Sustainable Approach for the Synthesis of Biopolycarbonates from

Aug 28, 2017 - The objectives of the present work were to (1) synthesize biopolycarbonates with desirable properties from CO2 and soybean oil under mo...
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Research Article pubs.acs.org/journal/ascecg

Sustainable Approach for the Synthesis of Biopolycarbonates from Carbon Dioxide and Soybean Oil Shaoqing Cui,† Yusheng Qin,‡ and Yebo Li*,†,§ †

Department of Food, Agricultural and Biological Engineering, Ohio Agricultural Research and Development Center, Ohio State University, 1680 Madison Avenue, Wooster, Ohio 44691-4096, United States ‡ Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, P. R. China § Quasar Energy Group, 8600 East Pleasant Valley Road, Independence, Ohio 44131, United States S Supporting Information *

ABSTRACT: A sustainable approach to produce biopolycarbonates was developed via copolymerizing carbon dioxide (CO2) and soybeanoil-based terminal epoxide (SOTE) monomers in the presence of catalyst SalenCoCl and cocatalyst bis(triphenylphosphine)iminium chloride (PPNCl). The SOTE monomers were obtained through the synthesis of soybean oil with recycled epichlorohydrin which can be produced from crude glycerol, a byproduct of biodiesel production. The characteristic profiles of SOTE monomers and biopolycarbonates demonstrated a 100% conversion and a biopolycarbonate yield of 83.3%. It was also found that the original functional double bonds on the linear fatty acid chains were maintained in the biopolycarbonate products, which affords a functional basis for various promising applications. The highest biopolycarbonate yield was obtained at a CO2 pressure of 4 MPa; SOTE monomers of 0.01 mol; catalyst loading SalenCoCl and cocatalyst PPNCl of 0.04 mmol each, which was around 1.2% (w/w) based on the total weight of reactant; reaction time of 24 h; and reaction temperature of 22 °C. A transparent biopolycarbonate film with stable thermal properties was obtained. With the incorporation of SOTE monomers into the synthesis of propylene oxide and CO2, competitive biopolycarbonates were obtained with strong tensile strength and enhanced elongation. This innovative green synthesis route of incorporation of CO2 to soybean oil provides a sustainable platform for the production of biopolycarbonates from biobased feedstocks and CO2 for various applications. KEYWORDS: Biobased, Polycarbonates, Soybean oil, Epoxide monomer, CO2 fixation

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INTRODUCTION

from CO2, and it has been produced on an industrial scale because of its relatively high tensile strength of approximately 4.7−21.5 MPa and moderate glass transition temperature (Tg) of approximately 25−70 °C.7,8 However, PO is a petroleumbased chemical, and thus synthesizing poly(propylene carbonate) still requires the use of limited natural resources. Therefore, the development of biobased polycarbonates via synthesis of biobased epoxides and CO2 is more attractive. A biopolycarbonate derived from limonene via the copolymerization of limonene oxide and CO2 was found to possess a high Tg.9,10 Building on this research, Hauenstein et al. synthesized a poly(limonene carbonate) with a high molecular weight, high Tg (130 °C), and one double bond per repeat unit, providing a platform for further modification.10,11 It is important to note that these works have focused

In recent years, the demand for the sustainable use of renewable natural resources as raw chemical materials has increased because of oil reserve depletion and environmental concerns. Carbon dioxide (CO2), a renewable natural C1 feedstock, is a promising low-cost and nontoxic resource for chemical synthesis, especially for polycarbonate polymer synthesis.1−4 Conventional polycarbonates produced using bisphenol A (BPA) are a class of thermoplastic polymers with high toughness, impact strength, and optical transparency and are, therefore, widely used in medical, optical, and electronic applications.5 However, bisphenol A based polycarbonates are toxic because they release toxic volatile compounds and polymers when used as food containers.6 An alternative route to develop polycarbonates that uses CO2 and epoxides has attracted considerable interest. For example, poly(propylene carbonate) synthesized through the copolymerization of CO2 and propylene oxide (PO) is a typical polycarbonate derived © 2017 American Chemical Society

Received: June 6, 2017 Revised: August 24, 2017 Published: August 28, 2017 9014

DOI: 10.1021/acssuschemeng.7b01819 ACS Sustainable Chem. Eng. 2017, 5, 9014−9022

Research Article

ACS Sustainable Chemistry & Engineering on developing hard polycarbonates with high molecular weight, high glass transition temperature, and strong toughness; however, minimal research has focused on soft-polymers with moderate thermal properties and attractive mechanical properties, despite these polymers being critical for biofilms and soft elastomers for medical applications. The inherent properties of an epoxide monomer are known to play an important role in the properties of its derived polymers.12,13 Limonene oxide, a cyclic aromatic epoxide, is inherently brittle, and thus its corresponding polymers exhibit hard mechanical properties and high glass transition temperatures. By contrast, polymers derived from epoxides with long linear chains display flexible and soft properties. Soybean oil, an easily available and low-cost commodity with a global production of approximately 51.5 million tons in 2015, is a promising renewable biofeedstock that possesses three fatty acid chains with several double bonds.14 Soybean-oil-based epoxides (SOBEs) with oxirane groups located internally along the fatty acid chain are versatile precursors for polymerization as they can be applied to the synthesis of plasticizers, epoxy resins, and polyurethanes.15−17 However, SOBEs were found to have relatively low reactivity due to the oxirane rings located internally along the long fatty acid chains. Zhang et al. has reported a polycarbonate with a low Tg from biobased fatty acids.18 In addition, thermosetting polymers derived from the aforementioned SOBEs were reported to have low thermal properties and reduced mechanical performance.19 To address this issue, our research group investigated a bioepoxide with a terminal oxirane group and internal double bonds derived from soybean oil via a modified synthetic route.19 The desirable soybean-oil-based terminal epoxide (SOTE) monomers are usually synthesized with epichlorohydrin (EPCH), which can be obtained from crude glycerol, a main byproduct of biodiesel production.20 To the best of our knowledge, there are no reports on the development of biobased polycarbonates from CO2 and soybean-oil-based epoxide monomers, especially an epoxide monomer with terminal oxirane rings, i.e., SOTE monomers. As illustrated in Scheme 1, a sustainable approach to produce biopolycarbonates was developed via copolymerizing CO2 and SOTE monomers, which were obtained through the synthesis of soybean oil with EPCH. The objectives of the present work were to (1) synthesize biopolycarbonates with desirable properties from CO2 and soybean oil under moderate reaction conditions; (2) characterize the produced SOTE monomers and biopolycarbonates; and (3) improve the elongation property of poly(propylene carbonate) through the incorporation of SOTE monomers into the polymerization process of PO and CO2. The innovative sustainable approach of incorporation of CO2 into soybean oil provides a platform for the production of green polycarbonates from biobased feedstocks and CO2 for various applications.

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Scheme 1. Synthesis of Biopolycarbonates from Soybean Oil and CO2

(triphenylphosphine)iminium chloride (PPNCl) was purchased from Alfa Aesar (Ward Hill, MA) and recrystallized from a mixture of dry methylene chloride and diethyl ether under nitrogen before use. All chemicals were of reagent grade unless otherwise noted. Synthesis of SOTE Monomers. The synthesis of SOTE monomers was conducted as described in our previous report.19 This procedure consisted of soap preparation and introduction of an epoxy group, as illustrated in Scheme 2. For soap preparation, 100.00 g of soybean oil was mixed with 100.00 g of water in a 1 L beaker and then was reacted with 50.00 g of sodium hydroxide (NaOH) solution (0.1 N) at 90 °C for 4 h. Then, the obtained soap was acidified for 1− 2 h by adding 70.00 g of sulfuric acid solution (H2SO4) (30%). Next, the fatty acid layer was obtained after washing three times with warm water using a separatory funnel. The purified soap was then dissolved in acetone at a mass ratio of 1:8. The solution of H2SO4 (30%) was added dropwise into the fatty acid/acetone medium with vigorous stirring for 4 h. Finally, the soap was obtained after filtering and drying at 100 °C under vacuum until a constant weight was reached. The SOTE monomers with terminal epoxide were obtained through the reaction of the prepared soap (50.00 g) and EPCH (50.00 g) at 100 °C for 1 h. Then, CTAB of 1.00 g was added, and the reaction mixture was reacted for approximately 1.5−2 h. After that, the reaction mixture was cooled to ambient temperature and centrifuged for 30 min at 4000 rpm to remove the unreacted solid soap from the suspension solution, and then the clean SOTE monomers were obtained after excess EPCH was removed using a vacuum rotary evaporator (Laborota 4001 OB, Hwidoph Instruments., Elk Grove Village, IL). Synthesis of Biopolycarbonates from SOTE Monomers and CO2. The synthesis of the biopolycarbonates from the SOTE monomers (0.01 mol, ∼3.10 g) and CO2 (22 °C, 4 MPa) was performed in a predried 10 mL autoclave in the presence of catalyst SalenCoCl (0.04 mmol, 28 mg,) and cocatalyst PPNCl (0.04 mmol, 23 mg). The percentage of catalyst was around 1.2% (w/w) based on the total weight of reactant. The autoclave was filled with CO2 at a pressure of 4 MPa. Then, the reaction was performed at three temperatures (22, 45, and 65 °C) and three reaction times (12, 24, and 48 h) with magnetic stirring at a stirring speed of 650 rpm. Solvent

MATERIALS AND METHODS

Materials. Soybean oil, consisting of fatty acids (99.0%), insoluble impurities (0.5%), and free acids (0.2%), was obtained from Provimi North America, Inc. (Brookville, OH). Dichloromethane (CH2Cl2), PO, ethanol, epichlorohydrin (EPCH), cetytrimethylammonium bromide (CTAB), and high-performance liquid chromatography grade tetrahydrofuran (THF) were purchased from Pharmco-AAPER (Shelbyville, KY). Standard polystyrene was purchased from Agilent Technologies (Santa Clara, CA). (R,R)-(−)-N,N-Bis(3,5-ditert-butylsalicylidene)-1,2-cyclohexanediaminocobalt(III)chloride (SalenCoCl) was prepared according to the previous literature. 19 Bis9015

DOI: 10.1021/acssuschemeng.7b01819 ACS Sustainable Chem. Eng. 2017, 5, 9014−9022

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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis Procedure of SOTE Monomers

extraction was applied on the obtained crude products which were dissolved in 5−10 mL of CH2Cl2 and then precipitated through dropwise addition of ethanol (50−100 mL) to remove the byproduct, catalysts, and some low-molecular-weight compounds. The purified biopolycarbonates were obtained after drying. The transparent biopolycarbonate films were obtained by applying the purified biopolycarbonates on a Teflon sheet (25 cm × 25 cm × 1 cm) using a film applicator at 6 miles gap clearance (square frame 4″, 0.5−6 miles, BYK Additives & Instruments Inc., Columbia, MD), and drying in air for 1 week. Synthesis of Biopolycarbonates from SOTE Monomers, PO and CO2. The synthetic procedure for the terpolymerization of the SOTE monomers, PO, and CO2 was similar to the above copolymerization procedure. The reactions were conducted with three volume ratios of SOTE-to-PO of 66.7:33.3, 50:50, and 33.3:66.7. Correspondingly, the contents of the SOTE monomers in these three experiments were 0.0072, 0.0054, and 0.0036 mol, and the contents of the PO were 0.0143, 0.0214, and 0.0286 mol, respectively. Poly(propylene carbonate) was prepared using only PO as the epoxide, i.e., SOTE-to-PO of 0:100. Similarly, after a 24 h reaction, the products were dissolved in CH2Cl2, precipitated with ethanol, and then dried to obtain the desired polycarbonates. Characterization of the Obtained Biopolycarbonates. Fourier transform infrared (FTIR) spectroscopy was performed on a Spectrum Two IR spectrometer (PerkinElmer Inc., MA) equipped with a universal attenuated total reflectance accessory. 1H nuclear magnetic resonance (1H NMR) spectroscopy was performed on a Varian INOVA 400 MHz spectrometer (Palo Alto, CA), using chloroform-d (CDCl3) as solvent. The average molecular weight and polydispersity index (PDI) of the polymers were determined by gel permeation chromatography (GPC) analysis, which was performed in THF at 35 °C with an elution rate of 0.8 mL/min on a Shimadzu LC-20 AB GPC system (Shimadzu, Columbia, MD) equipped with an RID-10A refractive index detector, an SPD-M20A prominence photodiode array detector, a Waters Styragel HR1 column (7.8 × 300 mm), and a Phenogel 5 m 10 E4 Å LC column (7.8 × 300 mm). Standard polystyrene samples were used to calibrate the GPC analysis. Thermogravimetric analysis (TGA) was performed on a Q50 thermogravimetric analyzer (TA Instruments, New Castle, DE) by heating polycarbonate samples from 50 to 600 °C at a rate of 10 °C/ min under nitrogen. Differential scanning calorimetry (DSC) was performed on a Q20 DSC analyzer (TA Instruments, New Castle, DE) equipped with refrigerated cooling. The glass transition temperature (Tg) was defined as the inflection point in the DSC thermogram obtained from the third heating scan. Mechanical properties, including the tensile strength, Young’s modulus, and elongation at break (%), were determined using an Instron universal testing machine (Model 4502, Instron Corp., Norwood, MA) with a crosshead speed of 25 mm/min. The biopolycarbonate yield (%) was calculated from the integrated area ratio between the cyclic carbonates (byproducts) peak and the crude biopolycarbonates peak 1HNMR spectrum.

involved two main stages: (1) The soap was obtained after soybean oil was reacted with sodium hydroxide in acetone as the solvent, and (2) the introduction of an epoxy group to the terminal site of the fatty acid chain was performed via the reaction of the soap (solid phase) with EPCH (organic liquid phase) in the presence of a phase transfer agent. It is worth noting that EPCH can be obtained using crude glycerol, a lowcost biofeedstock that is the main byproduct of biodiesel production.20 After the introduction of the epoxy group, the desired SOTE monomers were obtained with a satisfactory yield of up to 92.26%. The obtained SOTE monomers were characterized through FTIR spectra, as shown in Figure S1 in the Supporting Information [FTIR spectra of soybean oil (a) and SOTE monomer (b) illustrated in Figure S1, 1HMR spectroscopy of obtained SOTE monomers illustrated in Figure S2]. In comparison with the spectra of the soybean oil (Figure S1a), the SOTE monomer spectra (Figure S1b) exhibited two new characteristic absorbance peaks at 910 and 855 cm−1, which were attributed to an epoxy group and indicated that the epoxy groups were successfully introduced into the terminal sites of the fatty acid chains. A characteristic signal at 3008 cm−1, which represented the (CHCH) stretching of double bonds on the unsaturated fatty acids chains, was also observed, indicating that the original double bonds of the unsaturated fatty acid chains were maintained. These characteristic chemical structures were also demonstrated by 1HNMR spectroscopy. As shown in Figure S2 in the Supporting Information, the characteristic hydrogen atoms of the SOTE monomers were as follows: h, 5.35 ppm; c2, ∼4.4 ppm; c1, ∼3.9 ppm; b, ∼3.2 ppm; i, ∼2.85 ppm; a, 2.66 ppm; d, ∼2.36 ppm; g, ∼2.03 ppm; and e, ∼1.65 ppm. Again, these expected spectral results confirmed the successful introduction of the oxirane group and demonstrated the interior double bonds retained on the linear fatty acid chains, offering a promising potential site for further synthetic modification. Synthesis of Biopolycarbonates from SOTE Monomers and CO2. A sustainable approach for the synthesis of biopolycarbonates via copolymerization of CO2 with SOTE monomers is illustrated in Scheme 3. As exhibited in part a of Scheme 3, CO2 was inserted into the ring-opened SOTE monomers to form carbonate linkages, and the biopolycarbonate polymers then grew via the continuous insertion of CO2 into SOTE monomers, which was the desired route. Simultaneously, carbonate linkages can also undergo ringclosure to generate a cyclic carbonate which is considered to be a byproduct, and thus its production should be minimized, as shown in part b of Scheme 3. To achieve high selectivity of biopolycarbonates, in this study, two different catalyst systems involving zinc glutarate (ZnGA) and SalenCoCl were investigated in order to determine the most efficient one. Of

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RESULTS AND DISCUSSION Synthesis of SOTE Monomers from Soybean Oil. As shown in Scheme 2, the synthesis for the SOTE monomers 9016

DOI: 10.1021/acssuschemeng.7b01819 ACS Sustainable Chem. Eng. 2017, 5, 9014−9022

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ACS Sustainable Chemistry & Engineering Scheme 3. Synthesis of Biopolycarbonates from CO2 and the SOTE Monomers

Figure 1. FTIR spectra (a) and 1H NMR spectra (b) of biopolycarbonates synthesized from CO2 and SOTE monomers.

these two systems, the catalyst SalenCoCl with PPNCl as cocatalyst (molar ratio of 1:1) gave a better performance, enabling a carbonate linkage content of over 90%. The nature of the epoxide is another crucial factor that affects selectivity. A chemical structure with a geometric symmetry, such as cyclohexene oxide, favors polycarbonate formation, whereas an epoxy monomer showing an asymmetrically geometric structure, such as styrene oxide, preferentially forms cyclic carbonate.21,22 Compared with styrene oxide, the SOTE monomers might have relatively less geometric strain and thus show a higher selectivity toward biopolycarbonate formation. The steric hindrance of monomers also impacts the conversion rate and yield. For example, the SOTE monomer, which has a terminal oxirane ring, is less sterically hindered than the epoxide monomer with an interior ring, such as traditional epoxidized soybean oil. As a consequence, SOTE monomers with terminal epoxide rings achieved a high yield of biopolycarbonates (∼83.3%). The reason is that the nucleophilic attack mainly occurs at the methylene group of the terminal epoxide ring instead of at the carbon atoms of an internal epoxide ring. The major active carbon atom of the epoxide ring of the SOTE monomers was labeled with an arrow (site 1) in Scheme 3. The long chain connecting to the SOTE monomers significantly affects the physicochemical properties of the resulting biopolycarbonates, rendering it to be a unique and versatile platform for different applications as discussed in the following sections. Characterization of Biopolycarbonates. Biopolycarbonates were successfully synthesized from SOTE monomers and CO2 in the presence of SalenCoCl (PPNCl as cocatalyst) with a satisfactory yield and an average molecular weight of approximately 7400 g/mol after purification. As shown in the FTIR spectra in Figure 1a, after copolymerization, the absorbance bands at 915 and 855 cm−1 in the SOTE monomer spectra (Figure S1b in the Supporting Information), which corresponded to epoxide groups, disappeared completely on the spectrum of the obtained biopolycarbonate products,

indicating 100% conversion of SOTE monomers. The absorbance peaks of the carbonyl stretching mode shifted from 1739 cm−1 on the monomer spectra to 1743 cm−1 on the obtained biopolycarbonate spectrum (Figure 1a), suggesting the successful formation of biopolycarbonates. A small absorbance band at 1801 cm−1, attributed to the carbonyl groups of cyclic carbonate, indicated that a small amount of this byproduct was produced. The change of chemical structures was also confirmed by 1H NMR spectroscopy in Figure 1b. After copolymerization, the hydrogen signals of the epoxide structure indicated in Figure S2 (Supporting Information) disappeared; conversely, the hydrogen signals for the carbonate structure in the biopolycarbonates appeared at the following: a, ∼4.5 ppm; b, ∼5.12 ppm; and c, ∼4.23 ppm, which is consistent with the FTIR results. As byproducts and residual impurities substantially affect the properties of polycarbonates, e.g., transparency and purification, it is necessary to obtain the purified biopolycarbonate products.1 In this study, the cyclic carbonate byproduct and catalysts were removed after purification via precipitation in ethanol, and the obtained purified biopolycarbonates showed good transparency. Figure 2a shows the FTIR spectra of crude and purified biopolycarbonates. The double bond peaks at 3008 cm−1 were clearly found in the spectra of both the crude and purified biopolycarbonates, demonstrating that the original double bonds on fatty acid chains were kept in the biopolycarbonate product. Meanwhile, purification resulted in the average molecular weight of the polymer, with a PDI of 1.30, slightly increasing from 6200 to 7400 g/mol (Figure 2b) because of the removal of compounds at low molecular weight. These results demonstrated the exact alternating insertion of CO2 and the successful synthesis of biopolycarbonates via copolymerization of the SOTE monomers with CO2. In particular, the double bond functional groups maintained in 9017

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Figure 2. FTIR spectra (a) and GPC curve (b) of synthesized biopolycarbonates before and after purification.

Figure 3. Effects of catalyst loading, reaction time, and temperature on biopolycarbonates yield.

temperature), a reaction time of 24 h, and a maximum catalyst loading of 0.04 mmol. Properties of Biopolycarbonates. The thermal performance of polycarbonates is a crucial feature in engineering plastics, which depend on its chemical structure, molecular weight, and terminal groups. As shown on the DSC curve in Figure 4a, the glass transition temperature (Tg) was 12 °C, and the obtained biopolycarbonates were mostly amorphous. The Tg of the obtained biopolycarbonates was slightly lower than that of conventional poly(propylene carbonate),1,23 indicating moderate thermal properties. The low Tg might be caused by the long branched fatty acid chains, which may have inhibited the crystallization of polymers and led to the lower Tg. Another reason for low Tg might have been due to the presence of ether linkages (around 1−2%) in the polymer backbone, which were reported to have a negative effect on Tg.25 As shown in Figure 4b, the obtained biopolycarbonates showed three weight loss stages, starting at 220 and ending at 500 °C. The first degradation stage occurred at approximately 220−300 °C and was likely due to backbone scission. The subsequent two degradation stages involved the unzipping of the carbonate linkages. The physical and mechanical properties of the biopolycarbonates are listed in Table 1. Clearly, the obtained biopolycarbonates had good tensile strength (3.1 ± 1.2 MPa) and excellent elongation at break of 623%, and thus exhibited the properties of “soft materials”. The picture of the obtained crude biopolycarbonate product (Figure 4c) shows good

the biopolycarbonate product rendered it a unique platform for further versatile applications. Effects of Operating Parameters on Biopolycarbonate Yield. For a determination of the optimal reaction conditions, the effects of catalyst loading, reaction time, and temperature on biopolycarbonate yield were investigated. As shown in Figure 3a, increasing catalyst loading from 0.02 to 0.04 mmol increased the yield to 83.3%, whereas a further increase to 0.06 mmol slightly decreased the yield to 80.0%, possibly because excess catalyst loading led to excess nucleophiles, increasing the rate of the displacement of the carbonate intermediate bound to the catalyst nucleophile, and thus resulting in more ringclosure and reduced biopolycarbonate yield. The effect of reaction time on yield is shown in Figure 3b. Increasing the reaction time from 12 to 48 h caused the yield to increase and then decrease, with the highest yield occurring at a reaction time of 24 h. The reduced yield at a prolonged reaction time (48 h) might have been due to polymer degradation. As the temperature increased from 20 to 60 °C, the yield decreased sharply to 46.3%, indicating that the biopolycarbonate was optimally synthesized under a moderate reaction condition at room temperature (Figure 3c). Higher temperatures might have promoted cyclic carbonate byproduct formation and thus could have resulted in a low yield of biopolycarbonates. In addition, decreasing the reaction temperature to as low as 0 °C can favor the formation of polycarbonates from epoxides.24 Consequently, a satisfactory yield of biopolycarbonates (∼83.3%) was achieved at a temperature of 22 °C (room 9018

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Figure 5. FTIR of biopolycarbonates obtained from the terpolymerization of SOTE monomers, PO, and CO2. (Parts a, b, c, d, and e are the spectra of polycarbonates synthesized from terpolymerization of CO2, SOTE monomers, and PO with SOTE/PO at different volume ratios of 100:0, 66.7:33.3, 50:50, 33.3:66.7, and 0:100, respectively.)

different polycarbonates shifted from 1737 (poly(propylene carbonate)) to 1741.61−1743.13 (SOTE and PO monomer based biopolycarbonates) and then 1743.71 (SOTE monomers based biopolycarbonates) cm−1, indicating the successful formation of carbonate linkages from the epoxide rings of the PO and SOTE monomer units and the successful alternating insertion of CO2. Additionally, the two epoxide units were assumed to connect together to form a unique unit that then polymerized with CO2, thus generating the unique carbonate linkages observed in the range 1737−1743 cm−1. The typical chemical structures are indicated in Figure 5. The carbonyl stretching mode of the cyclic carbonate byproduct shifted from 1796 to 1801 cm−1, suggesting that the backbone was altered. The chemical structures were also characterized by 1H NMR spectroscopy, as shown in Figure 6. Obviously, the 1H NMR spectroscopy of biopolycarbonates synthesized from CO2 and SOTE monomers (Figure 6d) was different from that of polycarbonates synthesized from CO2 and PO (Figure 6e). The former spectroscopy (Figure 6d) shows the typical hydrogen signals of SOTE-based biopolycarbonates: h, ∼5.35 ppm; b, ∼5.13 ppm; a, ∼4.5 ppm; c, ∼4.23 ppm; i, ∼2.85 ppm; d, ∼2.36 ppm; g, ∼2.03 ppm; and e, ∼1.65 ppm. The typical hydrogen sites of PO-based polycarbonate (Figure 6e) were confirmed by m, ∼5.01 ppm, and k, ∼4.13.ppm. It is interesting to note that the spectroscopy of blended polycarbonates (Figure 6a−c), which were synthesized from mixed SOTE and PO monomer at different volume ratios (66.7:33.3, 50:50, and 33.3:66.7),

Figure 4. DSC (a) and TGA (b) curves of the biopolycarbonates synthesized from the SOTE monomers and CO2. Pictures c and d are the crude biopolycarbonate and purified biopolycarbonate-based film synthesized from the SOTE monomers and CO2. Picture e is the biopolycarbonate film derived from SOTE monomers, PO, and CO2.

elongation. Figure 4d shows the excellent transparency of obtained biopolycarbonate polymers. Synthesis of Biopolycarbonates from SOTE Monomers, PO, and CO2. PO is often used as an additive epoxide because of its high reaction activity with CO2 and its derived poly(propylene carbonate), which shows excellent biodegradability. However, its elongation at break is small under large molecular weight because of high hardness, as indicated in Table 1, which limits its use as a flexible material. Meanwhile, the biopolycarbonates derived from SOTE monomers showed high elongation at break. The chemical structures and thermal properties of the resulting terpolymerized biopolycarbonates from SOTE monomers, PO, and CO2 were characterized through FTIR and 1HNMR spectroscopy. As exhibited in the FTIR spectrum in Figure 5, the carbonyl vibrational band of the

Table 1. Molecular Weight and Mechanical Properties of the Biopolycarbonates Obtained from the Terpolymerization of CO2, SOTE Monomers, and PO polymer

Young’s modulus (MPa) a

bio-PC- S:P-100:0 bio-PC-S:P-66.7:33.3a bio-PC-S:P-50:50a bio-PC-S:P-33.3:66.7a bio-PC-S:P-0:100a

77.4 101.3 207 243 318

± ± ± ± ±

8.2 14.2 20.6 14.3 41.8

tensile strength (MPa) 3.1 4.7 14.8 16.9 22.7

± ± ± ± ±

1.2 1.4 2.1 2.1 2.5

elongation at break (%)

Mn (g/mol)

PDI

± ± ± ± ±

6213 10 381 17 430 30 055 98 735

1.16 1.08 1.06 1.03 2.07

623 430 352 253 54.7

34.3 26.9 24.7 17.6 7.2

Polymer names of “bio-PC-S:P-100:0”, “bio-PC-S:P-66.7:33.3”, “bio-PC-S:P-50:50”, “bio-PC-S:P-33.3:66.7”, and “bio-PC-S:P-0:100” are the biopolycarbonates synthesized from CO2, SOTE monomers, and PO with SOTE monomer/PO at different volume ratios of 100:0, 66.7:33.3, 50:50, 33.3:66.7, and 0:100, respectively. a

9019

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the continuous connection of SOTE monomers and CO2. A successful terpolymerization was demonstrated. The mechanical properties and molecular weight of the biopolycarbonates obtained from the terpolymerization of CO2, the SOTE monomers, and PO are listed in Table 1. With an increase in the SOTE monomer content from 0% to 66.7%, the average molecular weight of the obtained biopolycarbonates dramatically decreased from 31 000 to 6200 g/mol. Meanwhile, the elongation at break of poly(propylene carbonate) increased significantly from 54.7% to 430.0%, indicating the good elasticity of the synthesized biopolycarbonates. On the other hand, as the PO content increased up to 50%, the tensile strength of the blended biopolycarbonates increased from 3.1 to 14.8 MPa, and the elongation at break was kept at 352%, which is comparable with mechanical properties of some commercial petroleum-based polycarbonate products (tensile strength of 9.12 MPa and elongation at break of 147.5%).8 A high transparency of the obtained biopolycarbonates from SOTE monomers and PO was also demonstrated. Figure 4e shows a purified biopolycarbonate-based film derived from SOTE monomers (50% v/v), PO, and CO2. Figure 7 shows the changes in the Tg and thermal degradation after introducing the SOTE monomers at different ratios. As the ratio of the SOTE monomers increased from 0% to 50%, the Tg of the obtained biopolycarbonates decreased from 34.5 to 23.0 °C, (Figure 7a), and the thermal stability increased from 225 to 275 °C (Figure 7b). Moreover, the thermal stability of the blended biopolycarbonates was superior to that of poly(propylene carbonate), possibly because the addition of the SOTE monomers inhibited the generation of ether linkages and led to a higher thermal stability range. In summary, green biopolycarbonates, exhibiting competitive tensile strength and excellent elongation as well as high thermal stability, were achieved via synthesizing renewable CO2 and SOTE monomers, providing a new method that has the potential to replace petroleum-based propylene oxide.

Figure 6. 1H NMR spectra of biopolycarbonates obtained from the terpolymerization of SOTE monomers, PO, and CO2. (Parts a, b, c, d, and e are the spectra of polycarbonates synthesized from terpolymerization of CO2, SOTE monomers, and PO with SOTE/ PO at different volume ratios of 66.7:33.3, 50:50, 33.3:66.7, 100:0, and 0:100, respectively.)

exhibits unique characteristic hydrogen signals which are combination of the characteristic sites of individual SOTEbased polycarbonates and PO-based polycarbonates. The ratio between the integrated areas of peak m (∼5.01 ppm) to b (∼5.13 ppm) of the first three spectra (Figure 6a−c) seems to be in proportion to the molar ratios between SOTE monomer and PO, which was based on the spectrum of crude products. However, according to the results of solvent extraction, the byproducts were considered to be mainly from SOTE monomers as discussed above, assuming that the conversion of PO to polycarbonate was over 90% as aforementioned.26−28 Comparatively, the activity of PO to polycarbonates was relatively higher than that of SOTE monomer to polycarbonates because of the production of byproducts.29 These results confirmed a successful insertion of PO monomer into

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CONCLUSIONS A sustainable approach to synthesize biobased polycarbonates was successfully developed, with a yield of up to 83.3%, via copolymerization of SOTE monomers of 0.01 mol and CO2 in the presence of catalyst SalenCoCl (0.04 mmol) and cocatalyst

Figure 7. DSC (a) and TGA (b) curves of the biopolycarbonates obtained from the terpolymerization of SOTE monomers, PO, and CO2. (Curves a, b, c, d, and e are the curves of polycarbonates synthesized from terpolymerization of CO2, SOTE monomers, and PO with SOTE/PO at different volume ratios of 100:0, 66.7:33.3, 50:50, 33.3:66.7, and 0:100, respectively.) 9020

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Research Article

ACS Sustainable Chemistry & Engineering

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PPNCl (0.04 mmol) at room temperature after a reaction of 24 h. The targeted biopolycarbonates exhibited good thermal stability (T5% = ∼225 °C) and attractive mechanical properties, which indicated its potential for soft-material applications. More importantly, the original double bonds maintained on the linear chains of soybean oil were retained in the biopolycarbonates, rendering it a unique, promising, and versatile platform for various applications. Introduction of the SOTE monomers to the synthesis of PO-based polycarbonate system produced biopolycarbonates with desirable properties, including strong tensile strength, excellent elongation, and high transparency. The green synthesis route of incorporation of CO2 to soybean oil provides a platform for using various biobased feedstocks to produce biopolycarbonates that can be used for a number of applications. For further improvement of the mechanical properties of obtained biopolycarbonates, an introduction of epoxy curing agents into the copolymerization system will be investigated in our future study.

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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.7b01819. Additional FTIR and 1HNMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 330 263 3855. Fax: +1 330 263 3670. E-mail: [email protected]. ORCID

Yebo Li: 0000-0002-6198-5224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (2012-38202-19288) and united soybean board (1640-612-6287). The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b01819 ACS Sustainable Chem. Eng. 2017, 5, 9014−9022

Research Article

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DOI: 10.1021/acssuschemeng.7b01819 ACS Sustainable Chem. Eng. 2017, 5, 9014−9022