Thermoplastic Elastomers Derived from Menthide and Tulipalin A

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Thermoplastic Elastomers Derived from Menthide and Tulipalin A Jihoon Shin, Youngmin Lee, William B. Tolman,* and Marc A. Hillmyer* Department of Chemistry and Center for Sustainable Polymers, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *

ABSTRACT: Renewable ABA triblock copolymers were prepared by sequential polymerization of the plant-based monomers menthide and α-methylene-γ-butyrolactone (MBL or tulipalin A). Ring-opening transesterification polymerization of menthide using diethylene glycol as an initiator gave α,ωdihydroxy poly(menthide) (HO-PM-OH), which was converted to α,ω-dibromo end-functionalized poly(menthide) (Br-PM-Br) by esterification with excess 2-bromoisobutyryl bromide. The resulting 100 kg mol−1 Br-PM-Br macroinitiator was used for the atom transfer radical polymerization of MBL. Four poly(αmethylene-γ-butyrolactone)-b-poly(menthide)-b-poly(α-methylene-γ-butyrolactone) (PMBL-PM-PMBL) triblock copolymers were prepared containing 6−20 wt % PMBL, as determined by NMR spectroscopy. Small-angle X-ray scattering, differential scanning calorimetry, and atomic force microscopy experiments supported microphase separation in the four samples. The mechanical behavior of the triblocks was investigated by tensile and elastic recovery experiments. The tensile properties at both ambient and elevated temperature show that these materials are useful candidates for high-performance and renewable thermoplastic elastomer materials.

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renewable graft copolymers of PMBL and poly(lactide) (PLA; PMBL-g-PLA) also have been prepared as compatibilizers.23 Thermoplastic elastomers (TPEs) are important materials for myriad applications. They often comprise ABA triblocks in which incompatible A and B blocks are microphase separated. The mechanical characteristics of TPEs derive from the presence of B midblocks designed to be flexible under ambient conditions (i.e., Tg ≪ 25 °C) and hard (i.e., glassy or semicrystalline) A end-blocks that form physical cross-links.24 Petroleum-based styrenic block copolymers (SBCs) such as poly(styrene)-b-poly(isoprene)-b-poly(styrene) (SIS) and poly(styrene)-b-poly(butadiene)-b-poly(styrene) (SBS) represent an important class of ABA triblock TPEs.25−27 Recently, wholly or partially renewable ABA triblock TPEs have been studied employing amorphous and semicrystalline biobased PLA as the hard end-blocks (A). For example, we recently reported the renewable TPEs poly(lactide)-b-poly(isoprene)-bpoly(lactide) (PLA-PI-PLA)28,29 and poly(lactide)-b-poly(menthide)-b-poly(lactide) (PLA-PM-PLA).30−32 A number of other studies have been conducted on partially renewable TPEs with PLA end blocks.33−40 In addition, the preparation of well-defined AB diblock and ABA triblock copolymers containing poly(n-butyl acrylate) (PBA) as a soft phase (B) and PMBL as a hard phase (A) using atom transfer radical polymerization (ATRP) has been reported.14,41 Star block TPEs having middle soft PBA and outer hard PMBL with improved properties compared to their linear counterparts also have been described.42 Despite the wide variety of renewable

uture sustainable polymeric materials will rely on the discovery and development of biocompatible, biodegradable, and renewable alternatives to petroleum-based commodity materials.1−6 In particular, extensive research efforts have focused on polymers derived from natural sources (e.g., plants) both to address sustainability objectives and to generate new polymers with distinct or superior physical and chemical properties. Achieving these goals is highly desired for both large commodity and specialty materials. (−)-Menthol can be extracted from the plant Mentha arvesis and converted to derivatives by chemical or biochemical means. (−)-Menthone is one such commercially available derivative and can be converted into the seven-membered lactone (−)-menthide by a simple Baeyer−Villiger oxidation.7 αMethylene-γ-butyrolactone (MBL or tulipalin A) is a natural substance found in the common tulip Tulipa gesneriana L and has attracted particular attention.8,9 It can also be prepared by a two-step process from the widely available γ-butyrolactone.10 The related γ-methyl-α-methylene-γ-butyrolactone (MMBL) may be prepared from levulinic acid, which is in turn derived from cellulosic biomass.11 Both MBL and MMBL are renewable cyclic analogues of alkyl methacrylates such as methyl methacrylate (MMA). MBL can be polymerized by anionic and radical homo- and copolymerization techniques.8,12−17 Because of its nearly planar five-membered ring18 and its exomethylene double bond,19 MBL is more reactive than MMA. Poly(MBL) (PMBL) is a rigid thermoplastic,20 with good durability, a relatively high refractive index,21 and a glass transition temperature (Tg) of 195 °C.12 Copolymers and blends containing PMBL exhibit increased optical properties and resistance to chemical and physical stimulus.22 Rigid © 2012 American Chemical Society

Received: August 14, 2012 Revised: September 25, 2012 Published: October 12, 2012 3833

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Scheme 1

Table 1. Characterization Data for PM, Br-PM-Br, and PMBL-PM-PMBLa polymer PM(10) Br-PM-Br(10) PM(100) Br-PM-Br(100) PMBL-PM-PMBL(3−100−3) PMBL-PM-PMBL(5−100−5) PMBL-PM-PMBL(9−100−9) PMBL-PM-PMBL(13−100−13)

[M]0/[I]0 62

b

660b 120c 230c 300c 600c

Mnd (theo.; kg mol−1)

Mn (NMR; kg mol−1) e

10.0 10.3 103 103 108 111 119 127

9.8 9.8e 103e 101e 106f 110f 117f 125f

Mng (SEC; kg mol−1; Đ) 12.1 (1.07) 11.5 (1.07) 108 (1.06) 116 (1.06) 98.5 (1.12) 91.3 (1.13) h h

w

i

PMB

0.06 0.09 0.15 0.20

See Experimental Section for synthetic details. M is menthide and I is diethylene glycol (DEG). M is α-methylene-γ-butyrolactone (MBL) and I is dibromide-terminated poly(menthide) (Br-PM-Br) macroinitiator. dCalculated based on monomer conversion as determined by 1H NMR spectroscopy. eCalculated from relative integrations of PM repeating units and DEG for PM by 1H NMR spectroscopy. fCalculated from relative integrations of PMBL and PM repeating units for PMBL-PM-PMBL by 1H NMR spectroscopy. gDetermined by size exclusion chromatography (SEC) in CHCl3 at 35 °C relative to poly(styrene) standards. hNot soluble in CHCl3. iMass fraction of PMBL calculated from 1H NMR spectroscopy. a

b

c

temperature, expanding their palette of useful applications. Herein, we describe (a) a method for preparing Br-PM-Br macroinitiators (Mn = 10 and 100 kg mol−1) with low dispersities, (b) the use of such macroinitiators to prepare PMBL-PM-PMBL triblock copolymers, and (c) characterization of these triblocks using 1H and 13C NMR spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), atomic force microscopy (AFM), small-angle x-ray scattering (SAXS), and tensile testing.

TPEs, few possess thermal and mechanical properties comparable to SIS and SBS. We report herein the preparation and characterization of all renewable triblock TPEs, poly(α-methylene-γ-butylrolactone)b-poly(menthide)-b-poly(α-methylene-γ-butylrolactone) (PMBL-PM-PMBL), using the ring-opening transesterification polymerization (ROTEP) of menthide followed by ATRP of MBL. In previous work, we reported the tin-catalyzed polymerization of menthide (M) to give telechelic, α,ωdihydroxy functionalized PM with a high molar mass (Mn = 100 kg mol−1), low dispersity Đ = 1.07, and a low glass transition temperature (Tg) of −22 °C.30 We envisioned that a α,ω-dibromo functionalized PM (Br-PM-Br) macroinitiator could be prepared by esterification of telechelic PM with 2bromoisobutyryl bromide.43 Previous studies have shown that PLA is immiscible with PMBL23 and PM,30−32 and we expected that PM and PMBL would be incompatible given the polar nature of PMBL and the relatively nonpolar nature of PM. As a result, we postulated that TPEs with potentially useful properties could be generated with relatively small amounts of PMBL. Furthermore, the high Tg of PMBL could result in TPEs that retain their mechanical behavior at elevated

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RESULTS AND DISCUSSION Synthesis of Triblock Copolymers. As shown in Scheme 1,30 α,ω-hydroxy-functionalized PM was prepared by ROTEP of menthide using tin(II) ethylhexanoate (Sn(Oct)2) as the catalyst and diethyl glycol (DEG) as bifunctional initiator. Initial menthide/DEG ratios of 62:1 and 660:1 were chosen to target molar masses of 10.0 and 103 kg mol−1, respectively. Molar mass data obtained from 1H NMR spectroscopy and SEC data are given in Table 1. The terminal hydroxyl groups of the PM samples were functionalized with 2-bromoisobutyryl bromide. 1H NMR 3834

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spectroscopy analysis showed complete disappearance of the methine resonance at 3.33 ppm adjacent to the end hydroxyl group consistent with high levels of coupling. Unfortunately, the terminal methine and α-bromoester groups of the product were not observed, likely due to spectral overlap with PM(100) backbone resonances. We also failed to observe resonances in the 13C{1H} NMR spectrum for the α-bromoester group in BrPM-Br(100). However, Br-PM-Br(10), prepared under similar conditions, exhibited resonances in the 13C{1H} NMR spectrum at 171.3, 56.2, and 30.8 ppm corresponding to the carbonyl carbon, α-carbon, and methyl carbons of the terminal α-bromoester group (Figure 1). Using the relative integration

Figure 2. Size exclusion chromatography data (1 mg/mL chloroform) for (a) Br-PM-Br(100), (b) PMBL-PM-PMBL(3−100−3), and (c) PMBL-PM-PMBL(5−100−5) triblock copolymers.

1). The traces were unimodal with low Đ values, and no evidence of low molar mass PMBL homopolymer or intermolecular radical coupling (e.g., high molar mass shoulders) was apparent. These data demonstrate that the chain extension polymerization of PMBL is well controlled. However, the elution volumes for the two triblocks analyzed by SEC were only slightly lower than for the Br-PM-Br(100) macroinitiator sample. This slight shift resulted in apparent molar masses by SEC for the triblock samples that were lower than expected. We posit that this is due to low solubility and, thus, collapsed coil dimensions of the PMBL end blocks in chloroform. Thermal and Morphological Behavior. The thermal properties of the triblocks were assessed by differential scanning calorimetry (DSC, Figure 3). DSC traces of the triblock

Figure 1. Expanded region of the 13C{1H} NMR spectrum (125 MHz, CDCl3) of Br-PM-Br(10) after precipitation.

between the methylene protons of the DEG initiator fragment and the methine protons from the PM repeating units in the 1H NMR spectrum of Br-PM-Br(100), a Mn value of 101 kg mol−1 was determined for Br-PM-Br(100), nearly identical to that determined for PM(100), Mn = 103 kg mol−1. In addition, Mn and Đ for Br-PM-Br(100) and Br-PM-Br(10) measured using SEC with chloroform as the eluent and calibrated with polystyrene standards were 116 kg mol−1 (Đ = 1.06) and 12 kg mol−1 (Đ = 1.07), respectively. These values are comparable to those of the starting PM(100) (Mn = 108 kg mol−1, Đ = 1.06) and PM(10) (Mn = 12.1 kg mol−1, Đ = 1.07). Taken together, the data clearly indicate that the esterification process did not induce deleterious side reactions leading to cleavage or cross-linking of polymer chains. Using Br-PM-Br(100) as the macroinitiator, MBL was polymerized in DMF (60 °C, 15 h) using the CuCl/CuCl2/ bipyridine (bpy) system (Scheme 1).14,41−44 PMBL end blocks consisting of 6−20 wt % (Mn = 3−13 kg mol−1) were targeted. Upon completion of the reaction, new signals for the γmethylene protons and β- and β′-methylene protons of the PMBL end blocks appeared at 4.48 ppm and 2.00−2.50 ppm, respectively, in the 1H NMR spectrum (Figure S1). Reaction conversions of 40−60% were calculated from the integral ratio of the olefinic protons of the MBL monomer (5.74 or 6.07 ppm) and the methylene protons of PMBL (4.48 ppm) prior to precipitation. The PM/PMBL compositions were determined by comparing the integral values of the PM methine protons (4.75 ppm) and the PMBL γ-methylene protons (4.48 ppm; Figure S1). The purified triblock compositions ranged from 6− 20 wt % PMBL (Table 1). The 13C NMR spectrum also showed new resonances at 181.2, 66.0, and 45.2 ppm for the ring carbonyl, γ-methylene, and tertiary α-carbons of the PMBL end blocks (Figure S2). SEC analysis of the triblocks using chloroform as an eluent was challenging due the limited solubility of the copolymers. Molar mass data were only obtained for PMBL-PM-PMBL(3− 100−3) and PMBL-PM-PMBL(5−100−5) (Figure 2 and Table

Figure 3. DSC analysis for pure PMBL and PMBP-PM-PMBL triblock copolymers.

copolymers revealed two glass transition temperatures at −21 and 170−190 °C, consistent with microphase-separated PM midblock and PMBL end blocks. While the former Tg closely matched with that of PM(100),30 the latter is slightly lower than that of pure high molar mass PMBL (195 °C). 3835

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The thermal stability of the PMBL-PM-PMBL triblock copolymers was studied using thermal gravimetric analysis (TGA, Figure S3). Two stages are shown in the weight loss of each copolymer; the first weight loss is attributed to the decomposition of PM at 300−350 °C, and the second weight loss is from the decomposition of PMBL at 380−420 °C. These values are similar to those seen for the weight degradation of pure PM and PMBL (Figure S3). The surface morphologies of copolymer thin films prepared by spin coating (see Experimental Section) were investigated with atomic force microscopy (AFM). Figure 4 shows phase

Figure 5. One-dimensional SAXS profiles for PMBL-PM-PMBL triblock copolymers after annealing (200 °C for 3 days) at 25 °C.

ogy. The broad reflections at higher q values are consistent with form factor scattering from an array of spherical particles, supporting this assignment. The tensile properties of four triblock copolymers having 6, 9, 15, and 20 wt % PMBL were investigated (Table 2, Figure 6). At low strains, a linear response was observed in the stress and strain curve for all triblocks (Table 2, Figure 7). PMBL-PMPMBL(9−100−9) gave a Young’s modulus comparable to that of commercial poly(styrene)-poly(butadiene)-poly(styrene) (SBS) TPEs (>6.0 MPa).45 Beyond the low strain elastic region the PMBL-PM-PMBL(3−100−3) and PMBL-PMPMBL(5−100−5) samples demonstrated strains in excess of 1600%; mechanical failure of these triblocks was not observed because the elongation exceeded the capability of the instrument. PMBL-PM-PMBL(9−100−9) and PMBL-PMPMBL(13−100−13) also reach high strain at break values (ca. 700−800%). These ultimate elongations are comparable or superior to those of commercial TPEs46 and some PLA-PMPLA triblocks,30 and significantly higher than the elongations reported for PMBL-PBA-PMBL triblocks (50−100%, PBA = poly(n-butylacrylate); Table 2).41,42 Finally, tensile strengths for the PMBL-PM-PMBL samples increased significantly (4 to 13 MPa) with increasing PMBL content (6 to 20 wt %). These values are similar to those observed for some PLLA-PM-PLLA triblocks featuring large end-block wt% values (>30 wt %),32 but they are lower than those of commercial SBS TPEs (20−40 MPa).46 We also studied the recovery (i.e., true elasticity) of PMBLPM-PMBL(5−100−5) (Figure 8). Tensile bars were subjected to 20 loading/unloading cycles at a strain of 50% with a constant rate of displacement (5 mm min−1). To determine the elastic recovery and level of plastic deformation, the stress− strain profiles for each cycle were compared (Figure 8). The residual strain increased from cycles 1 and 3. However, the stress−strain behavior from cycles 3 and 20 remained constant. The triblock displayed low values of residual strain (εR = 5− 7%), confirming that these materials exhibit excellent levels of recovery at 50% strain.

Figure 4. AFM images for (a) PMBL-PM-PML(3−100−3), (b) PMBL-PM-PMBL(5−100−5), (c) PMBL-PM-PMBL(9−100−9), and (d) PMBL-PM-PMBL(13−100−13) as spun. Image size is 0.5 × 0.5 μm.

mode images of four PMBL-PM-PMBL “as-spun” samples. In the “as-spun” samples, hard PMBL domains appear as bright circular features, while the relatively soft PM domains appear as the darker matrix. The observed morphology of circular PMBL domains in a PM matrix is consistent with microphase separation of the copolymer. However, no clear dependence of the morphology on PMBL content in the copolymer was evident. After thermal annealing at 200 °C for 3 days, the triblock copolymers maintained a microphase-separated state. However, there was no improvement in the degree of longrange order or clear delineation of the ordered state morphology (Figure S4). The PMBL-PM-PMBL triblocks (after annealing 200 °C for 3 days) were also analyzed by small-angle X-ray scattering (SAXS) at ambient temperature (Figure 5). Principal reflections corresponding to microphase separation were observed, but well-defined secondary peaks were not. This result suggests poor long-range ordering, consistent with the AFM observations (Figure S4). The principal domain spacings of the annealed triblocks were 38, 51, and 61 nm for the 9, 15, and 20 wt % PMBL triblocks, respectively. Based on the above data from DSC, AFM, and SAXS and considering the low PMBL contents (6−20 wt %), we suggest that all the PMBLPM-PMBL triblocks adopt a disorganized spherical morphol3836

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Table 2. Tensile Properties of Renewable Triblock Copolymers ABA triblock copolymers

wt% A end block

PMBL-PM-PMBL(3−100−3) PMBL-PM-PMBL(5−100−5) PMBL-PM-PMBL(9−100−9) PMBL-PM-PMBL(13−100−13) PLA-PM-PLA(6−100−6)a PLA-PM-PLA(11−100−11)a PMBL-PBA-PMBL(3−27−3)b PMBL-PBA-PMBL(3−47−3)b

6 9 15 20 10 18 19 11

Young’s modulus E (MPa) 0.74 ± 1.51 ± 5.97 ± 17.3 ± 0.32 ± 0.45 ± 3.27d 0.75d

0.02 0.09 0.14 0.85 0.04 0.05

tensile strength σb (MPa)

strain at break εb (%)

3.9 ± 0.5c 4.1 ± 0.7c 10.6 ± 1.0 13.0 ± 2.1 0.02 0.03 1.9 0.7

>1800 >1600 800 ± 110 730 ± 150 1210 ± 30e 630 ± 60e 55 100

ref. this this this this 30 30 42 42

work work work work

a

PLA-PM-PLA: poly(D,L-lactide)-b-poly(menthide)-b-poly(D,L-lactide). bPMBL-PBA-PMBL: poly(α-methylene-γ-butyrolactone)-b-poly(n-butyl acrylate)-b-poly(α-methylene-γ-butyrolactone). cThese values are tensile strengths not at break point, but at maximized strain in this limited tensile test. dThese values were calculated from the plots of ref 41. eThese triblocks did not exhibit strain-hardening in the tensile tests and just showed necking or yielding.

Figure 8. Tensile recovery properties of PMBL-PM-PMBL(5−100−5) from 0 to 50% strain at 5 mm min−1 for 20 cycles: Four loading (filled symbols) and unloading (open symbols) for cycles 1 (■), 3 (●), 10 (◆), and 20 (★).

Figure 6. Stress−strain curves of (a) PMBL-PM-PMBL(3−100−3), (b) PMBL-PM-PMBL(5−100−5), (c) PMBL-PM-PMBL(9−100−9), and (d) PMBL-PM-PMBL(13−100−13). Experiments were conducted at room temperature and 5 mm min−1; ○, no failure; ×, failure.

Figure 7. Young’s moduli in stress−strain curves of (a) PMBL-PMPMBL(3−100−3), (b) PMBL-PM-PMBL(5−100−5), (c) PMBL-PMPMBL(9−100−9), and (d) PMBL-PM-PMBL(13−100−13).

Figure 9. Representative stress−strain curve of PMBL-PM-PMBL(5− 100−5) at elevated temperatures. The rate of displacement was 5 mm min−1 and ○ represents no failure during elongation.

The properties of PMBL-PM-PMBL(5−100−5) (as a representative sample) were also examined by tensile tests between 25−100 °C to evaluate the performance as the temperature was raised. As shown in Figure 9, elongations in excess of 1300% without failure were observed at all temperatures. At 1300% strain, the stress at 50 °C was nearly comparable to that at 25 °C at all strains. However, at 75 and 100 °C, the stress values at 1300% decreased to about 75 and 47% of the 25 °C values, respectively. Nonetheless, even at 100 °C, the mechanical behavior was respectable (stress at 1300% elongation ≈ 1.0 MPa). This behavior is notable because,

although styrenic ABA triblock copolymers (SBCs) such as SIS and SBS exhibit significant tensile strengths (10−30 MPa), the relatively low Tg of the polystyrene blocks (∼100 °C) prevents elastomeric performance at elevated temperature. Most efforts to substitute the polystyrene components with polymers having higher Tg do not comment on tensile performance at elevated temperature. An exception was poly(α-methylstyrene)-poly(butadiene)-poly(α-methylstyrene) (mSBmS, ∼45 wt % mS (Tg 165 °C)), for which the stress at 590% elongation was only 4.3 MPa at 100 °C compared to 22.7 MPa at 25 °C and 580% elongation.46−48 3837

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The X-ray source operated at a wavelength of 0.73 Å with a sample to detector distance of 4.6 m calibrated with silver behenate. The flight tube was evacuated. Two-dimensional diffraction images were recorded using a Mar 165 mm CCD X-ray detector. The twodimensional images were azimuthally integrated and reduced to the one-dimensional form of scattered intensity versus the spatial frequency q. (q = 4π sin θ/λ, where θ is one-half of the scattering angle) For SAXS measurement, polymer was casted from DMF and dried under vacuum. Tensile deformation experiments were carried out at room temperature on Rheometrics Scientific Minimat instrument for normal strain−stress properties and recovery tensile properties. Tensile strength at elevated temperature was determined by tensile tests on a TA Instruments ARES-G2 rheometer. Samples for tensile testing were prepared by solvent casting from DMF (10% w/v) in a PFA Petri dish to form a uniform sheet thickness (0.3−0.4 mm). After solvent evaporation at room temperature for 3 days, the samples were further dried at 100 °C for 12 h under reduced pressure. The dog-bone-shaped tensile bars were cut using a die (gauge length 5 mm or gage length 3 mm). Uniaxial extension was conducted at 5 mm min−1. Synthesis of α-Methylene-γ-butyrolactone (MBL). MBL was synthesized from γ-butyrolactone using a literature procedure.10 To a stirred suspension of sodium hydride (36 g of 60% oil dispersion, 0.6 mol) in 480 mL of diethyl ether, absolute ethanol (3.6 mL, 0.066 mol) was added under an Ar atmosphere. A mixture of ethyl formate (48.5 mL, 0.6 mol) and γ-butyrolactone (46.2 mL, 0.6 mol) was added to the suspension over the course of 2 h. The rate of addition was controlled to give a steady reflux and evolution of H2. After completing the addition, the resulting solution was stirred for a further 1 h. The light yellow solid was filled, washed with diethyl ether, and dried under vacuum to give α-methylene-γ-butyrolactone sodium salt as very fine particles (80.3g, Yield: 98%). The sodium salt (80.3 g, 0.6 mol) was added to a suspension of paraformaldehyde (81.1 g, 2.7 mol) and THF (900 mL) under Ar atmosphere. The resulting solution was refluxed for 1 h under nitrogen. After cooling to 10 °C, the crude product was treated with a mixture of 1 M aqueous potassium carbonate (K2CO3) solution (180 mL) and diethyl ether (600 mL), and the organic phase was separated, dried with MgSO4, and evaporated to dryness to afford a pale yellow oil. Purification by vacuum distillation yielded MBL (colorless liquid, 31.3 g, Yield: 53.2 g). Butylated hydroxytoluene (BHT) was added to prevent spurious free radical polymerization. 1H NMR (CDCl3) δ 6.26 (t, J = 2.8 Hz, 1H), 5.68 (t, J = 2.5 Hz, 1H), 4.38 (t, J = 7.3 Hz, 2H), 2.99 (m, 2H); 13C{1H} NMR (CDCl3) δ 174.7, 133.6, 122.1, 65.3, 27.3. Synthesis of α,ω-Dibromo End-Functionalized Poly(menthide) (Br-PM-Br) Macroinitiator. Poly(menthide)s, PM(100) and PM(10), having Mn value of 103 kg mol−1 (Đ 1.06) and 9.8 kg mol−1 (Đ 1.07) were prepared via ROTEP using Sn(Oct)2, as described previously.30 PM(100) (7 g, 0.14 mmol based on two end-hydroxy groups) was dried under vacuum for 24 h. PM(100) and a magnetic stirring bar was charged into a flame-dried Schlenk flask. Dry toluene (70 mL) was added to the flask and the mixture was stirred to dissolve PM(100) at room temperature for 2 h. After complete dissolution, triethyl amine (TEA) (234 μL, 1.68 mmol) was added dropwise using microsyringe. The flask was placed into ice bath and 2-bromoisobutyryl bromide (BIBB) ((208 μL, 1.68 mmol) in dry toluene (7 mL) was added dropwise over 30 min. Following addition, the temperature of the flask was slowly raised to 90 °C and the solution was left overnight.49 After cooling the reaction mixture to ambient temperature, additional toluene (70 mL) was added. The heterogeneous suspension was filtered with a filter paper. The dark brown filtrate was concentrated to about 10 mL using a rotary evaporator. The crude product was dissolved in methylene chloride (200 mL), washed successively with saturated NaHCO3 aqueous solution (150 mL) and water (2 × 100 mL), and dried over anhydrous MgSO4. The concentrated solution (ca. 10% w/v) was poured into cold methanol (− 70 °C) to precipitate the product. The precipitation process was repeated. The resulting pale brown solid was dried in vacuum oven at room temperature for 15 h (6.88 g recovered, 98% isolated yield based on product weight calculated by conversion). Br-

CONCLUSIONS Fully renewable ABA triblock copolymers derived from natural products and with excellent mechanical performance were synthesized and characterized. The hydroxy terminated poly(menthide) midblock was synthesized through controlled ROTEP of menthide with Sn(Oct)2. The dibromo endfunctionalized poly(menthide) (i.e., ATRP macroinitiator) was prepared via esterification using 2-bromoisobutyryl bromide. Subsequently, PMBL-PM-PMBL triblock copolymers were synthesized using copper-catalyzed ATRP of MBL. Four triblock copolymers of varied PMBL composition were prepared. NMR spectroscopy confirmed well-defined polymer architectures and SEC proved narrow molar mass distributions. Phase-separation was elucidated with DSC, AFM, and SAXS. Tensile experiments demonstrated impressive elongation, strain, and elastic recovery properties of PMBL-PM-PMBL TPEs. Even at elevated temperature the mechanical behavior of these samples was respectable. From these results, we expect fully renewable PMBL-PM-PMBL triblock copolymers to be potentially suitable for engineering thermoplastic elastomers.

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EXPERIMENTAL SECTION

Materials. All air- or moisture-sensitive compounds were handled under an inert atmosphere in a glovebox. Toluene was dried using sodium and benzophenone and distilled under reduced pressure. Diethylene glycol (Aldrich) used for polymerizations was distilled under reduced pressure over sodium. Tin(II) 2-ethylhexanoate (Aldrich) used for polymerization was also distilled using a Kugelrohr apparatus. Cuprous chloride (CuCl) was purified by dissolution in 36.5−38.0% HCl and precipitation in deionized water. The filtered solid was washed with ethanol and diethyl ether, then dried. (−)-Menthone (90%), γ-butyrolactone (≥99%), ethyl formate (97%), and m-chloroperoxybenzoic acid (m-CPBA, ≤77%) were used as received from Aldrich. All other solvents and reagents were used as received from commercial sources. Measurements. 1H and 13C NMR spectra (Varian INOVA-500) were employed at room temperature to confirm successful reaction and determine molar mass of polymer. Polymer samples were prepared in either CDCl3 (Cambridge) and DMF-d7 (Sigma-Aldrich) at a concentration of approximately 10 mg/mL for 1H NMR and 100 mg/mL for 13C NMR, respectively. Molar masses (Mn and Mw) were determined by SEC in chloroform (1.0 mL/min) at 35 °C versus polystyrene standards using a Hewlett-Packard high-pressure liquid chromatography system equipped with three Jordi poly(divinylbenzene) columns of 104, 103, and 500 Å pore size and a HP1047A differential refractometer. All GC-MS experiments were conducted on an Agilent Technologies 7890A GC system and 5975C VL MSD. DSC measurements were performed using a TA Instruments Discovery DSC under a nitrogen atmosphere. The polymer samples (7−9 mg) were heated to 220 °C, held for 1 min to avoid the influence of thermal history, cooled to −100 °C, held there for 1 min, and then reheated to 220 °C. The rates of heating and cooling were 10 °C/min. The values reported were obtained from the second heating cycle. An indium standard was used for calibration. TGA was performed on an analyzer system of TA Instruments under a nitrogen flow of 20 mL/min at a heating rate of 20 °C/min in a temperature range from 25 to 600 °C. Thin film surface morphology was investigated by tapping mode AFM (Agilent Technologies, Model 5500). Samples were prepared by spin coating of 1.5−2.0% (w/v) solution in dichloromethane on Si wafer (1 × 1 cm) at 2000 rpm. Polymer film thicknesses were approximately 100 nm, measured by ellipsometry. Prior to spin coating, the Si wafer was cleaned by sonication in acetone and isopropyl alcohol. Thermal annealing of the thin films was done in a vacuum oven at 200 °C for 3 days. SAXS measurements were performed at Argonne National Laboratory on the Advanced Photon Source (beamline 5 ID-D) maintained by the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT). 3838

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PM-Br(10) was also prepared from PM(10) under the same conditions. 1H NMR (CDCl3) for Br-PM-Br(100) δ 4.72 (m, 589H, Ha from the repeating unit of PM), 4.22 (q, J = 4.5 Hz, 4H, Hb from the incorporated initiator), 3.68 (t, J = 4.5 Hz, 4H, Hc from incorporated initiator), 2.30 (dd, J = 15.3 and 5.5 Hz, 589H from the repeating unit of PM), 2.07 (dd, J = 14.4 and 8.5 Hz, H from the repeating unit of PM), 1.94 (m, H from the repeating unit of PM), 1.82 (m, H from the repeating unit of PM), 1.53 (m, H from the repeating unit of PM), 1.33 (m, H from the repeating unit of PM), 1.18 (m, H from the repeating unit of PM), 0.94 (d, J = 6.6 Hz, H from the repeating unit of PM), 0.88 (dd, J = 6.8 and 2.0 Hz, H from the repeating unit of PM); 13C{1H} NMR (CDCl3) for Br-PM-Br(10) δ 172.8 (C1), 171.3 (C2), 80.2, 78.2 (C3), 69.0, 63.1 (C4), 56.2 (C5), 41.9, 41.4, 32.5, 32.4, 32.3, 31.2, 31.1, 30.8, 30.8 (C6), 30.3, 28.4, 28.3, 19.7, 19.7, 18.7, 18.6, 17.5, 17.4, 17.3. Synthesis of PMBL-PM-PMBL Triblock Copolymers. The general procedure to prepare the triblocks using ATRP of αmethylene-γ-butyrolactone (MBL) was as follows: The Br-PMBr(100) (1 equiv based on two end-bromide groups) was dried under vacuum for 24 h and charged into a Schlenk pressure vessel equipped with a magnetic stirring bar. The vessel was evacuated for 30 min, flame-dried, and backfilled with Ar. DMF was added to the vessel and the mixture was stirred to completely dissolve Br-PM-Br(100) at 60 °C for 1.5 h. MBL was passed through a basic dried alumina to remove inhibitor (BHT) prior to ATRP. After cooling the polymer solution to room temperature, the filtered MBL (120−600 equiv) and CuCl2 (0.2 equiv)/bpy (4.0 equiv) stock solution (DMF) were added to the vessel. The mixture was degassed by three freeze−pump−thaw cycles and filled with nitrogen. CuCl was added the frozen solution under nitrogen flow. The vessel was closed, degassed by three freeze− pump−thaw cycles, backfilled with nitrogen, and put into 60 °C oil bath. After 20 h, the reaction was stopped. DMF was added to dilute the polymer solution. A solution of the crude product was passed through an alumina column with DMF to remove Cu catalyst. The product was precipitated two times in excess methanol at room temperature. Finally, the product was dried under vacuum at 100 °C overnight. 1H NMR (DMF-d7) δ 4.75 (s, Ha from the repeating unit of PM), 4.48 (br, 2H, Hb from α-methylene of lactone rings in PMBL repeating units), 2.50−2.00 (br, 4H, Hc from β-methylene of lactone rings and Hd from methylene of backbone in PMBL repeating units), 2.37 (s, H from the repeating unit of PM), 2.17 (s, H from the repeating unit of PM), 1.96 (s, H from the repeating unit of PM), 1.86 (s, H from the repeating unit of PM), 1.66 (s, H from the repeating unit of PM), 1.55 (s, H from the repeating unit of PM), 1.40 (s, H from the repeating unit of PM), 1.22 (s, H from the repeating unit of PM), 0.99 (s, H from the repeating unit of PM), 0.93 (s, H from the repeating unit of PM); 13C{1H} NMR (DMF-d7) δ 181.2, 172.6, 78.2, 66.0, 44.4, 42.0, 32.9, 31.8, 30.7, 29.0, 20.1, 19.0, 17.8.

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra, TGA, and AFM profiles for PMBLPM-PMBL triblock copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Center for Sustainable Polymers at the University of Minnesota, a National Science Foundation supported Center for Chemical Innovation (CHE1136607). The authors greatly appreciate Dr. David Giles for helpful discussions and for some experimental support. 3839

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