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Structure/property relationships in copolymers comprising renewable isosorbide, glucarodilactone, Cite this: DOI: 10.1039/c7py00575j and 2,5-bis(hydroxymethyl)furan subunits†

Leon M. Lillie, William B. Tolman * and Theresa M. Reineke *

Carbohydrates and their derivatives have great potential as building blocks for the development of renewable materials that are cost and performance competitive with conventional petroleum-based materials.

D7ZLQ&LWLHVRQ To this end, the rigid carbohydrate-derived diols, glucarodilactone and isosorbide were functionalized with a castor oil derivative, 10-undecenoic acid, forming α,ω-dienes suitable for acyclic diene metathesis (ADMET) polymerization. Equimolar copolymerizations of these two monomers, glucarodilactone undecenoate (GDLU) and isosorbide undecenoate (GDLU), transform brittle homopolymers into elastic materials with shape memory capabilities. To understand the source of these properties, a series of copolymers consisting of a range of GDLU and IU compositions (100 : 0, 77 : 23, 76 : 24, 52 : 48, 48 : 52, 18 : 82, 0 : 100 mol percent GDLU to IU) was synthesized. The material and chemical properties were characterized by uniaxial tensile testing, small-amplitude oscillatory shear rheology, X-ray scattering and hydrolytic degradation testing. Small compositional changes in this family were shown to have a drastic impact on the observed mechanical performance and degradability of these materials. Rheological measurements of GDLU-containing copolymers found evidence for the presence of transient networking within these materials. We posit that the transient network within this material is responsible for the elasticity and shape memory abilities of the GDL-containing materials. In conﬁrmation of these properties, Received 6th April 2017, anovelα,ω-diene 2,5-bis(hydroxymethylfuran) undecenoate (BHMFU) was developed and served as a Accepted 11th May 2017 direct replacement for either GDLU or IU in copolymerizations. The replacement of GDLU resulted in a DOI: 10.1039/c7py00575j total loss of the elasticity and shape memory of the copolymers, supporting the role of GDLU in the rsc.li/polymers mechanical performance. 3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW Introduction sorbides and examined as adhesives.9 Incorporation of isosorbide facilitates polymers and blocks within these structures 3,6–10 Polyesters have attracted widespread attention due to their with high glass transition temperatures (Tg). tunable mechanical properties and potential susceptibility to In previous work, we have focused on incorporating a less- 1–4 hydrolytic degradation. Obtaining polyesters from novel bio- studied structural analog of isosorbide, D-glucaro-1,4:6,3-di- derived monomers is of great interest to meet consumer lactone (GDL, Scheme 1) into polyesters.11 Previous reports demands for high-performance and sustainable materials.5 exploring GDL have been limited to hydroxylated nylons,12,13 Isosorbide, a rigid bicyclic sugar derivative directly derived polyurethanes,14,15 and methacrylate-based thermosets.16,17 – from glucose, has found use in a wide array of materials.6 8 Its The latter work described the formation of a GDL-containing ability to serve as a direct replacement for rigid diols like α,ω-diene (GDLU) through the functionalization of GDL with bisphenol A (BPA) has encouraged its use for step growth poly- the biofeedstock 10-undecenoic acid. This sustainable merizations to create sustainable polycarbonates, polyesters, α,ω-diene and its congener, isosorbide undecanoate (IU), are and polyurethanes.6 Moreover, this structure has been functio- well suited for acyclic diene metathesis (ADMET) polymeriz- nalized and used to create block polymers with pendant iso- ation, which yielded linear polyesters of moderate molecular − weight (30–60 kg mol 1).11 In the case of GDLU, ADMET polymerization allowed for the polymerization chemistry to Department of Chemistry and Center for Sustainable Polymers, University of occur away from the dilactone core, mitigating potential degra- Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431, USA. dation that was observed with conventional condensation E-mail: [email protected], [email protected] 11,15,16 †Electronic supplementary information (ESI) available. See DOI: 10.1039/ polymerization methods. Yet, the reactivity of the intact c7py00575j GDL core served an important function, facilitating degra-

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and shape memory properties into this material class, and that these properties are critically dependent on the composition. Herein, we also demonstrate more economical and environmentally-friendly routes for the synthesis of GDLU and IU feedstocks.

Experimental Materials

Calcium D-saccharate tetrahydrate was obtained from Carbosynth Limited (Compton, Berkshire, UK). D-Glucaro- 1,4:6,3-dilactone was synthesized using previously reported 18 procedures. Deuterated chloroform (CDCl3) was obtained from Cambridge Isotope Laboratories (Tewskbury, MA, USA). 2,5-Bis(hydroxymethyl)furan was purchased from Polysciences Inc. (Warrington, PA, USA). 10-Undecenoyl chloride was

D7ZLQ&LWLHVRQ purchased from ACROS Organics, through Fisher Scientific (Pittsburgh, PA, USA). All other reagents were purchased from Sigma-Aldrich (St Louis, MO, USA). The purchased compounds were used directly without further purification. All glassware was oven-dried prior to use.

Scheme 1 Synthesis of monomers and polymers. Synthesis conditions: Characterization methods (a) esteriﬁcation of rigid diols with 10-undecenoic acid, 10-undecenoyl 1H NMR and 13C NMR spectra were obtained in deuterated chloride, or 10-undecenoic acid anhydride, (b) ADMET polymerization chloroform on a Bruker Avance HD-500. 1H NMR spectra were using Grubbs’ second generation catalyst (1.0 mol percent) and 1 mol% methyl 10-undecenoate in toluene (0.33 M) at 80 °C for 16 h under obtained with at least 64 scans with a 5 second acquisition vacuum. time and 10 second delay time. All spectra were referenced to

tetramethylsilane (TMS). Polymer molecular weights (Mw, Mn) and dispersities (Đ) were calculated after performing size dation upon exposure of the polymer to strong basic hydrolytic exclusion chromatography (SEC) using an Agilent 1260 Infinity conditions.11,15 Additionally, we showed in preliminary studies instrument with a Wyatt DAWN Heleos II 10-angle light that the thermal and mechanical behavior of these polyesters scattering detector at 662.6 nm and a Wyatt Optilab EX RI vary based on the identity of the building block (IU vs. GDLU, detector. The instrument was setup using Waters Styragel Scheme 1). Notably, we observed elasticity and shape memory HR6, HR4, HR1 columns with a tetrahydrofuran mobile phase −1

3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW in the 50 : 50 copolymer of GDLU and IU. at 25 °C with a 1.0 mL min flow rate. Inductively coupled

Herein, we present the results of studies aimed at answer- plasma mass spectrometry (ICP-MS) of P(GDLU52-co-IU48)was ing two main questions that arose from our previous work. performed by Robertson Microlit Laboratories. First, how does changing the ratio of GDLU and IU influence Thermogravimetric analysis (TGA) was performed on a TA the thermal, chemical, and mechanical properties of the Instruments Q500 under a nitrogen atmosphere with a heating − ADMET-derived copolymers? To address this question, we syn- rate of 10 °C min 1 using samples of 8–15 mg. Samples were thesized a family of copolymers with varying ratios of GDLU heated from room temperature (RT) to 550 °C. Differential and IU and characterized the analogs through thermo- Scanning Calorimetry (DSC) was performed on a TA gravimetric analysis, differential scanning calorimetry, size- Instruments Q-1000. Samples were tested in hermetically exclusion chromatography, uniaxial extensional testing, hydro- sealed aluminum pans. Each sample was equilibrated to − lytic degradation testing, small-angle oscillatory shear rheol- −50 °C and then heated to 125 °C at a rate of 10 °C min 1. The − ogy, and synchrotron X-ray scattering measurements. Second, samples were then cooled to −50 °C at a rate of −10 °C min 1. − we sought to answer the question: which carbohydrate build- Lastly, the samples were heated to 150 °C at 10 °C min 1. ing block was most important in promoting the elasticity and Glass transition temperatures and melting temperatures were shape-memory abilities observed with this class of materials? measured during the second heating ramp. To address this question, we replaced either GDL or isosorbide Tensile testing was performed on a Shimadzu AFS-X tensile with a different sustainable diol, 2,5-bis(hydroxymethyl)furan tester at room temperature with tensile bars possessing (BHMF), and synthesized the homopolymer of 2,5-bis(hydroxy- approximate gauge dimensions of 10 mm × 4 mm × 0.2 mm. − methyl)furan undecenoate (BHMFU), along with select compo- Samples were each extended at 50 mm min 1 and at least six sitions of BHMFU : GDLU and BHMFU : IU copolymers. We replicate runs were performed for each polymer sample. found that GDLU is responsible for imparting both elasticity Hysteresis testing was performed on the same Shimadzu AFS-X

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− tensile tester at 50 mm min 1 to either 67% or 200% strain, allowed to reflux for 2.5 h. The reaction was then removed then relaxed to zero percent strain at the same rate. This de- from the heating mantle and allowed to cool to RT. Excess formation was then cycled 20 times. Small-amplitude oscillatory acetic anhydride and acetic acid were removed with reduced shear experiments were performed on a Rheometrics Scientific pressure. No mixed anhydride formation was observed and ARES Classic rheometer with the 8 mm diameter parallel plate there was quantitative conversion of 10-UA during the reaction. geometry. This instrument has a forced convection oven with a The product was obtained as a light yellow oil (95 g, 95%). nitrogen atmosphere. Roughly 50 mg of each sample was loaded 1H NMR (500 MHz, chloroform-d) δ 5.79 (ddt, J = 17.0, 10.2, between the plates, to obtain a gap of approximately 1 mm, and 6.6 Hz, 2H), 5.04–4.86 (m, 4H), 2.50–2.33 (m, 4H), 2.03 (tdd, J = heated to 80 °C to form a uniform sample disk. Frequency 8.2, 6.0, 1.6 Hz, 4H), 1.65 (p, J = 7.3 Hz, 4H), 1.46–1.04 − sweeps were performed in the 0.1 ≤ ω ≤ 100 rad s 1 frequency (m, 21H). 13C NMR (126 MHz, chloroform-d) δ 169.72, 139.23, range at strains selected to maximize sample torque while 114.29, 77.42, 77.16, 76.91, 35.39, 33.89, 29.34, 29.25, 29.14, staying in the linear viscoelastic regime (1–10%). Temperature 28.99, 28.96, 28.95, 24.33. − − sweeps were performed at 3 °C min 1 at a frequency of 1 rad s 1 Improved synthesis of glucarodilactone undecenoate (GDLU) from 5 to 10 °C above the Tg to where the minimum torque limit was observed for the instrument (0.02 N). GDL (5.2 g, 0.03 mol, 1 equiv.) was added to an oven-dried Small-, middle-, and wide-angle X-ray scattering (SAXS, 250 mL RBF along with 10-undecenoic anhydride (22 g, MAXS, WAXS) experiments using synchrotron radiation were 0.062 mol, 2.1 equiv.). Acetonitrile (100 mL) was then added to

D7ZLQ&LWLHVRQ performed at the DND-CAT 5-ID-D beamline at the Advanced the RBF, forming a heterogeneous solution with GDL. The Photon Source located at the Argonne National Lab (Argonne, flask was sealed with a septum, placed under nitrogen, and IL). An X-ray wavelength of 0.729 Å was used and data were placed in an ice water bath prior to catalyst addition. collected using Rayonix area CCD detectors. 2D scattering Scandium triflate (0.15 g, 0.0003 mol, 0.01 equiv.) was patterns were integrated to obtain 1D plots of intensity versus measured into a scintillation vial and dissolved in 5 mL aceto- πλ−1 θ scattering wave vector q, where q =4 sin( /2). nitrile. The Sc(OTf)3 solution was added via syringe to the chilled reaction flask. The reaction, monitored via TLC (silica Synthesis of isosorbide undecenoate (IU) gel, CH2Cl2 eluent), was completed within 10 min. The solvent Isosorbide (15 g, 0.10 mol, 1 equivalent (equiv.)) and 10-un- was removed in vacuo to obtain a yellow viscous oil. The crude decenoic acid (40 g, 0.22 mol, 2.1 equiv.) were added to a GDLU was purified via column chromatography (silica gel, 500 mL round bottom flask (RBF). Toluene (100 mL) followed hexanes : DCM = 1 : 1) and was obtained as a white waxy solid by p-toluenesulfonic acid (1.0 g, 0.0050 mol, 0.050 equiv.) were (7.6 g, 51%). then added to the flask. The flask was equipped with a heating 1H NMR (500 MHz, chloroform-d) δ 5.86–5.75 (m, 2H), 5.49 mantle and Dean–Stark apparatus. The reaction was heated to (t, J = 6.1 Hz, 1H), 5.34 (d, J = 6.7 Hz, 1H), 5.12 (d, J = 5.2 Hz, reflux under stirring and the reaction was monitored via the 1H), 5.02–4.90 (m, 5H), 2.46 (dt, J = 18.6, 7.5 Hz, 4H), 2.04 (q, volume of water collected in the Dean–Stark apparatus. After J = 7.1 Hz, 4H), 1.72–1.58 (m, 4H), 1.33 (dd, J = 36.2, 5.7 Hz, completion of the reaction (approximately 3 h), the RBF was 22H). 13C NMR (126 MHz, chloroform-d) δ 173.45, 172.93, then removed from heat and allowed to cool to room tempera- 169.18, 169.00, 139.34, 139.26, 114.33, 114.27, 78.98, 77.41,

3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW ture. The remaining solvent was evaporated under reduced 77.36, 77.16, 76.91, 74.56, 73.07, 67.25, 33.92, 33.90, 33.45, pressure. The crude product mixture was purified via crystalli- 33.26, 29.35, 29.33, 29.23, 29.21, 29.17, 29.14, 29.01, 29.00, zation from cold hexanes (−20 °C) using a 1 : 9 crude product 24.75, 24.71. Melting point = 55.0 °C. to hexanes ratio. Crystallized IU was decolored via filtration through silica gel. The pure product was obtained as a color- Synthesis of 2,5-bis(hydroxymethyl)furan undecenoate less oil. Yield: 23 g (46%). (BHMFU) 1H NMR (500 MHz, chloroform-d) δ 5.81 (s, 1H), 5.17 (d, J = In a 500 mL RBF equipped with a magnetic stirring bar, 25.8 Hz, 1H), 4.97 (d, J = 34.2 Hz, 2H), 4.83 (s, 1H), 4.47 2,5-bis(hydroxymethyl)furan (2.2 g, 17 mmol, 1 equiv.) was dis-

(s, 1H), 3.88 (d, J = 73.2 Hz, 2H), 2.34 (d, J = 29.7 Hz, 2H), 2.04 solved in 50 mL anhydrous CH2Cl2. To this solution, triethyl- (s, 2H), 1.62 (s, 2H), 1.28 (s, 11H). 13C NMR (126 MHz, chloro- amine (4.4 g, 6.1 mL, 43 mmol, 2.5 equiv.) and DMAP (0.026 g, form-d) δ 173.30, 172.99, 139.27, 114.28, 86.09, 80.86, 77.97, 1.7 mmol. 0.01 equiv.) were added. The sample vial was sealed

77.41 (CDCl3), 77.16 (CDCl3), 76.91 (CDCl3), 73.85, 73.61, with a septum and purged with nitrogen for 5 min. 70.47, 34.27, 34.08, 33.90, 29.39, 29.37, 29.31, 29.28, 29.16, 10-Undecenoyl chloride (9.4 mL, 8.8 g, 43 mmol, 2.5 equiv.) 29.00, 24.98, 24.95. Melting point = 23.2 °C. was then added dropwise via syringe to the stirring reaction flask. The reaction was monitored via TLC and triethylamine Synthesis of 10-undecenoic acid anhydride hydrochloride precipitated from the solution as the reaction 10-Undecenoic acid (10-UA, 105 g, 0.57 mol, 1 equiv.) was progressed. After reaching completion (4 h) the reaction was measured in a 1 L RBF equipped with a magnetic stirring bar filtered and the solvent was removed from the filtrate under and then dissolved in acetic anhydride (265 mL, 2.8 mol, reduced pressure. The residue was purified via crystallization 4.9 equiv.). This solution was attached to a reflux condenser from cold hexanes (−20 °C) and cold filtration to yield BHMFU and heated to reflux using a heating mantle. The solution was as brilliant white flakes (4.7 g, 59%).

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1H NMR (500 MHz, chloroform-d) δ 6.36 (d, J = 1.1 Hz, 1H), acid-catalyzed reaction between isosorbide and 10-undecenoic 5.86–5.72 (m, 1H), 5.10–4.87 (m, 4H), 2.33 (t, J = 7.6 Hz, 2H), acid led to high conversions to IU (>95%). This reaction is free 2.03 (q, J = 7.1 Hz, 2H), 1.62 (p, J = 7.2 Hz, 2H), 1.44–1.15 (m, of major by-products, and upon recrystallization and color 12H). 13C NMR (126 MHz, chloroform-d) δ 173.34, 150.23, removal via rapid filtration through silica, highly pure IU was 139.12, 114.11, 111.34, 57.84, 34.08, 33.74, 29.23, 29.13, 29.02, obtained, albeit in moderate isolated yield (46%). 29.00, 28.84, 24.80. Melting point = 34.5 °C. Because of the enhanced reactivity of the dilactone core in GDLU, the direct reaction of GDL with 10-undecenoic acid in Representative ADMET polymerization procedure the presence of an acid catalyst presented diﬃculties. Instead, The monomer (2.8 mmol, 100 equiv.) and methyl 10-un- we found that the anhydride of 10-undecenoic acid, which is 19 decenoate (0.028 mmol, 1 equiv.) were loaded into an oven-dried readily obtained on a large scale, may be converted rapidly 3 50 mL RBF and dissolved in 5 mL anhydrous toluene. The (∼15 min) to GDLU using scandium(III) triflate as a catalyst. solution was purged with nitrogen for 10 min. Grubbs’ second While purification by column chromatography is still required, generation catalyst ([G2], 0.028 mol, 1 equiv.) was dissolved in it is rendered simpler by the cleanliness of the reaction, the 3.6 mL anhydrous toluene and added to the reaction flask. major impurity being the coproduct 10-undecenoic acid, The system was attached to a water-cooled condenser and which may be recycled. placed under dynamic vacuum (house vacuum). The reaction The novel α,ω-diene, 2,5-bis(hydroxymethyl)furan un- mixture was heated to 80 °C for 16 h. The polymerization decenoate (BHMFU), was also synthesized. The higher reactivity

D7ZLQ&LWLHVRQ mixture was then cooled to room temperature and quenched of the primary alcohols in BHMF allowed for rapid esterifica- with excess ethyl vinyl ether while stirring for 30 min. The tion with 10-undecenoyl chloride and triethylamine; this polymerization mixture was precipitated into cold methanol provided BHMFU in excellent purity and moderate yield three times (MeOH : crude polymer = 40 : 1) and dried in a (∼60%) after chromatographic workup. Fisher esterification vacuum oven overnight. Polymers were typically obtained in a with Dean–Stark distillation was attempted for the synthesis of greater than 70% yield. BHMFU, but significant decomposition of BHMF was observed while heating the reaction mixture to reflux. Hydrolytic stability testing Approximately 2 g of each polymer sample was dissolved in Polymer synthesis of the IU and GDLU copolymer family separate flasks containing 20 mL CH2Cl2 and dispensed across The monomers IU and GDLU were used to produce their 9 preweighed 20 mL scintillation vials using a 5 mL volumetric respective homopolymers and a set of GDLU and IU copoly- pipette. The solvent was removed overnight in a vacuum oven. mers with varying ratios of the two diﬀerent monomer units. The samples reduced down to a disk at the bottom of each vial ADMET polymerizations with Grubbs’ second generation cata- and the mass recorded. The 9 vials were split into 3 groups for lyst in toluene, (1 mol%) were performed at a concentration of each hydrolytic condition. With a 10 mL pipette, three vials in 0.33 M (80 °C, vacuum, 16 h). In the following, we describe the each group were charged with 10 mL of 0.25 M HCl, neutral homopolymers as P(monomer) and the copolymers as water, or 0.25 M NaOH. The samples were gently mixed with a P(monomer1x-co-monomer2y) where x and y describe the platform shaker and were exposed to each condition for 24 h percent composition of each respective comonomer. For 3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW at a time. After 24 h, each sample was decanted into a clean brevity, the copolymers described in Table 1 are labelled solely scintillation vial and rinsed with deionized water. Samples by their respective GDLU : IU ratio as determined by 1H NMR were allowed to dry overnight in the vacuum oven and were spectroscopy. Thus, for example, the sample labelled “77 : 23” promptly weighed after drying. The samples were then refers to P(GDLU77-co-IU23). All the polyesters were obtained as exposed to the same conditions over the course of six, 24 h time periods with fresh aqueous solutions added after each

day. Results were plotted as the percent insoluble mass after Table 1 Data summary for the P(GDLUx-co-IUy) copolymer system each 24 h period. a −1 a b b c Sample Mn (kg mol ) Đ Tg (°C) Tm (°C) Td (°C) P(GDLU) 61 1.8 32 59 206 Results and discussion 77 : 23 31 1.4 15 35 200 76 : 24 41 1.6 18 46 196 Monomer synthesis 52 : 48 53 1.8 7 24 203 48 : 52 18 1.1 2 20 207 In our previous report, GDLU and IU were prepared by the 18 : 82 17 1.3 −8 17 230 reaction of the bioderived diols GDL or isosorbide with P(IU) 56 1.8 −10 38 369 10-undecenoyl chloride in the presence of excess base.11 We a Number average molecular weight and dispersity determined via have improved this method by using direct condensation of SEC-MALLS in THF at 25 °C. b Glass transition temperature and melting GDL or isosorbide with 10-undecenoic acid or its anhydride, temperature were determined by diﬀerential scanning calorimetry c obviating the need for the acid chloride, high levels of added (DSC). Decomposition temperatures were determined at the 5% weight loss level by thermal gravimetric analysis (TGA). Copolymers base, and, in some cases, chromatographic purification steps. described in this table are labelled with their respective GDLU : IU Thus, azeotropic removal of water from the p-toluenesulfonic composition.

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brown solids, with the exception of P(GDLU18-co-IU82) which was a brown viscous liquid. All samples were found to be soluble in an array of organic solvents including acetone, tetra-

hydrofuran, CHCl3,CH2Cl2, and toluene. For comparison purposes, the following polymer samples were used from our pre- 11 vious reported work: P(GDLU), P(GDLU52-co-IU48), and P(IU). Thermogravimetric analysis, diﬀerential scanning calorimetry, MALLS-SEC, and uniaxial tensile testing results for these samples were characterized in the previous work and those results are included in the plots presented herein for comparison purposes.11

IU and GDLU copolymer family characterization Molecular weights and dispersities were determined via multi- Fig. 2 Diﬀerential scanning calorimetry of the P(GDLUx-co-IUy) matrix. − angle laser light scattering size exclusion chromatography Samples were heated from 50 °C to at least 100 °C, and the second heating traces are shown and were used for the measurement of glass (MALLS-SEC) in THF. Polymers were shown to exhibit mole- − transition temperature and melting temperature for these samples. cular weights ranging from 17 to 61 kg mol 1, with moderate D7ZLQ&LWLHVRQ dispersities in the range of 1.1–1.8 (Table 1). The thermal decomposition of the P(GDLUx-co-IUy) family The impact of compositional changes on mechanical pro- of polyesters is shown in Fig. 1, in which the single step perties was studied via tensile analysis (Fig. 3). Mechanical decomposition feature observed in P(IU) gives way to a more properties, shown in Table 2, were found to be highly depen- complex multi-step decomposition feature with increasing dent on the copolymer composition in the P(GDLUx-co-IUy) GDL-incorporation. Decomposition temperatures (Td)at5% system. As reported previously, P(IU) is a brittle material.11 The mass loss are reported in Table 1. P(IU) showed the greatest mechanical performance of the GDLU-containing copolymers thermal stability, with a Td of 369 °C. GDL-containing poly- was found to span multiple material regimes including elasto- esters had decomposition temperatures in the range of 196 °C to 230 °C, and decreasing the GDLU : IU ratio resulted in

more thermally stable polymers (higher Td). These observed decomposition temperatures are comparable to other aliphatic 20,21 polyesters such as P(3-hydroxybutyric acid) (Td = 275 °C). All values of the glass transition and melting temperatures were taken from the second heating of the sample. The poly-

esters shown in Fig. 2 have Tg values ranging from −10 °C to 32 °C, with the intermediate compositions following the Fox equation relationship for statistical copolymers (Fig. S24†).22 3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW As copolymers reach equal compositions of each monomer, the crystallinity was reduced; backbone compositional irregularity makes crystallization increasingly diﬃcult.23

Fig. 3 Uniaxial extensional tensile testing of the P(GDLUx-co-IUy) matrix. Samples were deformed at a uniform rate of 50 mm min−1 until break (denoted with X). Representative data from at least 5 replicates are shown for each composition.

Table 2 Tensile properties of the P(GDLUx-co-IUy) copolymer system

’ a σ b ε c Polymer Young s modulus (GPa) x (MPa) x (%) P(GDLU) 0.74 ± 0.03 28 ± 1 5.5 ± 0.1 P(GDLU76-co-IU24) 0.17 ± 0.02 15 ± 1 200 ± 30 P(GDLU52-co-IU48) 0.002 ± 0.001 24 ± 3 640 ± 60 P(IU) 0.059 ± 0.003 3.2 ± 0.1 9.2 ± 0.7

Fig. 1 Thermogravimetric analysis of the P(GDLUx-co-IUy) matrix. a Young’s modulus was calculated from the first 5% elongation during Degradation temperatures are recorded at 5% weight loss denoted by uniaxial tensile testing. b Average stress at break from at least c the black dashed line. Testing was performed under a N2 atmosphere. 5 measurements. Average maximum elongation at break.

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meric, ductile plastic, and glassy solid as a function of increas- of the aqueous solution and degrade completely within the

ing GDLU (Fig. 3). first day time period, but the ductile P(GDLU77-co-IU23) and The homopolymer, P(GDLU), displays a high modulus (0.74 glassy P(GDLU) repel the aqueous solution and degraded after GPa) and relatively small strain at break (5.5%) expected of a two and three days respectively. It should be noted that the glassy material.11 Copolymerization of GDLU with 24 mol% IU P(IU) homopolymer does not degrade under any of the con- reduces the modulus (0.17 GPa) and transforms the glassy ditions tested and the GDL-containing polyesters were found P(GDLU) homopolymer into a ductile material, with yielding to be relatively stable under neutral and acidic conditions as and strain hardening observed during testing (Fig. 3). seen in Fig. S34.† Furthermore, by copolymerizing GDLU with 52 mol% of IU, To understand the impact that six days of hydrolytic the material is further transformed into a stiﬀ elastomeric exposure had on the molecular weight and thermal behavior material with a low modulus (0.002 GPa) and excellent tough- of GDL-containing copolymers, the remaining insoluble mass ness.11 However, the mechanical capacity is lost with the in- was characterized by SEC and DSC (Fig. S35–40†). A significant

corporation of a greater IU content, where P(GDLU18-co-IU82) shift to lower molecular weights was observed for all copoly- was found to be molten at room temperature and, as a result, mers regardless of the testing conditions. The glass transition we were unable to perform tensile testing on this material. temperature did not change drastically after the hydrolytic stability testing, but the exposure to neutral and acidic GDL degradability enhancement aqueous conditions caused increased crystallinity or new

D7ZLQ&LWLHVRQ melting features to be observed in the copolymer samples. Our previous work reported the enhanced degradability from When exposed to aqueous conditions, we speculate that the direct incorporation of GDL into the backbone of this class GDL facilitates the degradation of our materials in two ways: of linear polyesters.11 Herein, we further study the hydrolytic (1) ring-opening of GDL breaks apart the polymer chains into stability of the new copolymers P(GDLU -co-IU ) and 77 23 oligomers allowing greater accessibility to the degradation P(GDLU -co-IU ) under neutral, acidic, and basic aqueous 18 82 media and (2) ring-opened GDL aids in acid catalysis for the conditions, as well as expanded characterization of P(GDLU - 48 hydrolysis of the remaining polymer backbone. Our previous co-IU ) under neutral and acidic hydrolytic degradation con- 52 characterization has supported these hypotheses, as we know, ditions. In the basic hydrolytic stability testing of GDL-contain- by NMR, that GDL-containing polymers completely degrade to ing materials, as shown in Fig. 4, the presence of GDL was the original material building blocks of isosorbide, glucaric required to facilitate degradation. However, we found that acid, and the 20-carbon metathesis bisacid product when decreasing the ratio of GDL in the polymer facilitated a exposed to basic aqueous conditions.11 decrease in the time required for complete degradation (0% insoluble mass). We attribute this behavior to the material state Network characterization and source during testing at room temperature. The molten P(GDLU -co- 18 To investigate the elasticity and shape memory abilities of IU75) and the rubbery P(GDLU48-co-IU52) better allow the influx 11 P(GDLU48-co-IU52), linear viscoelasticity (LVE) was measured using small-amplitude oscillatory shear. The LVE measure-

ments for P(GDLU48-co-IU52) are presented in Fig. 5 and the

3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW three important regimes are labelled as Zones 1–3. For an entangled thermoplastic, the rubbery plateau (Zone 1) usually

Fig. 4 Hydrolytic stability testing of the P(GDLUx-co-IUy) copolymer system under basic (0.25 M NaOH) aqueous conditions. Samples in triplicate were exposed to 0.25 M NaOH for 24 hour periods, dried and

insoluble mass was measured. Then the samples were exposed to a new Fig. 5 Linear viscoelasticity of P(GDLU52-co-IU48) compositions. Small- aqueous insult for the next period. This process was repeated over the amplitude oscillatory shear measurements were obtained at 1 rad s−1

course of six 24 hour periods. P(GDLU18-co-IU82) and P(GDLU52-co- from 1 to 10% strain to ensure that we remain in the linear viscoelastic IU48) degraded within one 24 hour period, P(GDLU18-co-IU82) is shown regime of the materials. Terminal ﬂow and rubbery plateau regimes are above as a dashed line for diﬀerentiation purposes. denoted as a label and gray shading.

Polymer Chemistry Paper

gives way to terminal flow (Zone 3) at lower frequencies or polymerization. Several ruthenium(II) removal routes including higher temperatures.22 We observed a deviation from this ruthenium scavengers, diﬀerent precipitation conditions, and expected rheological behavior (Zone 2) between the rubbery hydrogenation were attempted, but significant reduction of plateau (Zone 1) and the terminal flow (Zone 3) regimes, where the residual ruthenium levels was not observed. Thus, we the storage modulus (G′) and loss modulus (G″) are roughly believe that ruthenium may be chelated to our polymer struc- parallel and characterized by a slope of ∼1/2. While Zone 2 is ture and the ionic nature of this interaction could allow ruthe-

most pronounced in P(GDLU52-co-IU48) and observed to a nium to serve as a metal cation in ionic aggregates.

lesser extent in P(GDLU76-co-IU24) and P(GDLU). The LVE data As additional evidence for our network hypothesis, upon for compositions other than P(GDLU52-co-IU48) are presented characterization of the P(GDLUx-co-IUy) polymers by medium- in the ESI (Fig. S29†). Unlike the GDL-containing polyesters, angle X-ray scattering (MAXS), we observed a principle scatter- − P(IU) exhibits normal rheological behavior and no intermedi- ing peak (q*) of 1/d = 0.25–0.27 Å 1, indicative of the presence ate regime is observed between the rubbery plateau and term- of local density fluctuations with pair spacings of around inal flow regimes. 2.4 nm. The integer peak position ratio (2q* and 3q*) observed Similar rheological behavior to that shown in Fig. 5 has for the GDL-containing polymers, shown in Fig. 6, is indicative been observed in materials with transient networking.24 Zone of a layered morphology and has been observed previously in 2 has been characterized by a Rouse-like relaxation resulting systems with transient networking.28 A long range order does from the equilibrium relaxation of trapped chains between the not appear to be present in this system due to the lack of 24 D7ZLQ&LWLHVRQ temporary network points. Transient networks can have a higher ordered peaks. Since X-ray scattering arises from differ- large impact on the observed rheological behavior of a ences in the electronic density within the material, this tech- – material.25 30 Such transient networks are known to arise from nique would be unable to differentiate whether this scattering – either hydrogen-bonds or ionic interactions.24 29 By analogy, stems from hydrogen-bonded or ionic aggregates.22 we postulate that the intermediate regime, Zone 2, that we observe is the result of short-lived, transient interactions Copolymers containing BHMFU within P(GDLU52-co-IU48). We speculate that transient net- To better understand how the interplay between the carbo- works involving small amounts of hydrogen bond donors and hydrate-based building blocks of GDL and isosorbide led to acceptors could exist in the polymers we have prepared due to the elasticity and shape memory abilities of the copolymers, the thermal and hydrolytic instability of GDL. The GDL moiety we decided to replace GDL or isosorbide with a different has two major routes of decomposition that could lead to the renewable diol, 2,5-bis(hydroxymethyl)furan (BHMF). BHMF is liberation of hydrogen bond donors without complete material a reduced form of the widely studied compound, decomposition (Fig. S33†). These structures are capable of hydroxymethylfurfural.33 hydrogen bonding between themselves and other functional The homopolymer of BHMFU was synthesized, as well as moieties such as the ester backbone. None of these structures select compositions of copolymers with either GDLU or IU, as were observed by 1H NMR spectroscopy, thus the amount of such GDL decomposition is likely low. To impact the rheological behavior of the parent materials,

3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW transient networking must persist longer than the experimental timeframes. Weaker hydrogen bonding groups, such as acids, have been shown to have little impact on the bulk rheological properties of the resultant material due to their relatively short lifetime.25 Strong hydrogen bonded systems such as the ureidopyrimidinone-functionalized polymers reported by Feldman and co-workers have long-lived hydrogen-bonding that persists long enough to impact the rheological behavior.31 In ionomers, the ionic interaction between acid groups and a metal counterion causes nanoscale aggregates to form, serving as the linkages of the transient networks.32 Due to the nature of the bonding, ionic aggregates often have longer lifetimes than hydrogen bond networks.31 We thus hypothesized that in addition to pendant car- boxylic acids formed from partial GDL ring opening, the presence of the ADMET polymerization catalysts may also contri- bute to the observed transient network formation. After polymerization workup, we found that the materials contain Fig. 6 1D medium-angle X-ray scattering of the P(GDLUx-co-IUy) between 1500 and 1700 ppm of residual ruthenium as copolymer matrix at 25 °C. The principle scattering peak is labelled with measured by inductively coupled plasma mass spectrometry q* and higher order reﬂections are positioned at nq*. These samples (ICP-MS). This amounts to >80% of the catalyst used in the have been background subtracted. Plots are vertically shifted for clarity.

Paper Polymer Chemistry

Table 3 Data summary for the BHMFU : GDLU : IU copolymer system properties, such as elasticity and shape memory. It was shown that by copolymerizing GDLU with either IU or BHMFU, we a b b c Mn Tg Tm Td − were able to access a wide array of relevant mechanical pro- Polymer (kg mol 1) Đa (°C) (°C) (°C) perties while maintaining the hydrolytic degradation behavior P(BHMFU) 10 1.2 −55 59 270 seen previously in GDL-containing polymers.11,16 Moreover, a P(BHMFU -co-GDLU ) 55 1.2 3.0 25 197 35 65 new optimized synthesis for IU and GDLU was developed that P(BHMFU48-co-GDLU52) 34 1.5 −20 19 200 − ﬀ P(BHMFU52-co-IU48) 10 1.1 29 15, 26 284 allowed for a more facile, cost e ective, and cleaner synthesis of these useful monomers. a Number average molecular weight and dispersity determined via SEC-MALLS in THF at 25 °C. b Glass transition temperature and To understand the mechanical properties of elasticity and melting temperature were determined by diﬀerential scanning calori- shape memory observed in our previous work we examined the c metry (DSC). Decomposition temperatures were determined at the rheology of the copolymer samples. We speculate that the 5% weight loss level by thermal gravimetric analysis (TGA). observed behavior (Zone 2, Fig. 5) is due to a release equilibrium of trapped chains between transient network points in our material.24 Transient networking is further supported by Table 4 Tensile properties of BHMFU-containing copolymer systems the detection of small nanoscale aggregates by X-ray scattering. Young’s We further speculate that partial GDL decomposition may be a σ b ε c Polymer modulus (GPa) x (MPa) x (%) the source of the transient networks within the GDLU-contain-

D7ZLQ&LWLHVRQ ing polyesters, with possible complications due to residual P(BHMFU35-co-GDLU65) 0.018 ± 0.01 29 ± 6 480 ± 30 P(BHMFU48-co-GDLU52) 0.0040 ± 0.0006 2.5 ± 0.3 1400 ± 100 ruthenium (catalyst) species. Our future work is aimed at separating the possible eﬀects of residual ruthenium on the a Young’s modulus was calculated from the first 5% elongation during uniaxial tensile testing. b Average stress at break from at least complex rheological behavior of the GDL-containing polymers. 5 measurements. c Average maximum elongation at break. We are working towards this goal by synthesizing chemically- similar materials without ADMET polymerization and studying the implications on the physical and chemical properties of shown in Table 3. P(BHMFU) was found to be an even more these novel renewable materials. brittle material than P(IU), which prevented mechanical testing. Several compositions of BHMFU-containing copolymers were targeted to understand the impact of the replace- Acknowledgements ment of either isosorbide or GDL on the observed mechanical properties (Table 3). This project was funded through the National Science The BHMFU-containing copolymers exhibited a reduction Foundation under the Center for Sustainable Polymers (NSF ﬀ in Tg at the equimolar compositions, P(BHMFU48-co-GDLU52) project CHE-1413862). The authors thank Je rey Ting for his

and P(BHMFU52-co-IU48) when compared to P(GDLU52-co-IU48). assistance with the X-ray scattering experiments. We would This reduction led to a softening eﬀect on the mechanical pro- like to thank the following people for their valuable feedback

perties of P(BHMFU48-co-GDLU52) and P(BHMFU52-co-IU48) on the manuscript, Victoria Szlag, Joseph Hexum, and Jessica

3XEOLVKHGRQ-XQH'RZQORDGHGE\8QLYHUVLW\RI0LQQHVRW (Table 4). P(BHMFU52-co-IU48) was so soft that we were unable Thienes. This work was supported by the use of University of ’ to perform tensile testing on this sample. Since P(BHMFU48- Minnesota s Polymer Characterization Facility (UMN MRSEC,

co-GDLU52) retained measurable mechanical performance award number DMR-1420013). X-ray scattering experiments characteristics, an additional BHMFU-containing sample was were performed at the DuPont-Northwestern-Dow

synthesized, P(BHMFU35-co-GDLU65), that took into account Collaborative Access Team (DND-CAT Sector 5 of the Advanced

BHMFU’s impact on Tg. This sample had a Tg within 5 °C of Photon Source (APS) at Argonne National Laboratory.

P(GDLU52-co-IU48), and showed enhanced mechanical strength DND-CAT is supported by E. I. DuPont de Nemours & Co., The with an overall similar elastic and rheological behavior. Dow Chemical Company, and Northwestern University. Use of Thus, we found that BHMFU may serve as a suitable replace- the APS at Argonne National Laboratory was supported by the ment for isosorbide in copolymers with GDL (but not for GDL U.S. Department of Energy, Oﬃce of Science, Oﬃce of Basic in copolymers), further supporting our hypothesis that GDL is Energy Sciences, under Contract DE-AC02-06CH11357.) responsible for the mechanical strength and elasticity of the GDL-containing materials. Notes and references Conclusions 1 S. A. Miller, ACS Macro Lett., 2013, 2, 550–554. 2 S. A. Miller, Polym. Chem., 2014, 5, 3117. ADMET polymerization was used to form linear carbohydrate- 3 J. J. Gallagher, M. A. Hillmyer and T. M. Reineke, derived polyesters from GDLU, IU, and the novel BHMFU. ACS Sustainable Chem. Eng., 2015, 3, 662–667. These building blocks were then used to study the interplay 4 M. Nasiri and T. M. Reineke, Polym. Chem., 2016, 7, 5233– between the monomer structure and the polymer mechanical 5240.

Polymer Chemistry Paper

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