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A sustainable approach for the synthesis of biopolycarbonates from carbon dioxide and soybean oil Shaoqing Cui, Yusheng Qin, and Yebo Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01819 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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ACS Sustainable Chemistry & Engineering

A sustainable approach for the synthesis of bio-polycarbonates from carbon dioxide and soybean oil Shaoqing Cui1, Yusheng Qin2, Yebo Li1,3* 1

Department of Food, Agricultural and Biological Engineering, Ohio State University/Ohio

Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA 2

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese

Academy of Sciences, Renmin Street 5625, Changchun 130022, PR China 3

Quasar energy group, 8600 E. Pleasant Valley Rd, Independence, OH 44131, USA

* Corresponding Author. Tel.: + 1 330 263 3855; Fax: + 1 330 263 3670. E-mail address: [email protected] (Y. Li).

1

ACS Paragon Plus Environment


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Abstract A sustainable approach to produce bio-polycarbonates was developed via copolymerizing carbon dioxide (CO2) and soybean oil-based terminal epoxide (SOTE) monomers in the presence of catalyst SalenCoCl and co-catalyst 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 bio-polycarbonates demonstrated a 100% conversion and a bio-polycarbonate yield of 83.3%. It was also found that the original functional double bonds on the linear fatty acid chains were maintained in the bio-polycarbonate products, which affords a functional basis for various promising applications. The highest bio-polycarbonate yield was obtained at a CO2 pressure of 4 MPa; SOTE monomers of 0.01mol; catalyst loading SalenCoCl and co-catalyst 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 bio-polycarbonate film with stable thermal properties was obtained. With the incorporation of SOTE monomers into the synthesis of propylene oxide and CO2, competitive bio-polycarbonates 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 bio-polycarbonates from bio-based feedstocks and CO2 for various applications. Keywords: bio-based, polycarbonates; soybean oil, epoxide monomer, CO2 fixation

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Introduction In recent years, the demand for the sustainable use of renewable natural resources as raw chemical materials has increased due to oil reserves 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 polymers 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 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 petroleum-based chemical, thus synthesizing poly(propylene carbonate) still requires the use of limited natural resources. Therefore, the development of bio-based polycarbonates via synthesis of bio-based epoxides and CO2 is more attractive. A bio-polycarbonate 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

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important to note that these works have focused 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 bio-films 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 bio-feedstock 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 an alternating polycarbonate with a low Tg from bio-based 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 bio-epoxide with 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 EPCH, which can be

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obtained from crude glycerol, a main by-product of biodiesel production.20 To the best of our knowledge, there are no reports on the development of bio-based 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 bio-polycarbonates was developed via copolymerizing CO2 and SOTE monomers, which were obtained through the synthesis of soybean oil with epichlorohydrin. The objectives of the present work were to: (1) synthesize bio-polycarbonates with desirable properties from CO2 and soybean oil under moderate reaction conditions; (2) characterize the produced SOTE monomers and bio-polycarbonates; 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 bio-based feedstocks and CO2 for various applications. 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, USA). Dichloromethane (CH2Cl2), PO, ethanol, epichlorohydrin (EPCH), cetytrimethylammonium bromide (CTAB), and high-performance liquid chromatography grade tetrahydrofuran (THF) were purchased from Pharmco-AAPER (Shelbyville, KY, USA). Standard polystyrene was

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purchased

from

Agilent

Technologies

(Santa

Clara,

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CA,

USA).

(R,R)-(−)-N,N-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminocobalt(III)chloride (SalenCoCl) was prepared according to previous literature.19 Bis(triphenylphosphine)iminium chloride (PPNCl) was purchased from Alfa Aesar (Ward Hill, MA, USA) 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, 100.00g 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 1h. 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 6


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obtained after excess EPCH was removed using a vacuum rotary evaporator (Laborota 4001 OB, Hwidoph Instruments., Elk Grove Village, IL, USA).

Synthesis of bio-polycarbonates from SOTE monomers and CO2 The synthesis of the bio-polycarbonates from the SOTE monomers (0.01 mol, ~3.10 g) and CO2 (22°C, 4 MPa) was performed in a pre-dried 10 ml autoclave in the presence of catalyst SalenCoCl (0.04 mmol, 28 mg,) and co-catalyst 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°C, 45°C and 65°C) and three reaction times (12 h, 24 h and 48 h) with magnetic stirring at a stirring speed of 650 rpm. Solvent 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 bio-polycarbonates were obtained after drying. The transparent bio-polycarbonates films were obtained by applying the purified bio-polycarbonates on a Teflon sheet (25cm×25cm×1cm) using a film applicator at 6 miles gap clearance (Square Frame 4”, 0.5-6 miles, BYK Additives & instruments Inc., Columbia, MD, U.S), and were dried in air for one week.

Synthesis of bio-polycarbonates 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 7



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contents of the SOTE monomers in these three experiments were 0.0072 mol, 0.0054 mol and 0.0036 mol, respectively, and the contents of the PO were 0.0143 mol, 0.0214 mol and 0.0286 mol, respectively. Poly(propylene carbonate) was also 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 bio-polycarbonates Fourier transform infrared (FTIR) spectroscopy was performed on a Spectrum TwoTM IR spectrometer (Perkin Elmer Inc., MA, USA) 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, USA), 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, USA) equipped with an RID-10A refractive index detector, a 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 thermogravimeter (TA Instruments, New Castle, DE, USA) 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, USA) equipped with refrigerated cooling. The glass transition 8


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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, USA) with a crosshead speed of 25 mm/min. The bio-polycarbonate yield (%) was calculated from the integrated area ratio between the cyclic carbonates (by-products) peak and the crude bio-polycarbonates peak 1HNMR spectrum. Results and Discussion Synthesis of SOTE monomers from soybean oil As shown in scheme 2, the synthesis for the SOTE monomers 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 epoxy group to the terminal site of 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 low-cost bio-feedstock that is the main by-product 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 supporting information (Supporting Information. FTIR spectra of soybean oil (a) and SOTE monomer (b) illustrated in Figure S1, 1

HMR spectroscopy of obtained SOTE monomers illustrated in Figure S2.). In comparison

with the spectra of the soybean oil (Figure. S1(a)), the SOTE monomers spectra (Figure. S1(b)) exhibited two new characteristic absorbance peaks at 910 cm-1 and 855 cm-1, which were attributed to an epoxy group and indicated that the epoxy groups were successfully 9



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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 supplement file Figure. S2, the characteristic hydrogen atoms of the SOTE monomers were 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 bio-polycarbonates from SOTE monomers and CO2 A sustainable approach for the synthesis of bio-polycarbonates 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 bio-polycarbonate polymers then grew via the continuous insertion of CO2 into SOTE monomers, which was the desired route. Simultaneously, carbonate linkages can also undergo ring-closure to generate a cyclic carbonate which is considered to be a by-product thus its production should be minimized, as shown in part (b) of scheme 3. To achieve high selectivity of bio-polycarbonates, in this study, two different catalyst systems involving zinc glutarate (ZnGA) and SalenCoCl were investigated in order to determine the most efficient one. Of these two systems, the catalyst SalenCoCl with PPNCl as co-catalyst (molar ratio of 1:1) gave a better performance, enabling a carbonate linkage content of over 90%. The nature 10


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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 towards bio-polycarbonates 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 a terminal epoxide rings achieved a high yield of bio-polycarbonates (~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 bio-polycarbonates, rendering it to be a unique and versatile platform for different applications as discussed in the following sections. Characterization of bio-polycarbonates Bio-polycarbonates were successfully synthesized from SOTE monomers and CO2 in the presence of SalenCoCl (PPNCl as co-catalyst) with a satisfactory yield and an average molecular weight of approximately 7400 g/mol after purification. As shown in the FTIR spectra in Fig. 1(a), after copolymerization, the absorbance bands at 915 cm-1 and 855 cm-1 in the SOTE monomer spectra (Figure. S1 (b) in supplementary file), which corresponded to

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epoxide groups, disappeared completely on the spectrum of the obtained bio-polycarbonate 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 bio-polycarbonate spectrum (Fig. 1(a)), suggesting the successful formation of bio-polycarbonates. A small absorbance band at 1801 cm-1, attributed to the carbonyl groups of cyclic carbonate, indicated that a small amount of this by-product was produced. The change of chemical structures was also confirmed by 1H NMR spectroscopy in Fig. 1 (b). After copolymerization, the hydrogen signals of the epoxide structure indicated in Figure. S2 (supplementary file) disappeared; conversely, the hydrogen signals for the carbonate structure in the bio-polycarbonates appeared at a, ~4.5 ppm; b, ~5.12 ppm; and c, ~4.23 ppm, which is also consistent with the FTIR results. As by-products and residual impurities substantially affect the properties of polycarbonates, e.g., transparency and purification, it is necessary to obtain the purified bio-polycarbonate products.1 In this study, the cyclic carbonate by-product and catalysts were removed after purification via precipitation in ethanol, and the obtained purified bio-polycarbonates showed good transparency. Fig. 2 (a) shows the FTIR spectra of crude and purified bio-polycarbonates. The double bond peaks at 3008 cm-1 were clearly found in the spectra of both the crude and purified bio-polycarbonates, demonstrating that the original double bonds on fatty acid chains were kept in the bio-polycarbonate 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 (Fig. 2 (b)) because of the removal of compounds at low molecular weight. These results demonstrated the exact alternating insertion of CO2 and the

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successful synthesis of bio-polycarbonates via copolymerization of the SOTE monomers with CO2. In particular, the double bond functional groups maintained in the bio-polycarbonate product rendered it a unique platform for further versatile applications. Effects of operating parameters on bio-polycarbonate yield To determine the optimal reaction conditions, the effects of catalyst loading, reaction time, and temperature on bio-polycarbonate yield were investigated. As shown in Fig. 3 (a), increasing catalyst loading from 0.02 mmol 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 ring closure and reduced bio-polycarbonate yield. The effect of reaction time on yield is shown in Fig. 3 (b). Increasing the reaction time from 12 h 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 °C to 60 °C, the yield decreased sharply to 46.3%, indicating that the bio-polycarbonate was optimally synthesized under a moderate reaction condition at room temperature (Fig. 3 (c)). Higher temperatures might have promoted cyclic carbonates by-products formation and thus could have resulted in a low yield of bio-polycarbonates. 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 bio-polycarbonates (~83.3%) was achieved at a temperature of 22 °C (room temperature), a reaction time of 24 h, and a maximum catalyst loading of 0.04 mmol.

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Properties of bio-polycarbonates 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 Fig. 4 (a), the glass transition temperature (Tg) was 12 °C, and the obtained bio-polycarbonates were mostly amorphous. The Tg of the obtained bio-polycarbonates was slightly lower than that of conventional poly(propylene carbonate),1, 23 indicating moderate thermal properties. The low Tg might have been 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 Fig. 4 (b), the obtained bio-polycarbonates showed three weight loss stages, starting at 220 °C 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 bio-polycarbonates are listed in Table 1. Clearly, the obtained bio-polycarbonates 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 bio-polycarbonates product (Fig. 4 (c)) shows good elongation. Fig. 4 (d) shows the excellent transparency of obtained bio-polycarbonate polymers.

Synthesis of bio-polycarbonates from SOTE monomers, PO and CO2 PO is often used as an additive epoxide due to its high reaction activity with CO2 and its derived poly(propylene carbonate), which shows excellent biodegradability. However, its

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elongation at break is small under large molecular weight due to high hardness, as indicated in Table 1, which limits its use as a flexible material. Meanwhile, the bio-polycarbonates derived from SOTE monomers showed high elongation at break. The chemical structures and thermal properties of the resulting terpolymerized bio-polycarbonates from SOTE monomers, PO, and CO2 were characterized through FTIR and 1HNMR spectroscopy. As exhibited in the FTIR spectrum in Fig. 5 the carbonyl vibrational band of the different polycarbonates shifted from 1737 cm-1 (poly(propylene carbonate)) to 1741.61-1743.13 cm-1 (SOTE and PO monomer based bio-polycarbonates) and then 1743.71 cm-1 (SOTE monomers based bio-polycarbonates), 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 of 1737 cm-1 to 1743 cm-1. The typical chemical structures are indicated in Fig. 5. The carbonyl stretching mode of the cyclic carbonate by-product shifted from 1796 cm-1 to 1801 cm-1, suggesting that the backbone was altered. The chemical structures were also characterized by 1H NMR spectroscopy, as shown in Fig. 6. Obviously, the 1H NMR spectroscopy of bio-polycarbonates synthesized from CO2 and SOTE monomers (plot (d)) was different from that of polycarbonates synthesized from CO2 and PO (plot (e)). The former spectroscopy (plot (d)) shows the typical hydrogen signals of SOTE-based bio-polycarbonates: 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 (plot (e)) were confirmed by m, ~5.01 ppm and k, ~4.13.ppm. It is interesting

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to note that the spectroscopy of blended polycarbonates (plot (a), (b) and (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), exhibit unique characteristic hydrogen signals which are combination of the characteristic sites of individual SOTE-based 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 ((a), (b) and (c)) seems 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.

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Comparatively, the activity of PO to polycarbonates was

relatively higher than that of SOTE monomer to polycarbonates due to the production of by-products.

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These results confirmed a successful insertion of PO monomer into the

continuous connection of SOTE monomers and CO2. A successful terpolymerization was demonstrated. The mechanical properties and molecular weight of the bio-polycarbonates obtained from the terpolymerization of CO2, the SOTE monomers, and PO are listed in Table 1. With an increase in the SOTE monomers content from 0% to 66.7%, the average molecular weight of the obtained bio-polycarbonates dramatically decreased from 31000 g/mol 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 bio-polycarbonates. On the other hand, as the PO content increased up to 50%, the tensile strength of the blended bio-polycarbonates increased from 3.1 MPa to 14.8 MPa and the elongation at break was kept

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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 bio-polycarbonates from SOTE monomers

and

PO

was

also

demonstrated.

Fig.

4

(e)

shows

a

purified

bio-polycarbonate-based film derived from SOTE monomers (50% v/v), PO and CO2. Fig. 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 bio-polycarbonates decreased from 34.5 °C to 23.0 °C, (Fig. 7 (a)), and the thermal stability increased from 225 °C to 275 °C (Fig. 7 (b)). Moreover, the thermal stability of the blended bio-polycarbonates 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 bio-polycarbonates, 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. Conclusions A sustainable approach to synthesize bio-based 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 co-catalyst PPNCl (0.04 mmol) at room temperature after a reaction of 24 h. The targeted bio-polycarbonates 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 17



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maintained on the linear chains of soybean oil were retained in the bio-polycarbonates, 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 bio-polycarbonates 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 bio-based feedstocks to produce bio-polycarbonates that can be used for a number of applications. To further improve the mechanical properties of obtained bio-polycarbonates, an introduction of epoxy curing agents into the copolymerization system will be investigated in our future study.

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Supporting information

Figure. S1 FT-IR spectra of soybean oil (a) and SOTE monomer (b)

Figure. S2 1HMR spectroscopy of obtained SOTE monomer

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Acknowledgements This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (No. 2012-38202-19288) and united soybean board (No. 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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coupling of CO2 and epoxides under mild conditions. Appl. Organimet. Chem. 2011, 25, 424-428. DOI: 10.1002/aoc.1781 (27) Duan, J.; Wang, J.; Feng, L.; Wang, L.; Gu, X. Pressure dependence of the CO2/propylene oxide copolymerization catalyzed by zinc glutarate. J. Appl. Polym. Sci. 2010, 118, 366-371. DOI: 10.1002/app.32399 (28) Song, P.F.; Xiao, M.; Du, F.G.; Wang, S.J.; Gan, L.Q.; Liu, G.Q.; Meng, Y.Z. Synthesis and properties of aliphatic polycarbonates derived from carbon dioxide, propylene oxide and maleic anhydride. J. Appl. Polym. Sci. 2008, 15, 4121-4129. DOI: 10.1002/app.28449 (29) Cohen, C. T.; Coates, G. W. Alternating copolymerization of propylene oxide and carbon dioxide with highly efficient and selective (salen)Co(III) catalysts: effect of ligand and cocatalyst variation. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 5182– 5191. DOI: 10.1002/pola.21606

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Tables and Figures Table 1 Molecular weight and mechanical properties of bio-polycarbonates obtained from the terpolymerization of CO2, SOTE monomers and PO Scheme 1 Synthesis of bio-polycarbonates from soybean oil and CO2 Scheme 2 Synthesis procedure for SOTE monomers Figure1. FTIR spectra (a) and 1HNMR spectra (b) of bio-polycarbonates synthesized from CO2 and SOTE monomers Scheme 3. Synthesis of bio-polycarbonates from CO2 and SOTE monomers Figure 2. FTIR spectra (a) and GPC curve (b) of synthesized bio-polycarbonates before and after purification Figure 3. Effects of catalyst loading, reaction time, and temperature on bio-polycarbonates yield Figure 4. DSC (a) and TGA (b) curves of the bio-polycarbonates synthesized from the SOTE monomers and CO2. Picture (c) and (d) are the crude bio-polycarbonate and purified bio-polycarbonate-based film synthesized from the SOTE monomers and CO2. Picture (e) is the bio-polycarbonates film derived from SOTE monomers, PO and CO2 Figure 5. FTIR of bio-polycarbonates obtained from the terpolymerization of SOTE monomers, PO and CO2. (Symbol 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. ) Figure 6. 1HNMR spectra of bio-polycarbonates obtained from the terpolymerization of

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SOTE monomers, PO and CO2. (Symbol (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.) Figure 7. DSC (plot a) and TGA (plot b) curves of the bio-polycarbonates obtained from the terpolymerization of SOTE monomers, PO and CO2. (Symbol 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 (a), 66.7:33.3 (b), 50:50 (c), 33.3:66.7 (d) and 0:100 (e), respectively.)

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Table 1 Molecular weight and mechanical properties of the bio-polycarbonates obtained from the terpolymerization of CO2, SOTE monomers and PO Polymer *

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 *

Bio-PC-S:P-0:100 *

*

Young’s modulus

Tensile strength

Elongation

Mn (g/mol)

PDI

(MPa)

(MPa)

at break (%)

77.4±8.2

3.1±1.2

623±34.3

6213

1.16

101.3±14.2 207±20.6

4.7±1.4

430±26.9

10381

1.08

14.8±2.1

352±24.7

17430

1.06

243±14.3

16.9±2.1

253±17.6

30055

1.03

318±41.8

22.7±2.5

54.7±7.2

98735

2.07

Polymer name 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 bio-polycarbonate 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.

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Scheme 1 Synthesis of bio-polycarbonates from soybean oil and CO2

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Scheme 2 Synthesis procedure of SOTE monomers

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

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Scheme 3 Synthesis of bio-polycarbonates from CO2 and the SOTE monomers

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

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Figure 3. Effects of catalyst loading, reaction time and temperature on bio-polycarbonates yield

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

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Figure 5. FTIR of bio-polycarbonates obtained from the terpolymerization of SOTE monomers, PO and CO2. (Symbol 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. )

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Figure 6 1H NMR spectra of bio-polycarbonates obtained from the terpolymerization of SOTE monomers, PO and CO2. (Symbol (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. )

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Figure 7. DSC (plot a) and TGA (plot b) curves of the bio-polycarbonates obtained from the terpolymerization of SOTE monomers, PO and CO2. (Symbol 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 (a), 66.7:33.3 (b), 50:50 (c), 33.3:66.7 (d) and 0:100 (e), respectively.)

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For Table of Contents Use only

Synopsis Bio-polycarbonates possessing attractive optical, mechanical properties were synthesized from CO2 and soybean oil providing a platform for utilizing renewable bio-/feedstocks.

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Sustainable Approach for the Synthesis of ... - ACS Publications

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