Chiral Amplification in Polymer Brushes Consisting of Dynamic Helical

Sep 25, 2014 - Chiral Amplification in Polymer Brushes Consisting of Dynamic Helical Polymer Chains through the Long-Range Communication of Stereochem...
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Chiral Amplification in Polymer Brushes Consisting of Dynamic Helical Polymer Chains through the Long-Range Communication of Stereochemical Information Katsuhiro Maeda,* Shiho Wakasone, Kouhei Shimomura, Tomoyuki Ikai, and Shigeyoshi Kanoh Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan S Supporting Information *

ABSTRACT: The efficient hierarchical amplification of macromolecular helicity has been achieved through the longrange transfer of chiral information in polymer brushes consisting of a dynamically racemic helical poly(phenylacetylene) backbone and poly(phenyl isocyanate) pendants. The partial introduction of an optically active group at the pendant terminus, which was situated approximately 27 bond lengths (ca. >4 nm) away from the polyacetylene backbone, allowed for the transmission of stereochemical information to the polyacetylene backbone. This transfer of information induced a helical chirality in the polyacetylene backbone, which biased the helical handedness of the polyisocyanate pendants devoid of an optically active terminal group.

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INTRODUCTION Chiral amplification is regarded as an important phenomenon, which is inextricably associated with the origin of biomolecular homochirality in nature.1 Chiral amplification has attracted considerable attention from synthetic chemists interested in the development of ideal methods for the efficient synthesis of optically active chiral materials from small amounts of nonracemic compounds.2 Dynamic helical polymers, such as polyisocyanates and polyacetylenes, are composed of interconvertible right- and left-handed helices, which are separated by very few helical reversals, and these compounds have been reported to exhibit remarkable chiral amplification properties.3 For example, the copolymerization of achiral monomers with a small amount of optically active monomers can produce optically active polymers with a greater excess of one singlehanded helical conformation. This phenomenon was first reported in polyisocyanates by Green et al., who named it the “sergeants and soldiers effect”, where the optically active units (sergeants) controlled the helical sense of the achiral units (soldiers).4 This cooperative phenomenon has also been observed in supramolecular systems as well as other helical polymers.5 The introduction of an optically active group at the chain end of a dynamic helical polymer can also induce a preferred-handed helix.6 Similar levels of preferred-handed helicity induction can also be found in achiral oligopeptides, where the process occurs through an acid−base interaction known as the “chiral domino effect”.7 Long-range stereochemical communication over the nanometer scale has recently received considerable attention from scientists interested in mimicking biological systems.8 However, to the best of our knowledge there have been no reports in the literature to date pertaining to the amplification of macro© 2014 American Chemical Society

molecular helicity based on the sergeants and soldiers effect through long-range stereochemical communication. This is because it has been demonstrated that introduction of stereocenters into pendants at positions far removed from the polymer backbone results in an almost racemic helical conformation and optically active groups need to be introduced close to the polymer backbone in order to bias the helical handedness effectively.9 We previously reported the synthesis of helical polymer brushes composed of a poly(phenylacetylene) backbone and poly(phenyl isocyanate) pendants and found that the helical handedness of the polyacetylene backbone could be controlled through a covalent-bonding chiral domino effect by the introduction of an optically active group, such as (S)-2-(methoxymethyl)pyrrolidinyl ((S)-MMP), at the pendant terminus.10,11 In this study, we have synthesized a series of poly(phenylacetylene)based copolymer brushes bearing poly(phenyl isocyanate) pendants with an optically active group and an achiral group at their pendant termini (i.e., (poly(1Sr-co-21−r) and poly(1Rrco-21−r)) (Figure 1) and demonstrated the first efficient and clear example of the amplification of macromolecular helicity based on the sergeants and soldiers effect through long-range stereochemical communication. Furthermore, we found that the preferred-handed macromolecular helicity induced in the polyacetylene backbone effectively biased the helical handedness of the polyisocyanate pendants in these copolymer brushes without an optically active terminal group (Figure 2). Received: August 4, 2014 Revised: September 18, 2014 Published: September 25, 2014 6540

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Figure 1. Structures of macromonomers (macro-1S, macro-1R, and macro-2) and polymer brushes (poly-1S, poly(1Sr-co-21−r), poly-1R, and poly(1Rr-co-21−r)).

Figure 2. Schematic illustration of amplification of macromolecular helicity based on sergeants and soldiers effect through long-range communication of stereochemical information in copolymer brushes consisting of dynamic helical polymer chains (poly(1Sr-co-21−r)).

Scheme 1. Synthesis of Macromonomers (Macro-1S, Poly-1R, and Macro-2)

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absorption spectral measurements) and a JASCO 348WI apparatus (for CD spectral measurements). VCD spectra were measured in a 0.15 mm BaF2 cell with a Jasco JV-2001YS spectrometer equipped with a temperature controller (EYELA NCB-1200). Elemental analyses were performed by the Research Institute for Instrumental Analysis of Advanced Science Research Center, Kanazawa University, Kanazawa, Japan. Materials. Pyridine (Aldrich), pyrrolidine (Wako), and (S)- and (R)-MMP (TCI) were dried over calcium hydride and distilled under high vacuum. These amines were stored under nitrogen. Anhydrous tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and dichloromethane (CH2Cl2) were purchased from Kanto Kagaku. As the polymerization solvent, the as-purchased anhydrous THF was further dried over LiAlH4 under nitrogen and vacuum-transferred to a dry glass ampule just before polymerization. 3-Methoxyphenyl isocyanate (3MeOPI) (TCI) was dried over calcium hydride and distilled under high vacuum just before polymerization. Oxalyl chloride, tertbutyllithium (1.7 M in pentane), and anhydrous CHCl3 were

EXPERIMENTAL SECTION

Instruments. NMR spectra were recorded on JEOL ECA500 (500 MHz for 1H, 125 MHz for 13C) or LA400 (400 MHz for 1H) spectrometers (Tokyo, Japan) in CDCl3 using tetramethylsilane (TMS) as the internal standard. The number-average weight (Mn) and its distribution (Mw/Mn) of the polymers were determined by size-exclusion chromatography (SEC) on a Tosoh TSKgel Multipore HXL-M column (Tokyo, Japan) using chloroform (CHCl3) as the eluent at a flow rate of 1.0 mL/min. The molecular weight calibration curve was obtained using polystyrene standards (Tosoh). IR spectra were recorded with a JASCO Fourier transform IR-460 spectrophotometer (Hachioji, Japan). The laser Raman spectra were taken on a JASCO RMP-200 spectrophotometer. Absorption and circular dichroism (CD) spectra were measured in a quartz cell with a path length of 0.1, 0.5, or 1 cm on a JASCO V-570 spectrophotometer and a JASCO J-725 spectropolarimeter, respectively. The temperature (−10 to 40 °C) was controlled with a JASCO ETC 505T (for 6541

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purchased from Aldrich. 4-Ethynylbenzoyl chloride12 and Rh(nbd)BPh413 were synthesized according to the previously reported methods. Synthesis of Macromonomers. Macromonomers macro-1S, macro-1R, and macro-2 were synthesized by the anionic polymerization of 3MeOPI using the lithium amides of (S)-MMP (i.e., Li-(S)MMP), (R)-MMP (i.e., Li-(R)-MMP), and pyrrolidine (i.e., Lipyrrolidine) as initiators, respectively. The reactions were subsequently terminated by the addition of 4-ethynylbenzoyl chloride in a similar manner to that reported previously in the literature (Scheme 1).6b−d,10 The polymerization reactions were carried out in dry glass ampules under an atmosphere of dry nitrogen in THF at −98 °C. A typical polymerization procedure is described below. 3MeOPI (0.89 mL, 6.9 mmol) and THF (11.2 mL) were placed in a glass ampule equipped with a three-way stopcock using a syringe, and the resulting solution was cooled to −98 °C. The polymerization reaction was initiated by the addition of a 0.5 M solution of Li-(S)MMP in THF (1.4 mL), which was prepared by the dropwise addition of an equimolar amount of tert-butyllithium in pentane to a solution of (S)-MMP in THF at 0 °C. The resulting mixture was rapidly stirred for 50 min at −98 °C, and the polymerization was then terminated by the addition of an excess of 4-ethynylbenzoyl chloride (4.1 mmol) in a mixture of THF (1 mL) and pyridine (4.8 mmol). The resulting mixture was held at −98 °C for 15 min and then warmed to −78 °C, where it was held for 1 h to ensure complete termination. The mixture was then poured into a large volume of methanol (MeOH), which resulted in the precipitation of the polymeric product. The precipitate was collected by centrifugation and dried in vacuo at room temperature to give the product, which was dissolved in N,N-dimethylformamide (DMF) and stirred for 24 h at 40 °C to remove any −NH-terminated polymers that had not been capped with a 4-ethynylbenzoyl group. The resulting solution was poured into a large volume of hexaneethanol (3/1, v/v) to remove any depolymerization products. The precipitated macromonomer (macro-1S) was collected by centrifugation and dried in vacuo at room temperature overnight (0.41 g, 40% yield). The complete removal of −NH-terminated products was confirmed by the disappearance of the signal corresponding to the −NH group (ca. 10.5 ppm) from the 1H NMR spectrum of the macromonomer (Figure S1 in Supporting Information).6b−d,10 The degree of polymerization (DP) of the macromonomer (m) was determined to be 27 by 1H NMR analysis based on intensities of the peaks corresponding to the pendant phenyl proton resonances (4H, o + p + q + r, 5.6−7.2 ppm) relative to those of the aromatic proton resonances of the terminal phenylacetylene residue (4H, b + c, 7.7 and 7.4 ppm) (Figure S1A in Supporting Information). Two other macromonomers (i.e., macro-1R and macro-2) were synthesized in the same way as macro-1S using different initiators, and the results of these polymerization experiments are summarized in Table 1. Synthesis of Polymer Brushes. Polymer brushes poly-1S and poly-1R were synthesized by the homopolymerization of macro-1S and macro-1R, respectively, according to our previous procedure.10 Copolymer brushes poly(1Sr-co-21−r) and poly(1Rr-co-21−r) (where r

represents the number of macromonomers bearing a terminal chiral group) were synthesized by the copolymerization of macro-1S with macro-2 and macro-1R with macro-2, respectively, at various feed ratios using Rh(nbd)BPh4 as a catalyst (Scheme 2). A typical experimental procedure for the synthesis of poly(1Sr-co-21−r) is described below. Macro-1S (16 mg, 0.0038 mmol) and macro-2 (58 mg, 0.0113 mmol) were placed in a dry ampule, which was then evacuated on a vacuum line and flushed with dry nitrogen. This evacuation−flush procedure was repeated three times, and anhydrous THF (0.086 mL) was then added to the ampule to dissolve macro-1S and macro-2. The resulting solution was treated with a solution of Rh+(nbd)[(η6C6H5)B−(C6H5)3] (4.29 mM) in THF at 30 °C, to give a solution containing the macromonomer and rhodium catalyst at concentrations of 0.11 and 0.0016 M, respectively. The mixture was held for 5 h at 30 °C, and the resulting polymer brush was precipitated by the addition of a large volume of MeOH. The precipitate was collected by centrifugation before being dried overnight in vacuo at room temperature (70 mg, 96% yield). Poly(1S0.25-co-20.75) was found to be soluble in DMSO, THF, and CHCl3. The molecular weight (Mn) and molecular weight distribution (Mw/Mn) of poly(1S0.25-co-20.75) were estimated to be 1.5 × 105 and 3.17, respectively, based on SEC using a polystyrene standard and CHCl3 as the eluent. The stereoregularity of poly(1S0.25-co-20.75) was investigated by NMR and Raman spectroscopy. However, it was difficult to evaluate the stereoregularity of poly(1S0.25-co-20.75) from its 1H NMR spectrum (Figure S2 in Supporting Information) because the signals from the main chain protons, which can be particularly useful for assigning the conformation and configuration of the polyacetylene backbone,14 gave very weak intensities relative to those of the protons of the pendant polyisocyanates. The Raman spectrum of poly(1S0.25-co-20.75) provided some useful information and exhibited intense peaks at 1564 and 1332 cm−1, which were characteristic of cis-polyacetylenes and assigned to the CC and C−C bond vibrations, respectively. Peaks consistent with trans-polyacetylenes, however, were not observed in the Raman spectrum of poly(1S0.25 -co-20.75 ) (Figure S3A in Supporting Information).15 This result indicated that the poly(1S0.25-co-20.75) material synthesized in the current study possessed a highly cistransoidal structure. Other polymer brushes with different compositions were synthesized using the same procedure, and the polymerization results are summarized in Table 2. The cis-stereoregular poly(1S0.25-co-20.75) was placed in a mortar and ground with a pestle at room temperature for 10 min, which caused a pressure-induced cis-to-trans isomerization of the polyacetylene main chain to give trans-enriched poly(1S0.25-co-20.75) (g-poly(1S0.25-co20.75)). In the Raman spectrum of g-poly(1S0.25-co-20.75), new peaks appeared at 1513 and 1202 cm−1, which can be assigned to the CC and C−C bond vibrations in the trans-polyacetylene, respectively.

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RESULTS AND DISCUSSION Optically active (macro-1S and macro-1R) and inactive (macro2) macromonomers composed of a 3MeOPI chain bearing an enantiopure (S)- or (R)-MMP group and an achiral pyrrolidinyl group, respectively, at the initial chain end (α-end) and a polymerizable phenylacetylene moiety at the other end (ω-end) were synthesized according to the previously reported procedures (Scheme 1 and Table 1).6b−d,10 The degrees of polymerization (m) of macro-1S, macro-1R, and macro-2 were determined to be 27, 28, and 33, respectively, by 1H NMR analysis (Figure S1 in Supporting Information). The CD spectra of solutions of macro-1S and macro-1R in THF were mirror images of each other and showed intense induced CD (ICD) signals in the absorption region of the polyisocyanate backbone (ca. 265 nm). These signals indicated that macro-1S and macro-1R formed predominantly right- and left-handed helical conformations, respectively, which were biased by the optically active groups at their respective termini through a

Table 1. Polymerization Results for 3MeOPI in THF at −98 °C for 50 mina polymer b

run

initiator

sample code

yield (%)

mc

Mnd (103)

Mw/Mnd

1 2 3

Li-(S)-MMP Li-(R)-MMP Li-pyrrolidine

macro-1S macro-1R macro-2

40 39 51

27 28 33

3.0 3.1 3.5

1.2 1.2 1.2

a

Terminated by the addition of an excess of 4-ethynylbenzoyl chloride in a mixture of THF and pyridine. bHexane/ethanol (3/1, v/v) insoluble fraction after a holding period of 70 h in DMF at 40 °C. c Determined by 1H NMR spectroscopy in CDCl3. dDetermined by SEC (polystyrene standards) using THF as an eluent at 40 °C. 6542

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Scheme 2. Synthesis of Polymer Brushes Poly-(1Sr-co-21−r) and Poly-(1Rr-co-21−r)

Table 2. Copolymerization Results of Optically Active (Macro-1S or Macro-1R) and Optically Inactive (Macro-2) Macromonomers with Rh+(nbd)[(η6-C6H5)B−(C6H5)3] in THF at 30 °C for 5 ha polymer run

macro-1S (mol %)

1 2 3 4 5 6 7 8 9 10

100 75 50 25 10

macro-1R (mol %)

macro-2 (mol %)

r

sample code

yieldb (%)

Mnc (105)

Mw/Mnc

100 75 50 25 10

0 25 50 75 90 0 25 50 75 90

1 0.75 0.50 0.25 0.10 1 0.75 0.50 0.25 0.1

poly-1S poly(1S0.75-co-20.25) poly(1S0.5-co-20.5) poly(1S0.25-co-20.75) poly(1S0.1-co-20.9) poly-1R poly(1R0.75-co-20.25) poly(1R0.5-co-20.5) poly(1R0.25-co-20.75) poly(1R0.1-co-20.9)

98 96 94 93 97 92 91 94 93 88

1.2 1.4 1.5 1.5 1.1 1.1 1.2 1.4 1.5 1.4

3.5 3.1 3.3 3.2 3.2 3.2 3.3 3.7 3.4 3.9

a

[Macromonomer]/[Rh] = 100, [macromonomer] = 0.1 M. bMeOH-THF (2/1, v/v) insoluble fraction. cDetermined by SEC (polystyrene standards) with CHCl3 as the eluent.

Figure 3. (A) CD and absorption spectra of macro-1S (dotted red line), poly-1S (red line), poly(1S0.75-co-20.25) (blue line), poly(1S0.5-co-20.5) (orange line), poly(1S0.25-co-20.75) (green line), and poly(1S0.1-co-20.9) (purple line) in THF at −10 °C. The molar ellipticity (Δε) and molar absorption coefficient (ε) were calculated using the molar concentration of 3MeOPI (255−325 nm) and the corresponding macromonomers (>325 nm). (B) Changes in the CD intensity at 420 nm (red circle, polyacetylene) and 270 nm (blue square, polyisocyanate) of poly(1Sr-co-21−r) versus the content of the optically active units (r) in THF at −10 °C. The CD intensity at 270 nm (green triangle, polyisocyanate) of macro-1S is also plotted.

THF and afforded poly(1Sr-co-21−r) and poly(1Rr-co-21−r) with high molecular weight in high yields (>90%), as shown in Table 2. Poly-1S and poly-1R were also synthesized in the same way by the homopolymerization of macro-1S and macro-1R, respectively (Table 2). The cis-stereoregular structures of the obtained copolymer brushes were confirmed by laser Raman spectroscopy (Figure S3 in Supporting Information).15

covalent bonding chiral domino effect (Figures 3A and Figure S4A in Supporting Information).6b−d,16 Macro-2, however, which did not have an optically active group at its terminus, showed no ICD signal. The copolymerization reactions of macro-1S and macro-1R with macro-2 were investigated at different feed ratios using the zwitterionic rhodium(I) complex Rh+(nbd)[(η6-C6H5)B−(C6H5)3] (nbd = 2,5-norbornadiene) in 6543

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Figure 4. (A) VCD and IR spectra of poly-1S (red line), poly(1S0.75-co-20.25) (blue line), poly(1S0.5-co-20.5) (orange line), poly(1S0.25-co-20.75) (green line), poly(1S0.1-co-20.9) (purple line), poly-1R (dotted red line), and poly(1R0.25-co-20.75) (dotted green line) in THF at −10 °C. Poly-1S, poly-1R, poly(1Sr-co-21−r), and poly(1Rr-co-21−r) were used at a concentration of 25 mg/mL in THF. All of the spectra were collected for ca. 1 h at a resolution of 4 cm−1. (B) Changes in the VCD intensity of poly(1Sr-co-21−r) at 1753 cm−1 versus the content of the optically active units (r) in THF at −10 °C.

for poly(1Sr-co-21−r) at 270 nm were much more intense than those expected from the poly-1S (dotted blue line in Figure 3B). These results therefore indicated that a predominantly one-handed helical conformation was also induced in the polyisocyanate pendants, which corresponded to the macro-2 units without a terminal optically active group in the copolymer brushes. The vibrational CD (VCD) spectra of the copolymer brushes were also measured in THF at −10 °C to confirm that the ICD signals at 270 nm were occurring as a consequence of the helical screw sense preference of the polyisocyanate backbone, rather than changes in the conformation resulting from the positioning of the phenyl group against the polyisocyanate backbone. The VCD spectra of poly(1Sr-co21−r) and poly-1S exhibited bisignate VCD signals in the CO stretching band region of the polyisocyanate backbone, which reflected the preferred-handed helical structure of the polyisocyanate backbone (Figure 4A). Furthermore, the intensities (Δabs at 1753 cm−1) of these signals also exhibited a similar positive nonlinear relationship relative to the content of the optically active macro-1S units (Figure 4B). The VCD spectra of poly(1R0.25-co-20.75) and poly-1R were virtually mirror images of each other (Figure 4A). These VCD results provided strong evidence in support of the induction of preferred-handed helicity in the polyisocyanate pendants without a terminal optically active group, which corresponded to the macro-2 units. It was envisaged that the preferred-handed macromolecular helicity induced in the polyacetylene backbone would make a significant contribution to the induction of further helicity in the polyisocyanate pendants without an optically active group at their terminus in poly(1Sr-co-21−r) and poly(1Rr-co-21−r). To prove this, the cis-stereoregular poly(1S0.25-co-20.75) was converted to the trans-enriched poly(1S0.25-co-20.75) (g-poly(1S0.25-co-20.75)) by grinding,15b as confirmed by laser Raman spectroscopy (Figure S3B in Supporting Information), because it is known that the cis-stereoregular main-chain structure is essential for the formation of a preferred-handed helical conformation in poly(phenylacetylene)s.18 The resulting gpoly(1S0.25-co-20.75) almost lost its preferred-handed helical conformation, with the corresponding CD spectrum effectively showing the disappearance of the ICD signals resulting from the polyacetylene backbone regions (Figure 5). As expected, gpoly(1S0.25-co-20.75) showed a marked reduction in its ICD

The CD and absorption spectra of poly(1Sr-co-21−r) and poly-1S in THF at −10 °C are shown in Figure 3A.17 As previously reported for the homopolymer poly-1S,10 poly(1Srco-21−r) also exhibited intense ICD signals in the absorption region of the polyacetylene backbone (>325 nm) as well as the absorption region of the polyisocyanate backbone (325 nm).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, 1H NMR spectra of macromonomers, laser Raman, CD, and absorption spectra of poly(1Rr-co-21−r). This material is available free of charge via the Internet at http://pubs.acs.org.

signal at 270 nm, with the intensity of the signal being almost comparable to that of a mixture of macro-1S with macro-2 ([macro-1S]/[macro-2] = 0.25/0.75, mol/mol) under the same conditions. Furthermore, there was a marked decrease in the VCD intensity of the CO stretching band region associated with the polyisocyanate backbone of poly(1S0.25-co-20.75) following its conversion to g-poly(1S0.25-co-20.75), with the intensity becoming almost identical to that of the corresponding mixture of macro-1S with macro-2 (Figure 6). These results

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and by the Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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REFERENCES

(1) (a) Avalos, M.; Babiano, R.; Cintas, P.; Jimenez, J. L.; Palacios, J. C. Chem. Commun. 2000, 887−892. (b) Buschmann, H.; Thede, R.; Heller, D. Angew. Chem., Int. Ed. 2000, 39, 4033−4036. (2) (a) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38, 3419−3438. (b) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008− 2022. (c) Pijper, D.; Feringa, B. L. Soft Matter 2008, 4, 1349−1372. (3) (a) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860−1866. (b) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138−3154. (c) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013−4038. (d) Fujiki, M.; Koe, J. R.; Terao, K.; Sato, T.; Teramoto, A.; Watanabe, J. Polym. J. 2003, 35, 297−344. (e) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem. Rev. 2009, 109, 6102−6211. (4) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.; Oleary, D. J.; Willson, G. J. Am. Chem. Soc. 1989, 111, 6452−6454. (5) Palmans, A. R. A.; Meijer, E. W. Angew. Chem., Int. Ed. 2007, 46, 8948−8968. (6) (a) Okamoto, Y.; Matsuda, M.; Nakano, T.; Yashima, E. Polym. J. 1993, 25, 391−396. (b) Okamoto, Y.; Matsuda, M.; Nakano, T.; Yashima, E. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 309−315. (c) Maeda, K.; Matsuda, M.; Nakano, T.; Okamoto, Y. Polym. J. 1995, 27, 141−146. (d) Maeda, K.; Okamoto, Y. Polym. J. 1998, 30, 100− 105. (e) Nath, G. Y.; Samal, S.; Park, S. Y.; Murthy, C. N.; Lee, J. S. Macromolecules 2006, 39, 5965−5966. (f) Pijper, D.; Feringa, B. L. Angew. Chem., Int. Ed. 2007, 46, 3693−3696.

Figure 6. VCD and IR spectra of poly(1S0.25-co-20.75) before (red line) and after (blue line) grinding and the mixture of macro-1S and macro2 ([macro-1S]/[macro-2] = 25/75, mol/mol) (green line) in THF at −10 °C. The polymer brush and the mixture of macro-1S and macro-2 were prepared at a concentration of 25 mg/mL in THF. All of the spectra were collected for ca. 1 h at a resolution of 4 cm−1.

revealed that the induction of preferred-handed helicity in dynamically racemic polyisocyanate pendants without an optically active terminal group in the copolymer brushes was mainly caused by the induction of macromolecular helicity in the polyacetylene backbone, rather than by some form of chiral interaction with the neighboring preferred-handed polyisocyanate pendants bearing an optically active group at their terminal.19

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CONCLUSION In conclusion, we have demonstrated the amplification of macromolecular helicity based on the “sergeants and soldiers effect” through long-range stereochemical communication in 6545

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increased with an increase of r (Figure 3B and Figure S4B). We think that this slight increase in the ICD intensity at 270 nm is probably due to the induction of preferred-handed helicity in the polyisocyanate pendants without an optically active terminal through chiral interaction with the neighboring preferred-handed polyisocyanate pendants bearing an optically active group at their terminal.

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dx.doi.org/10.1021/ma501612e | Macromolecules 2014, 47, 6540−6546