Synthesis of Fully Biobased Polyesters from Plant Oil - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9793-9801

Synthesis of Fully Biobased Polyesters from Plant Oil Liejiang Jin,† Keyu Geng,†,‡ Muhammad Arshad,† Reza Ahmadi,† and Aman Ullah*,† †

Department of Agricultural, Food and Nutritional Science, University of Alberta, 4-10 Agric/For Centre, Edmonton, Alberta T6G 2P5, Canada ‡ Department of Polymer Science and Engineering, Zhejiang University, Zhe Da Road 38, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Self-metathesis of fatty acid methyl esters (FAMEs) from natural oils and commercial oleic acid was carried out using a microwave reactor in solvent-free conditions. Self-metathesis products were further identified and quantified by gas chromatography−mass spectroscopy (GC−MS) and gas chromatography−flame ionization detector (GC−FID). Conversion of ∼50% was achieved within a short span (∼2 min) in the presence of 0.05 mol % Hoveyda− Grubbs second generation catalyst (HG2) giving an equilibrium mixture of alkenes, α,ω-diester, and FAMEs. Highly pure dimethyl-9-octadecene-1,18-dioate (diester) was separated, and the desired quantity of it was reduced to 9octadecene-1,18-diol (diol). Condensation polymerization of diester and diol as monomers was performed using conventional heating, microwave irradiation, and microwaves coupled with conventional heating. Characterization and analysis of synthesized biopolyesters were carried out using different techniques including nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and tensile tests. Polyesters with the highest molecular weight of 337 kDa, ∼50 °C melting point, degradation temperature of about 400 °C, and the maximum strength of ∼5.5 MPa were obtained. These materials have great future potential to be used in different applications as a substitute of nonrenewable polyesters. KEYWORDS: Plant oil, Self-metathesis, Microwave irradiation, Aliphatic polyester, Renewable resources

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alkoxycarbonylation2,3 is effective to convert unsaturated FAMEs to compounds with esters on both ends. Other reactions, such as ozonolysis4,5 and the thiol−ene click reaction,6,7 have also been used to provide additional carboxyl acid, ester, or amine groups to fatty acids. Moreover, olefin self-metathesis,8 a type of reaction which allows the exchange of substituents on olefinic double bonds, is also a powerful tool for the preparation of bifunctional monomers. The olefin metathesis because of its operational simplicity, mild reaction conditions, and environment friendliness, is considered an attractive technique for synthetic transformations. The self-metathesis of unsaturated FAMEs was initially investigated by Boelhouwer’s group. Using WCl6 and Me4Sn as a catalytic system in chlorobenzene solution, methyl oleate and methyl elaidate were transformed to desired alkene and diesters with ∼50% conversion in 24 h.9 Solvent-free selfmetathesis was successfully carried out in a later stage and then commonly used. For instance, Mecking et al. obtained C20 diester with 33% yield from the self-metathesis of 10-

INTRODUCTION Fossil feedstocks, which are abundant and cheap, have been supplying the vast majority of raw materials for polymer production over the past few decades. However, fossil oil as a nonrenewable resource is gradually depleting, and its exploitation and combustion have negative impacts on the environment and ecosystem.1 As an alternative to avoid these disadvantages, renewable resources which meet the requirements of environmental protection and sustainable development are now being considered as building blocks for the preparation of polymeric materials and are attracting a growing interest from academia and industry. The largely available and easily accessible renewable resources are taken into consideration, such as plant oils. In Canada, millions of tons of plant oils are being produced every year, as renewable feedstocks which can be used for production of fine chemicals and polymers. Plant oils, including canola oil, have abundant unsaturated hydrocarbon chains. They are generally converted to fatty acids (FAs) or fatty acid methyl esters (FAMEs) for the simplicity of modification. Many reactions have been developed to modify FAs and FAMEs into α,ω-functionalized compounds as valuable monomers of polyesters, polycarbonates, and polyamides. For instance, © 2017 American Chemical Society

Received: May 28, 2017 Revised: September 12, 2017 Published: September 14, 2017 9793

DOI: 10.1021/acssuschemeng.7b01668 ACS Sustainable Chem. Eng. 2017, 5, 9793−9801

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Fatty acid methyl esters from different plant sources.

undecenoic acid at 48 °C within 4 h.10 Recently, Meier et al. reported a higher conversion of 98% of FAMEs from linseed and chia seed oil at 100 °C for 24 h in the presence of 0.15 mol % Grubbs first generation catalyst and 0.30 mol % 1,4benzoquinone (BQ) as olefin isomerization suppressant. However, for a relatively larger-scale batch of methyl linolenate of ∼30 g, a longer reaction time of 36 h and higher catalyst loading of 0.45 mol % (three different batches of 0.15 mol %) were required.11 With the advantages of low catalyst loading, and solvent-free and moderate conditions, olefin self-metathesis has extraordinary performance to build α,ω-functionalized acids or esters from plant oils, but catalyst deactivation due to longer reaction times still remains a challenge. Microwaves, having wavelengths between infrared and radio waves, have been widely used as an unconventional technique for the acceleration of various organic reactions from late 20th century.12,13 A few studies on olefin metathesis under microwave irradiation have been reported, particularly microwave-assisted ring-closing self-metathesis14,15 of several α,ωdienes and cross-metathesis16,17 of α-olefins. Reaction time was cut down to a few minutes or even seconds from a number of hours usually required by the conventional heating method without affecting the yield (>70%). However, microwaveenhanced olefin metathesis of natural resources has rarely been investigated. To the best of our knowledge, there are no reports on the microwave-assisted self-metathesis of plant oils such as canola and soybean oil. In addition to olefin metathesis, research work regarding polycondensation of fossil-fuel-based monomers using microwave technology has also attracted attention all over the world.18,19 Takeuchi et al. obtained poly(butylene succinate) (PBS) with a weight-average molecular weight (Mw) of 23 500 g/mol using microwave technology in a short time period (∼20 min).20 Recently, our group has reported cross-metathesis (ethenolysis and alkenolysis) of canola-oil methyl esters using microwaves under solvent-free conditions.21 In this study, for the first time we are reporting microwave-assisted selfmetathesis of FAMEs from plant oils for the production of diester. The desired quantity of this diester was reduced to diol, and both of these monomers (diester and diol) were used for the synthesis of biopolyesters, though a few examples of conventional polycondensation of plant-oil-derived diesters and diols have been successfully carried out.2,3,10 To the best of our knowledge, microwave-induced polymerization of these monomers has never been reported. The current study also includes the comparison of conventional and microwave-enhanced polycondensation of plant-oil-derived diester and diol under optimized conditions.

different chain lengths and unsaturation. Some of the common FAME types are depicted in Figure 1, containing saturated, monounsaturated, and polyunsaturated hydrocarbon chains. The self-metathesis of polyunsaturated POMEs produces several cyclo-olefins and short-chain olefins which are volatile, including cyclohexa-1,4-diene and hex-3-ene.11 Therefore, for minimization of product complexity by self-metathesis, the natural oil sources predominantly having oleic acid (c18:1) were selected for this study using self-metathesis under microwave conditions. For this purpose, first the fatty acids from oils were converted to monomeric methyl esters for the simplicity of quantification and elimination of the acidic group to quench its ability to be potentially involved in the deactivation of the catalyst. The compositions of POMEs were identified by GC−MS and quantified by GC−FID, as shown in Table 1. Polyunsaturated FAMEs (c18:2/c18:3) from all three sources are less than 17%, while the monounsaturated FAMEs (c16:1/c18:1) are the predominant compounds (>75%). Table 1. Composition of Different POMEs FAMEa A B C

c14:0

c16:0

3.4%

6.7% 7.4% 5.8%

c18:0

c16:1c

c18:1c

c18:2c

c18:3c

13.3% 4.4% 5.8%

3.0%

5.8%

77.0% 82.5% 79.2%

5.6%

a

A, canola-oil methyl ester; B, high-oleic soybean-oil methyl ester; C, methyl oleate, technical grade. Determined by GC−FID.

Hoveyda−Grubbs second generation catalyst (HG2) was particularly employed in this study for its high tolerance to polar and protic environments. The self-metathesis of pure methyl oleate using conventional heating has suggested that the increase in reaction temperature leads to a drop of observed conversion.22 Therefore, a medium temperature of 50 °C was selected in this work. A good conversion rate for the self-metathesis of canola-oil methyl esters was observed within a very short reaction time (2 min), while the product structures were deduced/characterized by their elution time, peak areas, and MS spectra (Figure 2). The unsaturated methyl esters in canola oil and their products after self-metathesis are depicted in Scheme 1. The products have been classified into three types: olefins with internal double bonds (a, b, and c); unsaturated FAMEs (d, e, and f); and unsaturated diester (g). Both cis and trans isomers were identified for compounds e, f, and g from GC−MS. Commonly, the cis compound has a larger dipole moment and higher polarity, so trans isomers are suggested to be eluted ahead of cis isomers on the basis of the separation principles of GC. This point of view was confirmed by comparing the elution time of c18:1t (trans) in the spectrum of metathesis products and c18:1c (cis) in FAMEs (Figure 2).

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RESULTS AND DISCUSSION Olefin Metathesis of FAMEs. Plant-oil methyl esters (POMEs) are generally mixtures of several FAMEs with 9794


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of analogical compounds (for olefins, FAMEs, and diesters, separately) were assumed the same; (3) the amount of generated olefins equals that of diesters. By considering all these factors, MRFs of the representative FAME (methyl oleate) and diester (dimethyl-9-octadecene-1,18-dioate isomers) were obtained after calibration, as presented in Figures S6 and S7. Conv(R) was determined using the following eq 1, where c represents the concentration for each of the components. Conv(R)% = =

c(olefins + diester) c(total products) c(a + b + c + g) × 100 c(a + b + c + d + e + f + g) (1)

Figure 2. GC−MS spectrum of canola-oil methyl esters (COMEs) and self-metathesis (SM) products.

In the case of canola-oil methyl ester, 50.6% Conv(R) was observed (Table 2). The reliability of the calculation was

Scheme 1. Self-Metathesis of Canola-Oil Methyl Esters

Table 2. Conversion of FAMEs by Self-Metathesis entry

FAMEsa

reaction timeb (min)

Conv(R) (%)

1 2 3

A B C

2 2 2

50.6 39.2 51.3

a

A, canola-oil methyl ester; B, high-oleic soybean-oil methyl ester; C, methyl oleate, technical grade. bConditions: 50 °C under microwave radiation at 0.05 mol % HG2 loading.

reconfirmed via yield calculation from an amplified reaction. A 14.5 g portion of dimethyl-9-octadecene-1,18-dioate [Conv(R) ≈ 43%] was separated after the self-metathesis reaction of 63 g of FAMEs. Considering the percentage of saturated FAMEs and loss of products during purification, the actual Conv(R) was supposed to be closer to the result calculated on the basis of GC−FID. Though the calculated conversions were in line with the reported results, it was still unknown whether the reaction was in an equilibrium state under microwave irradiation or not. For determination of the equilibrium point of microwave-assisted self-metathesis, aliquots were taken out for GC−FID analysis at different time intervals (2, 6, 10, 20, 30, and 60 min). The conversion of FAMEs into olefins and diesters was found to be around 50% after the chemical equilibrium was achieved. Moreover, Figure S8 also shows that olefin metathesis was completed within 2 min by microwave heating as compared to conventional heating which requires a number of hours. In this way, a rapid and efficient method has been established using microwaves. Further, FAME mixtures from two other sources were investigated for microwave-induced self-metathesis. Technicalgrade methyl oleate and high-oleic soybean-oil methyl esters were successfully converted into diester. For methyl oleate, around 50% Conv(R) was achieved (Table 2, entry 3), which is similar to the previously reported data on self-metathesis of methyl oleate. However, only 39.2% conversion was observed in the case of high-oleic soybean-oil methyl ester self-metathesis (Table 2, entry 2). Residual glycerol in FAMEs might be a conceivable reason for lower conversion, as the catalyst becomes deactivated after forming a Grubbs hydride complex with alcohol.27 As we have observed, crude FAMEs with residual glycerol have not displayed any conversion even after 20 min under microwave radiation.

Generally, during self-metathesis using conventional heating, the diesters with shorter or longer chain lengths may be generated because of olefin isomerization,23 leading to difficulty in separation of diester products. In this study, only peaks of C18 diester were identified. The absence of C17 or C19 diester peaks in the GC−MS spectrum strongly suggests that olefin isomerization did not take place during the microwave reaction. However, the existence of c18:1t (f) indicated that the reactants were not fully converted because of the equilibrium state of alkene self-metathesis. A conversion rate of ∼50% was widely observed from publications. For instance, Thomas A. FoGlia observed an exactly 50% conversion rate from olefinmetathesis of methyl oleate using Re 2 O3 ·Al 2O 3·Me4 Sn catalyst.24 Roland Winde reported a conversion rate of 49.6% for methyl oleate self-metathesis using a phoban-indenylidene ruthenium catalyst.25 Herein, to investigate the chemical equilibrium state directly, the real conversion [Conv(R)] was defined as the ratio of olefin and diester amounts to the total amount of products.26 For simplification of the conversion-rate calculation, three assumptions were established: (1) The mass response factors (MRFs) of double-bond isomers were considered the same; (2) MRFs 9795


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ACS Sustainable Chemistry & Engineering Effect of Catalyst Loading on Conversions. The effect of catalyst concentration on conversion rate of FAMEs (methyl oleate) for their self-metathesis was also investigated using different catalyst loadings of HG2 (0.05−0.005 mol %). HG2 loading was gradually reduced from 0.05 to 0.005 mol % (Table 3). Once the HG2 loading was decreased to 0.01 mol %, the

Table 4. Polycondensation of Diester and Diol by Conventional Heating

Table 3. Conv(R) of FAMEs (Methyl Oleate) after SelfMetathesis

weight having their melting temperatures (Tm’s) in the range 50−51 °C. Melting temperatures and melting enthalpy (ΔHm) of polyesters obtained by conventional heating were determined by DSC as shown in Table 4 and Table S1. A very slight difference in the Tm and ΔHm values of polyesters was observed, which means that there is no significant effect of catalyst nature and reaction time interval. However, the melting point of polyesters prepared with unsaturated long-chain monomers is lower than that of the saturated aliphatic polyesters. Mecking and his colleagues have prepared saturated long-chain C193 and C2010 polyesters and reported that both of these have 103 °C melting temperature, while the ΔHm values of those saturated polyesters were 144 and 178 J/g, respectively, which are much higher than those of our synthesized polyesters. The reported high ΔHm values indicate that these saturated polyesters possess a higher degree of crystallization, while, in our case, ΔHm is only 78−88 J/g (Table 4 and Table S1), which could be attributed to unsaturation of polyesters restricting the free motion of hydrocarbon chains. Therefore, the arrangement of unsaturated long chains in the microstructure is less organized than the saturated polyester, which leads to lower degree of crystallization and melting point. Microwave-Assisted Polycondensation. Microwave-induced polycondensation of diester/diol was carried out in a CEM Discover microwave reactor, using open-vessel mode under high vacuum (∼0.1 bar). Initially, trials using 2% Ti(OBu)4 were attempted, and only after few seconds of microwave irradiation, blue plasma occurred. The starting materials were charred, and glassware was cracked immediately (Figure S9a). Initially, it was assumed that under high pressure the microwave energy was mostly absorbed by the polymer debris attached on the vessel wall causing a rapid temperature rise of the glassware.18 Therefore, the vacuity was reduced (0.7 bar) to decrease the polymerization rate and avoid energy accumulation. Unfortunately, the experiment using Ti(OBu)4 as catalyst failed again with the occurrence of plasma. This was probably due to titanium causing sparking leading to the occurrence of plasma. Then, SnCl2 was tried as catalyst under ∼0.7 bar vacuum, and it worked well for polymer preparation without the occurrence of any plasma. Further microwave-assisted reactions were studied using SnCl2 under nitrogen atmosphere and in the absence of inert nitrogen at 0.7 bar vacuum (Table 5). The formation of

a

entry

HG2 loading (%)

reaction timea (min)

Conv(R) (%)

1 2 3 4

0.05 0.01 0.01 0.005

2 2 6 2

51.3 40.9 50.8 8.5

Conditions: 50 °C under microwave radiation.

required reaction time was prolonged from 2 to 6 min to achieve similar conversion (∼50.8%) as given in Table 3, entry 3. Further lowering of HG2 concentration to 0.005 mol % led to a sharp drop of conversion rate to 8.5% (Table 3, entry 4). Even with extended reaction time, the conversion was not improved. We hypothesized that a very low concentration of HG2 has completely been degraded/deactivated before the completion of reaction. Therefore, catalyst loading of 0.01 mol % was considered as a preferred concentration for selfmetathesis. Preparation and Characterization of Polyesters. Polyesters were prepared by polycondensation of diester (dimethyl-9-octadecene-1,18-dioate) and diol (9-octadecene1,18-diol) in the presence of SnCl2 or Ti(OBu)4 as catalyst at a suitable temperature as shown in Scheme 2, whereas diol was Scheme 2. Polycondensation of Diester (Dimethyl-9octadecene-1,18-dioate) and Diol (9-Octadecene-1,18-diol) in the Presence of Catalyst [SnCl2 or Ti(OBu)4]

obtained by reducing diester with lithium aluminum hydride (LiAlH4, see the Supporting Information). The preparation of polyester was carried out using three different approaches: (a) by conventional heating, (b) by microwave heating, and (c) by coupling microwave and conventional heating. Polycondensation by Conventional Heating. First, the polycondensation of diester and diol was carried out at 200 °C by conventional heating using two different catalysts [SnCl2 and Ti(OBu)4] for different time intervals to explore optimum conditions for polyester preparation as shown in Table 4 and Table S1. Polyesters obtained in the presence of both catalysts for different time intervals are of almost similar molecular

entry

catalyst

time (h)

Tm (°C)

ΔHm (J/g)

1 2

2% SnCl2 2% SnCl2

8 16

51.6 50.0

85.4 78.8

Table 5. Polycondensation of Diester/Diol Using 2% SnCl2 under Microwave Irradiation at 200°C entry

atmosphere

time (min)

Tm (°C)

ΔHm (J/g)

1 2 3

N2 N2 0.7 bar vacuuma

15 60 60

50.9 50.6 51.0

111.4 97.2 108.6

a

The reaction was performed under nitrogen atmosphere for 15 min on a microwave reactor before reducing the pressure to 0.7 bar.

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h to obtain a much stronger and hard polyester with a molecular weight approximately equal to that of the polyester from conventional heating (Figure S10). By using this coupled method, the polyester was obtained with a molecular weight close to that prepared by conventional heating, where total reaction time was cut down to half compared to the polycondensation reaction by conventional heating. Almost all of the polyesters prepared by different heating methods displayed glass transition temperatures at or below 0 °C. The polyester synthesized with the heating method had a Tg of 0 °C while the polyesters synthesized by microwaves and the coupled method using microwaves and conventional heating displayed a Tg of ∼−26 and −1 °C, respectively. The polyesters also showed similar melting temperatures (Tm’s) ranging from 50 to 51 °C as have been determined by DSC given in Figure 3. The enthalpy (ΔHm) of all synthesized polyesters was also determined from DSC. A higher value of melting enthalpy (ΔHm) for microwave-based polyesters was observed as compared to those obtained by the heating method. As we discussed above, ΔHm is a valuable index of degree of crystallization. Commonly, for polymers with same compositions and structures, the increase of molecular length will result in more difficulty of crystallization. Though a quantitative relation between Mw and ΔHm cannot be determined, it is generally considered that ΔHm will decrease with the rise of molecular weight. Using ΔHm as a reference of molecular weight, microwave conditions were optimized by adjusting reaction temperature and time (Table 6). At 200 °C, ΔHm decreased continuously with increase of reaction time and remained constant afterward. However, when the temperature was increased to 220 °C, a rise in ΔHm was observed from 30 to 60 min, which might have resulted from polymer degradation (Table 6). Thermal degradation of polymer under microwave conditions has been reported by a few groups.20,28,29 Hence, high temperature and long reaction times under microwave irradiation are not recommended for microwave-assisted polymerization. The prepared polyesters and monomers were characterized and evaluated with different analytical techniques such as ATR−FTIR, 1H NMR, GPC, TGA, DSC, DMA, and tensile testing. In addition, for polycondensation, the purity level and the ratio of each component are critical factors which directly influence the molecular weight of the eventual polymer. Prior to polymerization, the purity and structure of dimethyl-9octadecene-1,18-dioate (diester) and corresponding diol (9octadecene-1,18-diol) were confirmed by GC−MS, HRMS, and nuclear magnetic resonance (NMR) spectroscopy, using both 1 H and 13C NMR analyses (see Figures S1−S4). NMR and FTIR of different polyesters presented similar profiles; therefore the NMR and FTIR spectra of polyester (entry 2, Table 4) were plotted representatively. The 1H NMR spectrum of polyester is given in Figure 4. All significant peaks associated with different protons have been clearly labeled in the structure of polyester given in Figure 4. In addition to these significant peaks, two small peaks at chemical shifts of 3.66 (singlet) and 3.62 (triplet) ppm shown in the expanded region of Figure 4 correspond to terminal hydroxyl and methoxy protons of the polyester. The presence of all these significant peaks and the absence of the hydroxyl and diester methoxy protons of monomers indicated the successful preparation of polyesters. 13 C NMR results also support the formation of polyester as shown in Figure S5 (Supporting Information).

polymer was accomplished within 15 min in both cases (Figure S9b) compared to polymerization by conventional heating which took longer reaction times, while the melting point of polyesters from the microwave-induced method was also observed in the range 50−51 °C (Figure 3, Table 5). Slightly lower-molecular-weight polyester was obtained by microwave heating in comparison to those obtained by conventional heating (Figure S10)

Figure 3. DSC curves of polyesters from heating (Ht), microwave irradiation (MW), and coupled methods (MW+Ht). Curves have been offset for clarity.

The molecular masses, strength, and stiffness of polyesters prepared by conventional heating and by microwave irradiation were found to be different. It was observed that the polyesters by the microwave-assisted method have lower strength and stiffness as compare to those obtained by conventional heating, which could be attributed to entrapped residual methanol, a difference in the ratio of monomers used, and lower molar mass of polyesters. It was difficult to prepare the film of this polyester for mechanical study; therefore, a coupled method was established, where both microwave and conventional heating were used for polycondensation. Polycondensation by Coupling Microwave and Conventional Heating. In a coupled method, initially a polycondensation reaction was performed for 15 min on a microwave reactor under nitrogen atmosphere. After that, the reaction was continued at reduced pressure (0.7 bar) for particular time intervals (Table 6). These microwave-assisted polyesters were further heated to 200 °C on a hot plate using high vacuum (∼0.1 bar). The heat source was removed after 2.5 Table 6. Polycondensation of Diester/Diol with 2% SnCl2 (0.7 bar) under Microwave Conditions entry

T (°C)

timea (min)

Tm (°C)

ΔHm (J/g)

1 2 3 4 5 6 7

200 200 200 200 220 220 220

15 30 60 90 15 30 60

51.2 51.4 50.1 51.1 50.8 50.9 50.4

100.0 96.5 94.2 94.6 103.9 97.0 106.5

a

Each reaction was performed under nitrogen atmosphere for 15 min on a microwave reactor before reducing the pressure to 0.7 bar. 9797


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Figure 4. 1H NMR spectrum (400 MHz, C2D2Cl4) of polyester.

groups with higher intensity in the polyester spectrum confirm the successful polycondensation of diester and diol. Polyesters displayed very high molecular weight (Mw), when analyzed with the help of gel permeation chromatography (GPC) as shown in Figure S10 (Supporting Information). The polyester prepared by conventional heating displayed slightly higher molecular weight (Mw = 337 kDa, Mw/Mn = 7.3) as compared to the polyester obtained by microwave heating (Mw = 275 kDa, Mw/Mn = 8.8) and by coupling microwave and conventional heating (Mw = 315 kDa, Mw/Mn = 9.5). Because of an uncontrolled polymerization process, high polydispersity index (Mw/Mn) values were observed in all polyesters obtained by different methods. Furthermore, the thermal degradation behavior of diester, diol, and selected polyester (entry 2, Table 4) was investigated by thermogravimetric analysis (TGA). Polyesters obtained from heating, microwave, or the coupled method displayed similar TGA curves; therefore, only the curves of polyester from the heating method were plotted. Diester and diol displayed initial weight loss at a very low temperature (∼160 °C) as they exist as an individual molecule having less thermal stability and intermolecular forces. It is obvious from Figure 6 that, compared to diester, diol has less thermal stability as its volatilization/decomposition took place in the temperature range 160−265 °C, while the volatilization/decomposition of diester was observed in temperature range 160−285 °C. This difference in their stability/weight change behavior can be attributed to their divergent intermolecular interaction as they carry dissimilar functional groups, while the polyester obtained after condensation of diol and diester demonstrated higher thermal stability showing an initial weight loss starting at 375 °C. The polyester remains under continuous degradation up to 485 °C with a final weight loss of 100%. The higher thermal stability of polyester and its degradation at a higher temperature

Moreover, ATR−FTIR analyses of diol, diester, and polyesters were also performed to further confirm their structure as shown in Figure 5. The characteristic absorption

Figure 5. ATR−FTIR spectra of (1) dimethyl-9-nonadecene-1,18dioate, (2) 9-octadecene-1,18-diol, and (3) poly(1,18-9-nonadecendiyl-1,18-9-nonadecenedioate).

bands at 1735 (CO) and 1170 (COC) cm−1 of diester have completely disappeared in the spectrum of diol, which was obtained by reducing diester. In addition to that, the presence of a new band at 3350 cm−1 affiliated to the hydroxyl group indicates the complete conversion of diester into diol. The absence of the characteristic hydroxyl band and the presence of absorption bands associated with carbonyl and ether functional 9798



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CONCLUSION Fully renewable plant-oil-based polyesters were successfully prepared using conventional and microwave heating from monomers obtained by self-metathesis of FAMEs. Selfmetathesis of FAMEs from different sources was completed within 2 min, providing ∼50% conversion using the microwave technique. The products of self-metathesis were identified and quantified by GC−MS and GC−FID analysis. Similar conversion was achieved within 6 min, when HG2 catalyst loading was decreased from 0.05 to 0.01 mol %. Diester was separated from the self-metathesis products of FAMEs. Diol with high purity and yield was obtained via reduction of diester. Polycondensation of diester and diol was carried out by conventional heating, microwave irradiation, and coupling microwave and conventional heating. In all cases, the polyesters were prepared successfully, but with slightly different molecular weights. The polyesters were characterized by various analytical techniques including NMR, FTIR, GPC, DSC, TGA, DMA, and tensile testing. The polyesters obtained by the coupled method (microwave and conventional heating, Mw = 315 kDa) and by conventional heating (Mw = 337 kDa) were strong enough for film preparation to be used for mechanical study as compared to the polyester obtained entirely by microwave irradiation (Mw = 275 kDa). Higher elongation at break (11%) and lower storage modulus was observed for polyesters using the coupled method, while lower elongation at break (6.5%) and higher storage modulus was achieved for polyester by the conventional method. By using microwave irradiation, polyesters were obtained in 15 min, while the coupled microwave and conventional heating method afforded polyester with good strength in 4 h, which could only be achieved within 8 h by conventional heating.

Figure 6. Thermogravimetric analysis (wt % versus temperature) curves of diester, diol, and polyester.

corresponds to a high molecular weight and strength of polyester. Mechanical properties of polyesters were studied by preparing films of the powdered materials (hot pressed) using a Carver press. The polyester obtained by microwave heating was not strong enough to form a film. Therefore, the mechanical and tensile testing results of polyesters obtained by conventional heating and the coupled heating method are reported in the Supporting Information (Figures S11−S13). The polyester obtained by the coupled heating method (MW +Ht) demonstrated 2.3 MPa break stress, which is almost half of the maximum break stress (5.5 MPa) achieved with polyesters prepared by conventional heating (Figure S13). However, the elongation at break of polyester from the coupled heating method is up to 11%, which is almost double compared to that of the polyester obtained by the conventional heating method. The higher break stress and less elongation at break (strain) value for polyester obtained by the heating method could be attributed to its higher molecular weight making it more strong and rigid as compared to the polyesters from the coupled heating method. Dynamic mechanical studies of polyesters were conducted in two different temperature ranges because of the dissimilar behavior of polyesters as shown in Figure S12. Because of the higher rigidity of polyester from the heating method, it broke quickly below −50 °C, so the temperature range for its measurement was set from −50 to 60 °C. A slight difference was observed in the storage modulus of both types of polyesters. With the rise of temperature, storage modulus of both polyesters decreases gradually, and then, above 50 °C their storage modulus went down rapidly to zero. Overall, polyester from the heating method displayed a slightly higher storage modulus as compared to that obtained by the coupled method. A high decomposition temperature and medium strength was observed for the polyester that we synthesized. Nevertheless, the melting point of our synthesized polyester was lower compared to those of fossil-fuel-derived synthetic polyesters such as polyethylene terephthalate which has a melting temperature of ∼250 °C. The low melting point of this synthesized biopolyester may limit its high-temperature applications in its current state. Nevertheless, applications at low temperature, such as packaging, could be expected from the biopolyester. For high-temperature applications, further crosslinking of CC double bonds is likely to enhance the strength and thermal properties of polyester enormously, which could benefit potential applications in wider temperature ranges.

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EXPERIMENT SECTION

Materials. High-oleic soybean and canola oil samples were kindly provided by Bunge Canada. Oleic acid (technical grade) was supplied by Anachemie. Hoveyda−Grubbs second generation catalyst, lithium aluminum hydride, tin(II) chloride, titanium butoxide, silica gel (technical grade, 63−200 μm), 1,1,2,2-tetrachloroethane-d2, and chloroform-d were purchased from Sigma-Aldrich. Thin-layer chromatography plates (TLC silica gel 60 F254) were from Merck Group. Organic solvents, including tetrahydrofuran (THF), were dried by stirring over CaH2 for 3 days and then filtered in nitrogen atmosphere. Dichloromethane (DCM), methanol, acetone, ethyl acetate, and inorganic materials, such as potassium hydroxide, sodium sulfate, sodium hydroxide, and sodium sulfate, were purchased from SigmaAldrich. Hexane solvent was purchased from Caledon chemicals. Unless specified, these chemicals were used as received. General Consideration. Attenuated Total Reflectance−Fourier Transform Infrared (ATR−FTIR). Spectroscopic analyses were performed on a Nicolet 8700 spectrometer (Madison, WI). For FTIR analysis, the solid or liquid samples were placed directly onto the surface of the ATR crystal and analyzed. Nuclear Magnetic Resonance (NMR). 1H NMR and 13C NMR, analyses were carried out at room temperature on a Varian INOVA instrument at 400 and 100 MHz frequencies, respectively. Gas Chromatography−Flame Ionization Detector (GC−FID). A PerkinElmer GC Clarus 500 instrument equipped with fused silica capillary column SP2560 (100 m × 0.25 mm × 0.2 μm film thickness), detector 5975B Inert XL MSD, and flame ionization detector (FID) was used to identify and quantify fatty acid methyl esters (FAMEs) and products from olefin metathesis. A temperature of 280 °C was set for the detector and 240 °C for the injector. Hydrogen and air gases were used as flow phases at a rate of 450 and 45 mL/min, respectively. A 2 μL sample was injected, and a 20:1 ratio of split mode was 9799


Research Article

ACS Sustainable Chemistry & Engineering selected. Oven temperature was held at 45 °C for 4 min and then increased to 175 °C at a rate of 13 °C/min. After being held for 27 min, it was eventually increased to 215 °C at a heating rate of 4 °C/ min and maintained for 35 min. Gas Chromatography−Mass Spectrometry (GC−MS). GC−MS analyses were conducted on Agilent 6890N equipment. The used column and other conditions were set the same as for GC−FID. The molecular structures were identified by mass spectroscopy with a scanning range 50−600 amu at a rate of 2.66/s, while the microwaveassisted reactions were carried out in an open-vessel-mode microwave reactor (CEM Discover Labmate) equipped with an infrared temperature sensor (maximum pressure = 250 psi, maximum power = 200 W). Differential Scanning Calorimetry (DSC) Measurement. DSC measurements were performed on a TA Instrument (2920 Modulated DSC) device in nitrogen atmosphere. Samples (4.0−7.0 mg) were sealed in a pan and were placed into a DSC cell. The thermograms were recorded by heating the samples up to 150 °C at a rate of 3 °C/ min after equilibrating at −30 °C. DSC data from second heating cycle of samples was reported to eliminate the thermal history of the materials. Thermogravimetric Analysis (TGA). TGA was performed on a Q50 TGA instrument in the presence of nitrogen flow at a rate of 60 mL/ min. Samples with 5.0−20.0 mg were taken in a pan and placed in a furnace for analysis. The weight loss/thermal behavior of the samples was recorded by heating the samples up to 600 °C at a heating rate of 10 °C/min. Gel Permeation Chromatography (GPC). The average molecular weights (Mw’s) of prepared polyesters were determined by gel permeation chromatography. A Styragel HR 4E column (4.6 mm × 300 mm) and 2000 ELSD detector were equipped with the GPC instrument. The samples with a concentration of 0.5 mg/mL in THF were used, where THF was used as an eluent with a flow rate of 0.5 mL/min. A series of polystyrene standards were used to calibrate the instrument. Film Preparation. For polyester film preparation, the polyesters (∼1.5 g) were crushed in a mortar and then taken on steel plates for compression molding. A Carver press was used to press the samples, where a temperature of 80 °C and a pressure of 100 psi were applied for ∼8 min. The prepared polyester films were cut into a desired size before their mechanical study. Tensile Testing. The mechanical properties of polyester films were studied by a universal testing machine (autograph AGSX Shimadzu) at room temperature. Both ends of the film were fixed to a 50N load cell. The film was stretched at a rate of 0.90 mm/s until ruptured. Dynamic Mechanical Analysis (DMA). The viscoelastic properties of polyester films were measured using a TA Instrument (DMA Q800) device. At an oscillatory frequency of 1 Hz with applied deformation of 0.2% during heating, the measurement was carried out in a tensile mode where temperature was programmed from −90 to 60 °C at a rate of 2 °C/min. Microwave-Assisted Self-Metathesis of Different Methyl Esters. Fatty acid methyl esters (FAMEs) derived from oils (2 g) were placed into a 10 mL microwave tube equipped with a stirring bar. HG2 (0.05 mol %) catalyst was weighed and added into the reaction vessel in an inert atmosphere using a glovebox. A line of nitrogen flow was provided into the reaction vial, and the reaction was performed on a microwave reactor at a set temperature for the required time period. Product mixtures were passed through a small column of silica gel to remove the catalyst. A sample with a concentration of 0.5 mg/mL in dichloromethane was prepared and run for GC−MS and GC−FID analysis to characterize and quantify the products. The required product component dimethyl-9-octadecene-1,18-dioate (diester) was separated and purified with silica gel column chromatography using 2% ethyl acetate in hexane as an eluent. Polycondensation by Conventional Heating. Equal amounts of diester (0.5 mmol) and diol (0.5 mmol) were taken into a 10 mL glass tube, and then, 2.0 mol % catalyst, either SnCl2 or Ti(OBu)4, was added. Initially, at reduced pressure (∼0.7 bar) the reaction was run at 100 °C, and then, its temperature was raised gradually from 100 to 200

°C at a heating rate of 10 °C/20 min. The reaction was stopped after 16 h, and the polyesters obtained as a yellow solid were washed with THF and dried prior to any analysis. Microwave-Assisted Polycondensation. An equimolar ratio of diester and diol was taken in a 10 mL microwave glass vial, and after adding the desired amount of catalyst, the reaction vial was placed in a cavity of the microwave reactor. The microwave-assisted reaction was performed at 200 °C under nitrogen atmosphere for 15 min and then at reduced pressure (0.7 bar) for a certain time period (Table 5). The prepared polyester was washed with THF and dried before proceeding for different analyses. Polycondensation by Coupling Microwave with Conventional Heating. In the coupled method, first, polycondensation of diester and diol was carried out using a similar method to that mentioned above in the microwave-assisted synthesis. These microwave-assisted polyesters were further heated to 200 °C on a hot plate under vacuum (∼0.1 bar). The heat source was removed after 2.5 h. The obtained polyesters were thoroughly washed with THF and dried before proceeding for various analyses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01668. Methodology for preparation of compounds, HRMS, 1H NMR, 13C NMR, GC−MS and GC−FID data, table of polyester data, polyester film images, GPC, DMA, and mechanical testing results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (780) 492-4845. Fax: +1 (780) 492-4265. ORCID

Aman Ullah: 0000-0003-1801-0162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for the current work by The Alberta Innovates Bio Solutions (AI Bio). The plant oils were kindly provided by Bunge Canada, Dupont Pioneer, and Soy 2020.

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REFERENCES

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Synthesis of Fully Biobased Polyesters from Plant Oil - ACS Publications

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