Periodic ethylene-vinyl alcohol copolymers via ADMET polymerization: Synthesis, characterization, and thermal property

Polymer 54 (2013) 3841e3849

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Periodic ethylene-vinyl alcohol copolymers via ADMET polymerization: Synthesis, characterization, and thermal propertyq Zi-Long Li, An Lv, Lei Li, Xin-Xing Deng, Li-Jing Zhang, Fu-Sheng Du, Zi-Chen Li* Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 April 2013 Received in revised form 10 May 2013 Accepted 14 May 2013 Available online 21 May 2013

Periodic copolymers of ethylene and vinyl alcohol (EVA) were synthesized via acyclic diene metathesis (ADMET) polymerization of a series of structurally symmetric a,u-diene monomers that containing two vinyl acetate (VAc) units, followed by exhaustive hydrogenation and subsequent hydrolysis. This synthetic methodology provided a new type of EVA copolymer with defined sequence and ever highest VA content via ADMET. These polymer samples were characterized by gel permeation chromatography (GPC), NMR, and matrix-assisted laser desorption/ionization time-of-fight mass spectroscopy (MALDITOF-MS). Thermal properties of EVAc and EVA copolymers were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). EVAc copolymers exhibited two-stage decompositions on TGA traces and displayed a glass transition on DSC thermograms. Meanwhile, the EVA copolymers showed glass transition stages and sharp melting peaks on DSC thermograms, with the Tm values being comparable to that of high density polyethylene (HDPE). Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Ethylene-vinyl alcohol copolymer ADMET polymerization Sequence

1. Introduction Copolymer of ethylene and vinyl acetate (EVAc) is commonly used as hot melt adhesive, packing film, and rubbery toy. Due to its wide applications, EVAc copolymer is the world’s largest ethylene copolymer by volume, and is prepared industrially by radical copolymerization of the two monomers at high temperature and pressure [1,2]. Since ethylene and vinyl acetate exhibit nearly identical reactivity ratio under the radical polymerization conditions, the chemical composition of EVAc copolymers could be tuned over a broad range by simply changing the feed ratio. However, short branches are inevitably formed due to chain transfer reactions, thus disrupting the linearity of the polymer main chain. The structureeproperty relationship of thus obtained EVAc copolymers has been thoroughly investigated [3e6]. Ethylene and vinyl alcohol copolymer (EVA) is routinely obtained via saponification of the EVAc copolymer. Due to its excellent barrier property to prevent hydrocarbon or gas diffusion through membranes, EVA copolymers have been widely used as adhesives

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: þ86 10 6275 5543; fax: þ86 10 6275 1708. E-mail address: [email protected] (Z.-C. Li). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.05.037

and coatings [7]. Moreover, EVA copolymer represents one of the important hydrophilic synthetic resins for applications in biomedical fields [8e10]. However, EVA copolymers obtained by free radical copolymerization usually exhibit a limited degree of crystallinity and a wide melting temperature range due to their ambiguous primary structures. Palladium-catalyzed coordination copolymerization of ethylene and vinyl acetate was reported to generate linear EVAc copolymer with rather low VAc molar content [11]. Alternating EVAc polymer was attained via cationic grouptransfer polymerization (GTP) of 1,3-butadiene derivative followed by multi-step post-modifications, but the branched structures were still unavoidable [12]. Therefore, to get a more precise structureeproperty relationship of EVA copolymers and improve the material performance, research on control over the composition, tacticity, and sequence of EVA copolymers has become important. Metathesis has been extensively applied in polymer synthesis as one of the most efficient carbonecarbon bond formation reaction to afford periodic vinyl copolymers. For instance, Ramakrishnan et al. reported the ring-opening metathesis polymerization (ROMP) of alkylboron-substituted cyclooctene, subsequent oxidation and hydrogenation afforded EVA copolymers [13]. However, the placement of the pendent hydroxyl groups in the final polymer main chain is not in a regio-specific fashion, and the molar ratio of the two monomers was fixed. Later on, based on the same strategy, they conducted ROMP of different ring size cyclic olefins and

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obtained EVA copolymers with the VA content varying from 16 to 50 mol% [14]. However, irregular placement of pendent hydroxyl groups on the polyethylene backbone remained unsolved due to the non-selective nature of post-modification. Terpolymers of ethylene, vinyl alcohol, and other vinyl monomer units were synthesized using the same method [15,16]. Hillmyer and Grubbs et al. reported the direct ROMP of hydroxyl-functionalized cyclooctene using Ru-based Grubbs catalyst [17]. However, regioisomers were still contained in the repeating unit of the periodic copolymers. EVA copolymers with defined tacticity and predicted monomer sequence were achieved via ROMP by Grubbs et al. [18,19]. Rather high content of vinyl alcohol in the EVA copolymer was realized and the effect of stereochemistry on the thermal property was described. Very recently, Hillmyer et al. reported the synthesis of sequence-regulated and stereo-specific periodic vinyl copolymers containing vinyl acetate unit by the regio-selective ROMP of substituted cyclic alkenes [20,21]. This elegant work opened a new avenue for control over polymer microstructure from different levels, thus providing important information for thorough elucidating the structureeproperty relationship. Acyclic diene metathesis (ADMET) polymerization shows several advantages when adopted in periodic vinyl copolymer synthesis [22e26]. At first, compared with that of ROMP, monomer synthesis is generally simple especially when sequenced microstructure is to be built into the ADMET monomers. In order to eliminate the uncertainty of regioselectivity, the only requirement for monomer design is structural symmetry. Moreover, due to the high tolerance to polar groups of Grubbs catalysts, sequence information encoded within the monomer can be completely transformed into the repeating unit of the periodic copolymer via ADMET polymerization. Therefore, this is a universal synthetic pathway towards periodic vinyl copolymers via polycondensation. In addition, polar group content can be tuned by varying the length of methylene spacer that separating the functional group and the terminal alkene. In this way, the effect of specific polar group introduction on the thermal property can be elucidated, thus providing better understanding of the structureeproperty relationship. Accordingly, Wagener et al. described the synthesis of EVA copolymers using ADMET polymerization for the first time [27]. Vinyl alcohol content is another crucial parameter of EVA copolymer since the barrier property of the copolymer significantly decreases with reducing the hydroxyl group content [28]. However, monomers suitable for ADMET polymerization usually contain relatively long methylene spacers, thus making the hydroxyl group content inherently low. On the other hand, monomers bearing too short methylene spacers such as terminal allyl group are sluggish to polymerize due to the negative neighboring group effect [29]. Thus, we intended to solve this problem and get EVA copolymers with defined-sequence and tunable VA contents based on our previous works on ADMET polymerization [30e32]. In this contribution, six structurally symmetric a,u-diene monomers bearing one (M1) or two (M2-M6) vinyl acetate units were synthesized via Barbier reaction and subsequent acetylation. We ruled out direct polymerization of the hydroxyl-functionalized monomers due to the minimal solubility of the unprotected diols in common organic solvents suitable for ADMET, and oligomers were more likely to generate due to the sluggish diffusion in the highly viscous polymerization mixture. On the other hand, not as in the previous works to tune the length of methylene spacer separating the pendent group and the terminal alkene [22e26], the monomers (M2-M6) were different in the length of methylene spacer connecting the two vinyl acetate units. ADMET polymerization of the monomers and exhaustive hydrogenation were sequentially performed to afford periodic EVAc copolymers. It was found that intramolecular cyclization instead of ADMET polymerization

occurred for monomers M1 and M2. The other four monomers (M3eM6) were polymerized with Grubbs-II catalyst in the presence of benzoquinone (BQ) to yield EVAc copolymers in high yields and high molecular weights. After hydrogenation and hydrolysis of those EVAc copolymers, EVA copolymers with defined-sequence and tunable VA contents were obtained. The thermal properties of the EVAc and EVA copolymers were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). 2. Experimental 2.1. Materials 1,1,3,3-Tetramethoxypropane (>98.0%) was obtained from TCI. Trans-1,2-cyclohexanediol (98%), hepta-1,6-dien-4-ol (95%), and Grubbs catalysts (first and second generations) were purchased from Aldrich. Tripropylamine (TPA, 98%) and ethyl vinyl ether (stabilized with 0.1% N, N-diethylaniline) were used as received from Alfa Aesar. 4-Methylbenzenesulfonhydrazide (TSH, 97%), 2,5dimethoxytetrahydrofuran (mixture of cis- and trans-isomers, 99%), and 4-dimethylaminopyridine (DMAP, 99%) were from Acros. nButyraldehyde (98.5%) was purchased from Shanghai Shuangxiang Additives Factory. Acetic anhydride (98.5%) and Glyoxal solution (40%) were obtained from Sinopharm Chem. Reagent Co., Ltd. Glutaraldehyde solution (50%) was used as received from Beijing Yili Fine Chem. Co., Ltd. Allyl bromide (97.0%) and p-benzoquinone were from Beijing Chem. Reagent Co., Ltd. Ethyl acetate, toluene, and methanol were obtained from Beijing Tongguang Fine Chem. Co., Ltd. Tin dichloride dihydrate (98.5%), potassium iodide (A.R.), sodium hydroxide (NaOH), sodium carbonate (NaCO3), anhydrous sodium sulfate (Na2SO4), anhydrous calcium chloride (CaCl2), sodium periodate (NaIO4), silica gel, tetrahydrofuran (THF), petroleum ether, methylene dichloride (CH2Cl2), chloroform (CHCl3) were purchased from Beijing Chem. Works. Calcium hydride (CaH2) was obtained from Tianjin Xuan’ang Trade and Industry Co., Ltd. CH2Cl2, THF, and petroleum ether were refluxed with CaH2 for 8 h and then redistilled. CH2Cl2 was sonicated for half an hour before use. 2.2. Characterization Bruker ARX-400 spectrometer was utilized to measure the 1H (400 MHz) and 13C (100 MHz) NMR spectra. EVA copolymers were dissolved in d6-DMSO while all the other samples were dissolved in CDCl3 with tetramethylsilane (TMS) as the internal reference for chemical shifts. Number- and weight-average molecular weights (Mn and Mw) and polydispersity indices (PDI ¼ Mw/Mn) were determined by gel permeation chromatograph (GPC). The measurements were conducted in THF (flow rate: 1 mL/min) at 35  C with a Waters 1525 binary HPLC pump equipped with a Waters 2414 refractive index detector and three Waters Styragel columns A pore sizes). A family of narrowly (1  104, 1  103, and 500  dispersed polystyrenes was used as the standards, and Breeze 3.30 SPA software was applied to calculate the molecular weight and PDI. Electrospray ionization mass spectroscopy (ESI-MS) characterizations were conducted using a Bruker APEX-IV Fourier transform mass spectrometer in a positive ion mode. Matrix-assisted laser desorption/ionization time-of-fight mass spectroscopy (MALDI-TOF-MS) characterizations were performed on a Bruker BIFLEX-III MALDI-TOF mass spectrometer in a linear mode. Thermal gravimetric analysis (TGA) was carried out using a Q600-SDT thermogravimetric analyzer (TA Co., Ltd.) with nitrogen purging rate set at 50 mL/min. Measurements were conducted from room temperature to 600  C at a heating rate of 10  C/min. Calorimetric

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measurement was performed using a Q100 differential scanning calorimeter (TA Co., Ltd.) with nitrogen purging rate set at 50 mL/ min. The program was set to finish two cycles in the temperature range from 80e150  C for EVAc copolymers and from 0 to 180  C for EVA copolymers, respectively. The heating/cooling rate was set to 10  C/min. Data of the endothermic curve was acquired from the second scan. TA Universal Analysis software was used for data acquisition and processing in TGA and DSC measurements. IR spectra were recorded on a Bruker Vector-22 Fourier transform infrared spectrometer. Samples were dispersed by potassium bromide (KBr), and OPUS/IR software was applied to manipulate the spectra. 2.3. Synthesis of adipaldehyde The reported synthetic procedure was followed to prepare this compound [33]. Silica gel (15 g), NaIO4 (5.56 g, 26 mmol), deionized water (15 mL), and CH2Cl2 (50 mL) were added into a 500 mL round bottom flask. After vigorous stirring, a suspension was generated. A solution of trans-1,2-cyclohexanediol (2.32 g, 20 mmol) in CH2Cl2 (50 mL) was added dropwise to the suspension within 0.5 h. The suspension was vacuum filtered after 6 h, and the silica gel was washed with CH2Cl2 (100 mL  3). The organic filtrate was dried over anhydrous Na2SO4, filtered, and concentrated. Adipaldehyde was obtained as viscous colorless liquid in 99% yield. 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 9.75 (m, 2H), 2.47 (m, 4H), 1.66 (m, 4H). 13C NMR (100 MHz, CDCl3): d 201.69 (d, J ¼ 2.7 Hz), 43.00, 20.99 (Fig. S1). 2.4. Synthesis of the diol pre-monomers via Barbier reaction Take the synthesis of preM6 as an example. In a 500 mL round bottom flask, a solution of adipaldehyde (2.28 g, 20 mmol) in THF (15 mL) was added within 0.5 h at 35  C to a stirred mixture of SnCl2$2H2O (13.56 g, 60 mmol), KI (19.9 g, 120 mmol), and allyl bromide (5.2 mL, 60 mmol) in water (150 mL). The stirring was continued for 48 h at 30  C. The resulting mixture was neutralized with Na2CO3 solution and extracted with CH2Cl2 (3  150 mL). The organic phase was washed with Na2S2O3 solution (5% aq., 150 mL  3), dried with anhydrous Na2SO4, filtered, and concentrated to afford a yellowish oil, which was further purified by silica gel column chromatography (petroleum ether/ethyl acetate ¼ 10/4). The diol preM6 was obtained as a pale yellow liquid in 76% yield. 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 5.82 (m, 2H), 5.10 (dd, J ¼ 12.2, 5.8 Hz, 4H), 3.62 (d, J ¼ 1.9 Hz, 2H), 2.79 (s, 2H), 2.21 (m, 4H), 1.46 (m, 8H). 13C NMR (101 MHz, CDCl3): d 135.02, 117.48, 70.55 (d, J ¼ 9.2 Hz), 41.90, 36.57, 25.54 (d, J ¼ 11.1 Hz) (Fig. S2). ESI-MS: [M þ Naþ] ¼ C12H22O2Na, calcd: 221.15120, found: 221.15148. The syntheses of preM2-preM5 were essentially the same as for preM6 except that different starting materials were used (glyoxal for preM2, 1,1,3,3-tetramethoxypropane for preM3, 2,5dimethoxytetrahydrofuran for preM4, and glutaraldehyde for pre M5) [34,35]. 1 H NMR of preM2 (400 MHz, CDCl3), d (TMS, ppm): 5.85 (ttd, J ¼ 14.1, 7.0, 2.5 Hz, 2H), 5.11 (m, 4H), 3.69 (d, J ¼ 34.3 Hz, 2H), 3.51 (s, 2H), 2.29 (m, 4H). 13C NMR (101 MHz, CDCl3): d 134.83 (d, J ¼ 38.3 Hz), 117.35, 73.11 (d, J ¼ 52.1 Hz), 37.11 (d, J ¼ 180.4 Hz) (Fig. S3). 1 H NMR of preM3 (400 MHz, CDCl3), d (TMS, ppm): 5.81 (m, 2H), 5.12 (m, 4H), 3.92 (dddd, J ¼ 17.3, 15.0, 13.0, 8.8 Hz, 3H), 3.32 (s, 1H), 2.27 (ddt, J ¼ 13.5, 7.1, 6.7 Hz, 4H), 1.57 (m, 2H). 13C NMR (101 MHz, CDCl3): d 134.56 (d, J ¼ 36.9 Hz), 117.81 (d, J ¼ 11.3 Hz), 71.73, 68.03, 41.85 (dd, J ¼ 66.9, 23.4 Hz) (Fig. S4).

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1 H NMR of preM4 (400 MHz, CDCl3), d (TMS, ppm): 5.83 (ddt, J ¼ 17.1, 10.2, 7.2 Hz, 2H), 5.11 (m, 4H), 3.67 (m, 2H), 3.23 (m, 2H), 2.25 (m, 4H), 1.60 (m, 4H). 13C NMR (101 MHz, CDCl3): d 134.94, 117.68 (d, J ¼ 2.8 Hz), 70.89 (d, J ¼ 47.1 Hz), 42.01 (d, J ¼ 31.6 Hz), 32.95 (d, J ¼ 93.5 Hz) (Fig. S5). ESI-MS: [M þ Hþ] ¼ C10H19O2, calcd: 171.13796, found: 171.13757; [M þ Naþ] ¼ C10H18O2Na, calcd: 193.11990, found: 193.11947. 1 H NMR of preM5 (400 MHz, CDCl3), d (TMS, ppm): 5.83 (ddt, J ¼ 17.3, 10.3, 7.1 Hz, 2H), 5.09 (dd, J ¼ 13.1, 5.3 Hz, 4H), 3.62 (m, 2H), 3.26 (m, 2H), 2.23 (m, 4H), 1.48 (m, 6H). 13C NMR (101 MHz, CDCl3): d 135.08 (d, J ¼ 3.0 Hz), 117.33 (d, J ¼ 6.6 Hz), 70.52 (d, J ¼ 20.2 Hz), 41.95 (d, J ¼ 13.2 Hz), 36.38 (d, J ¼ 23.0 Hz), 21.56 (d, J ¼ 6.2 Hz) (Fig. S6). ESI-MS: [M þ Hþ] ¼ C11H21O2, calcd: 185.15361, found: 185.15357; [M þ Naþ] ¼ C11H20O2Na, calcd: 207.13555, found: 207.13558.

2.5. Synthesis of monomers via acetylation of the premonomers Monomer M1 was obtained by direct acetylation of the commercially available hepta-1,6-dien-4-ol in 99% yield. 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 5.75 (ddt, J ¼ 17.2, 10.2, 7.1 Hz, 2H), 5.08 (m, 4H), 4.97 (m, 1H), 2.32 (m, 4H), 2.03 (s, 3H). 13C NMR (101 MHz, CDCl3): d 170.49, 133.51, 117.76, 72.27, 37.96, 21.07(Fig. S7). All the other five monomers were synthesized by direct acetylating of the pre-monomers. Take the synthesis of M6 as an example. The diol preM6 (0.99 g, 5 mmol), acetic anhydride (3.06 g, 30 mmol), and DMAP (122 mg, 1 mmol) were sequentially weighed and transferred into a 100 mL round bottom flask. Then 30 mL CH2Cl2 was added to generate a homogeneous solution, which was vigorously stirred overnight at room temperature. The reaction mixture was neutralized with Na2CO3 solution, sequentially washed with deionized water and brine, dried with anhydrous Na2SO4, filtered, and evaporated to afford a yellowish liquid, which was further purified by silica gel column chromatography (petroleum ether/ethyl acetate ¼ 20/1). Monomer M6 was obtained as a colorless liquid in 99% yield. 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 5.74 (dq, J ¼ 10.0, 7.1 Hz, 2H), 5.06 (m, 4H), 4.90 (m, 2H), 2.29 (m, 4H), 2.03 (s, 6H), 1.53 (m, 4H), 1.32 (m, 4H). 13C NMR (101 MHz, CDCl3): d 170.59, 133.65, 117.55, 72.99 (d, J ¼ 3.4 Hz), 38.59 (d, J ¼ 3.0 Hz), 33.38, 25.05, 21.10. ESI-MS: [M þ Naþ] ¼ C16H26O4Na, calcd: 305.17233, found: 305.17192. Monomers M2eM5 were prepared in a similar way as colorless liquids in nearly quantitative yields. 1 H NMR of M2 (400 MHz, CDCl3), d (TMS, ppm): 5.72 (tdt, J ¼ 12.3, 10.3, 6.0 Hz, 2H), 5.07 (m, 6H), 2.35 (m, 4H), 2.05 (d, J ¼ 15.1 Hz, 6H). 13C NMR (101 MHz, CDCl3): d 170.03, 132.88 (d, J ¼ 43.6 Hz), 117.99 (d, J ¼ 39.8 Hz), 72.47 (d, J ¼ 24.9 Hz), 34.75 (d, J ¼ 111.2 Hz), 20.76 (d, J ¼ 4.5 Hz) (Fig. S8). ESI-MS: [M þ Naþ] ¼ C12H18O4Na, calcd: 249.10973, found: 249.10938. 1 H NMR of M3 (400 MHz, CDCl3), d (TMS, ppm): 5.73 (dddd, J ¼ 16.9, 14.0, 7.1, 2.7 Hz, 2H), 5.08 (m, 4H), 4.98 (dq, J ¼ 11.3, 5.8 Hz, 2H), 2.33 (m, 4H), 2.02 (m, 6H), 1.82 (m, 2H). 13C NMR (101 MHz, CDCl3): d 170.36 (d, J ¼ 8.3 Hz), 133.10 (d, J ¼ 7.7 Hz), 118.07 (d, J ¼ 10.6 Hz), 69.56 (d, J ¼ 124.2 Hz), 38.77 (d, J ¼ 69.1 Hz), 37.20 (d, J ¼ 35.5 Hz), 21.03 (d, J ¼ 11.7 Hz) (Fig. S9). ESI-MS: [M þ Naþ] ¼ C13H20O4Na, calcd: 263.12538, found: 263.12530. 1 H NMR of M4 (400 MHz, CDCl3), d (TMS, ppm): 5.73 (ddt, J ¼ 17.3, 10.3, 7.1 Hz, 2H), 5.08 (dd, J ¼ 13.2, 5.6 Hz, 4H), 4.89 (m, 2H), 2.30 (m, 4H), 2.03 (s, 6H), 1.58 (m, 4H). 13C NMR (101 MHz, CDCl3): d 170.55, 133.40 (d, J ¼ 2.5 Hz), 117.77 (d, J ¼ 1.3 Hz), 72.79 (d, J ¼ 25.0 Hz), 38.47 (d, J ¼ 3.7 Hz), 29.18 (d, J ¼ 9.0 Hz), 21.09 (d, J ¼ 10.8 Hz) (Fig. S10). ESI-MS: [M þ Hþ] ¼ C14H23O4, calcd:

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255.15909, found: 255.15906; [M þ Naþ] ¼ C14H22O4Na, calcd: 277.14103, found: 277.14106. 1 H NMR of M5 (400 MHz, CDCl3), d (TMS, ppm): 5.73 (ddt, J ¼ 17.2, 10.3, 7.1 Hz, 2H), 5.07 (dd, J ¼ 13.4, 5.8 Hz, 4H), 4.90 (m, 2H), 2.28 (m, 4H), 2.03 (s, 6H), 1.54 (m, 4H), 1.32 (m, 2H). 13C NMR (101 MHz, CDCl3): d 170.61, 133.56, 117.63 (d, J ¼ 1.2 Hz), 72.89, 38.54 (d, J ¼ 4.6 Hz), 33.22 (d, J ¼ 1.6 Hz), 21.05 (d, J ¼ 10.8 Hz) (Fig. S11). ESI-MS: [M þ Hþ] ¼ C15H25O4, calcd: 269.17474, found: 269.17457; [M þ Naþ] ¼ C15H24O4Na, calcd: 291.15668, found: 291.15644. 2.6. Synthesis of the mono-alkene compound M0 Hept-1-en-4-ol (regarded as preM0) was synthesized similarly as for preM6 by using n-butyraldehyde and allyl bromide in 92% yield. Acetylation of hept-1-en-4-ol afforded M0 in 99% yield. 1 H NMR of preM0 (400 MHz, CDCl3) d (TMS, ppm): 5.84 (m, 1H), 5.12 (m, 2H), 3.64 (dt, J ¼ 16.5, 5.8 Hz, 1H), 2.45 (s, 1H), 2.24 (m, 2H), 1.41 (m, 4H), 0.94 (m, 3H). 13C NMR (101 MHz, CDCl3): d 135.01, 117.49, 70.40, 41.89, 38.87, 18.76, 13.97 (Fig. S12). 1 H NMR of M0 (400 MHz, CDCl3), d (TMS, ppm): 5.75 (ddt, J ¼ 17.2, 10.2, 7.1 Hz, 1H), 5.06 (m, 2H), 4.93 (m, 1H), 2.31 (m, 2H), 2.02 (s, 3H), 1.53 (m, 2H), 1.32 (m, 3H), 0.91 (t, J ¼ 7.3 Hz, 3H). 13C NMR (101 MHz, CDCl3): d 170.50, 133.73, 117.36, 72.92, 38.62, 35.68, 20.99, 18.48, 13.76 (Fig. S13). 2.7. ADMET polymerization Take the synthesis of P6 as an example. Monomer M6 (282 mg, 1.0 mmol), p-benzoquinone (2.16 mg, 20 mmol), and Grubbs-II catalyst (8.4 mg, 10 mmol) were weighed and transferred sequentially into a 25 mL Schlenk flask equipped with a Teflon valve. Subsequently, 1.0 mL of CH2Cl2 (degassed by sonication for half an hour) was added to dissolve the mixture to obtain a homogeneous blue solution. The flask was connected to a reflux condenser which was equipped with an anhydrous CaCl2 loaded drying tube. Then the flask was placed in an oil bath set at 60  C. Nitrogen gas was continuously purged in through the Teflon valve during ADMET polymerization. As the polymerization proceeded, the solvent CH2Cl2 was gradually evaporated and the reaction mixture became highly viscous in a couple of hours as the magnetic bar was sluggish to stir. Polymerization was stopped after 120 h via adding CHCl3 (5 mL) followed by a large excess of ethyl vinyl ether. The concentrated dark blue polymer solution was poured into cold petroleum ether (about 100 mL) to generate a pale blue suspension. Dark blue precipitate was obtained via storage at 4  C overnight. After filtration and vacuum dryness, P6 was obtained in 95% yield as a brownish semi-solid. Unsaturated copolymers of P3-P5 were synthesized similarly, and the NMR spectra are shown in Figs. S14eS16. Cyclized monomers cM1 and cM2 could be obtained via a similar synthetic procedure with nearly quantitative yields. The RCM reactions were allowed to proceed for 24 h. The mixture became heterogeneous in that blue precipitate coexisted with a very small amount of liquid, which was not viscous. The mixture was dissolved in CHCl3 and filtered through a pad of silica. The silica gel was washed with CHCl3, and the combined organic phase was evaporated to generate pale yellow oil. 1H NMR of cM1 (400 MHz, CDCl3), d (TMS, ppm): 5.71 (d, J ¼ 7.7 Hz, 2H), 5.37 (t, J ¼ 6.9 Hz, 1H), 2.74 (dd, J ¼ 16.8, 6.9 Hz, 2H), 2.39 (d, J ¼ 18.0 Hz, 2H), 2.04 (d, J ¼ 6.2 Hz, 3H). 13C NMR (101 MHz, CDCl3): d 171.00, 128.27, 74.16, 39.68, 21.30 (Fig. S17). 1H NMR of cM2 (400 MHz, CDCl3), d (TMS, ppm): 5.59 (m, 2H), 5.12 (m, 2H), 2.31 (m, 4H), 2.05 (d, J ¼ 4.5 Hz, 6H). 13C NMR (101 MHz, CDCl3): d 170.32 (d, J ¼ 21.2 Hz), 123.54 (d, J ¼ 8.4 Hz), 69.34 (d, J ¼ 95.0 Hz), 29.14 (d, J ¼ 148.2 Hz), 21.01 (d,

J ¼ 8.1 Hz) (Fig. S18). ESI-MS: [M þ Naþ] ¼ C10H14O4Na, calcd: 221.07843, found: 221.07824. The GPC traces are displayed in Fig. S19. The synthesis of cM2 has been reported [36], and the NMR data are essentially the same. Dimerization of M0 was performed similarly to the RCM reaction mentioned above. Three reaction conditions were tested: (a) Grubbs-I catalyst; (b) Grubbs-II catalyst; (c) Grubbs-II catalyst with benzoquinone (Scheme S1). The amount of catalyst or benzoquinone was the same as that used in the ADMET polymerization described above. See Fig. S20 for the 1H NMR spectra of the olefin signals region and Fig. S21eS23 for the ESI-MS spectra of the products. 2.8. Exhaustive hydrogenation Take the synthesis of EVAc6 as an example. Polymer P6 (200 mg), TSH (614 mg, 3.3 mmol), and TPA (572 mg, 4.0 mmol) were weighed and transferred into a 100 mL round bottom flask. Subsequently, toluene (30 mL) was added to obtain a pale blue suspension. The flask was connected to a reflux condenser that equipped with an anhydrous CaCl2 loaded drying tube. Under vigorous stirring, the reaction proceeded at 130  C for 12 h, and tiny bubbles were observed during that time. Then, the reaction was stopped upon cooling to room temperature to generate an opaque solution with white precipitates. An equal supply of TSH and TPA was re-added, and the reaction was allowed to proceed for another 12 h at 130  C. The mixture was washed with water for three times, and then toluene in the organic layer was removed in vacuo. Chloroform (1 mL  3) was used to dissolve the crude hydrogenated product. The viscous solution was poured into cold petroleum ether (100 mL) to generate a pale brown suspension. Pale brown precipitate could be observed via storage at 4  C overnight. After filtration and vacuum dryness, EVAc6 was obtained in 90% yield. All the other EVAc copolymers were obtained in a similar way, and their NMR spectra are shown in Figs. S24eS26. 2.9. Hydrolysis Take the synthesis of EVA6 as an example. Polymer EVAc6 (150 mg), NaOH (0.4 g, 10 mmol), and CH3OH (10 mL) were added into a 25 mL round bottom flask. The reaction mixture was vigorously stirred at room temperature overnight and an opaque suspension was generated. The mixture was poured into cold deionized water (100 mL) and stored at 4  C for 3 h. The generated white precipitate was vacuum-filtered, washed with deionized water, and vacuum dried. Copolymer EVA6 was obtained in 90% yield as a white solid. All the other EVA copolymers were prepared via the similar synthetic procedure, and their NMR spectra are shown in Figs. S27e S29. 2.10. Synthesis of re-generated EVAc copolymers Take the synthesis of re-generated EVAc6 as an example. Copolymer EVA6 (50 mg), acetic anhydride (3 mL), DMAP (catalytic amount) were sequentially weighed and transferred into a 100 mL round bottom flask. Then CHCl3 (30 mL) was added to generate a pale yellow suspension, which was vigorously stirred and refluxed at 90  C overnight. The mixture was cooled to room temperature, neutralized with Na2CO3 solution, washed with deionized water, dried over anhydrous Na2SO4, filtered, and evaporated to afford highly viscous brownish liquid. The re-generated EVAc6 copolymer was obtained in 80% yield. All the other re-generated EVAc copolymers were synthesized similarly.

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3. Results and discussions 3.1. Monomer synthesis and characterization Grignard reaction between aldehyde and organomagnesium compound is a well-known CeC bond formation reaction [37]. Anhydrous anaerobic operation at low temperature is basically required. However, if one substrate is allyl bromide or its derivative, the reaction can be conducted even in water and in a one-pot manner at room temperature with the assistance of metals or metal salts. This can be roughly regarded as the characteristics of Barbier reaction [38]. We selected five linear aliphatic dialdehyde compounds (or their acetal precursors) to undergo Barbier reactions with allyl bromide in water at room temperature for 48 h in the presence of tin dichloride (SnCl2) and potassium iodide (KI). For acetal substrates, they hydrolyzed gradually in acidic medium so that dialdehyde formed in situ. Five structurally symmetric diols as pre-monomers were obtained with yields up to 83%. Subsequent acetylation of these diols generated the ADMET monomers M2-M6 bearing two vinyl acetate units in nearly quantitative yields (Scheme 1). M1 was obtained by direct acetylation of the commercially available hepta-1,6-dien-4-ol in 99% yield. All the monomers were characterized by ESI-MS, and 1H, 13C NMR spectra (Fig. 1 and Figs. S8eS11). These characterizations confirmed the expected monomer structures, monomers M1-M6 were all mixtures of stereoisomers. 3.2. ADMET polymerization, hydrogenation, and hydrolysis In general, Grubbs-I catalyst is preferentially chosen for ADMET polymerization due to its ability in suppressing olefin isomerization. However, a large number of olefins are inert for metathesis reaction if Grubbs-I catalyst is adopted [39]. Indeed, Grubbs-I catalyst is efficient enough for a,u-diene monomers with long methylene spacers in that negative neighboring group effect is not severe. For the monomers in this study, acetoxy group is very close to the terminal alkene so that steric repulsion cannot be overseen when Grubbs catalyst approaches are adopted. As we expected, moderate molecular weight products were generated if Grubbs-I catalyst was utilized for ADMET polymerization (data not shown). Therefore, more active Grubbs-II catalyst was chosen for ADMET polymerization in this study. In order to examine whether olefin isomerization takes place using Grubbs-II catalyst for ADMET polymerization, we synthesized a model compound hept-1-en-4-yl acetate (M0, see Scheme S1) through Barbier reaction between n-butyraldehyde and allyl bromide followed by acetylation. It was found that olefin isomerization was clearly resolved from the NMR (Fig. S20) and the ESI-MS (Figs. S21eS23) spectra of the dimerized product of M0. Nevertheless, this side reaction could be suppressed by adding two equivalents of benzoquinone (BQ) into metathesis system [40]. At first, we tried the ADMET polymerization of M1 and M2 in bulk, respectively. It was observed that the viscosities of the

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reaction mixtures did not increase as in the case of normal ADMET polymerization. After quenching the reaction, we filtered the mixtures through a pad of silica and analyzed the concentrated crude products. The NMR spectra (Figs. S17 and S18) revealed that only cyclic products were generated in both cases with no high molecular weight product as confirmed by the GPC traces (Fig. S19). Not surprisingly, ring-closing metathesis (RCM) reactions occurred to afford cM1 and cM2 due to the Thorpe-Ingold effect [41]. We then followed our previous reported conditions for the ADMET polymerization of the other four monomers M3-M6 [30,31]. The polymerization was conducted in CH2Cl2 at 60  C for 120 h under continuous N2 purge. After a couple of hours, the reaction mixtures became highly viscous, indicating formation of high molecular weight polymers. These unsaturated copolymers (designated as P3eP6, respectively) were separated and characterized (Scheme 2). Following the previously reported hydrogenation conditions [42], we got four saturated EVAc copolymers (from EVAc3 to EVAc6). Hydrolysis of these copolymers by saponification finally generated the EVA copolymers (from EVA3 to EVA6). For the copolymer sample EVA3, the theoretical vinyl alcohol molar content reaches 57%, being the EVA copolymer with ever highest vinyl alcohol content by ADMET polymerization. The molecular weight (Mn and Mw) and polydispersity index (PDI ¼ Mw/Mn) of all the unsaturated and EVAc copolymers were measured by GPC (Fig. 2). All the polymers had high Mn values ranging from 11 800 to 20 800. The PDI values of the EVAc copolymers were in the range of 1.68e1.92, in accordance with the step-growth mechanism. Elevated molecular weights and slightly reduced PDI values were observed after the unsaturated samples were hydrogenated, which can be attributed to the increase in theoretical molecular weights and the decreased molar fraction of low molecular weight components due to additional selective precipitation. The final EVA copolymers were not subjected to GPC measurements due to their insolubility in THF. However, these EVA copolymers could be re-acetylated and transformed into EVAc copolymers. The GPC traces of the re-generated EVAc copolymers were essentially the same, and the NMR spectra were also consistent with the original EVAc copolymers (data not shown). The unsaturated and EVAc copolymers were then characterized by NMR measurements. Fig. 3 shows the spectra of P6 and EVAc6, the NMR spectra of P3-P5 and the corresponding EVAc copolymers are shown in Figs. S14eS16 and Figs. S24eS26, respectively. The signals of terminal alkenes (5.0e5.1 and 5.7e5.8 ppm) of the monomers disappeared completely in the unsaturated copolymers, while the signals of internal alkenes (5.4e5.5 ppm) appeared, suggesting that the ADMET of these monomers were successful with the formation of high molecular weight copolymers. After exhaustive hydrogenation to transform the unsaturated copolymers into EVAc copolymers, the signals of the internal alkenes and the allylic protons (2.1e2.3 ppm) completely disappeared, indicating that 100% saturation of the internal alkenes was achieved using the selected method. In addition, in the NMR spectra of both the unsaturated copolymers and the EVAc

Scheme 1. Monomer Synthesis. Reagents and Conditions: (a) 3 equiv. allyl bromide, 3 equiv. SnCl2, 6 equiv. KI, H2O, 30  C, 48 h (b) 6 equiv. acetic anhydride, 0.2 equiv. DMAP, CH2Cl2, rt, overnight.

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Fig. 1. 1H and

13

C NMR spectra of M6 in CDCl3.

Scheme 2. EVAc and EVA Copolymers via ADMET Polymerization, Hydrogenation, and Hydrolysis. Reagents and Conditions: (a) Grubbs-II catalyst (1.0 mol %), BQ (2.0 mol %), CH2Cl2, N2 purge, 60  C, 120 h (b) 3.3 equiv. TSH, 4.0 equiv. TPA, toluene, 130  C, 24 h; (c) 5 equiv. NaOH, CH3OH, rt, overnight.

copolymers, the signals of a-methine (4.8e4.9 ppm) and methyl (2.0e2.1 ppm) protons of the acetoxy groups were maintained, thus the acetoxy groups can endure both the ADMET polymerization and hydrogenation. In the NMR spectra of the EVAc copolymers, the signals of the terminal methyl groups (0.9 ppm) could be resolved, thus permitting the calculation of absolute Mn values of the EVAc copolymers by comparing the peak area integration ratios between the a-methine protons of acetoxy groups and the terminal methyl protons. These calculated data are summarized in Table 1. The final EVA copolymers were generated from hydrolysis of their EVAc precursors. The 1H NMR spectrum of EVA6 in d6-DMSO is shown in Fig. 4. The 1H NMR spectra of the other EVA copolymers are shown in Figs. S27eS29. The signals of the hydroxyl groups and residual water in d6-DMSO were partially overlapped. The resolution of the main chain methylene protons is also low due to the

limited solubility of the copolymer in DMSO. Fortunately, the signals of the terminal methyl groups (0.9 ppm) could be clearly resolved, thus, the Mn values could be calculated based on the peak integration ratios between the a-methine protons of hydroxyl group (4.2 ppm) and terminal methyl protons. These data are also collected in Table 1, and they coincided well with those of the EVAc precursors. We also conducted IR characterization to confirm the existence of pendent hydroxyl groups in the EVA copolymers, and peaks at 1100 cm1 and 3300 cm1 were ascribed to the stretching vibrations of the CeO and OeH bonds, respectively (Fig. S30). As shown in Fig. 5, the fine chemical structures of the periodic EVAc copolymers were further characterized by MALDI-TOF-MS spectra. A series of peaks with regular intervals that equal to the theoretical repeating unit mass of each copolymer were observed in all these spectra, indicating the integrity of the periodic

Fig. 2. GPC traces of P3-P6 and the EVAc copolymers.

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Fig. 3. 1H and

13

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C NMR spectra of P6 and EVAc6 in CDCl3.

microstructures as expected. However, a minor series of peaks deviating from the major peaks by several methylene units due to the olefin isomerization could still be observed. Such a slight contamination of the isomerized copolymers may somehow affect the precise elucidation of the structureeproperty relationship. These observations also confirmed that olefin isomerization could

not be completely prevented in Grubbs-II catalyst assisted ADMET polymerization even if inhibitor such as benzoquinone was added [43e47]. Hence, in this study, the chemical structures of the copolymers can only reflect the average chemical composition in a single repeating unit. 3.3. Thermal properties

Table 1 Molecular weights and thermal data of the copolymers. Polymer P3 P4 P5 P6 EVAc3 EVAc4 EVAc5 EVAc6 EVA3 EVA4 EVA5 EVA6

Mn e e e e 12 800a 11 400a 18 200a 9 600a 7 100b 6 600b 9 500b 6 100b

Mnc 12 11 18 10 14 13 20 11 e e e e

200 700 200 200 500 300 800 800

Mwc 23 20 37 18 27 23 39 19 e e e e

700 900 300 000 300 000 900 800

Mw/Mnc 1.94 1.79 2.05 1.76 1.89 1.73 1.92 1.68 e e e e

Tg d ( C)

Tmd ( C)

DHmd (J/g)

Tde ( C)

e e e e 4.4 17.1 21.3 32.6 48.3 47.1 46.5 43.9

e e e e n.d. n.d. n.d. n.d. 144.7 145.3 135.1 136.9

e e e e n.d. n.d. n.d. n.d. 23.0 42.7 29.9 42.5

e e e e 301 312 317 317 210 282 308 299

a Calculated according to the peak area integration ratios between the a-methine protons of the acetoxy group (S1) and the main chain terminal methyl protons (S2) for each sample. Then Mn ¼ (theoretical repeating unit mass)  3S1/S2. b Calculated according to the peak area integration ratios between the a-methine protons of the hydroxyl group (S1) and terminal main chain methyl protons (S2) for each sample. Then Mn ¼ (theoretical repeating unit mass)  3S1/S2. c Determined by GPC (1 mL/min) using polystyrene calibration. d Determined by DSC, 10  C/min scan rate, the values recorded from the second scan data. Tm is defined as the peak value. e Determined by TGA, 10  C/min scan rate. Td is defined as the temperature at 5% weight loss.

The thermal stability of the EVAc and EVA copolymers were evaluated by TGA. As shown in Fig. 6, the TGA traces of all the EVAc copolymers displayed typical two-stage decomposition. The first stage occurred at the temperature range from 250  C to 350  C,

Fig. 4. 1H NMR spectrum of EVA6 in d6-DMSO.

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Fig. 5. MALDI-TOF-MS spectra of EVAc3 (A), EVAc4 (B), EVAc5 (C), and EVAc6 (D), respectively.

corresponding to the thermo-induced release of acetic acid from the EVAc copolymers [48,49]. The second stage occurred at above 380  C being the catastrophic decomposition, and the EVAc copolymers decomposed completely at about 480  C. For the four EVAc copolymers, the percentage weight loss due to acetic acid pyrolysis could be calculated, and the predicted values were marked in all the corresponding TGA traces, respectively. Much to our delight, each of the inflection points that connecting the two stages coincided well with the theoretical values, further confirming the integrity of periodic sequences of EVAc copolymers. Likewise, the TGA traces of the EVA copolymers also exhibited twostage decomposition (Fig. S31), with copolymer EVA3 being the most pronounced one. This can be explained as residual water entrapped within the EVA copolymer and/or the thermal elimination of water (due to intermolecular etherification), carbonyls, and

aromatic compounds (due to polymer chain pyrolysis) followed by sharp decomposition to 100% weight loss [50e53]. All the data are also collected in Table 1, where Td is defined as the temperature at 5% weight loss. Glass transition and melting behaviors were measured by DSC for the EVAc and EVA copolymers. EVAc copolymers only exhibited typical glass transition stages in the DSC thermograms of Fig. 7, and no melting peak or other transition was observed even when heated to 150  C. The Tm value of poly(vinyl acetate) is absent since this polymer decomposes before it melts. Wagener et al. reported that EVAc copolymers exhibited Tm values lower than 57  C [54]. However, in their copolymers, the repeating unit contained rather long methylene spacers so that polyethylene chains govern the crystallinity, therefore, the Tm values increased as increasing the ethylene content [55,56]. It can be obviously seen from Fig. 7 that the Tg values of the EVAc copolymers decrease with decreasing vinyl acetate contents (from EVAc3 to EVAc6, 4.4  C for EVAc3 and 32.6  C for EVAc6). This is reasonable if considering that the Tg values of polyethylene and poly(vinyl acetate) are 125  C [57,58] and 30  C [58], respectively. On the other hand, the EVA copolymers displayed both glass transition stages and melting peaks in the DSC thermograms. Based on the data collected in Table 1, the Tg values of the EVA copolymers increase slightly as increasing the vinyl alcohol content (from EVA6 to EVA3).

Fig. 6. TGA traces of the EVAc copolymers.

Fig. 7. DSC thermograms of the EVAc and EVA copolymers.

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Additionally, the Tm values of EVA3 and EVA4 are obviously higher than those of EVA5 and EVA6 due to the higher content of vinyl alcohol [12]. It is reported that a large number of copolymers containing ethylene and polar monomers exhibit lower Tm values than that of HDPE since the pendent group incorporation should disturb the main chain crystallinity. However, for EVA copolymers, extra hydrogen bonding between the hydroxyl groups should compensate such decrease in Tm values [59]. Therefore, it is not surprising that the Tm value of poly(vinyl alcohol) reaches 220  C [60]. In our case, the EVA copolymers showed Tm values from 135  C to 145  C, very close to the Tm value of HDPE [58]. This result suggests that these EVA copolymers could share the same processing technique as that applied for HDPE. Importantly, the melting peaks of these EVA copolymers are quite sharp compared to those of the industrially synthesized sequence random counterparts. The lower DHm values of EVA3 and EVA5 were ascribed to the hampered crystallinity caused by more severe olefin isomerization as discussed above. 4. Conclusions We have demonstrated a facile synthetic method for periodic EVAc and EVA copolymers with controllable sequence and composition via ADMET polymerization and clearly showed a relationship between the sequence and content of EA units and the thermo properties of the EVA copolymers. A series of a,u-diene monomers were obtained in high yields from commercial available materials. Among them, monomer M1 and M2 underwent RCM reaction instead of ADMET polymerization. Monomers M3eM6 could be polymerized by Grubbs-II catalyst in the presence of BQ to generate high molecular weight polymers, hydrogenation of which afforded a series of EVAc copolymers with regular sequence and tunable VAc contents. Subsequent hydrolysis of these EVAc copolymers generated EVA copolymers with EVA-3 containing the ever highest VA content (57 mol%) via ADMET polymerization. The VA contents play crucial roles in determining the thermo properties of the EVA copolymers, the Tg values decreased with lowering of the VA content, while all the EVA copolymers showed sharp melting peaks with the Tm values comparable to that of HDPE. Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 20534010, 21090351, and 21225416) and the National Basic Research Program of China (No. 2011CB201402). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2013.05.037. References [1] Nozakura SI, Morishima Y, Iimuro H, Irie Y. J Polym Sci Polym Chem Ed 1976;14:759e66. [2] Morishima Y, Nozakura S. J Polym Sci Polym Chem Ed 1976;14:1277e82. [3] Buerger DE, Boyd RH. Macromolecules 1989;22:2699e705. [4] Smith GD, Liu FG, Devereaux RW, Boyd RH. Macromolecules 1992;25:703e8.

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