Synthesis of Sequence-Specific Polymers with Amide Side Chains via

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Synthesis of Sequence-Specific Polymers with Amide Side Chains via Regio-/Stereoselective Ring-Opening Metathesis Polymerization of 3‑Substituted cis-Cyclooctene Kohei Osawa,† Shingo Kobayashi,*,†,‡ and Masaru Tanaka*,†,‡ †

Department of Biochemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan ‡ Institute for Materials Chemistry and Engineering, Kyushu University, CE41 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Highly regio-/stereoregular (trans-head-to-tail) polymers with amide side chains on every eighth backbone carbon were successfully synthesized by ring-opening metathesis polymerization (ROMP) of 3-substituted cis-cyclooctene (3RCOE) using Grubbs second-generation catalyst (G2). Regioregular linear ethylene−acrylamide copolymers were also prepared via hydrogenation of the obtained poly(3RCOE)s. The thermal properties and solubility of the obtained polymers were strongly influenced by the presence of amide hydrogen in the side chains, the presence of unsaturated bonds in the carbon backbone, and the side chain density. The presence of amide hydrogen in the side chains resulted in the formation of crystalline polymers and the lack of solubility of these polymers in common organic solvents. In contrast, the absence of amide hydrogen in the side chains led to the formation of amorphous polymers exhibiting good solubility in common organic solvents, and decreasing values of Tg were observed for amorphous polymers as a result of the saturation of double bonds in the backbone via hydrogenation.

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INTRODUCTION

poly(ethylene-co-functionalized monomer)s obtained have been reported to exhibit characteristic properties that correspond to their side chain structures or depend on the introduced ratio of the functionalized monomers.1,9,10 The copolymers mentioned above inherently possess randomly placed functionalized side chains; in other words, the polymers synthesized by copolymerization of two or more different monomers generally have an irregular monomer sequence. On the other hand, the development of organometallic catalysts over the past few decades has opened a window for the realization of structurally distinctive linear PE derivatives with precisely placed (regioregular) side chains through olefin metathesis reaction.2,3,11−19 Acyclic diene metathesis (ADMET) polymerization11,12 and ring-opening metathesis polymerization (ROMP)2,3,13−19 are the representa-

Important properties of vinyl polymers, such as thermal properties, solubility, and mechanical strength, are strongly influenced by a variety of structural factors in the polymer, including the order, distribution, and chemical structure of the side chains.1−3 Polyethylene (PE) is the vinyl polymer with the simplest structure, and it is known as one of the most commercially available thermoplastics due to its attractive features, such as its excellent chemical resistance to acids and alkalis, ease of molding, and cost-effectiveness.1,4 The introduction of functional groups into PE can be an important method of altering its properties, which leads to the production of novel PE derivatives that exhibit new physical and chemical properties. The synthesis of functionalized PE has generally been accomplished by the copolymerization of ethylene with vinyl monomers that have the functional groups as side chains (e.g., monosubstituted α-olefins). A wide range of functionalized monomers such as propylene,5 1-hexene-6-ol,6 and acrylamide7,8 have been successfully introduced, and the © XXXX American Chemical Society

Received: August 22, 2016 Revised: October 9, 2016

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DOI: 10.1021/acs.macromol.6b01829 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

the tacticity, molecular weight, and molecular weight distribution, and these studies are generally performed using living polymerization techniques.22−31 Contrastingly, there are few reports that describe the influence of the side chain arrangement in polyacrylamides on their physical properties. In this study, to investigate the monomer reactivity and the structure−property relationship, regio-/stereoregular polymers with precisely placed amide side chains were synthesized by the ROMP of 3RCOEs with N-isopropyl (iP), N,N-diethyl (DE), or N,N-dimethyl (DM) carbamoyl groups, which are the side chains corresponding to PNIPAAm, PDEAAm, and PDMAAm, respectively. The regioregular linear ethylene−acrylamide copolymers were synthesized via hydrogenation of the poly(3RCOE)s (Scheme 1).

tive olefin metathesis reactions used to produce the polymers and the reactions have been utilized to prepare model linear PE derivatives. In contrast to ADMET polymerization, which is a form of condensation polymerization, ROMP, which proceeds through a chain polymerization reaction, is a more powerful tool for the synthesis of model linear PE derivatives with a controllable molar mass.3,13 For example, Grubbs et al. reported the synthesis of regioregular polymers by the ROMP of symmetric monomers with odd-membered rings, such as cyclopentene and cycloheptene derivatives.14 In addition to the regioregular polymers, polymers with both high regioregularity and high stereoregularity have also been recently produced by the ROMP of cis-cyclooctene derivatives with a substituent at the 3-position (3RCOE) (Scheme 1).2,3,15,19 For

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Scheme 1. Synthesis of Linear Polyethylene Derivatives via ROMP Followed by Hydrogenation

EXPERIMENTAL SECTION

Molecular Characterization Method. All 1H NMR and 13C NMR spectra were recorded on a JEOL (Tokyo, Japan) ECX-500 MHz at room temperature in CDCl3 except for poly(3iPCOE) and hydrogenated poly(3iPCOE) (500 MHz 1H NMR; 125 MHz 13C NMR; TMS internal standard). 1H NMR and 13C NMR spectra of poly(3iPCOE) and hydrogenated poly(3iPCOE) were recorded at 120 °C in DMSO-d6. The thermal properties of the obtained synthesized compounds were determined by different scanning calorimetry (DSC). DSC was performed on a SII Nano-Technology Inc. (Tokyo, Japan) X-DSC7000. The molecular weight and PDI values of the polymers were determined by gel permeation chromatography (GPC). The GPC measurement was accomplished on a HPLC system utilizing a Tosoh (Tokyo, Japan) HLC-8220 GPC pump and three Tosoh TSKgels columns (super AW5000, super AW4000, and super AW3000), and the system was calibrated to polystyrene standards. THF was used as the eluent with a flow rate of 1.0 mL/min at 40 °C. Electrospray ionization mass spectrometry (ESI-MS) experiments were performed on a JEOL LMS-T100LC operated in the positive mode except for 2. The ESI-MS of 2 was performed in the negative mode. Elemental analysis was performed using a PerkinElmer, Inc. (Waltham, MA), CHNS/O 2400II analyzer. Materials. All reagents were used as purchased unless otherwise specified. Et2O, CH2Cl2, and CHCl3 were dried with molecular sieve 4 Å. COE, N,N-dimethylformamide (DMF), triethylamine, diethylamine, and tributylamine were purchased from Tokyo Chemical Industry. Ninhydrin and orthoperiodic acid were purchased from Wako Pure Chemical industries. Toluene, hexane, o-xylene, Et2O, CH2Cl2, CHCl3, and MeOH were purchased from Kanto Chemical. Oxalyl chloride and cis-4-octene were purchased from Alfa Aesar. Isopropylamine, dimethylamine hydrochloride, Grubbs second-generation catalyst (G2), ethyl vinyl ether, p-toluenesulfonyl hydrazide, and dibutylhydroxytoluene (BHT) were purchased from SigmaAldrich. SiliaMetS DMT was purchased from SiliCycle. Preparation and Analytical Data of 3-(2-Hydroxyindane-1,3dione-2-yl)-1-cyclooctene (1). Ninhydrin (25.3 g, 142.0 mmol), COE (78.5 g, 712.0 mmol), and toluene (600 mL) were added to a 1 L three-neck flask equipped with a Dean−Stark apparatus and then stirred at 140 °C for 40 h. The reaction mixture turned from deep green to brown as the reaction time lapsed, and the reaction was quenched by allowing the mixture to cool to room temperature. The mixture was washed with water and was dried over anhydrous magnesium sulfate. The concentrated solution of (1) (36.6 g, 134.8 mmol, 94.6% yield) was obtained as a pale yellowish solid (for characterization details, see Supporting Information pp S2−S6 and Figures S1−S5). 1H NMR δ (ppm): 1.10−1.75 (m, 8H), 1.98−2.23 (m, 2H), 2.86 (s, 1H), 3.14−3.20 (m, 1H), 5.60 (ddd, 1H, J = 1.2, 9.5, and 10.3 Hz), 5.85 (dt, 1H, J = 7.5 and 10.3 Hz), 7.85−7.91 (m, 2H), 7.95−8.01 (m, 2H). 13C NMR δ (ppm): 25.3, 26.4, 26.5, 29.3, 30.1, 43.1, 78.8, 123.6, 123.7, 125.7, 134.2, 136.5, 136.6, 141.1, 141.2, 200.3, 200.4. ESI-MS: m/z calcd for C17H18O3 [M + Na]+: 293.1149; found [M + Na]+: 293.1039.

example, Hillmyer et al. reported that polymers with high regio-/stereoregularity (head-to-tail (HT)/trans-double bond) were successfully synthesized by the ROMP of 3RCOEs (R = methyl, ethyl, hexyl, or phenyl) using the well-defined Grubbs second- (G2) and third-generation (G3) catalysts.2 Moreover, regioregular linear PE derivatives were obtained by hydrogenation of the regio-/stereoregular polymers. Contrastingly, regio-/stereoirregular polymers were obtained by the ROMP of cis-cyclooctene (COE) derivatives wiht a substituent at the 4or 5-position.13,16 In addition to alkyl substituents, a broad range of polar functional groups have been incorporated at the 3-position of COE derivatives, and the polymers with high regio-/stereoregularity were realized via the same methodology.3,16,19 However, the ROMP of 3RCOE with N-acetylamine was reported to have failed.3,20 Contrary to this, Sampson et al. reported that regio-/stereoregular polycyclobutenamers could be synthesized by ROMP of a cyclobutene derivative with a secondary amide at the 1-position.17 By comparing the chemical structure of the side chains in the studies by Hillmyer and Sampson et al., the amide nitrogen in N-acetylamine is bonded to the 3-position of COE in Hillmyer’s study, while the amide carbonyl of the substituent is linked to the 1-position of cyclobutene in the study by Sampson et al. By taking these two studies into account, regio-/stereoregular poly(cyclooctenamer)s with an amide side chains can be synthesized via the ROMP of 3RCOE with an amide substituent by linking the amide carbonyl group directly to the 3-position of COE; however, this synthesis has not ever been demonstrated. Polyacrylamides such as poly(N-isopropylacrylamide) (PNIPAAm), poly(N,N-diethylacrylamide) (PDEAAm), and poly(N,N-dimethylacrylamide) (PDMAAm) are good examples of side chains that can be incorporated. They have attracted a great deal of research interest, and a variety of studies on these polyacrylamides have been reported due to their high hydrophilicity and lower critical solution temperature (LCST).21 Most studies on the relationship between their controlled structures and physical properties have focused on B

DOI: 10.1021/acs.macromol.6b01829 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Preparation and Analytical Data of 3-Carboxy-1-cyclooctene (2). A mixture of 1 (36.6 g, 134.8 mmol) in diethyl ether (400 mL) was added to a 1 L three-neck flask, and orthoperiodic acid (72.5 g, 318.1 mmol) was added portionwise to the mixture at 0 °C. The reaction mixture was allowed to warm to room temperature and stirred for 5 h. The yellow reaction solution turned to a pale yellow suspension after orthoperiodic acid was added, and a white precipitate was formed. Only the supernatant liquid was collected by decantation, and it was washed with water and then extracted with 5% NaOH(aq). After the separation of the organic layer, the aqueous layer was acidified with 2 N HCl, and then the aqueous solution was extracted with hexane. After drying over anhydrous magnesium sulfate, the concentrated solution of 2 (16.4 g, 106.5 mmol, 75.0% yield) was obtained as a yellow liquid (for characterization details, see Supporting Information pp S7−S10 and Figures S6−S10). 1H NMR δ (ppm): 1.26−2.20 (m, 10H), 3.45−3.53 (m, 1H), 5.68 (ddd, 1H, J = 1.2, 9.2, and 10.3 Hz), 5.80 (dt, 1H, J = 7.5 and 10.3 Hz), 11.3 (br, 1H). 13C NMR δ (ppm): 25.3, 26.6, 29.2, 33.4, 42.6, 127.0, 131.6, 182.5. ESI-MS: m/z calcd for C9H14O2 [M − H]−: 153.0912; found: 153.0955. Preparation of 3-(Chlorocarbonyl)-1-cyclooctene (3). A mixture of 2 (16.4 g, 106.5 mmol) and DMF (a few drops) in CH2Cl2 (30 mL) was added to a 200 mL two-neck flask. Oxalyl chloride (15.3 g, 120.3 mmol) was added dropwise to the mixture using a dropping funnel, and the resulting mixture was stirred for 5 h at room temperature. The reaction mixture turned to a brown color from an orange color over time, and the reaction was quenched by removing the excess oxalyl chloride from the solution under vacuum. 3 was obtained as a brown liquid and was used for the preparation of 3RCOEs without purification and for molecular analysis. Preparation and Analytical Data of 3-(N-Isopropylcarbamoyl)-1cyclooctene (3iPCOE). A mixture of isopropylamine (6.5 g, 110.6 mmol) and triethylamine (11.4 g, 112.3 mmol) in Et2O (45 mL) was added to a 500 mL two-neck flask. 3 was added dropwise to the mixture using a dropping funnel for over 20 min at 0 °C, and then the mixture was stirred for 18 h at room temperature. The white precipitate obtained was removed from the reaction mixture by suction filtration. The filtrate was concentrated on a rotary evaporator, and a yellowish solid was obtained. The solid was dissolved in CH2Cl2; the solution was washed with water, 2 N HCl, and saturated sodium bicarbonate solution and then dried over anhydrous magnesium sulfate. After the solution was concentrated, the resulting crude product was purified by recrystallization with Et2O and CH2Cl2 (3:1) to obtain 3iPCOE (10.6 g, 54.1 mmol, 51.5% yield, Tm = 131 °C) as a white needle crystal (for characterization details, see Supporting Information pp S11−S14 and Figures S11−S15). 1H NMR δ (ppm): 1.12 and 1.13 (d, 3H and 3H, J = 6.3 Hz), 1.24−1.74 (m, 8H), 2.05− 2.19 (m, 2H), 4.01−4.11 (m, 1H), 5.29−5.43 (m, 1H), 5.60 (ddd, 1H, J = 1.7, 9.2, and 10.3 Hz), 5.77 (ddd, 1H, J = 1.2, 9.2, and 10.3 Hz). 13 C NMR δ (ppm): 22.8, 25.3, 26.5, 26.6, 29.3, 33.1, 41.2, 44.3, 128.8, 131.5, 174.2. ESI-MS: m/z calcd for C12H21NO [M + Na]+: 218.1516; found [M + Na]+: 218.1355. Anal. Calcd for C12H21NO: C, 73.80 H, 10.84; N, 7.17; O, 8.19. Found: C, 73.71; H, 10.72; N, 7.13; O, 8.84. Preparation and Analytical Data of 3-(N,N-Diethylcarbamoyl)-1cyclooctene (3DECOE). 3 synthesized above, diethylamine (10.6 g, 145.2 mmol), and triethylamine (13.2 g, 130.5 mmol) were used to prepare from 3DECOE. The resulting crude product was passed through acid and basic aluminum oxide packed column with Et2O. After purification by fractional vacuum distillation from CaH2, 3DECOE (11.1 g, 53.1 mmol, 49.1% yield, bp = 77−80 °C at 0.08 mmHg) was obtained as a colorless viscous liquid (for characterization details, see Supporting Information pp S15−S17 and Figures S16− S20). 1H NMR δ (ppm): 1.09 and 1.18 (t, 3H and 3H, J = 6.9 Hz), 1.34−1.88 (m, 8H), 2.08−2.16 (m, 2H), 3.15−3.46 (m, 4H), 3.51− 3.58 (m, 1H), 5.59 (ddd, 1H, J = 0.6, 9.2, and 10.3 Hz), 5.75 (ddd, 1H, J = 1.2, 9.2, and 10.3 Hz). 13C NMR δ (ppm): 13.2, 14.9, 25.2, 26.8, 27.0, 29.6, 33.2, 39.8, 40.4, 41.8, 129.8, 130.6, 174.4. ESI-MS: m/z calcd for C13H23NO [M + Na]+: 232.1672; found [M + Na]+: 232.1483. Anal. Calcd for C13H23NO: C, 74.59; H, 11.08; N, 6.69; O, 7.64. Found: C, 73.91; H, 11.43; N, 6.44; O, 8.87.

Preparation and Analytical Data of 3-(N,N-Dimethylcarbamoyl)1-cyclooctene (3DMCOE). A suspension of dimethylamine hydrochloride (17.9 g, 219.4 mmol) in CH2Cl2 (50 mL) was placed in a 500 mL two-neck flask. Triethylamine (59.8 g, 591.2 mmol) was added dropwise to the suspension using a dropping funnel for over 10 min at 0 °C to produce dimethylamine, and then the mixture was stirred for 3 h at 0 °C. 3 was then added dropwise to the mixture using a dropping funnel for over 15 min at 0 °C, and then the resulting mixture was stirred for 20 h at 0 °C. The yellow suspension obtained was washed with water, 2 N HCl, and saturated sodium bicarbonate solution and then dried over anhydrous magnesium sulfate. After the solution was concentrated, the resulting crude product was purified by passing it through a plugged silicagel column with hexane and Et2O followed by recrystallization with hexane to obtain 3DMCOE (8.1 g, 44.5 mmol, 40.8% yield, Tm = 56 °C) as a white needle crystal (for characterization details, see Supporting Information pp S18−S20 and Figures S21− S25). 1H NMR δ (ppm): 1.29−1.76 (m, 8H), 2.09−2.19 (m, 2H), 2.94 and 2.99 (s, 3H and 3H), 3.56−3.61 (m, 1H), 5.53 (ddd, 1H, J = 1.2, 9.2, and 10.3 Hz), 5.76 (ddd, 1H, J = 1.2, 8.6, and 10.3 Hz). 13C NMR δ (ppm): 25.1, 26.7, 26.9, 29.5, 32.6, 35.8, 37.0, 39.9, 128.9, 130.9, 175.0. ESI-MS: m/z calcd for C11H19NO [M + Na]+: 204.1360; found [M + Na]+: 204.1205. Anal. Calcd for C11H19NO: C, 72.88; H, 10.56; N, 7.73; O, 8.83. Found: C, 72.87; H, 10.60; N, 7.68; O, 9.18. General Polymerization Procedure. An example of the general polymerization procedure is described for the ROMP of 3iPCOE. A monomer solution of 3iPCOE (5.9 g, 30.0 mmol), G2 (10.2 mg, 12.0 μmol), and cis-4-octene (CTA, 15.5 mg, 138.0 μmol) in dry CHCl3 (2.0 M, 30.2 mL total volume) was prepared in a 100 mL two-neck flask according to the procedure reported by Kobayashi.2 The solution was degassed by three freeze−pump−thaw cycles before allowing the reaction to proceed under a nitrogen atmosphere, and then the solution was stirred at room temperature. The conversion from monomer to polymer was determined from the integration ratio of the monomer and polymer by 1H NMR spectroscopy. After reaching equilibrium, the reaction was quenched by adding ethyl vinyl ether (3.0 mL). The purification procedure of poly(3iPCOE) is different from that of poly(3DECOE) and poly(3DMCOE) because poly(3iPCOE) exhibits poor solubility in organic solvents at room temperature. An excess amount of CHCl3 was added to the reaction mixture of poly(3iPCOE) and stirred for 24 h at room temperature. The precipitate in the mixture was collected by suction filtration to obtain poly(3iPCOE) as a white solid. In the cases of poly(3DECOE) or poly(3DMCOE), the reaction mixture was diluted with CHCl3 followed by the addition of SiliaMetS DMT (metal scavenger, 20 equiv to catalyst), and then the mixture was stirred for 24 h. The catalyst residue was removed by suction filtration by using a glass filter. The solution was concentrated, and the crude products were dissolved in THF. The polymers were precipitated by pouring the solution into an excess amount of mixed solvent (hexane:ethanol = 9:1), and the precipitate was isolated by decantation. The isolated polymers were soaked overnight in pure water, and the water was removed by decantation. The polymers obtained were dissolved in THF and filtered using a syringe filter (pore size: 0.45; membrane: polytetrafluoroethylene; housing: polypropylene). After the solution was concentrated, poly(3DECOE) and poly(3DMCOE) were obtained as a colorless rubbery solids. Analytical Data of Poly(3iPCOE). For characterization details, see Supporting Information pp S21−S24 and Figures S26−S30. 1H NMR δ (ppm): 1.02−1.05 (m, 6H), 1.13−1.64 (m, 8H), 1.93 (m, 2H), 2.70 (m, 1H), 3.77−3.87 (m, 1H), 5.38 (dd, 1H, J = 7.5 and 15.5 Hz), 5.43 (dt, 1H, J = 6.3 and 15.5 Hz), 6.85−6.93 (m, 1H). 13C NMR δ (ppm): 22.7, 22.8, 26.9, 28.8, 29.3, 32.1, 32.8, 40.8, 50.0, 130.8, 131.3, 172.9. Anal. Calcd for C12H21NO: C, 73.85; H, 10.87; N, 7.14; O, 8.15. Found: C, 73.15; H, 10.87; N, 7.00; O, 8.16. Analytical Data of Poly(3DECOE). For characterization details, see Supporting Information pp S25−S27 and Figures S31−S35. 1H NMR δ (ppm): 1.08 and 1.15 (t, 3H and 3H, J = 7.1 Hz), 1.17−1.80 (m, 8H), 1.92−1.99 (m, 2H), 3.04 (m, 1H), 3.17−3.49 (m, 4H), 5.39− 5.43 (m, 2H). 13C NMR δ (ppm): 13.2, 15.0, 27.4, 29.3, 32.5, 33.4, 40.4, 41.7, 46.1, 129.8, 132.0, 173.4. Anal. Calcd for C13H23NO: C, C

DOI: 10.1021/acs.macromol.6b01829 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Synthesis of 3RCOE Derivatives

Table 1. Polymers from the G2-Catalyzed ROMP of 3RCOEs with Amide Groupsa,b Mn (kg/mol) R c

iP DEd DEd DMd DMd

M/G2 2500 2500 500 2500 500

conve (%) 74 36 95 53 85

yield (%) 58 −f 78 −f 79

calcd

1

H NMR

GPC

Mw/Mnh

t-HTi (%)

Tgj (°C)

Tmj (°C)

53 45 64 19 47

− −f 43 −f 29

− −f 1.49 −f 1.53

95 93 97 95 92

59 −f −4 −f 9

190 −k −k −k −k

39 42 42 36 36

g

g

a

cis-4-Octene and ethyl vinyl ether were used as the chain transfer agent (CTA) and polymerization terminator, respectively. bM/[CTA + G2] = 200. c1 M. d2 M. eConversion for 24 h reaction determined by 1H NMR (500 MHz, in CDCl3, TMS std). fNot determined. gNo data due to the poor solubility. hDetermined by GPC (in THF, PS std, at 40 °C). iTrans-head-to-tail content determined by 1H NMR. jDetermined by DSC. k Amorphous polymer. 74.61; H, 11.09; N, 6.67; O, 7.62. Found: C, 74.37; H, 12.61; N, 6.54; O, 8.10. Analytical Data of Poly(3DMCOE). For characterization details, see Supporting Information pp S28−S30 and Figures S36−S40. 1H NMR δ (ppm): 1.11−1.77 (m, 8H), 1.93−1.99 (m), 2.93 and 3.01 (s, 3H and 3H), 3.10−3.16 (m), 5.35 (dd, 1H, J = 8.1 and 15.6 Hz), 5.42 (dt, 1H, J = 6.4 and 15.6 Hz). 13C NMR δ (ppm): 27.3, 29.2, 29.3, 32.5, 32.9, 35.8, 37.2, 46.1, 129.0, 132.5, 174.1. Anal. Calcd for C11H19NO: C, 72.94; H, 10.60; N, 7.68; O, 8.78. Found: C, 72.47; H, 10.32; N, 7.09; O, 8.82. General Chemical Hydrogenation Procedure. An example of the chemical hydrogenation procedure is described for the hydrogenation of poly(3iPCOE). A mixture of poly(3iPCOE) (3.0 g, 15.4 mmol), p-toluenesulfonyl hydrazide (14.4 mg, 77.0 mmol), tributylamine (15.0 mg, 80.9 mmol), and BHT (8.8 mg, 39.9 μmol) in oxylene (70 mL) was added to a 300 mL two-neck flask, and the mixture was refluxed for 11 h at 140 °C (in the case of poly(3DECOE) and poly(3DMCOE), hydrogenation was conducted for 6 h at 100 °C). Reduction of the double bonds in poly(3RCOE)s was confirmed by 1H NMR spectroscopy, in which a disappearance of the olefinic signal was observed. The reaction was quenched by allowing it to cool to room temperature after the disappearance of olefinic signal was confirmed. The purification procedure of hydrogenated poly(3iPCOE) is different from that of hydrogenated poly(3DECOE) and poly(3DMCOE) because hydrogenated poly(3iPCOE) exhibits poor solubility in organic solvents. An excess amount of MeOH was added to the reaction mixture of hydrogenated poly(3iPCOE), and the mixture was stirred for 24 h at room temperature. Hydrogenated poly(3iPCOE) was collected as a white solid on a filter paper by suction filtration. In the case of hydrogenated poly(3DECOE) or poly(3DMCOE), the reaction mixtures were precipitated by pouring the mixtures into an excess amount of mixed solvent (hexane:ethanol = 9:1), and the polymers were isolated by decantation. The isolated polymers were soaked overnight in pure water, and the water was removed by decantation. The polymers obtained were dissolved in THF and filtered using a syringe filter (pore size: 0.45; membrane: polytetrafluoroethylene; housing: polypropylene). After the solution was concentrated, hydrogenated

poly(3DECOE) and hydrogenated poly(3DMCOE) were obtained as a colorless rubbery solid. Analytical Data of Hydrogenated Poly(3DECOE). For characterization details, see Supporting Information pp S31−S34 and Figures S41−S45. 1H NMR δ (ppm): 1.09 and 1.15 (t, 3H and 3H, J = 6.9 Hz), 1.17−1.26 (m, 10H), 1.32−1.63 (m, 4H), 2.45−2.50 (m, 1H), 3.30−3.39 (m, 4H). 13C NMR δ (ppm): 13.2, 15.1, 27.9, 29.6, 30.0, 33.5, 40.4, 41.9, 41.5, 175.6. Anal. Calcd for C13H25NO: C, 73.90; H, 11.94; N, 6.61; O, 7.55. Found: C, 73.57; H, 12.43; N, 6.49; O, 8.02. Analytical Data of Hydrogenated Poly(3DMCOE). For characterization details, see Supporting Information pp S35−S37 and Figures S46−S50. 1H NMR δ (ppm): 1.12−1.27 (m, 10H), 1.30−1.64 (m, 4H), 2.56−2.65 (m, 1H), 2.95 and 3.03 (s, 3H and 3H). 13C NMR δ (ppm): 27.8, 29.5, 29.6, 29.9, 33.2, 35.7, 37.5, 41.4, 176.6. Anal. Calcd for C11H21NO: C, 72.14; H, 11.57; N, 7.60; O, 8.68. Found: C, 71.66; H, 11.18; N, 7.41; O, 9.41.

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RESULTS AND DISCUSSION Monomer Synthesis. The monomer precursor 3 was prepared from COE32,33 through the introduction of ninhydrin, followed by acidolysis with orthoperiodic acid and chlorination with oxalyl chloride. Three 3RCOE derivatives were synthesized by the reaction of 3 with corresponding amines (Scheme 2). The synthesized 3RCOE derivatives were purified by column chromatography, fractional vacuum distillation, or recrystallization, and all the 3RCOEs were isolated with good yield values (41−51%). Polymerization. The 3RCOEs were polymerized with G2 at a ratio of [3RCOE]0/[G2 + CTA]0 ≈ 200 and [3RCOE]0 ≈ 1.0−2.0 M in CHCl3 at room temperature. cis-4-Octene and ethyl vinyl ether were used as the chain transfer agent (CTA) and the polymerization terminator, respectively. The results from ROMP are summarized in Table 1. In the case of 3iPCOE, the reaction mixture turned to a sol-like solution during the polymerization. In contrast, the ROMP of 3DECOE and 3DMCOE proceeded in a homogeneous manner with an increase in the viscosity of the solution, and the mixture D

DOI: 10.1021/acs.macromol.6b01829 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules changed color from pink to brown as time progressed. The structural changes upon ROMP of the 3RCOE derivatives were confirmed by 1H NMR spectroscopy. For example, the olefinic (Ha and Hb) and allylic (Hc) protons of all 3RCOEs shifted to upfield after ROMP (Figure 1 and Figures S51−S53).

Figure 3. Olefinic region of 1H NMR spectra of poly(3DMCOE) prepared with G2.

indicates that the polymer possessing a trans double bond was predominantly formed (Figure 3). Similar results were obtained by the 1H or 13C NMR spectroscopies of poly(3iPCOE) and poly(3DECOE), which suggests that the poly(3RCOE)s with high regio-/stereoregularity were successfully synthesized (see Supporting Information). In addition to clarifying the stereochemical possibilities of the synthesized poly(3RCOE)s, 1H NMR spectroscopy was used to determine the trans-HT content of each poly(3RCOE), and the trans-HT values for poly(3iPCOE), poly(3DECOE), and poly(3DMCOE) were 95, 97, and 92%, respectively (Table 1). The high regio-/stereoregularity of the poly(3RCOE)s obtained could be attributed to a distinctive reaction process for the ROMP of 3RCOE. Generally, the ROMP is known to proceed with the formation of metallacyclobutane (MCB) as a reaction intermediate via a [2 + 2] cycloaddition reaction between the olefin of the monomer and the metal alkylidene of the ruthenium catalyst. Cramer and Hillmyer reported that the ROMP of 3RCOE proceeded via the formation of a MCB with the monomer substituent positioned γ to the Ru center.2,15 This unique MCB formation is attributed to steric repulsion between the substituent at the 3-position of 3RCOE and the Nheterocyclic carbene (NHC) of G2.2,15 This reaction process likely contributed to formation of highly regio-/stereoregular poly(3RCOE)s in this study. Moreover, Hillmyer reported that regio-/stereoregularity of the poly(3RCOE)s trended to increase upon increasing the substituent size of the 3RCOEs. Comparing the trans-HT content values of poly(3RCOE)s synthesized in this study, the value increased in the order of DM < iP < DE, which implies that the size of the substituent in the monomer increased in the order of DM < iP < DE. In addition to the regio-/ stereoregularity of the obtained polymers, we expect that the substituent size of the 3RCOE derivatives would influence the polymerization rate of the 3RCOE derivatives. For the ROMP of 3DECOE, a higher feed molar ratio of G2 was required to achieve higher conversion compared with the ROMP of 3iPCOE (Table 1). This result suggests that the bulkier substituent of the 3RCOE would result in larger steric repulsion between the substituent of the 3RCOE and the NHC in G2, which would lead to the lower polymerization rate. On the other hand, the conversion of 3iPCOE (74%) is higher than that of 3DMCOE (53%) at the same feed molar ratio of G2. This result might be explained by considering the equilibrium reaction of ROMP.13,15 In the case of the ROMP of 3iPCOE, the resulting polymer was precipitated during the polymerization, and the reaction mixture turned to a slurry-like solution. The heterogeneity might decrease the concentration of the resulting polymer in the reaction solution, and the forward reaction of the ROMP could be promoted in the liquid phase.

Figure 1. Olefinic and allylic regions of 1H NMR spectra for 3DMCOE, poly(3DMCOE), and hydrogenated poly(3DMCOE).

The high regioregularity (HT) and stereoregularity (trans) of the obtained poly(3RCOE)s were confirmed by 1H and 13C NMR spectroscopies. Regio-/stereorandom polymers synthesized by ROMP of unsymmetrical substituted monomers exhibit eight olefinic signals in the 13C NMR spectra.34 Contrastingly, all poly(3RCOE)s synthesized in this study exhibited only two olefinic signals, which indicates that polymers with quite high regio-/stereoregularity (trans-HT or cis-HT)2 were obtained (Figure 2).

Figure 2. Olefinic region of 13C NMR spectra for poly(3RCOE)s synthesized by ROMP with G2. 1

H NMR spectroscopy was used to obtain the detailed information on two stereochemical possibilities (cis or trans). By paying attention to the olefinic region of poly(3DMCOE), for example, olefinic signals with doublet of doublets (dd) and doublet of triplets (dt) multiplicities were observed, which indicates that 3DMCOE was predominantly arranged in an HT fashion, and poly(3DMCOE) with side chains on every eighth carbon backbone was formed (Figure 3). Moreover, the magnitude of the coupling constants for two distinct olefinic signals in the 1H NMR spectrum (Jab = 15.6 Hz) is consistent with the trans configuration of the double bond,35 which E

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substituents introduced at the amide nitrogen of side chains affect the molecular mobility of polymers, and ethyl groups exhibit higher molecular mobility than methyl groups. Comparing the Tg values of unsaturated poly(3RCOE)s to those of saturated polymers, the hydrogenation of poly(3RCOE)s led to lower Tg values, which indicates that the formation of saturated bonds by hydrogenation would induce a high molecular mobility. Hillmyer et al. have also reported on the reduction of Tg value by the hydrogenation of poly(3RCOE)s.2,3 In contrast to poly(3DECOE) and poly(3DMCOE), poly(3iPCOE) and hydrogenated poly(3iPCOE) were crystalline polymers with Tm values of 190 and 218 °C, respectively. The presence of amide hydrogen in the side chain should induce intermolecular or intramolecular hydrogen bonding, and this likely results in crystallinity of the polymers. Similar results have been reported by Wagener et al.36,37 They reported that regioregular polymers with amide hydrogen in the side chains were synthesized by ADMET of symmetric α,ω-diene, and the most of the obtained polymers exhibited crystallinity. On the other hand, a lack of crystallinity was observed for polymers with a bulky substituent at the amide nitrogen, which indicates that the introduction of a bulky substituent at the amide nitrogen inhibits inter/intramolecular hydrogen bonding induced by the amide hydrogen. By referring to Wagener’s reports, poly(3iPCOE) might not exhibit crystallinity if its isopropyl moiety were changed into a bulkier substituent. Comparing the Tm value of poly(3iPCOE) with that of a hydrogenated variant, the hydrogenation of poly(3iPCOE) resulted in a higher Tm value, which suggests that the formation of saturated bonds would enhance the degree of crystallinity. Similar results were also reported by Hillmyer et al.2,3,16 Specifically, some of the poly(3RCOE) derivatives exhibited crystallinity after hydrogenation despite the amorphous property of the unsaturated poly(3RCOE) derivatives. These results likely originate from the enhancement of inter/ intramolecular interactions via hydrogenation of the poly(3RCOE)s. Solubility of Poly(3RCOE)s and Hydrogenated Poly(3RCOE)s. The solubility of prepared polymers was strongly influenced by the structure of the side chain and the backbone. Poly(3DECOE) and poly(3DMCOE) were soluble in common organic solvents (benzene, toluene, ethyl acetate, CH2Cl2, 1,4dioxane, THF, DMAc, DMF, NMP, MeOH, and o-xylene). Hydrogenated versions of poly(3DECOE) and poly(3DMCOE) also showed good solubility in THF, MeOH, and o-xylene. In contrast, poly(3iPCOE) was only soluble in highly polar solvents (DMSO, DMF, DMAC, and NMP) at 120 °C. Moreover, hydrogenated poly(3iPCOE) was not soluble in any highly polar solvent, even at 120−160 °C. This result suggests that the formation of saturated bonds via hydrogenation enhances the inter/intramolecular interactions of poly(3iPCOE). All the synthesized poly(3RCOE)s and hydrogenated poly(3RCOE)s were insoluble in water. Contrastingly, the polyacrylamides, such as PNIPAAm, PDEAAm, and PDMAAm, with relatively similar side chains to the poly(3RCOE)s, are known as water-soluble polymers.21 The nonsolubility of the poly(3RCOE)s in water would originate from the lower density of the side chains compared with that of polyacrylamides. From this result, the water solubility of the polymers can be controlled by tuning the density of the side chain. Moreover, the water insolubility of the polymers would contribute to the

The elimination of the polymer from the reaction system could result in the higher conversion of 3iPCOE. Chemical Hydrogenation. The poly(3RCOE)s were hydrogenated by using p-toluenesulfonyl hydrazide. In the cases of poly(3DECOE) and poly(3DMCOE), the reduction of double bonds via hydrogenation was highly selective without any side reactions on the side chains, and the disappearance of olefinic signals was observed by 1H NMR spectroscopy (Figure 1). On the other hand, the reduction of double bonds in poly(3iPCOE) could not be confirmed due to its nonsolubility in organic solvents. The results of hydrogenation are summarized in Table 2. Table 2. Characterization of Hydrogenated Polymersa 3RCOE

yield (%)

Tgd (°C)

Tmd (°C)

b

89 82 90

56 −9 2

218 −e −e

iP DEc DMc a

All hydrogenation was conducted for 6 h, and conversion of poly(3RCOE)s into hydrogenated polymers was >99%. bHydrogenation was conducted at 140 °C. cHydrogenation was conducted at 100 °C. dDetermined by DSC. eAmorphous polymer.

Thermal Properties of Poly(3RCOE)s and Hydrogenated Poly(3RCOE)s. Differential scanning calorimetry (DSC) was used to analyze the thermal properties of poly(3RCOE)s (Table 1) and hydrogenated poly(3RCOE)s (Table 2). Figure 4 shows the DSC profiles of the obtained polymers during the

Figure 4. DSC profiles of poly(3RCOE)s and hydrogenated poly(3RCOE)s during the heating process (10 °C/min): (1) poly(3iPCOE); (2) hydrogenated poly(3iPCOE); (3) poly(3DECOE); (4) hydrogenated poly(3DECOE); (5) poly(3DMCOE); (6) hydrogenated poly(3DMCOE).

heating process. Poly(3DECOE), poly(3DMCOE), hydrogenated poly(3DECOE), and hydrogenated poly(3DMCOE) were amorphous polymers with Tg values at −4, 9, −9, and 2 °C, respectively. By comparing the differences in the Tg value between unsaturated polymers or between saturated ones, lower Tg values were observed for polymers with the diethylamide group as the side chain. This result implies that F

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Education of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and the Center of Innovation Program from the Japan Science and Technology Agency (JST).

utilization of the polymers in water contacting conditions (e.g., coating agent). Although polyacrylamides have been studied as antifouling coating agent to utilize the polymers in biomedical applications,21,38 various complicated processes including copolymerization,21 grafting,38 and cross-linking39 were needed to use the polymers in the biological environment. Contrary to this, the polymers synthesized in this study would be used in the environment without the complicated processes due to their nonsolubility in water.

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CONCLUSION Highly regio-/stereoregular (trans-HT) polymers with amide side chains were successfully synthesized by the ROMP of 3RCOE derivatives using G2. Regioregular linear ethylene− acrylamide copolymers were successfully prepared by hydrogenation of the poly(3RCOE)s. The thermal properties and solubility of the prepared polymers were strongly influenced by the structure of their side chains and backbones. For amorphous polymers, a lower Tg value was observed for poly(3DECOE) compared with poly(3DMCOE), and a similar result was obtained for the hydrogenated variants. Moreover, the formation of saturated bonds by hydrogenation led to lower Tg values of poly(3DECOE) and poly(3DMCOE). In contrast to amorphous polymers, poly(3iPCOE) showed crystallinity, and a higher Tm value was observed after hydrogenation. With respect to the solubilities of poly(3DECOE) and poly(3DMCOE), both unsaturated polymers and saturated ones were soluble in common organic solvents. On the other hand, poly(3iPCOE) was only soluble in highly polar solvents at 120 °C, and a lack of solubility was observed for the hydrogenated version. Contrary to the common solubility in organic solvents, all polymers obtained were not soluble in water. The method used in this study may be a powerful tool for synthesizing regioselective linear ethylene−acrylamide copolymers as novel polyacrylamide derivatives. Additionally, the results contribute toward the development of a straightforward synthesis method for poly(ethylene-co-acrylate)s.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01829. Experimental details, characterization details data, and NMR spectra for obtained products (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.K.). *E-mail: [email protected] (M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K. gratefully acknowledges financial support from JSPS KAKENHI (No. 24750097 and No. 15K05512) from the Japan Society of the Promotion of Science (JSPS). K.O. acknowledges the Innovative Flex Course for Frontier Organic Material System (iFront) program fellowship from Yamagata University, Japan. M.T. acknowledges financial support from the Funding Program for Next-Generation World-Leading Researchers Researchers (NEXT Program) of the Ministry of G

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H

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