Synthesis and Chiroptical Properties of Helical Polyallenes Bearing

Oct 8, 2014 - Living polymerizations of l-1 and d-1 with allylnickel complex as a ... at least in the range of 0–55 °C. Although poly-l-1100 showed...
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Synthesis and Chiroptical Properties of Helical Polyallenes Bearing Chiral Amide Pendants Yuan-Yuan Zhu, Ting-Ting Yin, Xue-Liang Li, Ming Su, Ya-Xin Xue, Zhi-Peng Yu, Na Liu,* Jun Yin, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei 230009, China S Supporting Information *

ABSTRACT: Two allene derivatives, L- and D-N-(1-(octylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)benzamide (L-1 and D-1), bearing chiral amide pendants were designed and synthesized. Living polymerizations of L-1 and D1 with allylnickel complex as a catalyst afforded poly-L-1m and poly-D-1m with controlled molecular weights and narrow molecular weight distributions. These polymers were found to possess a stable helical conformation with a preferred handedness in aprotic solvents on the basis of their circular dichroism (CD) spectra and specific rotation as well as computer simulation. The helical conformation of the polymers was revealed to be stabilized by elongation of the repeating unit until the degree of the polymerization reaches 80. The slightly influence of temperature on the CD spectra of poly-L-1100 in CHCl3 indicated the helical conformation was quite stable at least in the range of 0−55 °C. Although poly-L-1100 showed similar CD spectra in different aprotic solvents, remarkable decrease was observed upon the addition of protic solvents such as methanol due to the weakened hydrogen bonding interactions between the adjacent repeating units. The poly-L-1100 behaves as a pHresponsive property; the helical structure of the main chain can be transformed to random coil by addition of trifluoroacetic acid to the THF solution which again switches back to helical conformation by neutralization with triethylamine. It was confirmed that the copolymerization of L-1 and D-1 obeyed the majority rule as supported by the nonlinear correlation between the enantiomeric excess of monomer 1 with the CD intensities of the generated copolymers. Atomic force microscope (AFM) and scanning electron microscope (SEM) studies revealed poly-L-1100 self-assembled into well-defined helical fibrils with distinct handedness.

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example, hydrogen bonding.3 A considerable number of studies have been conducted over the past few decades, however, to the best of our knowledge, the number of artificial polymers that are able to maintain a stable helical conformation in solution are still limited.4 Sterically restricted poly(methacrylate ester)s5 and poly(aryl vinyl)s,6 polyisocyanides,7 polyisocyanate,8 polycarbodiimide9 and polyacetylenes10 can be cited as the examples of such artificial helical polymers. In addition to their interesting helical structures, these polymers have exhibited broad applications in many fields including enantiomer separation, asymmetric catalyst, chiral recognition and liquid crystals.11 Therefore, the development of novel helical polymers with stable helical conformation is of great interest. Allene derivatives have cumulated double bonds and can be regarded as the isomers of propargyl derivatives. Take advantage of this characteristic, polymers with exomethylene substituents can be obtained through the selective polymer-

INTRODUCTION Many biomacromolecules, such as DNA and proteins possess chiral secondary structure, which plays important roles in realizing marvelous biological activities in living systems.1 In this context, the single-handed helical conformation is often found as one of the most essential higher-order structure. Inspired by the sophisticated biological helices and related unique functions, chemists have been challenged to develop artificial helical polymers and oligomers (foldamer).2 Synthetic polymers possessing stable helical conformation in solution, like biomacromolecules, are of great interest. Because they can display optical activity sole based on their main-chain helical conformation. The purposes for these researches are not only for mimicking the structure and function of the biological helices but also for their wide applications in materials science. Although some stereoregular macromolecules can take a helical conformation in the solid state, they cannot maintain the helical conformation in solution because of either the inversion of the helix or a change in the structure to random coils. A stable helical conformation requires a backbone that is sufficiently rigid to restrict the rotation. This usually is achieved by the noncovalent interactions between the side pendants, for © XXXX American Chemical Society

Received: September 13, 2014 Revised: September 29, 2014

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dx.doi.org/10.1021/ma5019022 | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Monomer L-1 and D-1, and the Corresponding Poly-L-1m and Poly-D-1m

allylnickel complex 2 as catalyst gave poly-L-1m and poly-D-1m in high yields with controlled molecular weights (Mns) and narrow molecular weight distributions (Mw/Mns). These polymers bearing chiral amide pendants adopt a stable helical conformation in aprotic solvents with a preferred handedness. The influences of the polymerization degree, temperature, and solvents on the stability of the helical conformation were investigated and the morphology of the aggregates of these polymers was also disclosed.

ization of either part (1,2- or 2,3-) of the cumulated double bonds.12 Allylnickel(II) complexes have been reported to promote the living/controlled polymerization of allene derivatives by the milestone works of Tomita and co-workers.13 We found that Ni(II)-terminated poly(3-hexylthiophene) can initiate living/controlled block copolymerization of hexadecyloxyallene, afforded well-defined block copolymer with controlled molecular weight and compositions.14 Although many researches have been performed in the past few decades, the investigations on the optically active, helical polyallene and its derivatives with preferred handedness is still very limited to date.15 Amides and peptides have been widely used for construction of various functional materials owning to their structure diversity, versatile chirality, as well as hydrogen bonding properties.16 Thus, a wide variety of amide-based synthetic polymers have been designed and synthesized. Some of those polymers exhibit unique optical properties based on higher order structures such as helix.17 For example, we found that poly(phenyl isocyanide)s bearing an L-alaine pendant with a long decyl chain can form a stable, rod-like helical conformation through strong intramolecular hydrogen bonding between amino-based pendants.18 Rowan et al. found that polyisocyanopeptide main chain adopt controlled helical conformation with twisted beta-sheet side group.19 Masuda and co-workers have developed poly(N-propargylamide)s with an excess of one-handed helical sense which was stabilized by the intramolecular hydrogen bonds between the pendants amide groups.20 In this contribution, two chiral allene derivatives L- and D-N(1-(octylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)benzamide (L-1 and D-1, Scheme 1) were designed and synthesized. Living polymerizations of L- and D-1 with

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EXPERIMENTAL SECTION

Measurements. The 1H and 13C NMR spectra were recorded on Bruker 600 or 400 MHz spectrometers. FT-IR spectra were recorded on PerkinElmer Spectrum BX FT-IR system using KBr pellets at 25 °C. Size exclusion chromatography (SEC) was performed on Waters 515 pump and Waters 2414 differential refractive index (RI) detector (set at 40 °C), and three linear Styragel HR1, HR2 and HR4 columns were used. The number-average molecular weight (Mn) and its distribution (Mw/Mn) data are reported relative to polystyrene standards. Tetrahydrofuran (THF) was used as eluent and the flow rate is 0.3 mL/min. Circular dichroism (CD) and UV−vis spectra were performed on JASCO J1500 and UNIC 4802 UV/vis double beam spectrophotometers, respectively. Quartz cells with 10.0 or 1.0 mm lengths were used in CD and UV−vis measurements. For the pHresponsive experiments, the sample solutions were incubated at room temperature for 5 min after the addition of TFA or TEA. Then CD and UV−vis spectra were recorded to obtain a constant date. The optical rotations were measured in a 10.0 cm quartz cell on a WZZ-2B polarimeter. Melting points of all samples were performed on a MelTemp apparatus and the temperatures are uncorrected. Atomic force microscopy (AFM) experiments were performed on a Digital Instruments Dimension 3100 Scanning Probe Microscope in tapping mode at room temperature. Samples for AFM observations were prepared by drop casting the solutions of poly-L-1100 onto precleaned silicon wafers (or mica as indicated). Scanning electron microscope (SEM) was carried out on a SU8020 operating at 15 kV accelerating B

dx.doi.org/10.1021/ma5019022 | Macromolecules XXXX, XXX, XXX−XXX

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was removed by evaporation under reduced pressure. The rest solution was neutralized with diluted aqueous HCl solution in small portions until the pH reached 2. The formed solid was filter and washed with water. After dried in vacuum and recrystallized with ethanol, 6 was isolated as white solid (7.04 g, 95% yield). 1H NMR (600 MHz, DMSO-d6, 25 °C): δ 12.68 (s, 1H, CO2H), 7.90 (d, J = 9.0 Hz, 2H, ArH), 7.06 (d, J = 9.0 Hz, 2H, ArH), 4.88 (s, 2H, OCH2), 3.61 (t, J = 1.8 Hz, 1H, CCH). Synthesis of L-7. EDC (2.13 g, 11.0 mmol) was added to the solution of L-4 (2.00 g, 9.99 mmol) and 6 (1.76 g, 9.99 mmol) in dichloromethane (20 mL). The resulting mixture was stirred at room temperature for 3 h. The reaction solution was then diluted with dichloromethane, washed with water and brine. The organic layer was dried over Na2SO4 and evaporated to dryness. The residue was purified by column chromatography (dichloromethane/methyl alcohol =30/1, v/v) to give L-7 as white solid (2.68 g, 75% yield). Mp: 123.8− 125.5 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ 7.78 (d, J = 8.4 Hz, 2H, ArH), 7.00 (d, J = 8.4 Hz, 2H, ArH), 6.74 (d, J = 7.2 Hz, 1H, NH), 6.24 (t, J = 4.2 Hz, 1H, NH), 4.74 (d, J = 2.4 Hz, 2H, OCH2), 4.64 (m, 1H, CH), 3.25 (m, 2H, NCH2), 2.54 (t, J = 2.4 Hz, 1H, C CH), 1.49 (m, 5H, CH2 and CH3), 1.26 (m, 10H, CH2), 0.86 (t, J = 6.6 Hz, 3H, CH3). 13C NMR (600 MHz, CDCl3, 25 °C): δ 172.57, 166.43, 160.03, 128.89, 126.87, 114.42, 77.78, 75.96, 55.68, 49.14, 39.51, 31.65, 29.30, 29.13, 29.04, 26.79, 22.48, 18.78, 13.94. [α]25D −4.7 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3280, 3270, 3100, 2960, 2920, 2861, 2188, 1650, 1630, 1579, 1506, 1467, 1446, 1372 cm−1. HRMS m/z: calcd for C21H31N2O3 [M + H]+, 359.2335; found, C21H31N2O3, 359.2274. Anal. Calcd for C21H30N2O3: C, 70.36; H, 8.44; N, 7.81. Found: C, 70.32; H, 8.34; N, 7.76. D-7 was prepared under the same synthetic procedure to that of L-7 described above, and the characterization data are displayed below. 1 D-7: M.P.: 125.2−126.3 °C. H NMR (600 MHz, CDCl3, 25 °C): δ 7.77 (d, J = 9.0 Hz, 2H, ArH), 7.00 (d, J = 9.0 Hz, 2H, ArH), 6.80 (d, J = 7.8 Hz, 1H, NH), 6.34 (t, J = 4.8 Hz, 1H, NH), 4.74 (d, J = 2.4 Hz, 2H, OCH2), 4.65 (m, 1H, CH), 3.25 (m, 2H, NCH2), 2.54 (t, J = 2.4 Hz, 1H, CCH), 1.49 (m, 5H, CH2 and CH3), 1.23 (m, 10H, CH2), 0.86 (t, J = 7.2 Hz, 3H, CH3). 13C NMR (600 MHz, CDCl3, 25 °C): δ 172.40, 166.47, 160.15, 128.89, 126.89, 114.58, 77.80, 76.03, 55.77, 49.16, 39.61, 31.74, 29.39, 29.20, 29.14, 26.85, 22.59, 18.79, 14.06. [α]25D +4.9 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3290, 3242, 3107, 2956, 2925, 2852, 2111, 1657, 1628, 1583, 1510, 1465, 1445, 1370 cm−1. HRMS m/z: calcd for C21H31N2O3 [M + H]+, 359.2335; found, C21H31N2O3, 359.2319. Anal. Calcd for C21H30N2O3: C, 70.36; H, 8.44; N, 7.81. Found: C, 70.39; H, 8.31; N, 7.85. Synthesis of L-1. Under dry nitrogen atmosphere, L-7 (2.68 g, 7.48 mmol) and potassium tert-butoxide (1.26 g, 11.22 mmol) were dissolved in THF (60 mL). After the reaction mixture was stirred overnight at room temperature, the reaction solution was diluted with ether, washed with water and brine. The combined organic layer was dried over Na2SO4 and evaporated to dryness. The residue was purified by column chromatography (ethyl acetate/petroleum ether =1/3, v/v) to give L-1 as a white solid (2.14 g, 80% yield). Mp: 108.1− 110.8 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ 7.77 (d, J = 8.4 Hz, 2H, ArH), 7.08 (d, J = 8.4 Hz, 2H, ArH), 6.84 (t, J = 6.0 Hz, 1H, OCH), 6.76 (d, J = 7.2 Hz, 1H, NH), 6.18 (t, J = 3.0 Hz, 1H, NH), 5.48 (d, J = 6.0 Hz, 2H, CCH2), 4.62 (m, 1H, CHCH3), 3.25 (m, 2H, NCH2), 1.49 (m, 5H, CH2 and CH3), 1.22 (m, 10H, CH2), 0.86 (t, J = 6.6 Hz, 3H, CH3). 13C NMR (600 MHz, CDCl3, 25 °C): δ 202.64, 172.53, 166.36, 159.76, 128.95, 127.95, 116.80, 116.06, 89.73, 49.20, 39.56, 31.69, 29.34, 29.17, 29.09, 26.83, 22.53, 18.83, 13.98. [α]25D −5.5 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3297, 3080, 2961, 2919, 2853, 1655, 1634, 1610, 1497, 1445, 1341 cm−1. HRMS m/z: calcd for C21H31N2O3 [M + H]+, 359.2335; found: C21H31N2O3, 359.2495. Anal. Calcd (%) for C21H30N2O3: C, 70.36; H, 8.44; N, 7.81. Found (%): C, 70.42; H, 8.31; N, 7.81. D-1 was prepared under the same synthetic procedure to that of L-1 described above, and the characterization data are displayed below. 1 D-1. Mp: 108.6−110.7 °C. H NMR (600 MHz, CDCl3, 25 °C): δ 7.77 (d, J = 8.4 Hz, 2H, ArH), 7.08 (d, J = 8.4 Hz, 2H, ArH), 6.84 (t, J = 6.0 Hz, 1H, OCH), 6.81 (d, J = 7.2 Hz, 1H, NH), 6.27 (t, J = 4.8 Hz,

voltage. An E600POL polarizing optical microscope (Nikon, Tokyo, Japan) equipped with a DS-5 M CCD camera (Nikon) connected to a DS-L1 control unit (Nikon) was used to perform the POM observations. Materials. All solvents were purified by the standard procedures before use. 4-Hydroxy benzoic acid, potassium carbonate, 3bromopropyne, n-octylamine, 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDC), N-benzyloxycarbonyl-L-alanine, N-benzyloxycarbonyl-D-alanine, potassium tert-butoxide, bis(1,5cyclooctadiene)nickel(0) (Ni(COD)2), allyl trifluoroacetate, triphenylphosphine were purchased from Aladdin, Sinopharm and SigmaAldrich Chemical Co. Ltd., and were used as received without further purification otherwise denoted. Synthesis of L-3. To a solution of N-benzyloxycarbonyl-L-alanine (2.60 g, 11.6 mmol) in dichloromethane (50 mL) was added EDC (2.30 g, 12.0 mmol) and n-octylamine (1.60 g, 12.4 mmol). After the mixture was stirred at room temperature for 3 h, the solvent was diluted with dichloromethane (100 mL) and washed with water and brine. The organic layer was dried over Na2SO4 and evaporated to dryness under reduced pressure. The residue was recrystallized from nhexane to give L-3 as a white solid (3.68 g, 96% yield). Mp: 96.0−97.8 °C. 1H NMR (600 MHz, CDCl3, 25 °C): δ 7.33 (m, 5H, ArH), 5.99 (s, 1H, NH), 5.11 (t, J = 15.0 Hz, 2H, CH2), 4.18 (t, J = 6.6 Hz, 1H, NH), 3.22 (m, 2H, NCH2), 1.47 (m, 2H, CH2), 1.38 (d, J = 7.2 Hz, 3H, CH3), 1.29 (m, 10H, CH2), 0.87 (t, J = 6.6 Hz, 3H, CH3). 13C NMR (600 MHz, CDCl3, 25 °C): δ 172.14, 155.98, 136.13, 128.47, 128.13, 127.91, 66.87, 50.50, 39.53, 31.72, 29.40, 29.17, 29.13, 26.79, 22.57, 18.73, 14.03. [α]25D −9.6 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3303, 3064, 2958, 2926, 2854, 1684, 1650, 1538, 1467, 1454, 1324, 1267, 1067 cm−1. HRMS m/z: calcd for C19H31N2O3 [M + H]+, 335.2335; found, C19H31N2O3, 335.2337. Anal. Calcd for C19H30N2O3: C, 68.23; H, 9.04; N, 8.38. Found: C, 68.27; H, 9.08; N, 8.41. D-3 was prepared under the same synthetic procedure to that of L-3. The characterization data for D-3 was showed below. 1 D-3. Mp: 94.0−96.6 °C. H NMR (600 MHz, CDCl3, 25 °C): δ 7.33 (m, 5H, ArH), 6.02 (s, 1H, NH), 5.10 (t, J = 14.4 Hz, 2H, CH2), 4.18 (t, J = 6.0 Hz, 1H, NH), 3.22 (m, 2H, NCH2), 1.47 (m, 2H, CH2), 1.38 (d, J = 7.8 Hz, 3H, CH3), 1.26 (m, 10H, CH2), 0.87 (t, J = 6.6 Hz, CH3). 13C NMR (600 MHz, CDCl3, 25 °C): δ 172.08, 155.97, 136.13, 128.50, 128.17, 127.96, 66.94, 50.55, 39.55, 31.73, 29.41, 29.17, 29.14, 26.79, 22.58, 18.69, 14.03. [α]25D +9.4 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3303, 3059, 2926, 2854, 1684, 1651, 1538, 1467, 1324, 1268, 1068 cm−1. HRMS m/z: calcd for C19H31N2O3 [M + 1]+, 335.2335; found, C19H31N2O3, 335.2333. Anal. Calcd for C19H30N2O3: C, 68.23; H, 9.04; N, 8.38. Found: C, 68.19; H, 9.14; N, 8.42. Synthesis of L-4. L-3 (3.68 g, 11.01 mmol) was dissolved in ethyl acetate (80 mL) and methyl alcohol (20 mL). To this solution was added palladium on activated carbon (10%, 0.38 g). The reaction mixture was then stirred at room temperature for 4 h under 1 atm of hydrogen. The reaction solution was filtered and the filtrate was evaporated to dryness under reduced pressure. The isolated crude product of L-4 was used directly in the next step without further purification. D-4 was prepared under the same synthetic procedure to that of L-4, and used directly in the next step without further purification. Synthesis of 5. 3-Bromopropyne (6.0 mL, 7.67 mmol) was added to a mixture of methyl 4-hydroxybenzoate (9.91 g, 6.52 mmol) and potassium carbonate (34.7 g, 25.1 mmol) in acetonitrile (32 mL) via a syringe. The resulting solution was stirred overnight under reflux. After cooled to room temperature and quenched with water, the solvent was extracted with ethyl acetate (20 mL × 3). The combined organic layer was washed with brine (10 mL × 1), dried over anhydrous Na2SO4 and concentrated to dryness. The isolated residue was further purified by silica gel chromatography to give 5 as colorless oil (9.29 g, 75% yield). 1H NMR (600 MHz, CDCl3, 25 °C): δ 8.08 (d, J = 9.0 Hz, 2H, ArH), 7.00 (d, J = 9.0 Hz, 2H, ArH), 4.75 (s, 2H, CH2), 3.89 (s, 3H, OCH3), 2.55 (s, 1H, CCH). Synthesis of 6. LiOH (3.68 g, 0.184 mol) was added to a solution of 5 (8.00 g, 42.1 mmol) in ethanol (40 mL) and H2O (8 mL). After the resulting mixture was stirred overnight under reflux, the ethanol C

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Figure 1. (a) SEC chromatograms of poly-L-1m prepared from L-1 with nickel complex 2 as catalyst in CH2Cl2 at room temperature with different initial feed ratios of monomer to catalyst. (b) Plots of Mn and Mw/Mn values of isolated poly-L-1m as a function of the initial feed ratio of L-1 to 2. Mn and Mw/Mn were determined by SEC with polystyrene standard (SEC conditions: eluent = THF, temperature =40 °C).

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1H, NH), 5.48 (d, J = 6.0 Hz, 2H, CCH2), 4.62 (m, 1H, CH), 3.25 (m, 2H, NCH2), 1.49 (m, 5H, CH2 and CH3), 1.25 (m, 10H, CH2), 0.86 (t, J = 7.2 Hz, 3H, CH3). 13C NMR (400 MHz, CDCl3, 25 °C): δ 202.68, 172.45, 166.39, 159.84, 128.95, 127.97, 116.85, 116.17, 89.80, 49.22, 39.62, 31.73, 29.39, 29.23, 29.20, 26.86, 22.57, 18.83, 14.02. [α]25D +5.6 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3303, 3070, 2959, 2924, 2853, 1659, 1631, 1603, 1500, 1440, 1342 cm−1. HRMS m/z: calcd for C21H31N2O3 [M + H]+, 359.2335; found, C21H31N2O3, 359.2319. Anal. Calcd for C21H30N2O3: C, 70.36; H, 8.44; N, 7.81. Found: C, 70.59; H, 8.30; N, 7.85. Allylnickel(II) Complex 2. This complex was prepared according to the reported literatures.11,12 A flame-dried Schlenk flask charged with allyl trifluoroacetate (0.05 mL, 0.36 mmol), Ni(COD)2 (0.10 g, 0.36 mmol), and toluene (14.0 mL) was sealed with a rubber septum. The mixture was reacted at room temperature for 20 min under dry N2 atmosphere. Then, PPh3 (1.0 M in toluene, 0.37 mL, 0.37 mmol) was introduced via a gastight syringe at 0 °C. The mixture was stirred at room temperature for 1 h to afford the initiator solution of 2 which was directly used for the polymerizations of L-1 and D-1 without further purification. Polymerization. All polymerizations were carried out in a Schlenk tube equipped with a three-way stopcock under dry nitrogen. Take the polymerization of L-1 as an example: a 10 mL oven-dried flask was charged with L-1 (0.05 g, 0.14 mmol), dry CH2Cl2 (1.4 mL), and a stir bar. After a solution of 2 (0.026 M in toluene, 0.05 mL) was added via a microsyringe, and the resulting mixture was stirring at room temperature for 0.5 h. The solution was then poured into a large amount of n-hexane, which caused a white solid to precipitate. The solid was then isolated via filtration, washed with n-hexane and dried under vacuum at room temperature to afford poly-L-1100 (47.5 mg, 95%). SEC: Mn = 36.9 kDa, Mw/Mn = 1.06. 1H NMR (600 MHz, CDCl3, 25 °C): δ 8.32−7.35 (br s, 3H), 7.04−5.84 (br s, 4H), 5.50− 4.50 (br s, 1H), 3.50−3.40 (br s, 4H), 1.98−1.54 (m, 5H, CH2 and CH3), 1.52−1.35 (m, 2H, CH2), 1.32−1.10 (m, 10H, CH2), 0.89− 0.75 (m, 3H, CH3). [α]25D −213.6 (c = 0.1, CHCl3). FT-IR (KBr, 25 °C): 3304, 3080, 2954, 2930, 2856, 1635, 1604, 1500 cm−1. Polymerization of D-1 was performed in a similar way, and the characterization data are summarized below. Poly-D-1100. 92% Yield. SEC: Mn = 37.0 kDa, Mw/Mn = 1.09. 1H NMR (600 MHz, CDCl3, 25 °C): δ 8.32−7.35 (br s, 3H), 7.04−5.84 (br s, 4H), 5.50−4.50 (br s, 1H), 3.50−3.40 (br s, 4H), 1.98−1.54 (m, 5H, CH2 and CH3), 1.52−1.35 (m, 2H, CH2), 1.32−1.10 (m, 10H, CH2), 0.89−0.75 (m, 3H, CH3). [α]25D +212.3 (c = 0.1, CHCl3). FTIR (KBr, 25 °C): 3304, 3080, 2954, 2930, 2856, 1635, 1604, 1500 cm−1. Computer Simulation. The molecular modeling and molecular mechanics calculations were performed according to the reported literature using materials Studio software (version 6.0; Accerlys Software Inc.) with the COMPASS force field implemented was used in the computer simulation.6c

RESULTS AND DISCUSSION Monomer Synthesis and Polymerization. Scheme 1 illustrates the synthetic procedures for monomer L-1 and D-1. The synthesis of L-1 was briefly described here. First, (S)-2(((benzyloxy)carbonyl)amino)propanoic acid reacted with octan-1-amine in CH2Cl2 with EDC as catalyst at room temperature afforded L-3 in 96% yield. Removed the carboxybenzyl group by hydrogenation reduction gave L-4 as colorless liquid in quantitative yield. Methyl 4-hydroxybenzoate reacted with 3-bromoprop-1-yne in reflux acetonitrile with potassium carbonate as base yielded 5, which was further saponification with LiOH led to the formation of acid 6 in 95% yield. Acid 6 coupled with L-4 with the EDC as catalyst generated L-7. L-1 was obtained in 80% yield by the isomerization of L-7 with potassium tert-butoxide as catalyst. The monomer D-1 was obtained under the same synthetic procedure to that of L-1. The structures of L-1, D-1, and the related intermediates were verified by 1H and 13C NMR, FT-IR spectroscopy, mass spectrometry, and elemental analysis. Both L-1 and D-1 behave good solubility in most common organic solvents such as THF, CHCl3, CH2Cl2, toluene, and methanol. 1H NMR spectra of L1 measured in CDCl3 at room temperature is shown in Figure S15 (see Supporting Information). A triplet at 6.84 ppm and a doublet at 5.48 ppm with 2:1 integral area ratio were respectively assigned to the resonances of the CH and CH2 protons of the allenyl group. The integral analysis of the allenyl signal to phenyl group was consistent to the protons ratio of the proposed structure, confirming the formation of the expected monomers. The two NH signals of L-1 appeared at 6.76 and 6.18 ppm in the 1H NMR spectrum. However, the chemical shifts of these protons are not fixed and showed concentration-dependent. For example, they shifted to downfield with increased concentration due to the formation of intermolecular hydrogen bonding. FT-IR spectra of L-1 and D-1 measured at room temperature using KBr plate also support the formation of intermolecular hydrogen bond because intense vibrations around 3300 and 2930 cm−1 were clearly observed (Figure S17, Supporting Information). The polymerization of L-1 was carried out in CH2Cl2 at room temperature by treated L-1 with allylnickel complex 2 with [L1]0/[2]0 = 100 under dry nitrogen atmosphere. Size exclusion chromatography (SEC) analysis of the isolated polymer indicated the polymerization was succeeded and gave an expected polymer poly-L-1100 (the footnote indicates the initial feed ratio of monomer to catalyst) in 95% yield. The SEC D

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Table 1. Polymerization Results of L-1 and D-1 with Allylnickel Complex 2 as Catalyst in CH2Cl2 at Room Temperaturea run

monomer

[α]25Db

[1]0/[2]0c

Mn (kDa)d

Mw/Mnd

yield (%)e

[α]25Db

Δε270f

1 2 3 4 5 6 7 8 9 10

L-1

−5.5 −5.5 −5.5 −5.5 −5.5 −5.5 +5.6 +5.6 +5.6 +5.6

20 40 60 80 100 120 40 80 100 120

7.49 14.4 22.4 29.0 36.9 43.8 14.2 28.1 37.1 42.9

1.06 1.05 1.07 1.05 1.06 1.08 1.04 1.06 1.09 1.08

93 93 96 94 95 94 93 93 92 90

−117.0 −163.0 −196.0 −207.0 −213.6 −215.8 +162.9 +206.0 +212.3 +213.3

−14.4 −20.9 −25.1 −27.5 −27.6 −27.6 +20.8 +27.5 +27.6 +27.6

L-1 L-1 L-1 L-1 L-1 D-1 D-1 D-1 D-1

a

The polymers were synthesized according to Scheme 1. bThe optical rotations of monomers and the respective polymers were recorded in CHCl3 at 25 °C (c = 0.1). cThe initial feed ratio. dThe Mn and Mw/Mn were determined by SEC and reported as equivalent to standard polystyrene. e Isolated yield. fThe molar circular dichroism were measured in CHCl3 at 25 °C (c = 0.5 mg/mL).

chromatogram of poly-L-1100 was depicted in Figure 1a, which showed a monomodal elution peak. The Mn and Mw/Mn of poly-L-1100 were respectively estimated to be 36.9 kDa and 1.06 based on SEC analyses with equivalent to polystyrene standard. Figure S21 (Supporting Information) shows the 1H NMR spectrum of the isolated poly-L-1100 measured in CDCl3 at room temperature. The resonance of allenyl groups of monomer L-1 disappeared completely after the polymerization, suggesting the conversion of the monomer to expected polymer. In addition, the sharp peaks of the other protons of L-1 became broad and relatively weak due to the restricted motion of polymer chain. The poly-L-1100 was found to consist of only the specific 2,3-polymerized unit as convinced by its 1H NMR spectra in CDCl3 at room temperature. That is, no peak assignable to exo-methylene moiety in the 1,2-polymerization was observed around 5.10 ppm in its 1H NMR spectrum. The specific formation of 2,3-polymerized unit is most probably due to the steric bulkiness of the substituent on the allene monomer. FT-IR spectrum of poly-L-1m revealed the formation of intramolecular hydrogen bonding because two intense peaks at 3300 and 2930 cm−1 were observed. The vibration at 2930 cm−1 was considerably increased as compared with the corresponding L-1, indicated the hydrogen bonding are intensified after the polymerization (Figure S22, Supporting Information). The living nature of the polymerization was supported by the polymerizations L-1 with allylnickel complex 2 under various initial feed ratios of [L-1]0/[2]0 in CH2Cl2 at room temperature. The SEC chromatograms of the isolated polymers are shown in Figure 1a. All the polymers exhibited monomodel elution peaks and shift to higher-molecular weight region with the increase of the initial feed ratio of [L-1]0/[2]0. As plotted in Figure 1b, a linear relationship between the Mn of the isolated polymers and the initial feed ratio of monomer L-1 to allylnickel initiator 2 was clearly observed. Therefore, a range of poly-L1ms with different Mn and narrow Mw/Mn ( 29.0 kDa (m > 80, Figure 4a). These results suggested that a helical conformation with preferred handedness of poly-L-1m is stabilized by the elongation of the repeating units. Accordingly, poly-D-1m also showed a similar correlation between the optical rotation and the CD intensities with the Mn and the degree of the polymerizations (Figure 4a). To investigate the stability of the helical conformation of the synthetic polymers, the CD and UV−vis spectra of poly-L-1100 in CHCl3 were measured at various temperatures. As shown in Figure 4b, the CD intensity around 270 nm slightly decreased as temperature increased from 0 to 55 °C, while the profiles of the UV−vis spectra were maintained. In addition, no changes on the CD patterns were observed when the solution was heated up from 0 to 55 °C. Once the temperature was recooled from 55 to 0 °C, the CD intensity was recovered. This result suggested that the helical structure of the backbone of these polymers is quite stable and has a dynamic nature. The slightly heating-induced changes on the CD intensity may come from the variation on the strength of intramolecular hydrogen bonding between the adjacent repeating units at elevated temperature. Due to the stability of the helical structure, there were no substantial changes on the CD and UV−vis spectra of poly-L-1100 when it was measured in different aprotic solvents such as THF, CHCl3, and CH2Cl2 at room temperature. However, addition of protic solvents with more polarity such as methanol to the THF solution of poly-L-1100 caused the considerable decrease in the CD intensity. As shown in Figure 4c, CD intensity at 270 nm of poly-L-1100 decreased from −129.5 to −44.6 with the content of methanol increased from 0 to 70% (c = 0.5 mg/mL, 25 °C). Further addition of methanol will cause the polymer precipitate. In the mixed solvents of THF and methanol, the polymer exhibited almost the same UV−vis absorption spectra, which suggested that no aggregation of the polymer took place in this case. The changes on CD intensity are probably due to the weakened intramolecular hydrogen bonding interactions induced by the protic solvent. Because the hydrogen bonding is sensitive to pH value of the solution, the helical structure of the synthetic helical polymers may exhibit pH-responsive property. To verify, various amount of trifluoroacetic acid (TFA) were added to a THF solution of poly-L-1100. CD and UV−vis spectra were measured after the solution was incubated at room temperature for 5 min to obtain constant data. As shown in Figure 4d, the CD intensity was gradually decreased with the addition of TFA and became constant with the TFA reached to 2.0 eq (relative to repeating units). Interestingly, added triethylamine (TEA) to the resulting solution of poly-L-1100 to neutralize the TFA, the CD intensity was recovered (Figure 4d). Probably the added TFA impaired the intramolecular hydrogen bonding of poly-L1100 and transformed the helical conformation of main chain to random coil. After the TFA was neutralized by TEA, the intramolecular hydrogen bonding was reformed and the helical conformation with the same handedness was formed again. However, the CD intensity could not reach to the initial value, suggesting a small amount of helix-inversion may take place during the acidication and neutralization process. These results

Figure 3. (a) CD and UV−vis spectra of poly-L-1100 in CHCl3 at 25 °C in various concentrations. (b) Plots of CD and UV−vis intensities at 270 nm with the concentration of poly-L-1100 in CHCl3 at 25 °C. (c) CD and UV−vis spectra normalized to the concentration of poly-L-1100 in CHCl3 at 25 °C in various concentrations.

nm increase with the increased concentration of poly-L-1100 and linear relationships were observed between the changes of CD and UV−vis with the concentration of the polymers increased from 0 to 0.7 mg/mL (Figure 3b). The CD and UV−vis spectra normalized to concentrations showed almost the same profiles (Figure 3c), which further confirmed the formation of helical conformation of the polymer backbone. Due to the living nature of the polymerization of L-1 and D-1 with allylnickel complex 2 as catalyst, poly-L-1m, and poly-D-1m F

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Figure 4. (a) Plots of optical rotation and molar circular dichroism of poly-L-1m and poly-D-1m in CHCl3 at 25 °C as a function of molecular weight (c = 0.5 mg/mL). (b) CD spectra of poly-L-1100 measured in CHCl3 at different temperatures. (c) CD spectra of poly-L-1100 measured in different solvents at 25 °C (c = 0.5 mg/mL). (d) CD spectra of poly-L-1100 in THF with the addition of different amount of TFA and TEA at 25 °C (c = 0.5 mg/mL).

indicated the one-handed preferred helical structure of the poly-L-1100 was pH-responsive and the optical activity can be easily controlled by just regulating the pH value of the solution. To examine the effect of the enantiomeric excess of monomer on the helical sense selectivity of the resulting polymer, a series polymers of poly(L-1-co-D-1) containing both L-1 and D-1 units was prepared from the polymerization of the mixture of L-1 and D-1 with different molar ratios while keeping the initial feed ratio of monomer to catalyst at 100. As summarized in Table S1 (Supporting Information), no significant differences in the molecular weight distribution were found in those copolymerizations even when monomer 1 having very low optical purity was used. This result suggested that the helical sense was not under kinetic control. The molar circular dichroism values at 270 nm of the resulting polymers were then plotted against the enantiomeric excess of monomer 1, which is shown in Figure 5. This figure clearly shows the

nonlinear dependence of the anisotropy factor on the enantiomeric excess of monomer, suggesting that a majorityrule effect is operational to such chiral amide-based helical polyallenes. Computer Simulation. To further verify the helical conformation of the main chain induced by the chiral amide pendants of the synthetic poly-L-1m and poly-D-1m, computer simulation was carried out. The COMPASS force field as implemented in the Materials Studio software (version 6.0; Accerlys Software Inc.) was used to carry out the molecular modeling and molecular mechanics calculations. Figure 6 shows

Figure 6. Computer-simulated helical conformations of poly-L-1m with top and side views.

top- and side-view of the three-dimensional optimal structure of a 20-mer model of poly-L-1m. It was found that the most stable conformation of poly-L-1m is 31 rod-like single-handed helical conformation. The linear polymer chain consisting of three parts: the periphery, the aromatic region, and the twisting mainchain core. This structure could be considered as columns like the helical polymer with similar structures.6c

Figure 5. Plot of molar circular dichroism of poly(L-1-co-D-1) as a function of enantiomeric excess of monomer 1. G

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Morphology. The self-assembly morphology of poly-L-1100 in solid state was first investigated using atomic force microscopy (AFM) by spin-casted a solution of poly-L-1100 in CHCl3 onto a clear, precleaned silicon wafers. After the sample was slowly dried in air, it was subjected to AFM observation. The tapping-mode images are shown in Figure 7, parts a and b.

possess a stable helical conformation with preferred handedness in aprotic solution as confirmed by their optical rotation and CD spectra as well as computer simulation. The helical conformation of these polymers was revealed to be stabilized by elongation of the repeating unit until the degree of the polymerization reaches about 80. It was demonstrated that the helical structure of the polymers was quite stable in aprotic solvents and behave pH-responsive property. Moreover, the copolymerization of L-1 and D-1 obeyed the majority rule as supported by the nonlinear correlation between the enantiomeric excess of monomer 1 with the CD intensities of the isolated copolymers. Further studies revealed that poly-L-1100 was self-assembled into well-defined helical fibril with distinct handedness. We believe the present studies will enrich the family of helical polymers and provide a new optically active material for future applications including enantiomer separation, asymmetric catalyst, and liquid crystals.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR, 13C NMR, and FT-IR spectra of L-1, D-1, and the related intermediates, 1H NMR and FT-IR spectra of poly-L1100 and poly-D-1100, and SEC chromatograms and the results of the copolymerization of L-1 and D-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. (a and b) AFM phase images of the self-assembled structures of poly-L-1100. (c) SEM image of the assembled structure of poly-L1100. (d) Polarized optical micrograph of poly-L-1100 in CHCl3 solution taken at room temperature.

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (Z.-Q.W.). *E-mail: [email protected] (N.L.). Notes

The authors declare no competing financial interest.

Well-defined nanofibrils with ca. 30 nm diameters and several micrometers persist length were clearly observed. All the polymers were found to self-assemble into left-handed helical fibrils with a helical pitch of ca. 40 nm, no right-handed helical structures were observed on the AFM images, probably due to the induction of the chiral amide pendants. However, AFM observations of the helical structures of poly-L-1100 were failed on mica substrate. Thus, AFM substrate may play important role on the formation of the well-defined morphologies. The morphology of the self-assembled structures of poly-L-1100 was further examined by scanning electron microscopy (SEM). As shown in Figure 7c, rod-like nanofibers with persist lengths more than several micrometers were observed from the thin film casted from CHCl3 solution of poly-L-1100 (c = 0.05 mg/ mL, 25 °C). The diameter of the fiber was estimated to be ca. 30 nm, which was consistent to the AFM observations. However, due to the resolution of the SEM in this case, the helical structure with distinct handedness could not be determined. As expected from the rigid rod-like features of the polymers with narrow Mw/Mns, the resultant poly-L-1100 exhibited liquid crystal phases as evidenced by their welldefined textures in concentrated CHCl3 solutions under a polarized optical micrograph at room temperature (Figure 7d).

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21104015, 21172050, 21371043, 51303044 and 21304027), Fundamental Research Fund for the Central Universities of China (2011HGRJ0005, 2012HGZY0012 and 2013HGCH0013) and the Natural Scientific Foundation of Anhui Province (1408085QE80). Z.Q.W. thanks the Thousand Young Talents Program for financial support. J.Y. expresses his thanks for Specialized Research Fund for the Doctoral Program of Higher Education (20130111120013) and Research Foundation for Returned Overseas Chinese Scholars of the Ministry of Education of China.

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REFERENCES

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CONCLUSIONS In summary, chiral amide-based allene-derivatives L-1 and D-1 were designed and synthesized. Living polymerizations of L-1 and D-1 with allylnickel complex 2 as catalyst afforded optically active poly-L-1m and poly-D-1m with controlled Mn and narrow Mw/Mn. The main chain of these polymers was found to H

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