Asymmetric anionic polymerization of n-substituted maleimides bearing an azo group with chiral anionic initiators

ABSTRACT Asymmetric anionic homopolymerizations of _N_-substituted maleimides bearing an azo group (RMI: R=4-(phenylazo)phenyl (PAPMI), 4-(phenylazo)-1-naphthyl (PANMI)) were performed with

_n_-BuLi or Et2Zn–chiral ligand ((1-ethylpropylidene)bis(4-benzyl-2-oxazoline) (Bnbox) or (−)-sparteine (Sp)) complexes to obtain optically active polymers. The optical activity of poly(RMI)

was influenced by _N_-substituent and polymerization conditions such as organometal type, structure of the chiral ligand, temperature and solvent type. The poly(PANMI) obtained with an

_n_-BuLi–Bnbox complex in tetrahydrofuran showed the highest specific rotation ([α]D25=+391.1°). _Trans–cis_ photoisomerizations of poly(PAPMI) and poly(PANMI) caused by ultraviolet (UV)

irradiation were observed from UV spectra. The rate of _trans–cis_ photoisomerization for poly(PAPMI) was faster than that for poly(PANMI). The Cotton effects for _trans_ isomers of

poly(PAPMI) and poly(PANMI) were relatively small, but those for _cis_ isomers of poly(PAPMI) and poly(PANMI) clearly exhibited a split circular dichroism curve. SIMILAR CONTENT BEING VIEWED

BY OTHERS SYNTHESIS AND CONTROLLED RADICAL POLYMERIZATION OF AXIALLY CHIRAL MONOMERS WITH A BINAPHTHYL SKELETON Article 13 August 2024 PROTON TRANSFER ANIONIC POLYMERIZATION WITH C–H BOND

AS THE DORMANT SPECIES Article 04 July 2024 STEREOCONTROLLED RADICAL POLYMERIZATION OF ACRYLAMIDES BY LIGAND-ACCELERATED CATALYSIS Article 14 December 2020 INTRODUCTION The conformational

transformation of polymers is attractive from the standpoint of chiroptical properties, and much research has been conducted on this transformation. In particular, polyacetylene1, 2, 3, 4

and polyisocyanate,5 with acidic or basic groups in their side chains, exhibit a very interesting property called ‘induced helix’. When optically active acids or bases exist in solution with

these polymers, the polymers can form one-handed helical conformations by acid–base complexation. This ‘induced helix’ was confirmed in polyacetylene having an optically active

substituent.4, 6 It is notable that the helical sense of these polymers can be controlled by the absolute configuration of the chiral acid and base. In addition, it is also known that

optically active polyisocyanates prepared from achiral monomers7, 8 with chiral anionic initiators and optically active monomers9, 10, 11 reveal a temperature-dependent helix–helix

inversion. Some polymers with azo groups in their side chains are interesting because conformational changes in these polymers are induced by the _trans_–_cis_ photoisomerization of azo

groups, which are unique chromophores because of their photosensitivity. There have been a number of reports on the conformational switching of optically active polymers bearing azo

chromophores. For example, helical polyisocyanate12, 13, 14, 15, 16 with an optically active azo pendant group exhibits a helix inversion triggered by the isomerization of the azo group.

Previously, Carlini and coworkers17, 18 reported radical copolymerizations of _N_-(4-phenylazo)-phenylmaleimide (PAPMI) with (+)-menthyl vinyl ether and (−)-menthyl vinyl ether and

chiroptical properties of the obtained copolymers. However, there are no reported investigations on homopolymerizations of _N_-substituted maleimide (RMI) bearing an azo chromophore by the

asymmetric anionic method. In this paper, asymmetric anionic polymerizations of two types of RMIs, PAPMI and _N_-(4-phenylazo)-1-naphthylmaleimide (PANMI), were carried out using chiral

complexes consisting of organometals and chiral ligands (Scheme 1). The formed polymer was characterized by gel permeation chromatography, D-line-specific rotation ([α]D25) and nuclear

magnetic resonance (NMR) spectra. The change in the chiroptical properties of the polymers during _trans_-to-_cis_ photoisomerization was investigated by ultraviolet (UV) and circular

dichroism (CD) spectra. EXPERIMENTAL PROCEDURE MONOMER PAPMI and PANMI were synthesized from maleic anhydride and the corresponding amine according to the reported methods.19, 20

_N_-(4-PHENYLAZO)PHENYLMALEIMIDE A solution of _para_-aminoazobenzene (1.0 g, 5.0 mmol) in dry benzene (20 ml) was added dropwise to a solution of maleic anhydride (0.5 g, 5.0 mmol) in dry

benzene (15 ml) at room temperature over a period of 30 min. Orange precipitates appeared during the reaction. The mixed solution was stirred for an additional 12 h. Precipitates were

collected by filtration and washed with benzene, water and methanol, in that order. The obtained _N_-(4-phenylazo)phenylmaleamic acid (PAPMA) was added to toluene (50 ml) and evaporated to

dryness to remove residual water by azeotropic distillation. PAPMA was immediately used for the next step without further purification (yield, 1.4 g, 4.7 mmol (94%)). PAPMA (1.4 g, 4.7 mmol)

in a dry benzene (100 ml) suspension was heated at 50 °C, after which ZnBr2 (1.1 g, 4.7 mmol) was directly added in one portion. The suspension was refluxed, and 1, 1, 1, 3, 3,

3-hexamethyldisilazane (HMDS, 1.5 ml, 4.7 × 1.5 mmol) in dry benzene (50 ml) solution was slowly added dropwise to the heated mixture over a period of 30 min, with vigorous stirring. The

reaction mixture was refluxed for an additional 4 h. After cooling to room temperature, the reaction mixture was filtered through filter paper, and the filtrate was washed with 0.5 N HCl aq.

(50 ml × 3), saturated NaHCO3 aq. (50 ml × 2) and saturated NaCl aq. (50 ml), in that order. The benzene solution was separated and dried over anhydrous MgSO4. The solution was concentrated

to dryness. The crude PAPMI was purified by column chromatography using benzene as an eluent to obtain pure PAPMI as a yellow solid (total yield, 1.2 g (87%); melting point, 160–163 °C;

proton NMR (1H NMR; _δ_ in p.p.m. from tetramethylsilane (TMS) in CDCl3): 6.96 (s, 2H, –CH=CH–) and 7.45–8.10 (m, 9H, aromatic protons); carbon-13 NMR (13C NMR; _δ_ in p.p.m. from TMS in

CDCl3): 169.16 (carbonyl groups), 134.34 (–C=C– in imide ring), 151.28, 131.27, 129.09, 126.78, 123.50 and 122.95 (phenyl groups)). _N_-(4-PHENYLAZO)-1-NAPHTHYLMALEIMIDE PANMI was

synthesized following a similar procedure as that used to synthesize PAPMI. Yellow powder (total yield, 91%; melting point, 169–171 °C; 1H NMR (_δ_ in p.p.m. from TMS in CDCl3): 6.96 (s, 2H,

–CH=CH–) and 7.45–9.00 (m, 11H, aromatic protons); 13C NMR (_δ_ in p.p.m. from TMS in CDCl3): 169.16 (carbonyl groups), 134.66 (–C=C– in imide ring), 152.52, 151.28, 132.20, 131.68, 131.95,

130.26 and 123.48 (phenyl and naphthyl groups)). CHIRAL LIGAND Bnbox ((1-ethylpropylidene)bis(4-benzyl-2-oxazoline)) was prepared from (_S_)-phenylalaninol and diethylmalonyl dichloride

according to the literature ([α]43525=−150.7°, _c_=1.0 g per 100 ml, _l_=10 cm, tetrahydrofuran (THF)).21, 22 (−)-Sparteine (Sp; Ishizu Seiyaku, Osaka, Japan) was distilled immediately

before polymerization ([α]43525=–10.3°, _c_=1.0 g per 100 ml, _l_=10 cm, THF). REAGENTS AND SOLVENTS Commercially available organometals, that is, _n_-BuLi and Et2Zn in an _n_-hexane

solution (Kanto Chemical, Tokyo, Japan), were used without purification. Solvents used for syntheses, polymerizations and measurements were purified in the usual manner. POLYMERIZATION

Anionic polymerization was carried out by adding a chiral complex to a monomer solution under a dry nitrogen atmosphere at 0 °C for 72 h. The polymerization was terminated by methanol

containing a small amount of HCl aq. The polymer was precipitated from polymerization solvent by dilution with a large amount of methanol. The precipitate was collected by suction filtration

and washed with methanol. The polymer was further purified twice by reprecipitation using THF–methanol or CHCl3–trifluoroacetic acid–methanol systems. The purified polymer was dried under

vacuum at room temperature for 2 days before characterization. SPECTROSCOPIC DATA OF POLY(PAPMI): Infrared (IR) (KBr) 1386 cm–1 (_ν_N=N) and 1705 cm–1 (_ν_C=O); 1H NMR (CDCl3) _δ_ 2.9–4.8

p.p.m. (br, –CH–CH–, 2H) and 6.0–8.2 p.p.m. (br, aromatic, 9H). 13C NMR (CDCl3) _δ_ 40.0–50.0 p.p.m. (br, –CH–CH–), 120.8, 123.1, 127.4, 129.0, 131.1, 149.2 and 151.9 p.p.m. (br, aromatic)

and 176.3 p.p.m. (br, C=O). SPECTROSCOPIC DATA OF POLY(PANMI): IR (KBr) 1363 cm–1 (_ν_N=N) and 1704 cm–1 (_ν_C=O); 1H NMR (CDCl3) _δ_ 3.5–4.6 p.p.m. (br, –CH–CH–, 2H) and 6.0–9.1 p.p.m. (br,

aromatic, 11H). 13C NMR (CDCl3) _δ_ 39.0–49.0 p.p.m. (br, –CH–CH–), 110.0, 123.3, 126.8, 129.9, 130.7, 148.4 and 152.9 p.p.m. (br, aromatic) and 176.3 p.p.m. (br, C=O). MEASUREMENTS Hg-

([α]43525) and D-line ([α]D25)-specific rotations were measured at 25 °C in THF or CHCl3–trifluoroacetic acid (9/1 (v/v)) using a JASCO DIP-140 (JASCO, Tokyo, Japan). 1H- (270 MHz) and 13C

NMR (68 MHz) spectra were obtained using a JEOL EX-270 (JEOL, Tokyo, Japan) and TMS as an internal standard. Gel permeation chromatography was performed using a Shimadzu chromatopac C-R7Ae

plus (Shimadzu, Kyoto, Japan) equipped with a Shimadzu SPD-10A UV detector (254 nm; Shimadzu) and a JASCO-OR 990 polarimetric detector (350–900 nm) (JASCO); THF was used as an eluent at 50 

°C to calculate the number-average molecular weight (_M̄_n) and molecular weight distribution (_M̄_w/_M̄_n) with polystyrene as a standard. UV and CD spectra were obtained in a quartz cell

at 25 °C with a Shimadzu UV-2200 (Shimadzu) and a JASCO J-20C apparatus (JASCO), respectively. RESULTS AND DISCUSSION POLYMERIZATIONS OF PAPMI The conditions and results of the

polymerizations of PAPMI with _n_-BuLi and alkyllithium–chiral ligand complexes are summarized in Table 1. Anionic polymerization of PAPMI without a chiral ligand produced the

methanol-insoluble polymer in a 12% yield (run 1 in Table 1). However, the conversion was 100%, and the monomer did not exist in the reaction solution after polymerization, as observed on

the basis of NMR spectra; that is, the methanol-insoluble part of the polymer was very low in yield and had an _M̄_n of 1300. The methanol-soluble part of the polymer was ∼88% of the yield

and had an _M̄_n <800. Using an organometal–chiral ligand complex as an initiator, the polymer yields improved, as shown in Table 1. A similar tendency was observed in the polymerizations

of RMI in many of our previous papers.23, 24 As a whole, the number-average molecular weights of the formed polymers were low (_M̄_n=900–4200), suggesting that the bulkiness of the

_N_-substituent inhibits the polymerizability. All polymerizations using chiral ligand complexes produced optically active polymers. The specific rotations of the poly(PAPMI)s prepared with

the alkyllithium–chiral ligand in toluene (runs 3, 5, 7 and 9 in Table 1) exhibited a sign opposite of that prepared with the same initiator in THF (runs 4, 6, 8 and 10). In addition, the

sign of the specific rotations of the poly(PAPMI)s obtained with the _n_-BuLi–chiral ligand in toluene or THF was opposite to that obtained with the _t_-BuLi–chiral ligand in the same

solvent. These results indicate that the sign of the optical activity depends not only on the chiral ligand but also on the solvent and the alkyl group bonded to lithium metal. The absolute

value of the specific rotation of the poly(PAPMI) formed in toluene was higher than that formed in THF, except for _t_-BuLi–Sp systems, suggesting that the use of a polar solvent such as THF

disturbs the asymmetric synthesis with _n_-BuLi–Sp, _n_-BuLi–Bnbox and _t_-BuLi–Bnbox complexes. Table 2 lists the conditions and results of the polymerizations of PAPMI using Et2Zn–chiral

ligand complexes. Compared with the results of Table 1, high-molecular-weight poly(PAPMI) was obtained by using Et2Zn–chiral ligand complexes (_M̄_n=1900–7000). All polymers prepared under

the conditions listed in Table 2 exhibited optical activity. The poly(PAPMI)s formed with Sp as a chiral ligand in toluene and THF showed dextrorotations ([α]D25=+8.6 and +24.4°). In

contrast, poly(PAPMI)s obtained with Bnbox were levorotatory ([α]D25=−313.3 to −124.1°), except for run 3 ([α]D25=+41.7°), and the absolute values were much greater than those of the

poly(PAPMI)s initiated with Et2Zn–Sp and alkyllithium–chiral ligand complexes. Et2Zn–Bnbox, in particular, is a suitable chiral anionic initiator for the asymmetric polymerization of PAPMI.

In all runs, the specific rotations of poly(PAPMI) obtained in THF were higher than those obtained in toluene, suggesting that the optimum symmetric reaction field was built by THF in

addition to the chiral ligand at the growing chain end.23, 24 When PAPMI was polymerized with Et2Zn–Bnbox in THF, the obtained polymers contained THF-insoluble parts (runs 5–7 in Table 2).

The polymers were completely dissolved in a mixed solvent of CHCl3 and a small amount of trifluoroacetic acid. In runs 5–7, the specific rotations of the methanol-insoluble polymers

containing THF-insoluble parts were much higher than those of the THF-soluble parts; that is, the THF-insoluble parts of poly(PAPMI) showed higher optical activity and poor solubility. This

probably indicates that THF-insoluble polymers have crystallinity, which is given by high stereoregularity, that is, optically active _threo_-diisotactic sequences. The molar ratio of

[Et2Zn]/[Bnbox] affected the specific rotation of the poly(PAPMI). The specific rotations of poly(PAPMI)s initiated with the molar ratio of [Et2Zn]/[Bnbox]=1.0/0.5 were higher than those

initiated with the molar ratio of [Et2Zn]/[Bnbox]=1.0/1.2. This suggests that a better structure of the initiator complex is formed by the ratio of 1.0/0.5; that is, the chiral complex

consists of two Et2Zn and one Bnbox molecule. POLYMERIZATIONS OF PANMI Table 3 summarizes the polymerization results of PANMI with alkyllithium–chiral ligand complexes. The yields and

number-average molecular weights were higher than those listed in Table 1 because PANMI has higher solubility than PAPMI. In all polymerizations using chiral ligands, the obtained

poly(PANMI) exhibited optical activity. The use of _n_-BuLi–Bnbox as an initiator produced highly optically active poly(PANMI)s ([α]D25=−186.8 and +391.1°). However, the poly(PANMI)s

obtained with _t_-BuLi–Bnbox (runs 9 and 10 in Table 3) exhibited very low specific rotations ([α]D25=+11.0 and −3.3°), despite possessing the same chiral ligand as in runs 7 and 8. This may

result from the fact that the _M̄_n of the polymers obtained with _t_-BuLi were 1000 to 1800, as shown in Table 3, which are relatively lower than those obtained with _n_-BuLi. The specific

rotations of the poly(PANMI)s initiated with alkyllithium–Bnbox in toluene showed an opposite sign to those of the poly(PANMI)s obtained with the same initiator in THF. On the basis of

these results, it was found that alkyl groups that coordinated to lithium, as well as to Bnbox, could influence the asymmetric synthesis during polymerization. The polymerization conditions

and results of PANMI using Et2Zn–chiral ligand complexes are listed in Table 4. Polymerizations using Sp as a chiral ligand produced levorotatory polymers. In contrast, poly(PANMI)s prepared

using Bnbox exhibited dextrorotations. These results are similar to those of the polymerization of PAPMI (see runs 3–7 in Table 2). That is, when Et2Zn is used as the initiator complex,

asymmetric synthesis is controlled only by the chirality of the chiral ligand. In the polymerization using Et2Zn–Bnbox, the specific rotations of the poly(PANMI)s formed with the molar ratio

of [Et2Zn]/[Bnbox]=1.0/0.5 ([α]D25=−156.3 and −190.7°) were higher than those formed with the ratio of 1.0/1.2 ([α]D25=−139.9 and −38.8°). Furthermore, there is a significant gap in the

specific rotations between poly(PAPMI)s (runs 4 and 6 in Table 4) formed by the different ratios in THF, indicating that the molar ratio significantly influences the asymmetric

polymerization of PANMI in THF. CHIROPTICAL PROPERTIES OF POLYMERS To investigate the optical rotation of every molecular weight part in the polymers, gel permeation chromatography

chromatograms were recorded by UV and polarimetric (_α_Hg) detectors. The obtained chromatograms are displayed in Figure 1. Chromatograms (A) and (B) in Figure 1 are due to the poly(PAPMI)s

obtained with _n_-BuLi–Sp (run 4 in Table 1, [α]D25=+7.8°) in THF and Et2Zn–Bnbox in THF (run 4 in Table 2, [α]D25=−124.1°), respectively. Chromatograms (C) and (D) are due to the

poly(PANMI)s formed with _t_-BuLi–Sp in THF (run 6 in Table 3, [α]D25=−108.7°) and Et2Zn–Bnbox in THF (run 6 in Table 4, [α]D25=−190.7°), respectively. In each polymer, the _α_Hg curve

corresponds to a UV curve. Moreover, the top of the _α_Hg curve is in fair agreement with that of the UV curve. From these results, it is clear that every molecular weight part in each

polymer possesses the same optical rotation; that is, the optical activity of the polymers is independent of the molecular weight. This also means that similar asymmetric addition reactions

constantly take place during the polymerization. Highly optically active poly(PAPMI) and poly(PANMI) may contain chiral conformations, such as partial helical sequences induced by high

continuity of _threo_-diisotactic units. To investigate this point further, the specific rotations of these polymers were measured at several temperatures. Figure 2 depicts the changes in

the specific rotations of poly(PAPMI) ((A), run 4 in Table 2, [α]D25=−124.1°) and poly(PANMI) ((B), run 6 in Table 4, [α]D25=−190.7°) with temperature. The specific rotations of these

polymers decreased with heating and increased with cooling. The gaps between the maximum and minimum specific rotations of poly(PAPMI) and poly(PANMI) were 27.0 and 17.7°, respectively, and

these changes were reversible. In Figure 2, the specific rotation of poly(PAPMI) showed a discontinuous change between 10 and 20 °C (A). On the other hand, as shown by (B), the degree to

which the specific rotation of poly(PANMI) changed between 10 and 30 °C was different from that between 30 and 50 °C. These changes are probably attributable to the relaxation of chiral

conformations such as helical sequences given by continuous (_S_, _S_)- or (_R_, _R_)-_threo_-diisotactic main chains. In addition, the discontinuity in Figure 2 indicates that the chiral

conformations of the polymers change dramatically in those temperature regions. PHOTOISOMERIZATIONS OF POLYMERS To analyze the photoisomerizations of the azo groups in PAPMI and poly(PAPMI),

UV spectra were measured before and after UV irradiation. Figure 3 shows UV curves (panels a–c) for PAPMI, the THF-soluble part of poly(PAPMI) formed with Et2Zn–Bnbox in THF (run 7 in Table

2; [α]D25=−78.6°) and poly(PAPMI) formed with Et2Zn–Bnbox in THF (run 4 in Table 2; [α]D25=−124.1°), respectively. The peak centered at 330 nm in each spectrum is ascribed to the π–π*

transition of the _trans_-azo aryl group. The absorbance at 330 nm in the UV spectra decreased with increasing irradiation time. Absorbance due to the π–π* transition of the _trans_-azo aryl

group of PAPMI and poly(PAPMI) almost disappeared after 40 min, and remained unchanged afterward, indicating that the _trans_-to-_cis_ photoisomerization was complete at 40 min. The rate of

_trans_-to-_cis_ photoisomerization was the same among PAPMI and poly(PAPMI)s with low- and high specific rotation. This indicates that the photoisomerization rate is not affected by

molecular weight and optical activity. Figure 4 presents the photoisomerization results for PANMI and poly(PANMI). UV curves (panels a–c) are due to PANMI, poly(PANMI) obtained with

Et2Zn–Bnbox in THF (run 6 in Table 4, [α]D25=−190.7°) and poly(PANMI) obtained with _n_-BuLi–Bnbox in THF (run 8 in Table 3, [α]D25=+391.1°), respectively. The peak centered at 380 nm is due

to the π–π* transition of the _trans_-azo aryl group, and its intensity decreased with UV irradiation time, indicating that the _trans_-azo forms were transformed into _cis_-azo ones by

photoisomerization. The photoisomerizations for PANMI monomer and poly(PANMI) were terminated at 120 and 180 min, respectively. The rate of _trans_-to-_cis_ photoisomerization for PANMI

monomer was evidently faster than that for its polymers. It can be presumed that an environment that is more crowded for the polymer than for the PANMI monomer prevents the _trans_-to-_cis_

photoisomerization. The photoisomerization rate was different among poly(PANMI)s. The photoisomerization rate for poly(PANMI) with a higher specific rotation (Figure 4c; [α]D25=+391.1°) was

slightly faster than that for poly(PANMI) with a lower specific rotation (Figure 4b; [α]D25=−190.7°). The differences in stereoregularity and/or conformation between the two poly(PANMI)s

probably influence the rate of photoisomerization. Compared with the results of the PAPMI series (Figure 3), the photoisomerization rates for the PANMI systems (Figure 4) were very slow.

This difference may result from bulkiness of the naphthyl groups bonded to the azo chromophores. To obtain information about the change in optical activity due to photoisomerization, CD and

UV spectra were measured in THF. Figures 5a and b show spectra of poly(PAPMI) prepared with Et2Zn–Bnbox in THF (run 4 in Table 2; [α]D25=−124.1°) and poly(PANMI) prepared with Et2Zn–Bnbox in

toluene (run 3 in Table 4; [α]D25=−139.9°), respectively. Before photoisomerization, poly(PAPMI) with _trans_-azo chromophores exhibited only a few CD peaks at ∼330 nm. However, after

photoisomerization, poly(PAPMI) with _cis_-azo chromophores showed obvious split CD patterns. The change in CD spectra was reversible. Poly(PANMI) bearing _trans_-azo groups exhibited a weak

and positive CD peak around the UV absorption band, but it changed into split CD patterns with higher molar ellipticity after photoisomerization. The change in CD spectra was also

reversible. The azo chromophores are apart from the chiral main chain; nevertheless, they exhibited a distinct CD band after photoisomerization. It seems that the _cis_-azo groups in the

polymers can form chiral conformations. In other words, chiral conformations such as helical conformations are induced by _trans_-to-_cis_ photoisomerization. There are few data to clearly

prove this point, but, as shown in Scheme 2, it can be explained on the basis of steric factors. Steric repulsion due to the _cis_-azo aryl groups among vicinal monomeric units is higher

than that due to the _trans_-azo groups so that _N_-pendant groups with _cis_-azo groups are as apart from each other as possible. Consequently, the polymer backbone with rich

_threo_-diisotactic sequences can form more helical conformations than the polymer having _trans_-azo aryl groups. CONCLUSIONS Asymmetric anionic polymerizations of achiral RMI-bearing azo

groups, PAPMI and PANMI, were achieved with organometal–chiral ligand complexes to obtain optically active polymers. The optical activity of poly(RMI) was greatly influenced by the

_N_-substituent and polymerization conditions. The poly(PAPMI) prepared with Et2Zn–Bnbox showed a levo-specific rotation of −313.3°. The poly(PANMI) obtained with _n_-BuLi–Bnbox in THF

showed the highest specific rotation of +391.1°. _Trans–cis_ photoisomerizations of the polymers caused by UV irradiation were observed by UV spectra. The intensity of the absorption band of

the _trans_-azo groups decreased progressively with UV irradiation time. The rate of _trans–cis_ photoisomerization for poly(PAPMI) was faster than that for poly(PANMI). There was no

difference in the rate of photoisomerization between polymers having different specific rotations. The rate of photoisomerization for poly(PANMI) was faster than that for PANMI. The

difference in the rate of photoisomerization between the monomer and polymer could be attributed to steric repulsion by polymeric effects. The Cotton effects for _trans_ isomers of

poly(PAPMI) and poly(PANMI) were relatively small, but those for _cis_ isomers of poly(PAPMI) and poly(PANMI) clearly exhibited a split CD curve, suggesting that the polymers with _cis_-azo

groups can form chiral conformations. REFERENCES * Yashima, E., Matsushima, T. & Okamoto, Y. Chirality assignment of amines and amino alcohols based on circular dichroism induced by

helix formation of a stereoregular poly((4-carboxyphenyl)acetylene) through acid-base complexation. _J. Am. Chem. Soc._ 119, 6345–6359 (1997). Article  CAS  Google Scholar  * Yashima, E.,

Goto, H. & Okamoto, Y. Induced helix of an aliphatic polyacetylene detected by circular dichroism. _Polym. J._ 30, 69–71 (1998). Article  CAS  Google Scholar  * Yashima, E., Maeda, K.

& Okamoto, Y. Memory of macromolecular helicity assisted by interaction with achiral small molecules. _Nature_ 399, 449–451 (1999). Article  CAS  Google Scholar  * Yashima, E., Maeda, K.

& Okamoto, Y. Synthesis of poly[_N_-(4-ethynylbenzyl)ephedrine] and its use as a polymeric catalyst for enantioselective addition of dialkylzincs to benzaldehyde. _Polym. J._ 31,

1033–1036 (1999). Article  CAS  Google Scholar  * Maeda, K., Yamamoto, N. & Okamoto, Y. Helicity induction of poly(3-carboxyphenyl isocyanate) by chiral acid-base interaction.

_Macromolecules_ 31, 5924–5926 (1998). Article  CAS  Google Scholar  * Yashima, E., Maeda, K. & Okamoto, Y. Helix-helix transition of optically active

poly((1R,2S)-N-(4-ethynylbenzyl)norephedrine) induced by diastereomeric acid-base complexation using chiral stimuli. _J. Am. Chem. Soc._ 120, 8895–8896 (1998). Article  CAS  Google Scholar 

* Okamoto, Y., Matsuda, M., Nakano, T. & Yashima, E. Asymmetric polymerization of aromatic isocyanates with optically active anionic initiators. _J. Polym. Sci. A Polym. Chem._ 32,

309–315 (1994). Article  CAS  Google Scholar  * Maeda, K. & Okamoto, Y. Helical structure of oligo- and poly(m-substituted phenyl isocyanate)s bearing an optically active end-group.

_Polym. J._ 30, 100–105 (1998). Article  CAS  Google Scholar  * Maeda, K. & Okamoto, Y. Synthesis and conformation of optically active poly(phenyl isocyanate)s bearing an

((S)-(α-methylbenzyl)carbamoyl) group. _Macromolecules_ 31, 1046–1052 (1998). Article  CAS  Google Scholar  * Maeda, K. & Okamoto, Y. Unusual conformational change of optically active

poly(3-((S)-sec-butoxycarbonyl)phenyl isocyanate). _Macromolecules_ 31, 5164–5166 (1998). Article  CAS  Google Scholar  * Maeda, K. & Okamoto, Y. Synthesis and conformational

characteristics of poly (phenyl isocyanate)s bearing an optically active ester group. _Macromolecules_ 32, 974–980 (1999). Article  CAS  Google Scholar  * Mülter, M. & Zentel, R.

Photochemical amplification of chiral induction in polyisocyanates. _Macromolecules_ 27, 4404–4406 (1994). Article  Google Scholar  * Mülter, M. & Zentel, R. Photochemical inversion of

the helical twist sense in chiral polyisocyanates. _Macromolecules_ 28, 8438–8440 (1995). Article  Google Scholar  * Mülter, M. & Zentel, R. Interplay of chiral side chains and helical

main chains in polyisocyanates. _Macromolecules_ 29, 1609–1617 (1996). Article  Google Scholar  * Mayer, S., Maxein, G. & Zentel, R. Correlation between the isomerization of side groups

and the helical main chain in chiral polyisocyanates. _Macromolecules_ 31, 8522–8525 (1998). Article  CAS  Google Scholar  * Fissi, A., Pieroni, O., Angelini, N. & Lenci, F.

Photoresponsive polypeptides. Photochromic and conformational behavior of spiropyran-containing poly(L-glutamate)s under acid conditions. _Macromolecules_ 32, 7116–7121 (1999). Article  CAS

  Google Scholar  * Angiolini, L. & Carlini, C. Optically active polymers containing side-chain photochromic moieties. Synthesis and chiroptical properties of copolymers of

trans-_N_-(4-azobenzene)maleimide with (–)-menthyl vinyl ether and (+)(S)-2-methylbutyl vinyl ether. _J. Polym. Sci. A Polym. Chem._ 29, 1455–1463 (1991). Article  CAS  Google Scholar  *

Angiolini, L., Caretti, D., Carlini, C., Altomare, A. & Solaro, R. Optically active polymers containing side-chain azobenzene moieties: photochromic and photoresponsive behavior of

copolymers of _N_-(4-azobenzene)maleimide with (−)-menthyl vinyl ether and (+)-(S)-2-methylbutyl vinyl ether. _J. Polym. Sci. A Polym. Chem._ 32, 2849–2857 (1994). Article  CAS  Google

Scholar  * Reddy, P. Y., Kondo, S., Toru, T. & Ueno, Y. Lewis acid-hexamethyldisilazane-promoted efficient synthesis of _N_-alkyl- and _N_-arylimide derivatives. _J. Org. Chem._ 62,

2652–2654 (1997). Article  CAS  Google Scholar  * Reddy, P. Y., Kondo, S., Fujita, S. & Toru, T. Efficient synthesis of fluorophore-linked maleimide derivatives. _Synthesis_ 1998,

999–1002 (1998). Article  Google Scholar  * Abiko, A. & Masamune, S. An improved, convenient procedure for reduction of amino acids to amino alcohols: use of sodium borohydride—sulfuric

acid. _Tetrahedron Lett._ 33, 5517–5518 (1992). Article  CAS  Google Scholar  * Denmark, S. E., Nakajima, N., Nicaise, O. J.- C., Faucher, A.- M. & Edwards, J. P Preparation of chiral

bisoxazolines: observations on the effect of substituents. _J. Org. Chem._ 60, 4884–4892 (1995). Article  CAS  Google Scholar  * Isobe, Y., Onimura, K., Tsutsumi, H. & Oishi, T.

Asymmetric polymerization of _N_-1-naphthylmaleimide with chiral anionic initiator: preparation of highly optically active poly(_N_-1-naphthylmaleimide). _Macromolecules_ 34, 7617–7623

(2001). Article  CAS  Google Scholar  * Isobe, Y., Onimura, K., Tsutsumi, H. & Oishi, T. Asymmetric polymerization of _N_-1-anthrylmaleimide with diethylzinc-chiral ligand complexes and

optical resolution using the polymer. _Polym. J._ 34, 18–24 (2002). Article  CAS  Google Scholar  Download references AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Applied

Chemistry, Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi, Japan Tsutomu Oishi, Motohisa Azechi, Kanako Kamei, Yukio Isobe & Kenjiro Onimura Authors *

Tsutomu Oishi View author publications You can also search for this author inPubMed Google Scholar * Motohisa Azechi View author publications You can also search for this author inPubMed 

Google Scholar * Kanako Kamei View author publications You can also search for this author inPubMed Google Scholar * Yukio Isobe View author publications You can also search for this author

inPubMed Google Scholar * Kenjiro Onimura View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHOR Correspondence to Tsutomu Oishi. RIGHTS

AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Oishi, T., Azechi, M., Kamei, K. _et al._ Asymmetric anionic polymerization of _N_-substituted maleimides

bearing an azo group with chiral anionic initiators. _Polym J_ 43, 147–154 (2011). https://doi.org/10.1038/pj.2010.126 Download citation * Received: 03 September 2010 * Revised: 25 October

2010 * Accepted: 26 October 2010 * Published: 08 December 2010 * Issue Date: February 2011 * DOI: https://doi.org/10.1038/pj.2010.126 SHARE THIS ARTICLE Anyone you share the following link

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content-sharing initiative KEYWORDS * asymmetric anionic polymerization * azo groups * chiral maleimide