Research Article pubs.acs.org/journal/ascecg
Degradable Thermosets Derived from an Isosorbide/Succinic Anhydride Monomer and Glycerol Perry A. Wilbon, Jeremy L. Swartz, Nina R. Meltzer, Jacob P. Brutman, Marc A. Hillmyer,* and Jane E. Wissinger* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *
ABSTRACT: Isosorbide is a renewable chemical of considerable interest as a monomer and monomer precursor due to its potential use in replacements for fossil-fuel derived polymers. In the present study, a facile microwave-assisted condensation of isosorbide with succinic anhydride was developed that dramatically reduced the reaction time. The resulting isosorbide disuccinic acid derivative (I-S-2) was polymerized under solvent-free conditions with glycerol to produce a renewable, cross-linked polyester with high modulus and appreciable thermal stability. Inclusion of 13 wt % or more of low molar mass hydroxy-telechelic poly(ethylene oxide) (PEO) (Mn = 300 g/mol) produced materials with a notable decrease in modulus and glass transition temperature. Degradation studies at 50 °C in acidic and basic solutions demonstrated the ability of the I-S-2 thermosets to be readily hydrolyzable. Furthermore, the resulting aqueous degradation solutions can be concentrated and reheated to produce new materials, albeit with a reduction in tensile properties. These I-S-2/glycerol thermosets represent economic, sustainable materials with tunable mechanical properties. KEYWORDS: Microwave-assisted reaction, Renewable, Isosorbide, Thermoset polymers, Glycerol, Poly(ethylene oxide)
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INTRODUCTION Thermosets are chemically cross-linked polymers with a threedimensional network, providing excellent mechanical strength, thermal stability, and solvent resistance.1 These polymers are used in numerous applications and make up almost one-third of the polymer industry.2 However, the desirable properties of thermosets are also responsible for their resistance to recycling and/or degradation. To overcome this deficiency, a range of reprocessable cross-linked polymers have been developed.3−9 However, it may be more desirable for a product to be degradable in situations where there is little incentive to recycle. Indeed, only about 14% of plastics are collected for recycling with only about 2% recycled into plastics of similar quality.10 Thus, there is a need for degradable polymeric materials and even more so for cross-linked polymers. Success in developing degradable plastics has been mainly based on renewably derived aliphatic polyesters such as poly(lactide).11−13 The polyester functionality of these thermoplastics is particularly attractive because of its susceptibility to enzymatic or hydrolytic degradation under physiological conditions. On the other hand, developing degradable materials that are durable enough for more demanding applications is also an important research endeavor. Therefore, an opportunity exists to develop resilient thermosets that are degradable at the end of their life cycle. An interesting chemical feedstock for degradable thermosets is isosorbide, a small molecule derived from glucose.14,15 Produced on an industrial scale, isosorbide has garnered much © 2017 American Chemical Society
attention as a renewable starting material for commercial polymer production.16,17 For instance, Mitsubishi produces an isosorbide-based polycarbonate called Durabio. Due to its excellent durability and optical clarity, Durabio can be used in applications such as front panels for smartphones.18,19 Numerous other isosorbide-based polymers exist in the literature, including polyesters, polyurethanes, and poly(meth)acrylates; many of these polymers are also hydrolytically degradable owing to the high polarity of the isosorbide moiety.14,15,20−23 The rigid, bicyclic structure of isosorbide tends to impart a high glass transition temperature (Tg) and modulus in the resulting materials. Due to these desirable properties, isosorbide has been proposed as a green alternative to bisphenol A.16,18 We describe a new, convenient, and efficient synthetic method for the condensation of isosorbide and succinic anhydride to give the corresponding diacid (I-S-2) using microwave technology as opposed to I-S-2 prepared via conventional heating, which requires long reaction times to achieve high conversions.24 We then produced degradable thermosets through the reaction of I-S-2 and glycerol. Furthermore, low molar mass telechelic poly(ethylene oxide) was incorporated into the materials to tune the material properties. Finally, we studied the mechanical properties, Received: June 26, 2017 Revised: August 13, 2017 Published: August 31, 2017 9185
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weighed (W2) again to determine the amount of insoluble material. Gel fraction was calculated as W2/W1. Hydrolytic degradation experiments were performed by immersing free films (0.70 g) in deionized (DI) water, 1 M NaOH and 6 M NaOH aqueous solutions, or 1 M HCl and 6 M HCl aqueous solutions at room temperature and 50 °C. The degradation times are the times for the samples to be completely dissolved in the solutions (forming transparent solutions). Microwave Synthesis of I-S-2. Monomer synthesis was performed in a CEM Discover SP microwave reactor equipped with a calibrated infrared (IR) temperature sensor (maximum pressure = 435 psig, maximum power = 300 W). A 35 mL microwave reaction vial was charged with isosorbide (6.2 g, 0.0425 mol), succinic anhydride (8.8 g, 0.0879 mol), and tin(II) 2-ethylhexanoate (0.17 g, 0.42 mmol). The vial was capped and flushed with nitrogen before being placed in a CEM Discover SP microwave synthesizer. The sample was then heated successively at 100 °C for 1 min, 110 °C for 1 min, and 140 °C for 3 min. Unreacted succinic anhydride was removed from the product by vacuum sublimation to yield a clear, viscous oil at room temperature in >99% yield. 1H NMR integration indicated ∼6% unreacted isosorbide still remained. IR (neat): νmax = 1729, 1707 cm−1; 1H NMR (400 MHz, CDCl3): δ = 6.92 (s, 2H), 5.22 (s, 1H), 5.19 (q, J = 5.2 Hz, 1H), 4.85 (t, J = 5.2 Hz, 1H), 4.49 (d, J = 4.6, 1H), 4.00−3.92 (m, 3H), 3.84 (dd, J = 9.9, 5.1 Hz,1H), 2.71 (s, 4H), 2.69−2.62 (m, 4H) ppm; 13C NMR (100 MHz, CDCl3): δ = 177.30, 177.72, 171.62, 171.26, 85.79, 80.73, 78.25, 74.29, 73.21, 70.39, 28.89, 28.86, 28.83, 28.61 ppm; MS (ESI-TOF, m/z) [M + Na+] expected = 369.0792; found = 369.0788. Polymerization of I-S-2 and Glycerol. Crude I-S-2, which still contained the tin catalyst, (2.0 g, 5.8 mmol) and glycerol (0.35 g, 3.9 mmol) were placed in a 4.4 × 1.3 cm aluminum pan and heated on a hot plate at 80 °C until a homogeneous liquid was obtained. The reaction was then heated in an oven under a nitrogen atmosphere at 100 °C for 1 h, 120 °C for 2 h, and 150 °C for 2 h. The reaction was postcured for 12 h at 160 °C in a drying oven. To produce dogbones for mechanical testing, the polymers were soaked in acetone for 2 h at room temperature. The softened samples were cut with a tensile bar dye and allowed to dry for 12 h at 160 °C in a drying oven. Polymerization of I-S-2, Glycerol and Poly(ethylene oxide). The following is an illustrative example: crude I-S-2 (2.0 g, 5.8 mmol), glycerol (0.28 g, 3.1 mmol), and poly(ethylene oxide) (0.35 g, 1.2 mmol) were placed in an aluminum pan and heated on a hot plate at 80 °C until a uniform liquid was obtained. The reaction was then heated under a nitrogen atmosphere at 100 °C for 1 h, 120 °C for 2 h, and 150 °C for 6 h. The reaction was postcured for 12 h at 160 °C in a drying oven. To produce dogbones for mechanical testing, the polymers were soaked in acetone for 2 h at room temperature. The softened samples were cut and allowed to dry for 12 h at 160 °C in a drying oven. The ratio of −COOH/−OH was 1:1 for all samples. The amount of PEO varied with targeted incorporation of 7, 13, and 24 wt %. (See SI for polymerization conditions for 7 and 24 wt % polymer samples)
hydrolytic degradability, and reprocessing potential of the isosorbide-based materials.
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EXPERIMENTAL SECTION
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CHARACTERIZATION
Materials. All materials were used as received unless otherwise noted. Isosorbide with polymer grade purity (∼99%) was provided by Archer Daniels Midland Company. Succinic anhydride and glycerol with assayed purity of ∼99% were purchased from Fisher Scientific. Poly(ethylene oxide) (300 g/mol) and tin(II) 2-ethylhexanoate (Sn(Oct)2) were purchased from Aldrich. General-purpose aluminum weighing dishes (I.D. × D. 4.4 × 1.3 cm) were purchased from VWR.
1
H NMR and 13C NMR spectroscopy experiments were performed in CDCl3 (99.8%, Cambridge Isotope Laboratories) on a Bruker Avance III HD nanobay AX-400 spectrometer (Billerica, MA) at 400 and 100 MHz, respectively, equipped with a 5 mm BBO SmartProbe and referenced to the CHCl3 peak at 7.26 ppm. FT-IR was performed using a Bruker Alpha Platinum ATR spectrometer (Billerica, MA). High-resolution mass spectrometry was performed using a Bruker BioTOF II (Billerica, MA) in positive mode ESI. Uniaxial tensile testing was conducted using dogbone shaped tensile bars (ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and a gauge length of 14 mm) on a Shimadzu Autograph AGS-X Series tensile tester (Columbia, MD) at 22 °C with a uniaxial extension rate of 5 mm/min. Young’s modulus (E) values were calculated using the Trapezium software by taking the initial stress−strain curve from 0 to 1% strain. Reported values are the average of at least five samples. Dynamic mechanical thermal analysis (DMTA) was performed on a TA Instruments RSA G2 analyzer (New Castle, DE) utilizing dogbone shaped tensile bars (ca. 0.5 mm (T) × 3 mm (W) × 25 mm (L) and a gauge length of 14 mm). The axial force was adjusted to 0 N and a strain adjust of 30% was set with a minimum strain of 0.05%, a maximum strain of 5%, and a maximum force of 1 N in order to prevent the sample from buckling or going out of the specified strain. Furthermore, a force-tracking mode was set such that the axial force was twice the magnitude of the oscillation force. A temperature ramp was then performed from −10 to 200 °C at a rate of 5 °C/min, with an oscillating strain of 0.05% and an angular frequency of 6.28 rad S-3 s −1 (1 Hz). The Tg was calculated from the maximum value of the loss modulus (G″). The molar mass between cross-links (Mx) was estimated using eq 1:
E′(T ) = 3G′(T ) = 3RTνe =
3ρRT Mx
(1)
where E′ and G′ are the storage modulus from tensile and shear rheology, respectively, R is the universal gas constant, T refers to the absolute temperature in the rubbery region (ca. 298 K), νe is the crosslink density, and ρ is the density of the cross-linked materials which was determined by measuring the volume and mass of each polymer sample in triplicate. Note: In the glassy regimes, the materials often resulted in overload of the transducer, resulting in low accuracy in this regime as indicated by the noise in the data up to the glass transition. Thermal properties were evaluated using differential scanning calorimetry (DSC) on a TA Instruments Discovery DSC (New Castle, DE). Approximately 5 mg of sample were loaded into hermetically sealed aluminum pans under a nitrogen purge of 50 mL min−1. Materials were heated to 120 °C to erase thermal history, cooled to −80 °C at 10 °C min−1, and heated to 120 °C at 10 °C min−1. Glass transition temperatures (Tg) are reported upon the second heating curve. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 (New Castle, DE) at a heating rate of 10 °C min−1 from 25 to 550 °C. The gel content of the polymer samples was determined using acetone extraction. Cured polymer samples (with a thickness of 1.5− 1.8 mm) weighing between 0.1 and 0.3 g (W1) were subjected to Soxhlet extraction by acetone for 24 h, after which they were dried and
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RESULTS AND DISCUSSION I-S-2 was previously utilized as an intermediate in polyurethane synthesis and as a tackifier.23,25 However, the synthesis required long reaction times of up to 24 h at 120 °C and resulted in a final product with discoloration due to deleterious side reactions. We sought to improve upon the reaction conditions and investigated the use of a microwave reactor for the synthesis of I-S-2. Microwave-assisted organic synthesis is commonly used in the pharmaceutical industry and provides two major advantages over traditional heating methods: (1) more efficient energy transfer in the reaction and (2) more uniform heating throughout the reaction vessel.26 Remarkably, the quantitative conversion of isosorbide and a slight excess of succinic anhydride to I-S-2 occurred in 10 min when utilizing a microwave (Scheme 1). However, the product obtained was discolored (light brown). We discovered that if the reaction was 9186
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purification (i.e., the crude reaction mixture) did not significantly affect the properties of the resulting thermosets. Initially, glycerol was chosen as the cross-linker as it is renewably derived and economical (Scheme 2).27−29 Oligomeric PEO was explored as an additive with glycerol in an attempt to modify the mechanical and thermal properties of the resulting thermoset (Scheme 2). Oligomeric PEO was chosen due to its well-known hydrophilic nature, which would facilitate mixing with the hydrophilic system, and its low transition temperature that would likely enhance the toughness of the final thermoset. The ratio of total hydroxyls (including the glycerol and PEO) to carboxylic acids was maintained at 1:1 in all experiments. The polymerization was initially performed under air; however, this resulted in highly discolored films, likely owing to the long curing time. (While the microwave proved to be suitable for the production of these materials, we were unable to easily produce films suitable for mechanical analyses using our equipment.) Performing the polymerization under N2 resulted in clear, colorless films. However, when the polymerization was heated directly to the final curing temperature (150 °C) under N2, discoloration was still observed. We determined that gradual ramping of the curing temperature from 100 °C (1 h) to 120 °C (2 h) to 150 °C (2 h) and finally to 160 °C (12 h) resulted in clear and colorless films. Isosorbide and I-S-2 will rapidly undergo degradation reactions at elevated temperatures, resulting in brown discoloration; however, the thermosets containing isosorbide have significantly improved thermal stability. Therefore, we posit that incorporation of the isosorbide moieties into the network increases their resistance to thermal degradation and discoloration. In samples containing PEO, the cure time at 150 °C was increased to achieve high gel fractions; the gel fractions were all above 0.94 (Table 1). To confirm the full conversion of reactive moieties, FT-IR spectra of the resulting films were acquired (Figure 2). The broad peak around 3000 cm−1, corresponding to the −OH stretching from the monomers, was significantly reduced when comparing the monomer to the polymer. This indicated high conversion of the acid groups to esters during the polymerization. TGA was also used to monitor the polymerization by quantifying the amount of water released (Figure S2). Slightly more mass was lost than expected which is likely due to residual water in the glycerol. Regardless, it still provides further evidence of high conversion. Tensile testing was used to determine the mechanical properties of the cross-linked samples. As seen in Table S1
Scheme 1. Synthesis of I-S-2
carried out under an inert atmosphere (N2) a clear and colorless product was formed with no change in the overall yield (Figure 1 and S1). The reaction could be further
Figure 1. I-S-2 synthesis under air (left) and under nitrogen (right).
optimized with the inclusion of 1 mol % Sn(Oct)2 which reduced the reaction time for quantitative conversion to 5 min and was advantageous when carried forward to the subsequent polymerization as no additional catalyst is required for subsequent polymerization. Attempts to perform the polymerization without catalyst were largely unsuccessful, often leading to discolored, brittle materials. The step condensation polymerization of I-S-2 with polyols was carried out in the bulk, in the presence of catalytic Sn(Oct) 2. Using I-S-2 directly as synthesized without Scheme 2. Polymerization of I-S-2 with Glycerol and PEOa
a
Structures are representative. 9187
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ACS Sustainable Chemistry & Engineering Table 1. Thermal and Mechanical Properties of I-S-2 Based Thermosets Wt% PEO
Gel fraction
Density (g/mL)a
E′ (MPa)b
Mx (kg mol‑1)c
Tg,DMTA (°C)d
Tg,DSC (°C)e
Td,Air (°C)f
Td,N2 (°C)f
0 7 13 23
0.98 0.98 0.96 0.97
1.25 1.33 1.36 1.38
7.9 6.8 4.6 3.8
1.59 1.90 2.69 3.24
68 54 34 21
55 45 29 7
312 284 323 290
313 307 324 308
Determined by way of triplicate mass and volume measurements. bPlateau modulus at 100 °C. cCalculated using E′ at 100 °C and eq 1. Determined from the maximum of the loss modulus (E″) from DMTA. eTaken on the second heating ramp at a rate of 10 °C min−1. fDefined as the temperature at which 5% mass loss is observed. a
d
due to lowering the amount of glycerol cross-linker as well as higher ductility of PEO (Figure 3). These results were confirmed by DMTA, whereby increasing the loading of PEO resulted in a decrease in the plateau modulus and, thus, the cross-link density (Figure S3). DMTA also confirmed the crosslinked nature of these materials as there was no reduction in modulus well past the Tg of each material. As expected, increasing amounts of PEO in the cross-linked polymer structure also led to a decrease in the Tg of the materials as determined by DMTA and DSC (Table 1 and Figure S4). Interestingly, the Td of the materials did not vary with any noticeable trend as a function of the PEO content (Figure S5). Thus, it is possible to effectively alter the mechanical and thermal properties of these materials for their specific application without undesirable decreases in their thermal stability. To determine the hydrolytic degradability of these materials, samples were soaked in various aqueous solutions and heated at 50 °C until the polymer films were completely tramsformed to a clear solution, with no visible pieces of film observable (Figure 4, Table S2). Nearly all the samples degraded in under
Figure 2. FTIR (offset to be visualized) of I-S-2 (black) and I-S-2/ glycerol/0 wt % PEO thermoset (red).
and Figure 3, the tensile properties were modified by varying the amount of PEO incorporated. Polymers composed of only I-S-2 and glycerol were the stiffest as the mechanical properties are dominated by the rigid, bicyclic structure of isosorbide while also having the highest cross-link density. The addition of PEO led to a significant decrease in the stiffness presumably
Figure 4. Degradation of I-S-2/glycerol sample with 7 wt % PEO in 1 M HCl at 50 °C.
5 h, while many of them achieved full degradation in less than 1 h. Acid degradation was slower for each polymer; however, it is generally accepted that ester bonds are more susceptible to hydrolytic cleavage under basic conditions. Furthermore, the hydrolytic degradation of polymers occurred in DI water over approximately 1 month. The amount of PEO also influenced the degradation times; increasing the amount of PEO led to significantly faster degradation rates which was likely due to a decrease in the cross-link density and an increase in the hydrophilicity of the polymer. The degradability of the materials was also tested at room temperature. The times to full degradation were much longer; however, the materials were still able to degrade over a two-month time period under acidic and basic conditions (Table S2). The hydrolytic susceptibility of these materials in ambient conditions could prove extremely useful for a variety of short-term applications. Based on a study involving the reprocessing of polyester thermosets via solvolysis in ethylene glycol, we sought to determine if our materials could undergo a similar process in water utilizing a microwave as the heating source.30 Thus, we
Figure 3. Comparisons of tensile stress−strain behavior for polymer samples with varying weight percent of PEO at 22 °C with a uniaxial extension rate of 5 mm/min. 9188
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ACS Sustainable Chemistry & Engineering utilized the microwave reactor to heat the 0 wt % PEO sample in water at 98 °C; the material required approximately 2.5 days at this temperature before it achieved full dissolution. The resulting solution was placed in an aluminum pan, and the water was allowed to evaporate at 60 °C. A 1H NMR spectrum of the hydrolysis product was indicative of a mixture of I-S-2, glycerol, succinic acid, isosorbide monoester, and isosorbide. Ideally, the hydrolysis product would solely consist of glycerol and I-S-2 such that a similar value material could be made from the degraded material; we hypothesized that the isosorbide ester would be far more difficult to cleave due to steric hindrance. However, the 1H NMR spectrum suggested that while the glycerol ester was more labile, some of the isosorbide esters were also cleaved (Figure S6). Regardless, we attempted to repolymerize the hydrolysis mixture under the same conditions that we produced the original material. Upon completion of the polymerization there was a noticeable mass loss (55% mass recovery), as well as a marked color change (Figure 5). Adding Sn(Oct)2 prior to polymerization decreased
Figure 6. Tensile stress−strain behavior for original (black) and reprocessed polymer (red) samples with zero weight percent PEO at 22 °C with a uniaxial extension rate of 5 mm/min.
acidic conditions, and increasing the PEO content further increased their rate of degradation. Furthermore, the hydrolysis mixture could be dried and repolymerized to form a new material, albeit with different tensile properties. The above results reveal the effectiveness of using isosorbide to synthesize degradable polymers and provide a green chemistry approach to producing biobased polymeric materials.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 5. 0 wt % PEO thermoset before reprocessing (left) and after reprocessing (right).
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02096. DMTA, TGA, DSC, and hysteresis data (PDF)
the mass loss (75% mass recovery), although there remained a noticeable change in color of the final polymer. We hypothesize that the mass loss and color change are likely due to the poor reactivity of isosorbide to esterify with carboxylic acids, which would allow the isosorbide to degrade before reincorporation into the network. Indeed, TGA of pure isosorbide reveals that thermal decomposition occurs around 150 °C (Figure S7). Examining the mechanical properties of the reprocessed material revealed a significant loss in the tensile properties (Figure 6 and Table S3). This could also be attributed to the degradation of isosorbide upon reprocessing. Removing the rigidity imparted by the isosorbide monomer should make the reprocessed material more flexible, which is observed in the noticeable decrease in the stress at break (Table S3). Regardless, the ability to reprocess these materials is still advantageous, even if it results in materials of lower quality.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.A.H.). *E-mail:
[email protected] (J.E.W.). ORCID
Marc A. Hillmyer: 0000-0001-8255-3853 Jane E. Wissinger: 0000-0002-9240-3629 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSF under the Center for Sustainable Polymers, CHE-1413862. Part of this work was carried out in the College of Science and Engineering Polymer Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Number DMR-1420013. We would also like to thank Dr. Erik Hagberg of ADM for providing the isosorbide used in this study.
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CONCLUSIONS In summary, we have reported an efficient and straightforward synthesis of isosorbide based thermoset polymers. Using a microwave reactor, the dicarboxylic acid monomer (I-S-2) composed of isosorbide and succinic anhydride was synthesized in high yield in under 5 min. This monomer was reacted with glycerol to produce a completely biobased thermoset polymer. These polymers displayed a high modulus attributed to the rigid structure of the isosorbide moiety. Oligomeric PEO (Mn = 300 g/mol) was added to the polymerization to increase the flexibility of the polymer films and decrease the cross-link density. All of the polymers could degrade under basic and
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