Introduction

In nature, macromolecules such as peptides, nucleic acids and polysaccharides have highly ordered structures, mostly forming left- or right-handed helicies. These structures are optically active and are crucial for the special functions of these molecules such as molecular recognition and catalysis. To artificially form a one-handed helical structure, it is necessary to synthesize an optically active polymer by asymmetric polymerization.1, 2, 3, 4

We have previously investigated asymmetric anionic polymerizations of N-substituted maleimides (RMI) with organometal/chiral ligand complexes.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 As RMI consists of a 1,2-disubstituted structure, poly(RMI) can be presumed to produce four types of structures: threo-diisotactic, threo-disyndiotactic, erythro-diisotactic and erythro-disyndiotactic (Scheme 1). Erythro-diisotactic and erythro-disyndiotactic structures cannot be formed with cis-additional reaction because of steric hindrance among monomeric units due to carbonyl groups in imide rings.16 Trans-additional structures, that is, threo-diisotactic and threo-disyndiotactic structures, can be formed in the polymerization of RMI. Threo-disyndiotactic structures show no optical activity because (S, S)- and (R, R)-configurational pairs exist at equivalent levels in the polymer main chains. Poly(RMI) with a threo-diisotactic structure can show optical activity when one of the asymmetric carbon pairs in the monomeric unit exists in excess. Furthermore, the high continuity of the same configurational pair ((S, S) or (R, R)) in the threo-diisotactic structures may allow for a helical conformation. Asymmetric induction of the polymer main chains was affected to a large extent by N-substituents, polymerization solvents, polymerization temperatures and other polymerization conditions. In our previous studies, we systematically investigated the polymerization of RMIs with various N-substituents. The RMI with a 1-naphthyl or (S)-methylbenzyl group as an N-substituent showed the highest specific optical rotation in chloroform ([α]435=+762.3°, +551.7°).

Scheme 1
figure 1

Structure of poly(RMI).

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Crown ether is a cyclic ether derivative and has attracted a great deal of attention as a host molecule that can incorporate metal cations (M+) and alkyl ammonium salts (R-NH3+) such as amines and amino acids as guest molecules. These complexes are formed by dipole–dipole interactions between crown ether and cations. Recently, polymers bearing various crown ethers were investigated. For example, Yashima17, 18, 19, 20, 21 and Kakuchi et al.22, 23, 24, 25 reported that poly(phenylacetylene) and poly(isocyanate) bearing aza crown ether or benzo crown ether as host molecules induced a one-handed helical structure on addition of alkyl ammonium salts, amino alcohols and chiral amino acids. However, there have been no reports on the asymmetric polymerization of maleimide bearing crown ether derivatives.

In this study, maleimide bearing N-4′-benzo 15-crown-5 ether (B15C5MI) was synthesized from maleic anhydride and the corresponding amine. Asymmetric anionic polymerizations of B15C5MI were performed with an organometal and a chiral ligand complex as initiators (Scheme 2). Random or block copolymerizations of B15C5MI were also carried out with N-1-naphthylmaleimide (1-NMI) or (S)-(–)-methylbenzylmaleimide ((S)-MBZMI) (Scheme 3). Chiroptical properties and structures of the polymers were investigated by 13C nuclear magnetic resonance (NMR), gel permeation chromatography and circular dichroism (CD) measurements.

Scheme 2
figure 2

Asymmetric anionic polymerization of B15C5MI.

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Scheme 3
figure 3

Asymmetric anionic copolymerization of B15C5MI with 1-NMI and (S)-MBZMI.

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Experimental procedure

Materials

Solvents used for syntheses, polymerizations and measurements were purified in the usual manner and distilled over sodium metal. n-Butyllithium (n-BuLi) (Kanto Chemical, Tokyo, Japan, in n-hexane, 1.55 mol l−1) was used as purchased. Diethylzinc (Et2Zn) (in n-hexane, 1.04 mol l−1) was kindly supplied by Tosoh Corp. (Tokyo, Japan). (–)-Sparteine ((–)-Sp) (Tokyo Chemical Industry, Tokyo, Japan) was used after purification by distillation under reduced pressure ([α]435=−10.3°, c=1.0 g per 100 ml, l=10 cm, tetrahydrofuran (THF)). (S, S)-(1-Ethylpropylidene)-bis(4-benzyl-2-oxazoline) ((S, S)-Bnbox) was prepared as reported previously ([α]435=−123.5°, c=1.0 g per 100 ml, l=10 cm, THF).26, 27, 28 2, 2′-Azobisisobutylonitrile (Ishizu Seiyaku, Osaka, Japan) was used after recrystallization from methanol.

Monomer synthesis

The synthetic route of the monomer is shown in Scheme 4.

Scheme 4
figure 4

Synthesis of B15C5MI.

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4′-Aminobenzo 15-crown-5 ether

4′-Nitrobenzo 15-crown-5 ether (Tokyo Chemical Industry, 5.1 g, 16.3 mmol) was dissolved in ethyl acetate (400 ml) in a Schlenk reaction tube, and then 10 wt % palladium-activated carbon (Wako Pure Chemical Industries, Osaka, Japan; 0.51 g) was added. The solution was evacuated by aspirator and replaced by hydrogen gas. After stirring under a hydrogen atmosphere for 1 week, the reaction tube was evacuated by aspirator, replaced by nitrogen gas and then filtered to remove palladium-activated carbon. The filtrate was concentrated under reduced pressure to yield a red–purple liquid (yield: 4.6 g (99.4 %), 1H NMR (δ in p.p.m. from tetramethylsilane in CDCl3): 3.75 (s, 8H, other -O-CH2-), 3.84-3.93 (m, 4H, Ph-O-CH2-CH2-), 4.03-4.12 (m, 4H, Ph-O-CH2-CH2-), 6.21 (dd, 1H, Ar), 6.28 (d, 1H, Ar), 6.74 (d, 1H, Ar)).

N-4′-Benzo 15-crown-5 maleamic acid

A solution of 4′-Aminobenzo 15-crown-5 ether (4.6 g, 16.2 mmol) in dry THF (100 ml) was added dropwise to a solution of maleic anhydride (1.9 g, 19.4 mmol) in dry THF (100 ml) at room temperature, and then the mixture was stirred for 18 h. Yellow precipitates were collected by suction filtration to obtain N-4′-Benzo 15-crown-5 maleamic acid. N-4′-Benzo 15-crown-5 maleamic acid was dried under vacuum at room temperature for 2 days (yield: 5.1 g (82.7%), 1H NMR (δ in p.p.m. from tetramethylsilane in CDCl3): 3.72 (s, 8H, other -O-CH2-), 3.82-3.92 (m, 4H, Ph-O-CH2-CH2-), 3.98-4.10 (m, 4H, Ph-O-CH2-CH2-), 6.40 (d, 1H, -CH=CH-CONH-), 6.44 (d, 1H, -CH=CH-COOH), 6.77 (d, 1H, Ar), 6.98 (dd, 1H, Ar), 7.29 (d, 1H, Ar), 9.24 (s, 1H, -CH=CH-CONH-)).

B15C5MI

Acetic anhydride (30 ml) and anhydrous sodium acetate (0.13 g, 1.3 mmol) were combined in a flask. N-4′-Benzo 15-crown-5 maleamic acid (5.1 g, 13.4 mmol) was added, and the resulting suspension was dissolved by stirring and heating for 1 h. The reaction mixture was cooled almost to room temperature and then poured into a mixture of ice water (300 ml) and dichloromethane (200 ml). After acetic acid was neutralized with NaHCO3, the precipitated product was removed by suction filtration. The filtrate was extracted three times with dichloromethane (200 ml × 3) and the organic phase was washed with saturated NaHCO3aq (200 ml) and water (200 ml × 3) in sequence and then dried over Na2SO4. The organic phase was concentrated under reduced pressure to yield a yellow solid (yield: 3.0 g (61.0%), m.p.: 149.5–150.5 °C, 1H NMR (δ in p.p.m. from tetramethylsilane in CDCl3): 3.70–3.80 (m, 8H, other -O-CH2-), 3.85–3.95 (m, 4H, Ph-O-CH2-CH2-), 4.10-4.20 (m, 4H, Ph-O-CH2-CH2-), 6.80 (s, 2H, -CH=CH-), 6.88 (d, 1H, Ar), 6.90 (d, 1H, Ar), 6.95 (d, 1H, Ar), 13C NMR (δ in p.p.m. from tetramethylsilane in CDCl3): 69.5–69.1 (Ph-O-CH2-CH2-), 71.1–70.5 (other -O-CH2-), 77.5–76.5 (Ph-O-CH2-CH2-), 112.4 (Ar), 113.9 (Ar), 119.2 (Ar), 124.3 (Ar), 134.1 (-CH=CH-), 148.9 (Ar), 149.3 (Ar), 169.7 (C=O)).

N-1-Naphthylmaleimide

1-NMI was synthesized from maleic anhydride and 1-naphthylamine according to a previously reported method.29, 30

(S)-(–)-N-α-Methylbenzylmaleimide

(S)-MBZMI was synthesized from maleic anhydride and (S)-(-)-1-phenylethylamine according to a previously reported method.29

Anionic homopolymerization

Monomer and chiral ligand were placed in a Schlenk reaction tube and a pear-shaped flask, respectively. The flasks were evacuated by vacuum pump and then replaced by nitrogen gas. Polymerization solvent was added to each vessel by a syringe under nitrogen atmosphere. Organometal (n-BuLi or Et2Zn) in an n-hexane solution was introduced into the chiral ligand solution with a syringe to prepare the initiator complex. The monomer solution was maintained at the polymerization temperature. The complex solution was added by a cannula in a stream of nitrogen to initiate polymerization. Polymerization without chiral ligand was initiated by directly adding organometal in an n-hexane solution to a monomer solution with a syringe. After the prescribed time (24 or 72 h), polymerization was terminated with a small amount of methanol containing two drops of 6 N aqueous hydrochloric acid. The polymerization solution was concentrated. Dichloromethane was added to the residue, and the solution was washed using 0.1 N aqueous hydrochloric acid. After the organic layer was concentrated, the residue was purified by three cycles of reprecipitation from THF-methanol. The pure polymers were dried in vacuo at room temperature for 2 days.

Anionic random copolymerization

The mixtures of B15C5MI and 1-NMI or (S)-MBZMI were placed in a Schlenk reaction tube. The tube was evacuated by vacuum pump and then replaced by nitrogen gas. Polymerization solvent was added to each vessel using a syringe under nitrogen atmosphere. From that point, the reaction proceeded in the same manner as the homopolymerization.

Anionic block copolymerization

After 1-NMI or (S)-MBZMI was homopolymerized with the organometal/chiral ligand complex for 6 h, a B15C5MI monomer solution was added to the solution and block copolymerization was initiated.

Radical polymerization

Radical polymerization was conducted in a sealed glass tube using 2, 2′-azobisisobutylonitrile as an initiator under nitrogen atmosphere. All reagents were added to the glass tube, and the mixture was degassed three times before sealing the tube under vacuum. Polymerization was conducted in an oil bath regulated at 60 °C for 24 h. The polymerization was terminated by cooling the reaction mixture to 0 °C. The obtained polymer was purified in a method similar to that of anionic polymerization.

Measurement

Specific optical rotations were measured in THF or chloroform at 25 °C using a quartz cell (l=5 cm) by a JASCO DIP-1030 polarimeter (JASCO, Tokyo, Japan). CD and ultraviolet (UV) spectra were recorded in THF or chloroform at 25 °C using a quartz cell with a path length of 0.1 mm by a JASCO J-805 spectropolarimeter. The number-average molecular weights (Mns) of the polymers were measured with gel permeation chromatography with a Shimadzu LC-10A instrument (Shimadzu Corp., Kyoto, Japan) equipped with an SPD-10A UV detector (Shimadzu Corp.), a JASCO OR-990 polarimetric detector and a C-R7Ae plus data processor. THF or chloroform was used as the eluent. The molecular weight calibration curve was obtained with polystyrene standards. 1H and 13C NMR spectra were obtained by JEOL-EX270 (JEOL, Tokyo, Japan).

Results and Discussion

Homopolymerization of B15C5MI

Radical or asymmetric anionic polymerizations of B15C5MI were performed under several conditions. Table 1 summarizes the results of these polymerizations of B15C5MI. When only n-BuLi was used, the yield of poly(B15C5MI) obtained in toluene (Run 2 in Table 1) was lower than that in THF (Run 3 in Table 1), indicating that the polarity of the solvent significantly affects the yield of the polymerization of B15C5MI with n-BuLi. A similar phenomenon was found for the polymerizations of 1-NMI.10 When the concentration of the initiator solution was 10 mol%, polymerization did not proceed, but a concentration of 20 mol% yielded a polymer. The yields of the methanol-insoluble part of poly(B15C5MI)s obtained with n-BuLi/ligand complexes were higher than those with Et2Zn/ligand complexes. This may result from the differences in electronegativity and valence between n-BuLi and Et2Zn, because the occurrence of anionic polymerization is generally influenced by the electronegativity of the metal. The Mn and yields of polymers obtained with the organometal/ligand complexes were higher than those obtained with only organometal. These results confirmed that the catalytic activity of the organometal/ligand complex was higher than that of organometal only. That is, the coordination of the organometal by the ligand would contribute to the enhancement of catalytic activity.31 To investigate the interaction between crown ether and lithium or zinc in the polymerization, the 1H NMR spectra of the monomer solution added to lithium chloride or zinc chloride were measured. No interaction was observed between crown ether and metal in the 1H NMR spectra. Therefore, it can be concluded that lithium and zinc did not affect the polymerization of maleimide with crown ether. Poly(B15C5MI) was synthesized with Mn=1500–5400 by anionic polymerization. Previous papers reported Mn values from 1600 to 3400 of maleimide polymers with bulky substituents (N-substituted: 9-fluorenyl, diphenylmethyl, triphenylmethyl and cyclododecyl),6, 7 similar to the molecular weights reported here for poly(B15C5MI)s. Therefore, these results suggest that the bulky substituent affected the polymerization ability of maleimide.

Table 1 Polymerizations of B15C5MIa
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Optical properties of poly(B15C5MI)

All polymers obtained with organometal/chiral ligand complexes were optically active. The poly(B15C5MI) initiated by the Et2Zn/(S, S)-Bnbox complex showed the highest specific optical rotation of the obtained polymers. The absolute values of the specific optical rotation of poly(B15C5MI)s formed with Et2Zn/ligand complexes were higher than those with n-BuLi/ligand complexes. Polymerization was slow when Et2Zn was used as an initiator, which could indicate that the stereoregularity of the polymer was improved because the growth end attacked a monomer selectively. The specific optical rotations of the polymers synthesized with Et2Zn/(–)-Sp and Et2Zn/(S, S)-Bnbox initiators were negative and positive, respectively. In our previous study, poly(N-phenylmaleimide)s with a phenyl group as the N-substituent initiated with Et2Zn/(–)-Sp had a negative specific optical rotation, and that initiated with Et2Zn/(S, S)-Bnbox had a positive specific optical rotation.31 Therefore, the polymerization mechanism of poly(B15C5MI)s tended to be similar to that when poly(N-phenyl maleimide) was used as the chiral ligand.

Structures of poly(B15C5MI)

13C NMR spectra of poly(B15C5MI) obtained with various initiators were measured. The methine carbons of the main chain with a phenyl group as the N-substituent and carbonyl groups of the maleimide ring showed peaks at about 38–48, 110–150 and 176 p.p.m., respectively. According to the 13C NMR spectra of poly(CHMI) obtained with anionic initiators, the methine carbons of the main chain exhibited two peaks at 40 and 43 p.p.m. assigned to threo-diisotactic and threo-disyndiotactic structures, respectively.32 The threo-diisotactic and threo-disyndiotactic structures of the poly(B15C5MI)s obtained by anionic polymerization were indistinguishable by the 13C NMR spectra, but the peaks of those obtained by radical polymerization appeared around 39 and 42 p.p.m., respectively. The content of the threo-diisotactic structure in the polymer obtained by radical polymerization was higher than that obtained by anionic polymerization.

Copolymerization of 1-NMI and B15C5MI

In our previous report,9, 10 homopolymerizations of 1-NMI were performed with an organometal and a chiral ligand. As a result, poly(1-NMI) with the specific optical rotation of 762.3° was obtained with Et2Zn/(S, S)-Bnbox in THF at room temperature for 72 h. This polymer showed the highest specific optical rotation among all maleimide polymers. Accordingly, asymmetric anionic copolymerizations of B15C5MI and 1-NMI were performed with Et2Zn/(S, S)-Bnbox in THF at room temperature for 72 h. This polymerization condition is the same as the poly(1-NMI) condition that yielded the highest specific optical rotation. Table 2 shows the results of copolymerizations of B15C5MI and 1-NMI. In the case of both the random copolymer and the block copolymer, a higher copolymer yield was obtained when 1-NMI was 95 mol% in the feed monomer (Runs 2 and 4 in Table 2). These results suggest that steric hindrance by a bulky benzocrown ether group of B15C5MI may have affected the copolymerization ability. Comparing block copolymers with random copolymers, the yields and Mn values of block copolymers (Runs 3 and 4 in Table 2) were higher than those of random copolymers (Runs 1 and 2 in Table 2) under the same conditions. The results of block copolymerizations suggested that 1-NMI was polymerized first because of its high polymerization ability. Therefore, copolymerization ability depends on the polymerization ability of 1-NMI.

Table 2 Asymmetric anionic random copolymerizations or block copolymerizatons of 1-NMI (M1) and B15C5MI (M2)a with Et2Zn/(S, S)-Bnbox (1.0/0.5)b in THF at r.t. for 72 h.
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Optical properties of poly(1-NMI-co-B15C5MI)

The specific optical rotation of the copolymer differed depending on the composition of the copolymer. Increasing the 1-NMI content in the copolymer tended to yield a positive specific optical rotation. In the homopolymerization of 1-NMI, poly(1-NMI) obtained with Et2Zn/(S, S)-Bnbox as an initiator had a large positive specific optical rotation (+762.3°).10 Therefore, these results demonstrate that the optical rotation of the copolymer depends on the 1-NMI content.

Figure 1 shows CD and UV spectra of the copolymers. The UV spectra of the copolymer showed peaks around 210–240 nm and 260–310 nm, which could be attributed to the 1Bb (long axis) and 1La (short axis) electron transition of naphthalene chromophore in the N-substituent, respectively.9 Split CD spectra were observed around UV absorption bands on the basis of the π–π* transition of the naphthyl group. Split CD spectra of all copolymers showed a positive first and a negative second cotton effect. These results indicate that copolymers possess a positive exciton chirality. In other words, the transition dipole moment of each naphthyl group is arranged clockwise for the (+)-copolymer.9

Figure 1
figure 5

CD and UV spectra of poly(1-NMI-co-B15C5MI): (a) block copolymer (M1/M2=98:2, [α]435=+46.1°, Run 4 in Table 2), (b) block copolymer (M1/M2=86:14, [α]435=+15.6°, Run 3 in Table 2), (c) random copolymer (M1/M2=87:13, [α]435=+20.2°, Run 2 in Table 2) and (d) random copolymer (M1/M2=78:22, [α]435=−7.9°, Run 1 in Table 2).

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To investigate the dependence of the optical activity of the polymer on molecular weight, gel permeation chromatograms were traced with UV and polarimetric detectors connected in series. Figure 2 shows typical chromatograms. The top and bottom curves were obtained by polarimetric and UV detections, respectively. For the poly(1-NMI-co-B15C5MI) obtained by copolymerization of 1-NMI and B15C5MI (Run 4 in Table 2), the polarimetric chromatogram was in good agreement with the UV spectrum, indicating that every poly(1-NMI-co-B15C5MI) possessed equivalent optical rotation.

Figure 2
figure 6

Gel permeation chromatography (GPC) curves for poly(1-NMI-co-B15C5MI) obtained by block copolymerization (M1/M2=98/2, [α]435=+46.1°, Run 4 in Table 2).

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Copolymerization of (S)-MBZMI and B15C5MI

In our previous report,8 homopolymerizations of (S)-MBZMI were performed with an organometal and a chiral ligand. As a result, when only n-BuLi and Et2Zn were used, the yields, Mn values and specific optical rotations of the polymers were 38.9–76.4%, 3100–17 900 and −106.9 to +66.6°, respectively. When a complex of organometal and chiral ligand was used, the same parameters were 54.8–100%, 2600–74 800 and −77.8 to +551.7°, respectively. We found that the chiral ligand improved the polymerization ability and stereoselectivity of the polymer main chain. This polymer showed the highest specific optical rotation (+551.7°) among all polymers synthesized using chiral maleimides. Hence, asymmetric anionic copolymerizations of B15C5MI and (S)-MBZMI were performed with Et2Zn/(−)-Sp in toluene at 0°C for 72 h. These polymerization conditions are the same as those that yielded the highest specific optical rotation of poly((S)-MBZMI). Table 3 shows the results of copolymerizations of B15C5MI and (S)-MBZMI. These results suggest that the copolymerization of B15C5MI and (S)-MBZMI tended to produce yields and Mn values similar to that of B15C5MI and 1-NMI.

Table 3 Asymmetric anionic random copolymerizations or block copolymerizatons of (S)-MBZMI (M1) and B15C5MI (M2)a with Et2Zn/(−)-Sp (1.0/1.2)b in toluene at 0 °C for 72 h
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Optical properties of poly((S)-MBZMI-co-B15C5MI)

The random copolymers and block copolymers synthesized here had negative and positive specific rotations, respectively.

Figure 3 displays the chromatograms of poly((S)-MBZMI-co-B15C5MI). The left chromatogram is from a random copolymer (Figure 3a) and the right chromatogram is from a block copolymer (Figure 3b). For the random copolymer (Run 1 in Table 3), the polarimetric chromatogram corresponded with the UV spectrum. This suggests that every poly((S)-MBZMI-co-B15C5MI) had an equivalent optical rotation, whereas the block copolymer had a positive optical rotation at a high molecular weight and a negative optical rotation at a low molecular weight. When Et2Zn/(−)-Sp was used as an initiator, poly((S)-MBZMI) had a positive optical rotation8 and poly(B15C5MI) had a negative optical rotation. The results for the block copolymer could be because the high-molecular-weight copolymers had a high (S)-MBZMI content, whereas the low molecular weight had a low (S)-MBZMI content.

Figure 3
figure 7

Gel permeation chromatography (GPC) curves for poly((S)-MBZMI-co-B15C5MI) obtained by (a) random copolymerization (M1/M2=96/4, [α]435=−20.3°, Run 2 in Table 3) and (b) block copolymerization (M1/M2=97/3, [α]435=+34.3°, Run 4 in Table 3).

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Effect of adding (R)-cyclohexylamine to achiral poly(B15C5MI)

Figure 4 shows the CD and UV spectra of poly(B15C5MI) (Run 11, Table 1) with (R)-cyclohexylethylamine ((R)-Cyc) in CH3CN/chloroform (1/1). Increasing the amount of (R)-Cyc, the cotton effect due to the carbonyl group of maleimide around 230–250 nm increased significantly. Therefore, these results suggest that the conformational chirality of the polymer main chain was changed by adding (R)-Cyc. In addition, to investigate the interaction between the polymer and ammonium salt, the CD and UV spectra of the polymer solution containing the (R)-Cyc/HClO4 complex were measured in CH3CN/chloroform (1/1). The cotton effects of the polymer did not change when chiral ammonium salt was added. Chiral ammonium salts were previously shown to have an effect on polymers such as poly(phenylacetylene) and poly(isocyanate), which have flexible main chain structures.17, 18, 19, 20, 21, 22, 23, 24, 25 In this study, we investigated the effect of adding a chiral ammonium salt to the maleimide polymer with a rigid main chain structure. Because the main chain structure is rigid, there is no space between the bulky crown ether side chains. Therefore, the chiral ammonium salt may have been unable to coordinate with the crown ether because of the steric hindrance between side chains.

Figure 4
figure 8

CD and UV spectra of poly(B15C5MI) (Run 11 in Table 1) in the presence of (R)-Cyc.

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Conclusions

  1. 1)

    Optically active poly(B15C5MI)s were obtained by asymmetric anionic polymerizations with organometal/chiral ligand complexes.

  2. 2)

    The effect of chiral ligand on the polymerization of poly(B15C5MI)s tended to be similar to that of poly(N-phenylmaleimide).

  3. 3)

    The polymer obtained by radical polymerization was confirmed to have peaks indicating a threo-diisotactic structure by 13C NMR spectra.

  4. 4)

    The polymerization ability and optical rotation of copolymers depended on those of 1-NMI and (S)-MBZMI.

  5. 5)

    The results of adding of (R)-Cyc to poly(B15C5MI) obtained by radical polymerization, suggest that the conformational chirality of the polymer main chain was changed by adding (R)-Cyc.