temperature (Tc) values between 13 and 16 (±1) °C which are weakly depending on the size of the molecules. nHB = nHB0 + a(1 − exp(−T /Tc))

Prolonged and weak one- to three-step glass transitions at low temperature were detected in both the DSC cooling and heating thermograms of the oligomers. The glass transition temperature shifted continuously to higher temperatures as the molecular size of the oligomers increases, suggesting an increased hindrance to regular packing. Two overlapping exotherms were observed in the DSC cooling thermograms indicating the crystallization of two close but different phases. The melting of these two phases was clearly recorded in the form of two overlapping endotherms in the cooling cycles of the oligomers. The DSC heating thermograms of the dimer and trimer displayed recrystallizations from the melt phase reminiscent of triolein and suggesting that mobility in the melt is sufficient enough to allow such transformations only for the smaller oligomers. In fact all characteristic temperatures of crystallization and melting scale indicate a strong correlation with the number of monomers (n) in the oligomer. The peak temperature of the exotherms as well the endotherms versus n, for example, were readily fitted with exponential rise to a maximum functions. The application of the Herschel−Bulkley model to shear stress/shear rate data of the oligomers revealed a flow behavior which depends on molecular size and temperature. Overall, the oligomers are thinning at low temperature and become Newtonian at higher temperature. The temperature at which this “transition” occurs increases with molecular size. The viscosity−temperature data of the oligomers collected in the Newtonian region were analyzed by application of a generalized form of the van Velzen model to the data. The model fitted very well to the experimental data and yielded parameter values which well reflected the increasing complexity of the molecules. For example, the magnitude of m, the coefficient of this model which is related to the complexity of the molecule, decreased with the number of monomers from dimer to hexamer, reflecting the increasing difficulty of transfer of the molecules through the liquid matrix. The interplay between the size and cis-/transconfiguration of the oligomers was found to be critical in the thermal as well as the flow behavior. The physical properties of the oligomers examined in this study are obviously affected by the transconfiguration as can be inferred from linear trends observed in their variation with relative number of trans- double bonds. The heptamer (n = 7) appeared to be the molecule at which the effect of size is dominating. This study indicates that there is a limit at which one can increase the number of monomers to significantly change the physical properties of oligomers. Note there is not experimental evidence to invoke symmetry considerations apart from the slight off trend displayed by the heptamer.

(3)

If one assumes a value of 1.00 ± 0.01 to be valid as the power index for a Newtonian flow, Tc may be considered as the thinning−Newtonian “transition” temperature. 4.5.2. Viscosity versus Temperature of the Oligomers. The viscosity versus temperature curves of oligomers determined using the temperature rate method are presented in Figure 4a. The values of viscosity obtained with this method agree very well (within the experimental error) with those obtained in the Newtonian domain from shear rate/shear stress measurements. At any given temperature, the viscosity increased exponentially with increasing number of monomers (n), as shown in Figure 4b. The experimental viscosity−temperature data were analyzed using a generalized form of the van Velzen equation (GvVE, eq 4). ⎛ 1 ⎞ ln(η) = A⎜ −1 + m ⎟ ⎝ T ⎠

(4)

The GvVE is especially interesting as the parameters of the model are physically meaningful: the parameter A relates directly to the magnitude of the viscosity of the liquid and the parameter m is related to the complexity of the molecule and the strength of the intermolecular forces involved during sheared flow. Figure 5a and b show the variation of coefficient m and parameter A as a function of number of monomer (n). As expected from the overall magnitude of viscosity, parameter A increased with number of monomers except for the heptamer (Figure 5a). Also, except for the heptamer, the magnitude of m decreased sharply (Figure 5b) indicating the increasing difficulty of transfer of molecules through the liquid matrix. The rheology of the oligomers is obviously affected by the number of trans- double bonds. A linear trend was observed for the variation of the GVvE parameter A and coefficient m versus relative transconfiguration from dimer to hexamer (not shown). This highlights the interplay between the size of the molecule and its transconfiguration, with the latter dominating in oligomers smaller than heptamer. The heptamer (n = 7) may be the oligomer at which the trans- character no longer dominates in defining the viscosity behavior as also suggested by the trends observed in crystallization and melting characteristics (see discussion in Section 3.3.1). One, however, cannot exclude that the viscosity versus temperature of this oligomer is simply not suitably fitted by the model.

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ASSOCIATED CONTENT

S Supporting Information *

5. CONCLUSIONS Seven self-metathesis-like oligomers of triolein were synthesized by Steglich esterification from three oleic acid derivatives by Steglich esterification reactions and fully characterized by 1H NMR, 13C NMR, mass spectra, and GPC. The onset temperature of degradation of the oligomers measured at 10% weight loss by TGA was as high as 350 °C, suggesting excellent thermal stability. Further analysis of TGA indicated that the degradation of the oligomers occurs in three main steps starting with the decomposition of the ester groups and ending with the fatty acid chains.

Scheme S1. Structures of oligomers. Scheme S2. Synthesis route of the oligomers. Table S1. Column chromatography method, yields of the synthesis reactions of the oligomers and intermediate products, and 1H NMR, 13C NMR, and MS characterization data. Table S2. Proton ratio of the oligomers. The presented ratio is calculated based on the proton amount of OCCH2 found in 1H NMR. Calc.: Theoretical data calculated based on the molecular formula of the oligomer; Exp.: experimental data calculated based 1H NMR spectra. Table S3. ESI-MS and GPC results. FW: calculated formula weight; MW: Molecular weight as determined by Electrospray 2217



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(15) Song, D.; Narine, S. S. Synthesis and characterization of highly functionalized symmetric aromatic hexa-ol intermediates from oleic acid. Chem. Phys. Lipids 2008, 155 (1), 43−47. (16) Yue, J.; Narine, S. S. Separation and quantification of vegetable oil based polyols by high performance liquid chromatography with evaporative light scattering detection. J. Am. Oil Chem. Soc. 2007, 84 (9), 803−807. (17) Yue, J.; Narine, S. S. Synthesis of aromatic triols and triacids from oleic and erucic acid: Separation and characterization of the asymmetric and symmetric isomers. Chem. Phys. Lipids 2008, 152 (1), 1−8. (18) Zuo, J. Q.; Li, S. J.; Bouzidi, L.; Narine, S. S. Thermoplastic polyester amides derived from oleic acid. Polymer 2011, 52 (20), 4503−4516. (19) Li, S.; Hojabri, L.; Narine, S. Controlling Product Composition of Metathesized Triolein by Reaction Concentrations. J. Am. Oil Chem. Soc. 2012, 1−13. (20) Banks, R. L.; Beailey, G. C. Olefin disproportionation–a new catalytic process. Ind. Eng. Chem. Prod. Res. Dev. 1964, 3, 170−173. (21) Mol, J. C. Industrial applications of olefin metathesis. J. Mol. Catal. A: Chem. 2004, 213 (1), 39−45. (22) Rybak, A.; Fokou, P. A.; Meier, M. A. R. Metathesis as a versatile tool in oleochemistry. Eur. J. Lipid Sci. Technol. 2008, 110 (9), 797− 804. (23) Warwel, S.; Bruse, F.; Kunz, M. New polymers from plant oils as raw material. Fresenius Environ. Bull. 2003, 12 (6), 534−539. (24) Mol, J. C. Application of olefin metathesis in oleochemistry: An example of green chemistry. Green Chem. 2002, 4 (1), 5−13. (25) Warwel, S.; Bruse, F.; Demes, C.; Kunz, M. Polymers and polymer building blocks from meadowfoam oil. Ind. Crops Prod. 2004, 20 (3), 301−309. (26) Van Dam, P. B.; Mittelmeijer, M. C.; Boelhouwer, C. Metathesis of unsaturated fatty acid esters by a homogeneous tungsten hexachloride−tetramethyltin catalyst. J. Chem. Soc. Chem. Commun. 1972, 1221−1222. (27) Van Dam, P.; Mittelmeijer, M.; Boelhouwer, C. Homogeneous catalytic metathesis of unsaturated fatty esters: New synthetic method for preparation of unsaturated mono-and dicarboxylic acids. J. Am. Oil Chem. Soc. 1974, 51 (9), 389−392. (28) Rybak, A.; Meier, M. A. R. Acyclic diene metathesis with a monomer from renewable resources: Control of molecular weight and one-step preparation of block copolymers. Chemsuschem 2008, 1 (6), 542−547. (29) Hojabri, L.; Kong, X. H.; Narine, S. S. Fatty acid-derived dilsocyanate and biobased polyurethane produced from vegetable oil: Synthesis, polymerization, and characterization. Biomacromolecules 2009, 10 (4), 884−891. (30) Hojabri, L.; Kong, X. H.; Narine, S. S. Functional thermoplastics from linear diols and diisocyanates produced entirely from renewable lipid sources. Biomacromolecules 2010, 11 (4), 911−918. (31) Fokou, P. A.; Meier, M. A. R. Acyclic triene metathesis polymerization with chain-stoppers: molecular weight control in the synthesis of branched polymers. Macromol. Rapid Commun. 2008, 29 (19), 1620−1625. (32) Fokou, P. A.; Meier, M. A. R. Use of a renewable and degradable monomer to study the temperature-dependent olefin isomerization during ADMET polymerizations. J. Am. Oil Chem. Soc. 2009, 131 (5), 1664−1665. (33) Henna, P.; Larock, R. C. Novel thermosets obtained by the ringopening metathesis polymerization of a functionalized vegetable oil and dicyclopentadiene. J. Appl. Polym. Sci. 2009, 112 (3), 1788−1797. (34) Lu, Y. S.; Larock, R. C. Novel polymeric materials from vegetable oils and vinyl monomers: preparation, properties, and applications. ChemSusChem 2009, 2 (2), 136−147. (35) Refvik, M. D.; Larock, R. C.; Tian, Q. Ruthenium-catalyzed metathesis of vegetable oils. J. Am. Oil Chem. Soc. 1999, 76 (1), 93−98. (36) Tian, Q. P.; Larock, R. C. Model studies and the ADMET polymerization of soybean oil. J. Am. Oil Chem. Soc. 2002, 79 (5), 479−488.

ionization mass spectrometry (ESI-MS), and Mn: Number average molecular weight as determined by gel permeation chromatography (GPC) with polystyrene as standards. Table S4. Thermal stability data of the oligomers. T5, T10: temperatures (±3 °C) at 5% and 10% weight loss, respectively. TI, II, III: temperature (±3 °C) intervals of decomposition for steps I, II, and III, respectively. WLI, II, III: weight loss (±0.5%) in steps I, II, and III, respectively. Figure 1. 1H NMR spectrum of quatrimer. Figure 2. 13C NMR spectrum of quatrimer. Figure 3. Derivative of TGA (DTG) of oligomers. The data were collected using a heating rate of 10 K/min under nitrogen atmosphere. This information 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.

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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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Synthesis and Physical Properties of Triacylglycerol Oligomers

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