Production of 9-Hydroxynonanoic Acid from Methyl Oleate and

Feb 14, 2008 - The feasibility of a previously established method based on ozonolysis and hydrogenation reactions for the production of 9-hydroxynonan...
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Biomacromolecules 2008, 9, 949–953

949

Production of 9-Hydroxynonanoic Acid from Methyl Oleate and Conversion into Lactone Monomers for the Synthesis of Biodegradable Polylactones Guoguang Liu, Xiaohua Kong, Hayley Wan, and Suresh Narine* Alberta Lipid Utilization Program, Department of Agricultural Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, University of Alberta, Edmonton, Alberta T6G 2P5, Canada Received November 6, 2007; Revised Manuscript Received December 20, 2007

The feasibility of a previously established method based on ozonolysis and hydrogenation reactions for the production of 9-hydroxynonanoic acid from oleic acid has been demonstrated. Metal catalyzed lactonization conditions have been used to convert 9-hydroxynonanoic acid into 1,11-dioxacycloicosane-2,12-dione, which is a potential monomer in the synthesis of polylactones. The structure of 9-hydroxynonanoic acid and 1,11dioxacycloicosane-2,12-dione has been confirmed by 1H NMR, 13C NMR, and FTIR. In addition, 9-hydroxynonanoic acid was analyzed by high-resolution mass spectroscopy and 1,11-dioxacycloicosane-2,12-dione was analyzed by GC-MS. Aliphatic poly(nonanolactones) have been synthesized via ring-opening polymerization of the dilactone. The structure and number average molecular weight (Mn) of the poly(nonanolactones) have been calculated by 1H NMR and GPC. The physical properties of these poly(nonanolactones) have been characterized by modulated differential scanning calorimetry (MDSC) and thermogravimetric analysis (TGA).

Introduction Polyester products have long been synthesized from raw materials derived from petrochemical reserves and have been extensively studied and mass produced. The current depletion of these reserves and the corresponding increases in costs has generated a need for a more renewable and cost-effective source for polymer starting materials. Most of the currently utilized material sources, such as thermoplastics and fibers, are not biodegradable and have raised concerns about environmental contamination and pollution. Therefore, there is an additional need to find alternative starting materials that can be used to synthesize new monomers and produce biodegradable aliphatic polyesters with comparable properties. Biodegradable aliphatic polyesters such as polycaprolactone (P(-CL)) have been widely used in medical applications, for example, in drug delivery, due to their environmentally friendly properties and their compatibility with human tissues.1 P(-CL) has a low glass transition temperature (Tg) and high permeability and is one of the most frequently used compounds in drug delivery systems. Aliphatic polyesters can also play an important role in the development of biodegradable materials possessing properties that make them especially suitable for agriculture, industrial, and medical applications. Traditionally, polyesters have been synthesized using melt condensation reactions. They can also be synthesized using acid catalysts or other additives to activate the carboxylic acid and facilitate the polymerization process. Phase transfer catalysts2 and some environmentally friendly processes have also been previously reported.3 Ring-opening polymerization (ROP) reactions can provide more convenient and economic routes in the synthesis of target polyester polymers when compared to using traditional step polymerization methods.4–6 ω-Hydroxyl fatty acids (Figure 1) are compounds that possess hydroxyl and carboxylic acid functionalities at the terminal of * Corresponding author. E-mail: [email protected]. Telephone: 1-780-492-9081. Fax: 1-780-492-7174 .

Figure 1. General structure of a ω-hydroxyl fatty acid (n ) the number of methylene groups + 4).

Figure 2. Structure of nonanolactone.

the fatty acid chain, making such molecules potentially useful intermediates in organic and polymer synthesis.7 ω-hydroxyl fatty acids are interesting from several standpoints: They are used in the preparation of mixed diesters, e.g., methyl 9-acetoxynononoate, which is a potential reactant in transesterification reactions.8 They can also be applied in the preparation of cosmetic formulations.9 ω-Hydroxyl fatty acids are also intermediates in the synthesis of phospholipids, which have been used as membrane model systems for investigating biochemical interactions.10 In addition, they are valuable intermediates for the synthesis of very useful large-membered ring lactones, some of which exhibit flavor properties or are useful monomers for the synthesis of biodegradable polyesters and polylactones.11 Nonanolactone (Figure 2), a 10-membered ring lactone, is an important intermediate and a useful monomer for producing aliphatic polyesters or polylactones, particularly by ROP. Literature methods for the production of nonanolactone from 9-hydroxynonanoic acid have reported yields that range from low to mediocre.12–16 In many cases, the monolactone was not formed exclusively but the dilactone (1,11-dioxacycloicosane2,12-dione) was commonly obtained (Scheme 1). Some processes, although providing reasonable yields of nonanolactone, e.g., 87%, were found to be very time-consuming and required the use of a mechanically driven syringe pump in conjunction with the use of expensive catalysts such as scandium triflate.16

10.1021/bm7012235 CCC: $40.75  2008 American Chemical Society Published on Web 02/14/2008

950 Biomacromolecules, Vol. 9, No. 3, 2008 Scheme 1. Synthesis of Monolactone and Dilactone from 9-Hydroxynonanoic Acid

In this report, we demonstrate that 9-hydroxynonanoic acid, which is a precursor to the desired monomer, can be obtained in high yield and purity from oleic acid using cost efficient catalysts and mild reaction conditions. To our knowledge, this is the first time that 9-hydroxynonanoic acid has been converted into the dilactone in high yield and purity using cheap and environmentally friendly catalysts and a time-efficient methodology. We show that the dilactone can be used as a monomer in ring-opening polymerization reactions to provide potentially useful polyester materials. The catalysts involved in the ROP reactions are commercially available as well as cheap and environmental friendly.

Experimental Section Materials. Oleic acid (90% purity) was purchased from SigmaAldrich. Methyl oleate was synthesized from oleic acid, anhydrous methanol, and catalytic iodine (A.R. grade, all purchased from SigmaAldrich). Sodium thiosulfate, Raney nickel 2800 (slurry in water), ethanol, THF, toluene, DCM, aluminum isopropoxide (99%), and lanthanum chloride were obtained from Sigma-Aldrich Company. Ethyl acetate and diethyl ether were obtained from Fisher. All the other chemicals and solvents were reagent grade or better. Equipment. Gas Chromatography (GC) System. Gas chromatograms were obtained on a Varian 3500 capillary GC equipped with a flame ionization detector (GC-FID), Varian 8200 auto sampler, and a BP20025 column (30m × 0.25 mm, i.d. 0.25 µm). The injector and the detector temperature were fixed at 250 °C. The temperature of the column was initially set at 50 °C and then increased to 250 °C in two successive ranges: from 50 to 90 °C at a rate of 25 °C/min and from 90 to 250 °C at a rate of 10 °C/min. Gas Chromatography–Mass Spectroscopy Detector System (GC-MS). An Agilent Technology 6890 capillary GC equipped with a 5975B inert XL mass spectroscopy detector (GC-MSD) with a molecular ion range of 1000 was used for analysis. Agilent Technologies 7683 B series auto sampler injector, which can inject a 1.0 µL sample with a split ratio of 80:1, and an HP-5 column (30m × 0.25 mm, i.d. 0.25 µm) were used to identify the lactone products. The temperature of the column was initially set at 70 °C and held for 0.5 min, then the temperature was set to increase to 250 °C at a rate of 10 °C/min and the temperature was held for 20 min. FTIR, NMR, and Mass Spectrometry. FTIR spectra were measured with a Mattson Galaxy series FT-IR 3000 spectrophotometer. 1H NMR and 13C NMR were recorded at larmor frequencies of 400 and 100 MHz, respectively, using a Varian UNITY 400 NMR spectrometer (Varian, Inc., CA). Deuterated chloroform (CDCl3) was used as solvent. Mass spectra were acquired on a Mariner biospectrometry workstation (PerSeptive Biosystems, Inc., MA). Molecular Weight Measurements by GPC and 1H NMR. The numberand weight-average molecular weights (Mn and Mw, respectively) were determined by gel permeation chromatography (GPC). Studies by GPC were carried out using an Agilent G1311A quaternary pump, G1362A refractive index detector, and a PL gel column (5 µm mixed-D). Chloroform was used as the eluent at a flow rate of 1.0 mL/min. Sample

Liu et al. concentrations of 0.4% (w/v) and injection volumes of 10 µL were used. Polystyrene standards were used to generate a calibration curve. Molecular weights of polyesters were also measured by end group analysis with 1H NMR. The progress of polymerization of the hydroxylesters was followed by the steady increase of the signal intensity of the two end groups: the isopropyl group adjacent to the ester functionality, and the methylene group adjacent to the hydroxyl group. Modulated Differential Scanning Calorimetry (MDSC). The TA 2920 modulated DSC system from TA Instruments was used to analyze the thermal transitions of the polyester. The procedure to record the crystallization and melting curves was as follows: Initially the sample was kept at 20 °C for 3 min to reach its equilibrium state and then was heated to 120 °C at a rate of 20 °C/min to erase its thermal history. To record the crystallization curve, the sample was cooled down to -60 °C at a constant rate of 5 °C/min and kept at this temperature for 3 min to allow for the completion of the crystallization. The sample was then heated to 120 °C at a constant rate of 10 °C/min to record the melting curve. The relative crystallinity of the polyesters was calculated according to the following equation: wc ) (∆Hf/∆Hf0) × 100, where wc is the relative crystallinity, ∆Hf is the heat fusion of the polyesters, and ∆Hf0 is the heat of fusion of 100% crystalline polyester, which was 204.5 J/g.17 ThermograVimetric Analysis (TGA). TGA was carried out on a TGA Q50 thermogravimetric analyzer (TA Instruments) following the ASTM D3850-94 standard. The sample was ground to a powder, and approximately 10 mg of the specimen was loaded into an open platinum pan, which was preflamed. The samples were heated from 25 to 600 °C under dry nitrogen at a constant heating rate of 10 °C/min. Synthetic Procedures: Preparation of 9-Hydroxynonanoic Acid from Methyl Oleate Using the Ozonolysis/ Hydrogenation/ Saponification Based Methodology. 1. Ozonolysis. To a three-necked flask, 20.0 g of methyl oleate and 100 mL of anhydrous ethanol were added. The flask was fed with a magnetic stirrer, inlet for ozone, and outlet for gas. Ozone was produced by an ozone generator (Azcozon model RMV16-16 from Azco Industries Ltd., Canada) using cylinder oxygen as a feeding gas. The reaction was performed at -4 °C (ice-salt bath) at flow rate of 5 L/min of oxygen with an agitation rate of 100 rpm for 27 min. The concentration of ozone was 62.0 g/m.3 After 27 min, the ozone generator was stopped and the reaction vessel was purged with nitrogen (N2) for 3 min to remove any residue of ozone in the reactor vessel. To the ozonide product, 800 mL of ethyl acetate was added and the ozonide product was used for hydrogenation. 2. Hydrogenation. Raney nickel catalyst (5.0 g, slurry in water) was added to the ozonolysis product in a hydrogenation vessel (2 L, Parr Instrument Co.) fitted with a magnetic drive. The reaction vessel was charged with hydrogen gas at 100 psi and a temperature of 70 °C. After 3 h, the hydrogen flow and heat were stopped and the temperature of the reaction vessel was allowed to cool to room temperature. The reaction vessel was finally purged with nitrogen gas to remove any residue of hydrogen gas. The resulting mixture was filtered by a Buchner funnel. The filtrate was then transferred to a flask, and solvent was removed by rotary evaporation to yield 20.2 g of oily residue. 3. Saponification. The crude hydrogenated product (20.2 g) was saponified using 100 mL of sodium hydroxide solution (8.0%) for 3 h. Afterward, the solution was cooled to room temperature. The resulting mixture was then washed with ether (3 × 50 mL). The combined ether layers were concentrated to give 7.2 g of n-nonanol (74%). The aqueous layer was cooled down to 0 °C and acidified with concentrated hydrochloric acid (10.5 mL). The acidified mixture was extracted with ether (3 × 50 mL). The organic layers were combined and washed with brine until neutral. The solution was finally dried with anhydrous sodium sulfate. Sodium sulfate was filtered off, and the filtrate was concentrated with a rotary evaporator. Crude desired product was obtained as a white solid (10.4 g). The product was further purified by

Production of 9-Hydroxynonanoic Acid from Methyl Oleate recrystallization from diethyl ether to give 8.7 g of pure 9-hydroxynonanoic acid with a yield of 74%. Typical Procedure for the Lactonization of 9-Hydroxynonanoic Acid Using Hafnium Chloride. A dried 25 mL three-necked flask was equipped with a Teflon-coated magnetic stir bar, a 15 mL pressure equalized addition funnel (containing a cotton plug and molecular sieves 3Å (6.0 g)), and a reflux condenser. 9-Hydroxynonanoic acid (4.98 g, 28.6 mmol), HfCl4 (225.0 mg, 0.7 mmol), and 10 mL of xylene were added to the flask. After 24 h of refluxing, the resulting mixture was cooled to ambient temperature and dissolved in 8 mL of chloroform. Acetone was added to precipitate the product, which was obtained as a yellowish powder in almost quantitative yield (98%). Ring-Opening Polymerization of the Dilactone (1,11-Dioxacycloicosane-2,12-dione) Catalyzed by Lanthanum Chloride. A three necked round-bottom flask, previously flamed and purged with dry nitrogen several times, was fixed with a condenser with a dry nitrogen inlet. The dilactone monomer (522.0 mg, 1.7 mmol), LaCl3 (6.1 mg, 0.3 mmol) and 10 mL dry toluene were added to the flask. The reaction was refluxed for 24 h under a nitrogen atmosphere. The reaction was then cooled to room temperature and the reaction mixture dissolved in CHCl3. Polymer was precipitated with excess methanol. After washing with methanol several times and drying under vacuum, 0.3 g of solid was obtained. Ring-Opening Polymerization of the Dilactone (1,11-Dioacycloicosane-2,12-dione) Catalyzed by Aluminum Isopropoxide. To a flame-dried round-bottom flask, Al(O-i-Pr)3 (19.0 mg, 0.1 mmol) and dilactone (1.0 g) was added under a nitrogen atmosphere. Dry toluene (10 mL) was then added through a rubber septum using a syringe. After 24 h at 80 °C, the polymerization reaction was stopped by adding a 3-fold molar excess of 0.5 M HCl. Initiator residues were removed by two successive extractions with an aqueous solution of 0.1 M EDTA, followed by repeated washing of the organic phase with water. The polymer was recovered by precipitation in methanol and was dried under reduced pressure at room temperature until no change in weight was observed and 0.4 g of solid was obtained.

Results and Discussion Methyl oleate, a precursor in the synthesis of 9-hydroxynonanoic acid, was synthesized according to a literature procedure using oleic acid, methanol, and catalytic iodine.18 9-Hydroxynonanoic acid was obtained from methyl oleate in moderate yield (74%) and high purity (98%) following an ozonolysis/hydrogenation/saponification procedure. This procedure is based on previously established literature methods.19,20 n-Nonanol, a byproduct of the process and an expensive solvent, was obtained in moderate yield (82%) with high purity (98%). The structure of 9-hydroxynonanoic acid has been confirmed by HR-MS, FTIR, 1H NMR, and 13C NMR. HR-MS: ESI-MS using negative method: m/z 173.11722 ([M - H]-). FTIR: 3334, 2931, 2852, and 1692 cm-1. 1H NMR (CDCl3 400 MHz) δ (ppm): 2.35 (m, 2 H, CH2COOH), 3.65 (t, J ) 7.0 Hz, CH2OH), 1.65 (m, 2 H, CH2CH2OH), 1.55 (m, 2 H, CH2CH2COOH), 1.35 (s, 8 H, -CH2CH2CH2CH2-). 13C NMR (CDCl3 100 MHz) δ (ppm): 173.36, 62.94, 33.97, 32.59, 29.11, 29.12, 28.91, 25.59, 24.63. This methodology has been carried out on scales of up to 100.0 g of oleic acid as starting material. In an attempt to obtain nonanolactone from 9-hydroxynonanoic acid, melt condensation reactions as well as a few different methods involving cost efficient acid catalysis were also carried out in order to produce nonanolactone from 9-hydroxynonanoic acid. It was found that chain ester mixtures were obtained in most cases instead of the desired nonanolactone. It has been reported that medium sized rings (9-, 10-, and 11-membered rings) are generally difficult to synthesize due to ring strain.21

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When hafnium chloride (a commercially available reagent) was used to catalyze the lactonization reaction of 9-hydroxynonanoic acid in xylene under refluxing conditions, a dilactone product (1,11-dioxacycloicosane-2,12-dione) was obtained in high yield (98%) and high purity (98%). The structure of 1,11dioxacycloicosane-2,12-dione (obtained after a 24 h reaction) was characterized by GC-MS, FTIR, 1H NMR, and 13C NMR analysis. A summary of all spectra data for the dilactone follows. GC-MS: EI- MS (70 eV) m/z 312.2 (M+). FTIR: 2929, 2853, 1733, 1160, and 1216 cm-1. 1H NMR (CDCl3 400 MHz) δ (ppm): 1.30 (16 H, -CH2CH2CH2CH2COO-), 1.60 (8 H, -CH2CH2COO- and –COOCH2CH-), 2.30 (4 H, t, J ) 7.2 Hz, -CH2COO-), 4.07 (4 H, t, J ) 6.8 Hz, -COOCH2-). 13 C NMR (CDCl3 100 MHz) δ (ppm): 173.90 (-CH2COOCH2-), 64.35 (-COOCH2-), 34.33, 29.14, 28.62, 25.87, and 24.94. Having been unsuccessful at obtaining the desired monolactone, it was decided to attempt ring-opening polymerization on the obtained dilactone instead. Two different polymerization catalysts (lanthanum chloride and aluminum isopropoxide) were tested and produced poly(nonanolactones) A and B, respectively. The structures of poly(nonanolactones) A and B have been identified by proton NMR and their physical properties have been characterized using GPC, DSC, and TGA. Properties of Poly(nonanolactones). Poly(nonanolactones) A and B were synthesized by ring-opening polymerization reactions of the dilactone monomer catalyzed by lanthanum chloride and aluminum isopropoxide, respectively. The 1H NMR spectra of poly(nonanolactones) A and B are shown in parts a and b of Figure 3, respectively. In the 1NMR spectrum of poly(nonanolactone) A, there is a triplet at 3.65 ppm that corresponds to the methylene group adjacent to the hydroxyl group. The hydroxyl groups in the above poly(nonanolactone) are the terminating groups, which indicate that poly(nonanolactone) was formed. In the 1H NMR spectrum of poly(nonanolactone) B, the signals at 3.65 and 5.05 ppm have an integration ratio of 2 and correspond to the methylene group (-CH2OH, triplet) adjacent to the hydroxyl group and the isopropyl ester group ((CH3)2C-OCO-, septuplet), both of which are terminating groups. These terminal groups indicate that poly(nonanolactone) was formed. The structures of poly(nonanolactones) A and B are shown in Figure 4. The number-average molecular weights (Mn) of the poly(nonanolactones) were determined by 1H NMR end group analysis22 and GPC. The results are listed in Table 1. The number-average molecular weight values determined by GPC are relative to those of polystyrene, but the Mn values determined by NMR end group analysis are absolute. On average, Mn, determined by 1H NMR end group analysis was slightly lower than the values obtained by GPC, but the trends were the same. The Mn of poly(nonanolactone) A was higher than that of poly(nonanolactone) B. Thermal behaviors of both poly(nonanolactones) were characterized by MDSC. A weak glass transition was observed for poly(nonanolactone) B, whereas no obvious glass transition was observed for poly(nonanolactone) A. This might be due to the fact that heat capacity change over the glass transition region of these types of materials was too small to be detected. From the melting profiles of poly(nonanolactone) A and B, the melting point, Tm, and heat of fusion, ∆Hf, of each product were determined and summarized in Table 1. Tm of poly(nonanolactone) A is slightly higher than that of poly(nonanolactone) B, while ∆Hf and wc of poly(nonanolactone) A are slightly lower than those of poly(nonanolactone) B. These results suggest that these two poly(nonanolactones) have similar thermal behavior.

952 Biomacromolecules, Vol. 9, No. 3, 2008

Liu et al.

Figure 3. 1H NMR spectra of poly(nonanolactones) A and B.

hand, polymers with long chains cannot be accommodated effectively into the crystals and therefore have a large portion of chains in the amorphous phase resulting in lower crystallinity. Figure 4. Structures of poly(nonanolactones) A and B.

In addition, the wcs of both poly(nonanolactones) is much lower than that of the dilactone starting material (36%). This could be due to the fact that small chains generate more regular crystal packing with small portions of amorphous material. On the other

TGA curves of the two poly(nonanolactones) and their derivatives (DTGA) are shown in Figure 5. All the decompositions started at approximately 250 °C, losing weight very slightly until 300 °C, where a rapid drop followed and ended at approximately 500 °C. The shapes of the weight loss curves of both poly(nonanolactones) were similar in the temperature range of 250–410 °C and different in the 410–500 °C temperature range.

Production of 9-Hydroxynonanoic Acid from Methyl Oleate

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Table 1. Physical Properties of Poly(nonanolactones) A and B Mw (g/mol)

Mn (g/mol) poly(nonanolactone) A poly(nonanolactone) B a

Obtained from GPC.

b

a

b

7000 /4200 5300a/3600b

a

1000 8000a

PDI

Tg (°C)

Tm (°C)

∆Hf (J/g)

Wc (%)

-42.3 ( 0.2

69.2 ( 0.1 69.1 ( 0.2

33 ( 2 39 ( 2

16 ( 1 19 ( 1

a

1.4 1.5a

Calculated by 1H NMR.

terminal groups of these two poly(nonanolactones). Work is ongoing in this laboratory to further establish the physical properties of these poly(nonanolactones) as well as synthesize derivatives of the dilactones for further ROP reactions to yield other poly(nonanolactones). In the long term, this methodology will have application in the area of utilizing canola oil or soapstock/deodorizer distillates as renewable starting material sources in addition to drug delivery and time release systems. Acknowledgment. The financial support of NSERC, Bunge Oil, Alberta Crop Industry Development Fund, Alberta Agricultural Research Institute, Alberta Canola Producers Commission, AVAC Ltd., and Archer Daniels Midland are gratefully acknowledged. We are also grateful to Ereddad Kharraz and Niranjan Purohit for their help in the laboratory. Figure 5. TGA and DTGA data for poly(nonanolactones) A and B.

DTGA data reveal three main degradation processes, one correlated with the first 10–20% of the weight loss, the second with 70–80% of the weight loss, and the third with the remaining weight loss. Poly(nonanolactone) A had its fastest rate of loss at 380 °C, and poly(nonanolactone) B had its fastest rate of loss at 390 °C. According to the thermal degradation mechanism of P(-CL),–25 which has similar structure with poly(nonanolactones) A and B, the first stage of decomposition might be due to oligomer degradation and the random degradation of ester chains. The second step of decomposition might result from volatile products such as water and carbon dioxide. The third step of decomposition could be caused by depolymerization of the polymer chain via an unzipping mechanism, which requires the presence of a hydroxyl group in order to occur. The degradation tendency of poly(nonanolactones) A and B is similar to that of the P(-CL) as shown from the TGA and DTGA curves. Slightly different degradation features might result from the different terminal groups of these two poly(nonanolactones), as the terminal groups in poly(nonanolactone) A are a hydroxyl group and carboxylic acid group, while the terminal groups in poly(nonanolactone) B are a hydroxyl group and an isopropyl group. In summary, 9-hydroxynonanoic acid has been synthesized with high purity and yield from methyl oleate via an ozonolysis/ hydrogenation/saponification based methodology. Dilactone, 1,11-dioxacycloicosane-2,12-dione, was synthesized for the first time from 9-hydroxynonanoic acid using cost efficient catalysts. The obtained dilactone has then been used as a monomer and subjected to ring-opening polymerization conditions to yield poly(nonanolactones) A and B whose structures and average molecular weights have been determined by proton NMR using end group analysis and GPC. The thermal behavior and thermal degradation behavior of the two poly(nonanolactones) are quite similar, with slight differences that could result from different

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