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CHAPTER 15

SYNTHESIS AND FUNCTION OFCHIRAL p-CONJUGATED POLYMERSFROM PHENYLACETYLENES

TOSHIKI AOKI, TAKASHI KANEKO, and MASAHIRO TERAGUCHI

15.1 INTRODUCTION

p-Conjugated polymers such as polymers from phenylacetylenes have attracted

much more attention among numerous scientists and engineers than other conven-

tional polymers like vinyl polymers because they offer many possible useful

properties owing to their p-electrons such as electric conductivity, optical nonlinear

susceptibility, electroluminescence, molecular magnetism, photoluminescence, and

so on. A famous p-conjugated polymer is nonsubstituted polyacetylene, its well-

known electrically conductive polymer discovered by Professor Shirakawa who

received the Nobel Prize for Chemistry in 2000. The chemical structure of this

p-conjugated polymer, which is synthesized by addition polymerization of acety-

lene [1], is simple, allowing for no variation. To introduce variations such as

functional groups and chiralities into the structure and to improve the properties

of the nonsubstituted polyacetylene, other synthetic methods (i.e., new monomers

and new initiators) have been developed by excellent researchers (e.g., Prof. Masuda

and Prof. Grubbs and Prof. Schrock who received the Nobel Prize for Chemistry in

2005). Therefore, the availability of many kinds of conjugated polymers has been

increasing. In particular, new initiators of addition polymerizations of substituted

acetylenes, including phenylacetylenes, were intensively studied byMasuda et al. [2].

In this chapter, the synthetic methods of two types of chiral p-conjugatedpolymers are described from phenylacetylenes we reported on. One is the

p-conjugated polymer obtained by the addition polymerization of substituted pheny-

lacetylenes that contains alternating double bonds in the backbone and are usually

called “substituted poly(phenylacetylene)s” (the systematic name is “substituted poly

(1-phenylvinylene)s”) (Scheme 15.1a). The other type of p-conjugated polymer from

Polymeric Chiral Catalyst Design and Chiral Polymer Synthesis, First Edition. Edited by Shinichi Itsuno.� 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.

423

phenylacetylenes is obtained from halo-substituted phenylacetylenes by condensation

polymerization (Scheme 15.1b). In this case, the substituted phenylacetylene mono-

mers need to have a halogen atom present on the aromatic ring. The resulting

p-conjugated polymers, which contain alternating triple and double bonds, also can

be called “substituted poly(phenylacetylene)” but usually are called by their systematic

name, “substituted poly(phenyleneethynylene)s” (Scheme 15.1b). The polymers are

also synthesized from two monomers—dihalobenzene and diethynylbenzene [3, 4].

Poly(phenylacetylene)s prepared by addition polymerization by using a rhodium

complex as an initiator have highly controlled chemical structures, such as a high cis-

content and helical main chains. In addition, if the helical sense of the p-conjugatedpolymers is controlled, then the polymer backbone itself becomes optically active.

Suchone-handed helical polymerswere researchedbyAkagi [1] andYashimaet al. [5].

The backbone chirality of the p-conjugated polymers can be detected directly by

measuring their circular dichroism (CD) behavior because the main chain itself is a

chromophore. The synthetic methods of chiral poly(phenylacetylene)s by the addition

polymerization that we reported will be mentioned in Section 15.2.

Poly(phenyleneethynylene)s can also form precise molecular architectures, such

as foldamers, macrocycles, and dendrimers, through control of the linkage position

of the aromatic units. In addition, if the helical senses of the foldamers are controlled,

then the polymer backbones themselves become optically active. The description of

synthesis of chiral poly(phenyleneethynylene)s by condensation polymerization is

included in Sections 15.3.2.2 and 15.4.2.

These unique chiral p-conjugated polymers could have possibly many kinds of

functions. For example, they can be used as chiral catalysts (Section 15.2.4.1) or as

optical resolution membranes (Section 15.2.4.2). In addition, the polymers that have

a unique regulated structure can react in a topochemical manner (Section 15.2.4.3).

PhH

PhenylacetylenePhH

n

nPh- = phenyl group

Poly(phenylacetylene)

Addition polymerization

PhH

Halophenylacetylene

n

Poly(phenyleneethynylene)

Condensation polymerization

X Phn

(a)

(b)

-Ph- = phenylene group

SCHEME 15.1. Synthesis of two types of chiral polymers from phenylacetylenes:

(a) by addition polymerization of phenylacetylenes and (b) by polycondensation of halo-

phenylacetylenes.

424 SYNTHESIS AND FUNCTION OF CHIRAL p-CONJUGATED POLYMERS

Chapter 15 describes the author’s investigation into synthesis and the properties of

chiral p-conjugated polymers from phenylacetylenes (i.e., poly(phenylacetylene)s

and poly(phenyleneethynylene)s).

15.2 HELIX-SENSE-SELECTIVE POLYMERIZATION (HSSP) OFSUBSTITUTED PHENYLACETYLENES AND FUNCTION OF THERESULTING ONE-HANDED HELICAL POLY(PHENYLACETYLENE)S

15.2.1 Synthesis of Chiral p-Conjugated Polymers fromPhenylacetylenes by Asymmetric-Induced Polymerization (AIP)and Helix-Sense-Selective Polymerization (HSSP) of Chiraland Achiral Phenylacetylenes

There are two main synthetic methods to obtain chiral polymers by addition

polymerization of phenylacetylenes. They are asymmetric-induced polymerization

(AIP) and helix-sense-selective polymerization (HSSP), which we found first as

shown in Scheme 15.2. In this chapter, AIP is mentioned concisely in Sections

15.2.1.1, 15.2.2.4, 15.3.2.1, and 15.4.2 because some other reviews are available

[1–5],whereasHSSP is explainedmore precisely inSections 15.2.2–15.2.4 and15.4.3

because the method, which we reported first [12], is unique and no reviews on this

subject have been published.

15.2.1.1 AIP of Chiral Phenylacetylenes. In this section, monomers suitable

for AIP of chiral substituted acetylenes containing chiral phenylacetylenes that we

Achiral catalyst

CHC One-handed helical polymer

with chiral groups

Chiral monomer Chiral polymer

Bulky

chiral group

Asymmetric-induced polymerization(AIP)

T. Aoki et al., Chem. Lett., 2009(1993), Macromolecules, 29,

4192(1996), Macromolecules, 32, 79(1999), J. Polym. Sci., A40,

1689(2002).

Polymerization

CHC

Chiral catalyst

One-handed helical polymer

without chiral groups

Chiral polymerAchiral monomer

Helix-sense-selective polymerization(HSSP)

T. Aoki et al., J. Am. Chem. Soc., 125, 6346(2003), Chem.Lett., 34,

854(2005), Macromolecules, 40, 7098(2007). Macromol. Chem. Phys., 210,

717(2009). Macromolecules, 42, 17(2009). Polymer, 51, 2460(2010).

Polymerization?

SCHEME 15.2. Concepts of AIP and HSSP of poly(phenylacetylene)s.

HELIX-SENSE-SELECTIVE POLYMERIZATION 425

reported [6–11] are discussed. During the course of an investigation into the

synthesis of a new enantioselective permeability membrane material, the authors

accidentally discovered an AIP in which a one-handed helical chirality was induced

in the main chain during polymerization of a phenylacetylene with a bulky chiral

L-menthoxycarbonyl group, p-{L-(-)-menthoxycarbonyl}phenylacetylene [6]. The

polymer could be fabricated into a self-supporting membrane that showed a CD

spectrum similar to that of the solution. The membrane showed enantioselectivity in

permeation. After this discovery, the authors synthesized and polymerized many

other phenylacetylenes that had a chiral substituent to check whether any main-chain

chirality was induced. As a result, many chiral monomers were suitable for the the

AIP, and the resulting chiral polymers were applicable to optical resolution

membranes as shown in Scheme 15.3.

For example, the homopolymrs of (-)-p-(dimethyl(10-pinanyl)silyl)phenylacety-

lene obtained with a Rh complex showed strong CD absorptions similar to that of

p-{L-(-)-menthoxycarbonyl}phenylacetylene [6], whereas p-{L-(-)-2-methylbutyl-

carbonyl}phenylacetylene and p-{tetramethyl-3-(10-pinanyl)disiloxanyl}phenyla-

cetylene showed weak CD absorptions [4]. The position and size of the chiral

substituents is important for asymmetric induction in AIP.

Interestingly, the homopolymers of (-)-p-(dimethyl(10-pinanyl)silyl)phenyla-

cetylene prepared using WCl6 showed much weaker CD absorptions than those

prepared by a Rh complex [4]. (-)-o-(Dimethyl(10-pinanyl)silyl)phenylacetylene

was synthesized and polymerized using WCl6 [4, 6]. The resulting polymer with a

pinanylsilyl group at the ortho position showed weak CD absorptions similar

to poly((-)-p-(dimethyl(10-pinanyl)silyl)phenylacetylene) prepared with WCl6.

Poly((-)-p-(dimethyl(10-pinanyl)silyl)phenylacetylene)s made using a rhodium

complex had a high cis content, whereas those made with WCl6 did not. Therefore,

cis-trans regularity was an important factor for making one-handed chiral conforma-

tions.To investigate the effects of the chemical structures of the chiral groups, including

the number and position of the chiral groups in the monomers, on the induction

of chirality in the main chain during polymerization and on the degree of enantios-

electivity in the permeation of the polymeric membranes, oligosiloxanylphenylace-

tylenes with one or two bulky chiral pinanyl groups at the 1-, 3-, and/or 5-position

Si

CH3

PiCH3

CH3Si CH3

CH3

Pi

R

Si

CH3

CH3

CH3

CH3

Si CH3

CH3

OSi

R1 R2

H

*

*

nn

Pi- =

n

*

n

R- = -H, -CH3, -C6H5

R1- = -Pi, -CH3, R2- = -CH3, -Pi

SCHEME 15.3. Examples of one-handed helical poly(phenylacetylene)s with chiral pendant

groups for optical resolution membranes prepared by AIP.


of an oligosiloxane chain were polymerized with a Rh complex to produce high

molecular weight polymers [8]. With the exception of polymers with bulky sub-

stituents, polymers with a chiral pinanyl group at the 1-position of an oligosiloxanyl

group showed high molar ellipticity in the main-chain region in the CD spectra. The

exceptional polymers with bulky substituents had ultraviolet (UV)-vis absorptions at

longer wavelengths. However, the polymers from monomers with a chiral pinanyl

group at the 3- or 5- position of an oligosiloxanyl group showed almost no CD

absorptions. The flexible Si-O spacer did not transmit the chiral information to

the main chain [8]. Although the polymers of p-{pentamethyl-1,3-bis(10-pinanyl)

trisiloxanyl}phenylacetylene and p-{tetramethyl-1,3,5-tris(10-pinanyl)trisiloxanyl}

phenylacetylene had a pinanyl group at the 1 position, they showed almost no CD

absorptions. In this case, their UV-vis absorptions shifted to longer wavelengths than

those of the corresponding polymers that had disiloxanyl groups. However, the case

of polyphenylacetylenes with a branched oligopinanylsiloxane(p-[tris{dimethyl(10-

pinanyl)siloxy}silyl]phenylacetylene) showed CD absorptions despite not having

pinanyl group at the 1-position. This may be because their bulky substituents changed

the main-chain structure.

Other than the phenylacetylenes described, the polymerization of other chiral

disubstituted acetylenes (Scheme 15.3) by using another catalyst such as WCl6yielded chiral p-conjugated polymers with one-handed helical main chain.

The detail of modified AIP, AIP followed by polymer reaction in membrane state

(RIM) we developed to obtain one-handed helical poly(phenylacetylene)s that do not

have chiral pendant groups is described in Section 15.3.

15.2.1.2 HSSP of Achiral Phenylacetylenes. As described in the previous

section (15.2.1.1), chiral-substituted polyphenylacetylenes have been synthesized by

polymerization (AIP) of monomers with bulky chiral substituents. However, after the

chiral side groups were removed by hydrolysis or reduction, the resulting polymer

did not show CD absorptions in the main chain region in solution. A chiral helical

nonsubstituted polyacetylene had been synthesized by polymerization of acetylene

in a chiral nematic reaction field [1]. However, there were no reports of obtaining

chiral helical substituted polyacetylenes that had no other chiral moieties in the side

or end groups.

Therefore, in Section 15.2, we describe details of HSSP that can produce one-

handed helical substituted poly(phenylacetylene)s that had no other chiral moieties

in the side and end groups.

15.2.2 HSSP of Three Types of Monomers (RDHPA, RDAPA,and RDIPA, Scheme 15.4a)

The authors have found a simple and novel synthetic method (HSSP) for obtaining

such a chiral polymer from an achiral substituted acetylene monomer using a chiral

catalytic system. In addition, the helical conformation was stable in solution. This is

the first example of helix-sense-selective polymerization of substituted acetylenes

whose chiral helicity is stable in solution without the aid of other chiral substituents

or other small molecules [12].


C

OH

OH

OR

(a)

HC : RDHPA

C

C N

C N

HC :RDAPA

O

O

H

R'

H

R'

O

OH

C(CH3)3

C(CH3)3

C(CH3)3

C(CH3)3

OR

C

N

N

HC :RDIPAOR

OR'

OR'

OH

OH

OSiCH3

CH3

O SiCH3

CH3

O SiCH3

CH3

CH3 : S3BDHPA

OH

OH

O : DoBDHPA

OC12H25

OH

OH

OC12H25 : DoDHPA

:HGPA

C

OCH3

OCH3

ORHC

(b)

C

N C

N C

HC

H

H

O

R'

O

R'

C

Si OH

Si OH

OC12H25HC

CH3

CH3

CH3

CH3

OR

C

N

N

HC

OR

OR

C

C O

C O

HC

O

O

C12H25

C12H25

CNH

HCOH

R-, R'- = alkyl group

:1

:2

:3

:4

:5

:6

C

OCH3

OCH3

ORHC

(b)

C

N C

N C

HC

H

H

O

R'

O

R'

C

Si OH

Si OH

OC12H25HC

CH3

CH3

CH3

CH3

OR

C

N

N

HC

OR

OR

C

C O

C O

HC

O

O

C12H25

C12H25

CNH

HCOH

R-, R'- = alkyl group

:1

:2

:3

:4

:5

:6

SCHEME 15.4. Monomers (a) suitable and (b) unsuitable for HSSP.


In Section 15.2.2, the method of HSSP we found first as well as the monomers

suitable for HSSP are discussed [12–22]. Modification of HSSP is also discussed in

Section 15.2.3 [23–25]. In Section 15.2.4, application of HSSP is described.

15.2.2.1 HSSPofPhenylacetylenesHavingTwoHydroxylGroups(RDHPA)[12, 13, 17–19]. The authors polymerized an achiral phenylacetylene that had

two hydroxyl groups (RDHPA, Scheme 15.4a) and a dodecyloxy (DoDHPA,

Scheme 15.4a), a dodecyloxybenzyl (DoBDHPA, Scheme 15.4a), or hexamethyl-

trisiloxanylbenzyloxy group (S3BDHPA, Scheme 15.4a) using a chiral catalytic

system consisting of a rhodium dimeric complex, [Rh(nbd)Cl]2 (nbd¼2,5-

norbornadiene), as a catalyst and a chiral amine, (R)-1-phenylethylamine (R)-

PEA), as a cocatalyst. All polymers showed Cotton effects at wavelengths

around 430 nm and 310 nm where there are no UV absorptions of RDHPA and

(R)-PEA (Figures 15.1 and 15.2).

The absorption band at 430 nm is assigned to the conjugated main chain, and the

peak at 310 nm may result from a chiral position between adjacent pendant groups.

The intensity of the band at 430 nm was similar, and the peaks at 310 nm were a little

stronger compared with other one-handed helical poly(phenylacetylene)s that had

chiral side groups prepared by AIP. Therefore, the authors realized the first HSSP of a

substituted acetylene using a chiral catalyst. However, no helix-sense-selective

polymerizations occurred in the case of achiral phenylacetylenes, a monomer with

two methoxy groups instead of two hydroxyl groups (1, Scheme 15.4b), and

p-trimethylsilylphenylacetylene, which does not have any hydroxy groups

(Scheme 15.4b).

The polyphenylacetylene without chiral substituents derived from the chiral

homopolymer prepared by AIP showed no CD in solution. However, the chiral

structure of polyDoDHPAwas stable in chloroform at room temperature for 5 months.

Moreover, even when the solutionwas heated to 50 �C, almost no changewas detected

in the CD. This stability may be caused by intramolecular hydrogen bonds between

hydroxy groups in different monomer units. To confirm this theory, the authors

FIGURE 15.1. CD and UV-vis spectra of one-handed helical polyphenylacetylenes prepared

by HSSP of RDHPA. (A): a: poly(S3BDHPA), b: poly(DoBDHPA) prepared by using (R)-PEA

in chloroform at 20 �C; (B): poly(S3BDHPA) prepared by using (S)-PEA in DMSO/CHCl3having different contents (vol.-%) of DMSO (a: 0, b: 9, c: 23, d: 33, e: 44, f: 50, g: 52); (C): poly

(DoBDHPA) prepared by using (R)-PEA in DMSO/CHCl3 having a different content(vol.-%)

of DMSO (a: 0, b: 5, c: 10). Riprinted from [19]. Copyright(2009), with permission from

Elsevier.


measured CD and infared (IR) data in various two-component solvents that have

different polarities. As the content of the polar dimethyl sulfoxide(DMSO) solvent

component increased, the CD signal became smaller and then disappeared (Fig-

ures 15.1 and 15.2). At the same time, the OH absorption at 3300 cm�1 shifted to a

longer wavelength, indicating that the hydrogen bonds had weakened [12]. Therefore,

the intramolecular hydrogen bonds were effective in stabilizing the chiral structure.

The authors have also examined the role of the chiral amine cocatalyst in HSSP.

Several chiral amines were effective for the polymerization reaction, and their

effectiveness depended on their bulkiness and ability to coordinate to rhodium. To

determine the structure of the true active species in the catalytic system, the authors

have synthesized a new chiral rhodium complex that has two chiral amines as ligands

([Rh(nbd) ((R)-PEA)2]þBF4

�). The isolated chiral complex also catalyzed the

helix-sense-selective polymerization. These findings suggest that a chiral rhodium

complex that has two chiral amines may be the true active species when using the

catalytic system consisting of [Rh(nbd)Cl]2 and a chiral amine [13].

15.2.2.2 HSSP of Phenylacetylenes Having Two Amido Groups(RDAPA) [20, 21]. It was of interest to see whether acetylene monomers other

than RDHPA, could be adapted for HSSP. To achieve such a polymerization of other

Wavelength (nm)

300 400 500 600

[θ]d

eg .

cm2 /d

mol

)

0

5000

12000

ε (l

. m

ol –1

. cm

–1)

35000

20000

-20000

0

265

a

bcd

e

a

b c d

e

a

e

b cd

FIGURE 15.2. Change ofCDandUV-vis spectra of one-handed helical polyphenylacetylenes

prepared byHSSP of DoDHPA in various solvents: (a) in CCl4, (b) in CCl4/DMSO(50/1), (c) in

CCl4/DMSO(30/1), (d) in CCl4/DMSO(20/1), and (e) in CCl4/DMSO(10/1). (Reprinted with

permission from [12]. Copyright 2003 American Chemical Society.)


achiral monomers by the chiral catalytic system, new achiral phenylacetylenes with

two N-alkyamido groups (RDAPA, Scheme 15.4a) were designed as monomers. The

N-alkylamido groups in RDAPAwere thought to play an important role, causing the

formation of intramolecular hydrogen bondings between the amido groups.

Consequently, the polyRDAPA obtained using the chiral catalytic system showed

Cotton signals in CD measurements. This result proved that the monomer with two

amide groups could polymerize in a helix-sense-selective fashion. To confirm the

importance of hydrogen bonding for this helix conformation, a polar solvent was

added to the CD solution once again. As a result, the CD intensity of polyRDAPA

decreased significantly.

A homopolymer of another achiral phenylacetylene (Scheme 15.4b), that has two

alkyl ester groups (4 in Scheme 15.4b) that cannot make hydrogen bonds instead of

N-alkylamido groups was obtained in a similar chiral condition but showed no CD

signal. From the previous experiments, it became clear that the hydrogen bonds in the

helix conformation of the polyRDAPA are crucially important [20, 21] similar to that

of polyRDHPA. Although some monomers have two groups that can make hydrogen

bonds, such as two N-phenylamido groups (2) instead of N-alkylamido groups in

RDAPA and two hydroxysilyl groups (6), they were not suitable for HSSP. This

finding suggests that the position of the functional groups that can make hydrogen

bonds is important to maintain a one-handed helical conformation in the resulting

polymers. In addition, because a monomer with only one hydroxyl group that can

make hydrogen bonds (5) was not suitable for HSSP, two groups that can make

hydrogen bonds per monomer were necessary.

In summary, monomers such as ethers (1 and 3) and ester (4) derivatives, which

were not suitable for HSSP, do not have functional groups that can make hydrogen

bonds. However, because we found that some monomers with no functional groups

making hydrogen bonds were suitable for HSSP, they are described in

Section 15.2.2.3.

15.2.2.3 HSSP of Phenylacetylenes Having Two Imino Groups(RDIPA). All aforementioned monomers suitable for HSSP need functional

groups that can make hydrogen bonds. However, some HSSP active monomers

do not have such functional groups like RDIPA [22] and HGPA [14] as shown in

Scheme 15.4a. These monomers cannot make hydrogen bonds but do have a bulky

substituent that may keep an one-handed helical conformation. In these RDIPAs,

long alkyl chains were necessary to yield soluble one-handed helical polymers and to

avoid exchange reactions during polymerization described in Section 15.2.3.2.

The requirement of bulky groups for HSSP active monomers is not clear at

present, and only a few examples are available so far [14–16]. The details of other

examples are described in Section 15.4.3.

15.2.2.4 AIP of Chiral Phenylacetylenes Followed by Removing theChiral Groups by RIM. Other than HSSP, which can produce one-handed helical

poly(acetylene)s without the coexistence of any other chiral groups, we found

another method referred to as modified AIP. After obtaining one-handed helical


poly(substituted acetylene)s with many chiral pendant groups by AIP, the chiral

groups were removed completely by RIM (polymer reaction in membrane state). The

detail is described in Section 15.3. As another example of RIM, selective

cycloarmatization (SCAT) is described in 15.2.4.3.

15.2.3 Modified HSSP

15.2.3.1 Self HSSP (SHSSP). In Section 15.2.2 on HSSP, we described only

achiral monmers such as RDHPA. Now we will describe new chiral RDHPA-type

monomers that have two hydroxyl groups and a chiral group(R). Three novel

chiral RDHPA-type monomers, phenylacetylenes that have a chiral

octyloxyethylaminomethyl or hydroxyethylaminomethyl group derived from

an L-aminoalcohol and two hydroxymethyl groups, were synthesized and

polymerized by two achiral catalysts (nbd)Rhþ[h6-(C6H5)B�(C6H5)3] and [Rh

(nbd)Cl]2/triethylamine (TEA) as well as a chiral catalytic system ([Rh(nbd)Cl]2/

(S)- or (R)-phenylethylamine ((S)- or (R)-PEA)). All the resulting polymers

showed Cotton effects at wavelengths around 430 nm. This observation indicated

that they had an excess of one-handed helical backbones. Positive and negative Cotton

effects were observed for the polymers (poly(RVDHPA)) with an L-valinol residue

produced by using (S)- and (R)-PEA as a cocatalyst, respectively, although the

monomers had the same chirality (Scheme 15.5a). However, the two polymers

(poly(RPDHPA) and poly(RADHPA), which have L-phenylalaninol residue or an

O

OH

OH

NH OR

AIP

*

Chiral catalytic system

Achiral catalyst

O

OH

OH

NH OR

*

HSSP

No HSSP

AIP

Achiral catalyst

(a)

(b)

R- = -H or alkyl group

RPDHPA

RVDHPA

SCHEME 15.5. Monomers (a) suitable and (b) unsuitable for both HSSP and AIP.


L-alaninol residue obtained by using (S)- and (R)-PEA as a cocatalyst, showed CD

absorptions with identical signs (Scheme 15.5b)). Therefore, we found that the chiral

monomer with an L-valinol residue(RVDHPA) was suitable for both modes of

asymmetric polymerization (i.e., HSSP with the chiral catalytic system and AIP

with the achiral catalysts (Scheme 15.5a)) [23]. However, the other monomers that

have an L-phenylalaninol residue (RPDHPA) or an L-alaninol residue (RADHPA)

were not suitable for HSSP because the helix sense could not be controlled by the

chirality of PEA (Scheme 15.5b)).

During the aforementioned experiments, we observed some unexpected behavior

in the asymmetric polymerizations of RPDHPA and RADHPA. Despite the bulkier

chiral group in RPDHPA, the CD peaks of the resulting polymers were lower than

those of RADHPA. To explain the unexpected behaviors in the asymmetric poly-

merizations of the two chiral monomers with a chiral bidentate ligand, a novel third

mechanism of asymmetric polymerization (i.e., self helix-sense-selective polymer-

ization (SHSSP)) was proposed.

A chiral ligand group in the monomers worked as a chiral substituent for chiral

induction in AIP as well as a chiral cocatalyst in HSSP. If the effect of the chiral group

on the chiral induction in AIP is low, then it functions only as a chiral cocatalyst for

HSSP. Therefore, the monomer works as a chiral cocatalyst of HSSP of the monomer

itself. We call it SHSSP. For example, RADHPAwas the best in the three monomers

for SHSSP (Scheme 15.6a).

15.2.3.2 Pseudo HSSP (PHSSP). RDIPA that has imino groups was HSSP

suitable as described in Section 15.2.2.3. However, some of them showed an

exchange reaction of the imino group with amines added as a cocatalyst. In that

case, because the imino groups were substituted by chiral amines, which were added

as a chiral cocatalyst, they were not suitable for the HSSP. However, if the formed

chiral imine can be resubstituted by the corresponding achiral amine, which was the

same as that in the monomer, then the corresponding polymer of the monomer can be

obtained. Therefore, this polymerization consisting of polymerization and a

O

OH

OHCH3

ORNH

* N

N

OH

OH

(a) (b)

R- = -H or alkyl group

RADHPA

SCHEME 15.6. Monomers suitable for modified HSSP: (a) SHSSP and (b) PHSSP.


simultaneous exchange reaction followed by a reexchange reaction seems to be

similar to HSSP because the corresponding polymer of the starting achiral monomer

is obtained. Therefore, we call it “pseudo helix-sense-selective polymerization”

(PHSSP) [24]. To promote PHSSP, monomers should be selected that have a

substituent that can be exchanged easily. For example, when monomers that have

no substituents at 4-position as shown in Scheme 15.6b) were used, PHSSP could

occur smoothly. We found that the monomer shown in Scheme 15.6b) was suitable to

PHSSP [24].

15.2.4 Functions of One-Handed Helical PolyphenylacetylenesPrepared by HSSP

Because the resulting chiral polymers from HSSP of RDHPA have unique structures,

such as cis-cisoidal conformation, double-strand helical structure, one-handed

helicity maintained by hydrogen bonds Figure 15.3, extremely high molecular

weights, good self-membrane-forming ability, and so on, it showed many unusual

properties (Scheme 15.7). In this section, some of these properties are described

concisely.

15.2.4.1 Chiral Cocatalysts of HSSP. Several kinds of chiral compounds

such as chiral amines, L-aminoalcoholes, and a polymer that has chiral amino groups

were suitable for HSSP. We found that the chiral polymers prepared by HSSP of

RDIPA were also effective as a cocatalyst of HSSP of RDIPA and RDHPA. For

example, one kind of RDIPA was suitable for HSSP of the other kind of RDIPA

(Scheme 15.8) [25].

FIGURE 15.3. Possible conformation of one-handed helical poly(RDHPA) prepared by

HSSP.


SCHEME15.7.Uniqueproperties

ofone-handed

helical

poly(phenylacetylene)sprepared

byHSSPofRDHPA.

435

15.2.4.2 Optical Resolution Membranes. In this section, enantioselective

permeations through the resulting polymeric membranes prepared by AIP and HSSP

are presented [5, 11]. To realize good optical-resolution membrane materials, the

authors thought that the polymers should possess the following properties: (1) The

polymers should have a self-membrane–forming ability, thereby eliminating

domains and defects that have no recognition ability. (2) The polymers should

have a high content of chiral structures to enhance the amount of chiral recognition

sites. The authors reported that poly[(-)-1-{dimethyl(10-pinanyl)silyl}-1-propyne]

prepared by AIP yielded a self-supporting membrane that showed enantioselective

permeation for many kinds of racemates. Notably, the enantioselectivity was almost

100% in the initial period. Because the polymer powder had no enantioselectivity in

adsorption, the enantioselectivity appeared only in permeation (diffusion) through

the pure membrane. Subsequently, the authors selected other poly(substituted

aromatic acetylene)s, especially poly(substituted phenylacetylene)s, many of

which were newly synthesized by homopolymerization of the corresponding new

chiral monomers (¼AIP), for use as enantioselective membranes (Scheme 15.3). The

reasons for the selection of chiral-substituted poly(phenylacetylene)s were as

follows: (1) Many kinds of substituted aromatic acetylenes can polymerize to

yield soluble high-molecular-weight polymers, even if the monomers bear bulky

groups. Poly(phenylacetylene)s tend to have good self–supporting-membrane-

forming abilities because of their high molecular weights. Even if it has an

extremely large and bulky group such as macromonomers shown in Scheme 15.9,

it will result in good self-supporting membranes. Therefore, the polymerizable group,

phanylacetylene, is thought to be suitable to yield polymers that can be fabricated to

membrane materials. For example, polymers from some macromonomers

(Scheme 15.9) were soluble and had high molecular weights. Therefore, they have

a self-membrane-forming ability. (2) When substituted phenylacetylenes have chiral

substituents, the monomers can polymerize to yield one-handed helical polymers.

Therefore, the membranes can have a high chiral content.

OC12H25

N

N

OR

OR

OC12H25

N

N

OH

OH

OC12H25

N

N

OH

OH

RhCl

RhCl

OC12H25

OC12H25

N

N

OR

OR

N

N

OR

OR

Catalyst

Cocatalyst

R- = alkyl group

SCHEME 15.8. HSSP by using one-handed helical polyphenylacetylenes prepared by HSSP

as a chiral cocatalyst.


Si

Si

CH

3

H3C

CH

3

CH

3S

iC

H3

CH

3

O

Siloxan

e m

acrom

on

om

er

CH

3

Si

CH

3

OSi

H3C C

H3

CH

3

n

Den

doron

macrom

on

om

er

Si

Si

CH

3

H3C

CH

3

CH

3

Si

Si

CH

3

H3C

CH

3

CH

3

n

n

SCHEME

15.9.Exam

plesofmacromonomersthat

haveaphenylacetyleneas

apolymerizable

group.

437

15.2.4.2.1 Optical Resolution Membrane from Chiral Polymers by theHSSP. In our laboratory, many kinds of chiral poly(substituted acetylene)s have

been synthesized by AIP (Scheme 15.3) and applied for optical resolution

membranes [5–11]. However, because the chiral poly(substituted acetylene)s

prepared from chiral monomers had two kinds of chiral recognition sites, (i.e., a

chiral main chain and chiral pendant groups), we could not confirm the optical

resolutionabilitiesof the chiralhelicalmainchain.Recently, asdescribedpreviously,we

succeeded in obtaining a chiral helical poly(substituted phenylacetylene) that has an

asymmetricstructureonlyinthemainchainbyHSSP[12].Thispolymerwassynthesized

byHSSP of achiral 4-dodecyloxy-3,5-bis(hydroxymethyl)phenylacetylene (DoDHPA)

using a chiral catalytic system consisting of [Rh(2,5-nornornadiene(¼NBD))Cl]2 and

chiral phenylethylamine ((R)- or (S)-PEA). In addition, the polymer showed a good

membrane-forming ability, and its membrane exhibited enantioselective permeatbility.

The fact directly indicated the effectiveness of the main chain’s chirality on

enantioselectivities. To our knowledge, this was the first example to confirm the

effectiveness (Figure 15.4). However, because enantioselectivity of the chiral

membrane from poly(DoDHPA) was not so high, achiral ligands were introduced to

its pendants that can interact more effectively with permeants to improve

enantioselectivity [26].

15.2.4.3 Selective cyclic aromatization(SCAT). The resulting polymers

from HSSP of RDHPA had a highly ordered structure such as cis-cisoidal

conformation, double-strand helical structure, one-handed helicity, and so on

(Figure 15.3). As a result, the polymer membrane showed an unexpected reaction

when it was exposed in light. The SCAT produced the corresponding cyclic trimer,

0

20

40

60

80

100

120

140

0 200 400 600 800 1000

Permeation time (h)

QT

rp (x

10

–6

g)

FIGURE 15.4. Enantioselective permeation thorough membranes from one-handed helical

polyphenylacetylenes prepared by HSSP of poly(DoDHPA). Plots of quantity (QTrp) of

permeated R-(þ)-(.)- and S-(-)-(*)-tryptophan(Trp) from 0.50wt% racemic aq. solution

v. s. permeation time.


1,3,5-trisubstituted benzene almost quantitatively. In addition, the resulting

membrane maintained self-membrane–forming ability despite the low molecular

weight. The SCAT so far was observed only for the polymers obtained by HSSP of

RDHPA. This may be because the reaction largely depends on the configuration and

conformation of the starting polymers.

15.3 CHIRAL DESUBSTITUTION OF SIDE GROUPSIN MEMBRANE STATE

15.3.1 Polymer Reaction in Membrane State(RIM)

In general, two methods exist to synthesize chiral polymers whose chiral structures

are present alone in the main chain as asymmetric carbons and/or as a one-handed

helical conformation. One is HSSP, using a chiral catalytic system as a chiral source,

of achiral monomers that have no chiral groups and the other is AIP, using achiral

catalysts, of monomers that have a chiral group as a chiral source.

One-handed helical poly(substituted aromatic acetylene)s obtained by HSSP or

AIP are useful for enantioselectively permeable membranes and chiral polymer

catalysts because the polymers can have a high chiral content [4, 5, 11].

Recently, we proposed a new method for the synthesis of one-handed helical

polymers without the coexistence of any other chiral moieties [9, 27]. In this method

(AIP-RIM) a chiral compound is used as a starting monomer in AIP followed by the

desubstitution of the chiral groups from a one-handed helical polymer membrane

(RIM) (Scheme 15.10).

The most important feature of this method is that the desubstitution (a polymer

reaction) of the chiral groups was carried out with the polymer as a solidmembrane. In

themembrane, quantitative desubstitution was realized with maintenance of the one-

handed helical conformation. It is interesting that the “membrane state” acted almost

like a protecting group. We call the method RIM (polymer reactions in a solid

membrane), and in Sections 15.3.2.1 and 15.3.2.2, the desubstitution of chiral groups

of one-handed helical polymers from phenylacetylenes is described as examples of

RIM. In addition, an application of RIM to the synthesis of chiral porous materials

from polystyrene that has chiral groups is described in Section 15.3.2.3.

15.3.2 Reaction in One-Handed Helical Polymer Membranes: Synthesisof One-Handed Helical Polymers with no Chiral Side Groups and noChiral Carbons

15.3.2.1 Poly(phenylacetylene) Membranes. Poly[(–)-1-p-[dimethyl(10-

pinanyl)silyl]phenyl-2-phenylacetylene](poly(p-PSDPA)) obtained with a TaCl5–

n-Bu4Sn catalyst is soluble in common solvents such as toluene and chloroform

and possesses high molecular weight (>1� 106) and displays strong CD signals

because of the one-handed helical main chain structure [7]. It is known that

aryltrimethylsilanes undergo scission of the aryl–Si bond in the presence of

proton acids. The membrane (thickness 0.89mm) of poly(p-PSDPA) was

prepared by casting a toluene solution of the polymer onto a quartz plate, and

CHIRAL DESUBSTITUTION OF SIDE GROUPS IN MEMBRANE STATE 439

desilylation was carried out by immersing the membrane in a mixture of hexane/

CF3COOH (1:1 v/v) for 1 day (Scheme 15.11). The completion of the reaction was

confirmed by the IR spectrum [27].

The resultant polymer (thickness 0.65mm), poly(diphenylacetylene(DPA)),

showed a specific rotation ([a]Dþ 5590�) even larger than the value ([a]Dþ 2380�)of poly(p-PSDPA) as well as intense CD signals comparable with those of

poly(p-PSDPA) in the 350–450 nm region. The large [a]D value and CD signals

of poly(DPA) compared with those of poly(p-PSDPA) partly should be caused by the

shrinkage of the membrane upon desilylation. These results indicate that poly(DPA)

retains practically the same one-handed helical conformation in the main chain as in

*

chiral monomer

achiral monomer

AIP-RIM

achiral catalyst

chiral catalyst

HSSP

*

*

*

*

*

one-handed helix

(only one chiral source: main chain)

RIMone-handed helix

(two chiral sources: main chain

and side groups)

*

Y

ZX

RIM

Y

ZX

1) membrane formation

2) removal of chiral groupsAIP

: polymer matrix

chiral pore

membrane

removal of polymer

side groups

membranefunctional membrane

addition of

functional groups

removal of chiral groups

SCHEME 15.10. Concepts of RIM, AIP-RIM, and HSSP.


poly(p-PSDPA). In other words, the main chain of poly(DPA) can exist in a

sufficiently stable helical conformation with a large excess helix sense in the solid

state irrespective of the absence of any chiral pendant groups.

Diphenylacetylenes that have one or two pinaylsilyl groups (p-PSDPA, BPSDPA)

polymerized by AIP yield chiral helical polymers with various chiral pinanyl

contents (Scheme 15.12) [9]. Then, fabrication of the chiral poly(diphenylacetylene)

membranes was achieved via Si–C scission catalyzed by trifluoroacetic acid. As a

result, chiral desubstitution in the membrane state proceeded quantitatively despite

the reaction being heterogeneous. Completion of the reactions was confirmed by IR

spectra of the membranes before and after the reaction. Thus, the characteristic

absorptions at 3060 (ds,aliphatic C–H), 1250 (ds, SiC–H), 1119 (nas, Si–CH3), 855 (nas, Si–

CH3), and 812 (ns, Si–CH3

) cm�1 completely disappeared in the IR spectrum of a

polymer membrane, which agreed well with that of poly(diphenylacetylene) syn-

thesized directly by polymerization of DPA. CD spectra of the desubstituted polymer

CF3COOH

C CSi

SiCH3

CH3

CH3 C C

Copoly (BPSDPA/p-PSDPA)

Poly(DPA)

CH3

C C n

SiCH3

CH3

C C m

SiCH3 CH3Si

CH3

CH3C CSi

CH3

CH3

in hexane, r.t.

+

TaCl5-n-Bu4Sn

BPSDPA

p-PSDPA

AIP RIM

m+n

SCHEME 15.12. AIP-RIM of p-PSDPA and BPSDPA.

SiCH3 CH3

CF3COOH C CC C

nn

in hexane, r.t., 1day

Ph Ph Ph

Si Si SiR RR

Ph Ph PhCF3COOH

in membrane

R = CH2

poly(p-PSDPA)

C CSiCH3

CH3

Poly(DPA)

TaCl5-n-Bu4Sn

p-PSDPA

AIP RIM

SCHEME 15.11. AIP-RIM of p-PSDPA and molecular-scale voids produced by RIM.


membranes showed Cotton effects in the UV-vis region despite the absence of chiral

pendant groups. This fact indicates that the desubstituted polymer membranes retain

the same chiral helical conformation as in the original polymer. We can conclude that

we had obtained polymer membranes that have a chiral structure that have only

a chiral main chain as the chiral source. Also, enantioselective permeations of

the polymeric membranes were investigated. In the permeation of (�)-tryptophan

(0.5wt% aqueous solution) through the depinanylsilylated poly(p-PSDPA), an

enantioselectivity (aR) of 2.89 (48.5%ee) was observed. In addition, the enantio-

selective pervaporation of (�)-2-butanol was achieved (aR¼ 3.83 [58.6%ee]).

Similarly, the Si–C bonds of the chiral pinanylsily groups of a chiral helical poly

(phenylacetylene)(p-PSPA) were cleaved quantitatively in the membrane state. The

desubstituted polymer had a CD signal in the UV-vis region despite the lack of any

chiral pendant groups, so that a chiral structure resulting from only a main chain was

confirmed.

At the next level of investigation, the scission of a Si–O bond was examined as a

second approach for in situ chiral desubstitution in a membrane. Thus, poly

(phenylacetylene)s membranes that have chiral pinanylsilyl groups tethered through

Si–O bonds (PSOMPA, PSOPA) as well as an induced chiral helical structure were

treated with trifluoroacetic acid to examine the effects in situ desubstitution

(Scheme 15.13) [10]. The results indicated that chiral desubstitution in the membrane

state proceeded quantitatively despite the heterogeneous reaction. The IR spectra of

the desubstituted polymers showed absorptions at 3300 cm�1 (ns, O-H), indicating theformation of the hydroxyl groups, and the absorptions at 3010 (ns, aliphatic C-H), 1210

(ds, SiC-H), 1150 (nas, Si-CH3), 925 (nas, Si-CH3

), and 900 (ns, Si-CH3) cm�1 present in the

IR spectra of the original polymer membranes, which indicated the presence of

pinanylsilyl groups, completely disappeared. The desubstituted polymer membranes

showed CD signals in the UV-vis region similar to those of the original polymer

despite the absence of the chiral substituents—a fact indicating that the main chains

of the polymers retained their chiral helicity.

SiCH3 CH3

CF3COOH C CH

C Cnn

poly(p-PSPA)

CHC SiCH3

CH3 [Rh(nbd)Cl]2-Et3N

p-PSPA

H

CHC CH2 SiCH3

CH3

Om

m = 0 : PSOPA

= 1 : PSOMPA

[Rh(nbd)Cl]2-Et3N

CH2

C CnH

O Si

CH3

CH3m

CF3COOH

C CH n

CH2

OHm

AIP RIM

AIP RIM

SCHEME 15.13. AIP-RIM of phenylacetylene monomer (PSOPA, PSOMPA, and p-PSPA).


The enantioselective permeation of an aqueous solution of a racemic phenylala-

nine (PHE) through these desubstituted polymer membranes was examined. Similar

to the original polymer membranes, all desubstituted polymer membranes showed

(R)-isomer enantioselectivity in permeation. This result directly indicates the

importance of the contribution of the chiral main chain for enantioselective perme-

ation. Enhancements of the permeation rate were observed in the permeation through

the desubstituted polymer membranes. These results show that molecular-scale voids

generated by depinanylsilylation were retained and were effective in the enhance-

ment of permeation. This method shows great promise to improve membranes that

suffer from low permeability.

15.3.2.2 Poly(phenyleneethynylenes) Membranes. One-handed helical

poly(phenyleneethynylenes) that have only a second-order structure as a chiral

source were synthesized by in situ desubstitution [28]. That is, we synthesized a

chiralpoly(phenyleneethynylene)s thathadopticallyactivementhoxycarbonylgroups

by polycondensations of (þ)-menthyl 3,5-diethynylbenzoate with (þ)-menthyl

3,5-diiodobenzoate using PdCl2(PPh3)2–PPh3–CuI (PMtMt) (Scheme 15.14),

investigated the main-chain conformation of the obtained polymer in solution and

inmembrane by CDmeasurements, and then the in situ desubstitution of the polymer

was performed.

PMtMt was obtained in a good yield (yield: 74%,Mw¼ 160� 103,Mw/Mn¼ 2.8)

and showed the largest Cotton signal in the UV region. Because PMtMt had a enough

molecular weight, self-supporting membranes can be fabricated using the solvent-

castingmethod. IntheCDspectrumofPMtMttakeninchloroform/benzene(30:70 v/v)

at�10�C, large Cotton effects were observed in the absorption region of the polymer

O O

O O

THF, MeOH, KOH

O O- K+

O O- K+

HCl aq.

O OH

O OH

n n n

r.t., 3 daysr.t.

a few minutes

O O

I I

O O

+

O O

O O

n

PdCl2(PPh

3)2, CuI, PPh

3

PMtMt

PMtMt

RIM

toluene/Et3N

SCHEME 15.14. Synthesis of one-handed helical PMtMt (AIP) and RIM of PMtMt.


backbone, indicating an excess of one-handed helical conformation. However, as the

ratio of chloroform was increased, the intensity of the Cotton signals decreased, and

finally, the Cotton signals completely disappeared in chloroform.

A thin membrane was prepared on a quartz disc using the spin-coating method.

When toluene was used as a casting solvent, the membrane obtained showed a clear

Cotton effect that was almost the same as that observed in solution but was

accompanied by a slight blue-shift. However, no Cotton signal was observed when

a membrane cast from chloroform solution was measured.

RIM for the PMtMt membrane was examined as Scheme 15.14. Desubstituion

was carried out by immersing a PMtMt membrane into an alkaline solution at room

temperature for 3 days. Then the membrane was washed with methanol and treated

with dilute hydrochloric acid to protonate the carboxylate groups. Finally, the

membrane was washed with methanol again and dried under reduced pressure.

In situ desubstitution of PMtMt proceeded a quantitatively yield. Confirmation of the

complete reaction was attempted by comparing IR spectra of the membranes before

and after the reaction. Thus, a broad and intense O–H stretching absorption (ns, O-H)in the region of 3500–2500 cm�1 originated from carboxyl groups appeared, and

absorptions at 2955 (ns, aliphatic C-H) cm�1 derived from menthyl groups was reduced

in the IR spectra of membrane after the reaction, but the broad absorption of ns, O-Hwas unclear because of overlapping. Therefore, we tried to check the completion of

the reaction by gravimetry. As a result, the weight loss of the membrane upon

desubstitution supported the completion of the reaction. For example, PMtMt was

9.57mg; desubstituted PMtMt was 4.81mg; calculated was 4.88mg.

To measure CD spectra of the desubstituted PMtMt membrane, a thin membrane

was prepared using the spin-coating method on a quartz disc with a toluene solution

and then treated as described previously. The desubstituted PMtMt membrane

showed CD signals similar to those of the original polymer in the UV-vis region.

This result indicates that the desubstituted PMtMt retains the same preferential one-

handed helical conformation as those in the original polymers despite the absence of

chiral pendant groups in the membranes.

15.3.3 Reaction in Polystyrene Monolith: Synthesisof Chiral Porous Materials

Two chiral styrene monomers that have a (–)-dimethyl(10-pinanyl)silyl group and a

(–)-menthoxycarbonyl group, (–)-PSSt and (–)-MtSt, respectively, were synthesized

and then the preparation of chiral polystyrene monoliths was examined by W/O

emulsion polymerization of (–)-PSSt/divinylbenzene or (–)-MtSt/divinylbenzene.

Then, the removal of the chiral substituents, (e.g., (–)-dimethyl(10-pinanyl)silyl

groups and (–)-menthyl groups), from the present monoliths was carried out, and the

imprinting effect of the formed chiral cavity was investigated by enantioselective

adsorption (Scheme 15.15) [29].

Then, the preparation of chiral styrene monoliths using (–)-PSSt and (–)-MtSt was

carried out by using emulsion polymerization of a water-in-oil system that was

prepared with a sun-and-planet–type blender.


All polymerizations yielded cross-linking polystyrene gels in quantitative yields.

The sizes and the total area of the pores were measured by the mercury intrusion

method. Sharp distribution curves for the pore size were obtained, and all obtained

chiral polystyrene monoliths had 2–3mm of the pore radius. The total pore areas of

the chiral monoliths were in the range of 1.15–3.33m2/g, which was larger than that

of the conventional porous polystyrene beads.

In the next stage, we examined the depinanylsilylation of poly[(–)-PSSt] and

de-menthylation (hydrolysis) of poly[(–)-MtSt] in gel state.

The depinanylsilylation of poly[(–)-PSSt] was carried out by immersing it in

CF3COOH. Depinanylsilylation proceeded quantitatively, and completion of the

reactions were confirmed by IR spectra before and after the reaction. Thus, the

characteristic absorptions at 1250 (ds, SiC–H), 1116 (nas, Si–CH3), 855 (nas, Si–CH3

), and

812 (ns, Si–CH3) cm–1 completely disappeared, and the absorption at 3032 (ds, aliphatic

C–H) cm�1 increased in the IR spectrum of de-poly[(–)-PSSt], which agreed well with

the results of polystyrene synthesized directly by radical polymerization. Also,

hydrolysis of poly[(–)-MtSt] was accomplished as follows: A piece of poly

[(–)-MtSt] was heated in KOH aq./2-propanol for 3 days at reflux conditions. Then,

the monolith was acidified by exposure into concentrated HCl aq. In the IR spectra of

de-poly[(–)-MtSt], a broad and strong absorption, resulting from –OH stretching of a

carboxyl group appeared, and the spectra agreed well with that of poly(4-vinylben-

zoic acid) as synthesized.

The conversions of depinanylsilylation and de-menthyl determined by weight

reduction were 96% and 84%, respectively.

R

SiCH3

CH3

O

OR =

(-)-PSSt

n m n m

poly[(-)-RSt]

monolith

de-poly[(-)-R'St]

monolith

R R'

+

AIBN

sorbitan monooleate

(as an emulsifier)

H2O

CF3COOH

or

KOH aq.

Monomer Water

W/O emulsion Porous monolith

Polymer

Pore

,

(-)MtSt

R' = H , COOH

1) Still standing

Polymerization

2) Removal of

the water

RIM

SCHEME 15.15. Synthesis and RIM of chiral poly[(–)-RSt] monoliths.


With the mercury-intrusion method, the macroporous structures of de-poly

[(–)-PSSt] and de-poly[(–)-MtSt] were characterized. The density of the monoliths

was slightly decreased after de-substituents, and total pore area increased (e.g., from

1.15 to 48.5m2/g for poly[(–)-PSSt). In contrast, the average pore radius decreased in

the original monolith. This result indicated that the molecular-scale voids were

generated by the de-substituent in the monolith.

The enantioselective adsorption experiments of a racemic trans-stilbene oxide

were examined using the chiral polystyrene monoliths obtained.

At first, the chiral substituents containing monoliths (i.e., poly[(–)-PSSt] and poly

[(–)-MtSt]) were subjected to an enantioselective adsorption experiment. Then, both

monoliths showed enantioselectivity to adsorb the (S, S)-isomer preferentially. The

highest value of selectivity (a(s,s)) was of 1.49 (19.7% ee) when poly[(–)-PSSt] and

acetone as a solvent for adsorption/desorption were used.

Second, to clarify the chiral imprint effect in the present monoliths, no chiral

group containing monoliths, (i.e., de-poly[(–)-PSSt] and de-poly[(–)-MtSt]) were

subjected to enantioselective adsorption experiment.

In the results, wewere rewarded with the desired result. That is, de-poly[(–)-PSSt]

and de-poly[(–)-MtSt] showed enantioselectivity in adsorption despite no chiral

group contents of the monoliths being available. de-Poly[(–)-PSSt] and de-poly

[(–)-MtSt] preferentially adsorbed the (S, S)-isomer as well as original monoliths.

Using the de-poly[(–)-PSSt], the desirable result was obtained in that the adsorption

quantity was increased as a(s,s) remained at a similar level. However, when the de-

poly[(–)-MtSt] was used, the adsorption quantity was decreased and a(s,s) was

increased in comparison with poly[(–)-MtSt].

15.4 SYNTHESIS OF CHIRAL POLYRADICALS

15.4.1 Molecular Design of Optically Active Helical Polyradicals

Organic radicals are usually known as unstable transient intermediates in organic

reactions. However, some organic radicals such as radical crystals of galvinoxyl [30]

and steric hindered nitroxyl [31] are so stable that they remain in ambient atmosphere

from a few months to a few years. Polymers bearing numerous free radical groups,

so-called polyradicals, were used for polymer antioxidants, redox resin, and spin

labeling [32]. Furthermore, they have recently been regarded as building blocks to

construct a molecule-based ferromagnet [33]. Poly(phenylacetylene)s and poly(1,3-

phenyleneethynylene)s have been well investigated as a backbone structure of the

polyradical for magnetic materials [34]. However, stereoregularities have not been

mentioned. The optical activity of helical p-conjugated polyradicals will be possiblecandidates in combination with optic, electronic, and magnetic properties. Our goal

is to develop new polyradicals with electronic, magnetic, and chiroptical properties

through the fusion of optically active helical polymers with the control of hierarchi-

cal structures (Scheme 15.16).

To synthesize such noble chiral polymers, we used two methods, AIP (see

Section 15.2.1.1) and HSSP (see Section 15.2.2). In this section, two examples of


synthetic methods of new polyradicals were used, one is AIP of monomers that have a

radical groupwith comonomers with a chiral group as a chiral source (15.4.2), and the

other isHSSPofmonomerswitharadicalgroupusingachiralcatalyticsystem(15.4.3).

15.4.2 Copolymerization of the Monomers Possessing Radicaland Chiral Moieties

Chiral polyradicals were obtained simply by the (co)polymerization of monomers

possessing radical and chiral moieties. For example, an optically active helical poly

(phenylacetylene) poly(p-PSPA-co-GPA) was synthesized by copolymerization of

(4-ethynylphenyl)hydrogalvinoxyl (HGPA) and a chiral acetylene monomer p-PSPA

using [Rh(nbd)Cl]2 catalyst (Scheme 15.17) [35]. The copolymer poly(p-PSPA-co-

HGPA) was obtained in good yield with a high molecular weight (Mn� 105). The

compositions of the copolymers were almost the same as the feed compositions of

monomers. The CD spectra of the copolymers were different from that of the mixture

of poly(p-PSPA) and poly(HGPA) and was the same as that of poly(p-PSPA). A

bathochromic shift of the absorption edge was observed in the UV-visible absorption

spectra with increasing amounts of HGPA. In all CD spectra of the copolymers,

positive Cotton effects were observed in the absorption region (450 – 550 nm) of the

backbone chromophore, and the peaks also shifted to a longer wavelength with the

bathochromic shift of the absorption edge, indicating an excess of one-handed helical

polyacetylene backbone. With a higher HGPA composition, a split-type CD signal

appeared and increased its intensity in the absorption region (420 nm) of the hydro-

galvinoxyl chromophore. In the CD spectra of the polyradical poly(p-PSPA-co-GPA)

containing more than 20 mol% of p-PSPA, the Cotton effect was observed in the

absorption region (470 nm) of the galvinoxyl radical chromophore and in (450 –

550 nm) the backbone chromophore. This result indicates the maintenance of the

excess of a one-handed helix even after the oxidation reaction as well as the successful

synthesis of an optically active helical polyradical. Another optically active helical

poly(phenylacetylene) poly(HGPA-co-PEAGPA) was synthesized by the copolymeri-

zation of HGPA and PEAGPA (Scheme 15.17), which was synthesized to bond the

chiral group to HGPA, using a [Rh(nbd)Cl]2 catalyst [36]. The Cotton effect indicated

that the helix sense of poly(HGPA-co-PEAGPA)s could be controlled by the compo-

sition of HGPA and PEAGPA in the copolymers.

X

X

X

Optically active helical polymer

Optical activity

Chiral polyradical

Magneto-chiral dichroism

Polyradical

Magnetic property

SCHEME 15.16. Molecular design of optically active helical polyradicals.

SYNTHESIS OF CHIRAL POLYRADICALS 447

Anopticallyactivepoly(1,3-phenyleneethynylene)GPSPEbearinggalvinoxylunits

and dimethyl(10-(1S)-pinanyl)silyl groups was synthesized via the synthesis of the

hydrogalvinoxyl precursor polymerHGPSPE by condensation polymerization of (3,5-

dihalogenophenyl)hydrogalvinoxyl and 1,3-diethynylbenzene monomer bearing di-

methyl(10-(1S)-pinanyl)silyl groups using a Pd(PPh3)4 catalyst (Scheme 15.18) [37].

p-PSPA + HGPA

n

R

HGPA + PEAGPA

HGPA:

GPA:

PEAGPA:

poly(HGPA-co-PEAGPA )

poly(GPA-co-PEAGPA )

R =

X = H

X =

*

*

X =O

NH

alkaline

K3Fe(CN)6 aq.

R =

poly(p-PSPA-co-GPA )

H[Rh(nbd)Cl]2

R

Si

CH3

CH3

CH2

O

OXp-PSPA

poly(p-PSPA-co-HGPA )

PbO2

SCHEME15.17. Copolymerization of the phenylacetylenemonomers possessing radical and

chiral moieties.

*

X = H

X =

*

n

*

+

*

I

I

O

OH

HGPSPE:

Si

O

OX

Si

GPSPE:

Pd(PPh3)4,

CuI

PbO2

SCHEME 15.18. Synthesis of the optically active poly(1,3-phenyleneethynylene)s HGPSPE

and GPSPE.


The molecular weight of the polymer HGPSPE (Mw¼ 1.7� 105, Mw/Mn¼ 3.7) was

improved by using the diiodomonomer instead of the dibromomonomer. The obtained

polymer was soluble in chloroform, tetrahydrofuran, ethyl acetate, benzene, and

toluene but was insoluble in acetone, diethyl ether, DMSO, DMF, alcohols, and

aliphatichydrocarbons.ThepolyradicalGPSPEwasobtainedbyoxidizing thepolymer

HGPSPE by treating of the polymer solution in degassed chloroformwith fresh PbO2.

TheCD spectra ofHGPSPE andGPSPEweremeasured invarious solutions. In theCD

spectrumofHGPSPEtaken inethylacetate solution, clearCottoneffectswereobserved

in the absorption region of the backbone andhydrogalvinoxyl chromophore, indicating

an excess of one-handed helical foldamer conformation. However, the formation of

GPSPEunfortunatelybecame insoluble inethyl acetate.Yet,Cottoneffectswerehardly

observed in chloroform,THF, and benzene solution for bothHGPSPEandGPSPE.But

the CD signals appeared in CD spectra of the polymer and polyradical by adding of

methanol to the chloroform solution, accompanying a bathochromic shift of absorption

maxima of galvinoxyl chromophore and a hypochromic effect of absorption around

300 nm. An optically active poly(1,3-phenyleneethynylene)-based polyradical

NNMtPE bearing nitronylnitroxide units and chiral menthyloxycarbonyl groups was

directly synthesized by condensation polymerization of (3,5-diiodophenyl)nitronyl-

nitroxide and homochiral menthyl 1,3-diethynylbenzoate using a Pd(PPh3)4 catalyst

(Scheme 15.19) [38]. The polyradical NNMtPE was obtained in good yield (77%,

Mw¼ 2.6� 103, Mw/Mn¼ 1.3). In the CD spectrum of NNMtPE taken in benzene

solution, clear Cotton effects were observed in the absorption region of the backbone,

indicating an excess of one-handed helical foldamer conformation, whereas no Cotton

effects were observed in chloroform solution. The CD signal intensity of NNMtPE in

benzene increased and decreased with the addition of methanol and chloroform,

respectively. An optically active poly(binaphthyl-6,6’-diylethynylene-1,3-phenyle-

neethynylene)GPENpwith pendant galvinoxyl residueswas synthesized via synthesis

of thehydrogalvinoxylprecursorpolymerHGPENpbycondensationpolymerizationof

(1,3-diiodophenyl)hydrogalvinoxyl and 6,6’-diethynyl-2,2’-dihexyloxybinaphthyl

using a Pd(PPh3)4 catalyst (Mw¼ 4.6� 104,Mw/Mn¼ 3.0) (Scheme 15.20) [39]. The

polymer resulted in the corresponding polyradical with a high spin concentration by

treating the polymer solution with PbO2. In the CD spectra of the polymer and

polyradical taken in various solution, clear Cotton effects were observed in the

*

n

*

I

I

NN O

ON

N O

O

* **

*

+

NNMtPE

OO

Pd(PPh3)4,

CuI

OO

SCHEME 15.19. Synthesis of the optically active poly(1,3-phenyleneethynylene) NNMtPE.


absorption region of the binaphthyl chromophore, whereas no Cotton effect was

observed in that of the galvinoxyl chromophore.

15.4.3 Synthesis of Chiral Polyradicals via HSSP of Achiral Monomers

Although the copolymerization of a radical monomer and a chiral monomer is a

simple approach to obtain the chiral polyradicals, the chiral monomer reduces the

number of radical units in the polymer, which leads to a reduction of spin interaction.

Therefore, HSSP using chiral catalysts or chiral initiators is a possible method to

obtain various one-handed helical polyradicals because the process demands no

chiral moiety in the monomer. This fact will result in increased flexibility of

monomer design, besides obvious economic implications as a result of using only

a catalitic amount of chiral compounds that are often expensive. Recently, optically

active helical poly(HGPA) [14–16], poly(HGDHPA) [17, 40], poly(HNHHPA), poly

(EHNHHPA), and poly(NHHPA-co-DoDHPA) [37] bearing stable radical precursor

have been synthesized by HSSP of the corresponding achiral phenylacetylenes, using

the rhodium complex catalyst, in the presence of chiral 1-PEA (Scheme 15.21).

HSSP of the monomer HGPAwas carried out in the presence of the [Rh(nbd)Cl]2catalyst and (R)- or (S)-PEA. A red solid polymer was obtained in low yield with an

average molecular weight of ca. 104, but in the CD spectrum taken in chloroform

solution, clear Cotton effects were observed not only in the absorption region (450 –

600 nm) of the backbone chromophore but also in that of the hydrogalvinoxyl

chromophore (420 nm). The CD spectra of the polymers obtained by polymerization

using (R)- and (S)-PEA as a solvent resulted in mirror images of each other. These

results indicate an excess of one-handed helical polyacetylene backbone. The Cotton

effect in 420 nm showed split-type CD signals that were attributed to exiton coupling

between the hydrogalvinoxyl chromophores. The sign of the CD signal in the

absorption region (420 nm) of the hydrogalvinoxyl chromophore was calculated

from the polymer geometry, and the result suggests that an excess of right-handed

helix (P-helix) was induced by polymerization in the presence of [Rh(nbd)Cl]2

GPENp:

OC6H13OC6H13

H

Hn

OC6H13OC6H13

O OH

II +

O OX

X = H

X =

HGPENp:

Pd(PPh3)4,

CuI

PbO2

SCHEME 15.20. Synthesis of the optically active poly(binaphthyl-6,6’-diylethynylene-1,3-

phenyleneethynylene) HGPENp and GPENp.


catalyst and (R)-PEA [15]. When excess amounts of (R)-PEA were added to the

solution of achiral or racemic poly(HGPA) that were obtained by polymerization

using triethylamine or racemic PEA as solvents, no Cotton effects were observed.

Therefore, it was confirmed that HSSP occurred in the presence of the chiral PEA.

Because the helical polyacetylenes possessing no chiral moieties except for helicity

have two conformational enantiomers whose states are thermodynamically equiva-

lent to one another, the conformational rotations of the polymers should lead to the

full racemization of polymers on the intrachain and/or interchain. However, the

optically active poly(HGPA) had an excess of one-handed helix kinetically stabilized

by the achiral bulky side groups. The CD signals of optically active poly(HGPA)

O

OX

HGDHPA: X = HDoDHPA

=

GDHPA:

HGPA

HGDHPA

HNHHPA

EHNHHPA

NHHPA + DoDHPA

poly(GPA)

poly(GDHPA)

poly(HNHPA)

poly(EHNHPA)

poly(NHPA-co-DoDHPA)

HGPA:

GPA:

O

O

OX

EHNHHPA:

HO

HO

poly(HGPA)

poly(HGDHPA)

poly(HNHHPA)

poly(EHNHHPA)

poly(NHHPA-co-DoDHPA)

Oxidation

NHHPA:

N

R

EHNHPA:

HO

HNHHPA:

OX

R = OC6H13

HNHPA:

R = OCH2CH(C2H5)C4H9

R = H

OC12H25

HO

HO

n

X =

Rh complex cat.

(R)- or (S)- PEA

X = H

X =

X = H

X =

X = H

X =

X = H

X =NHPA:

SCHEME 15.21. HSSP of the achiral phenylacetylenes bearing a stable radical precursor,

using the rhodium complex catalyst, in the presence of chiral 1-PEA.


remained for a long time in chloroform solution, and the half-life of the CD signal

intensity was more than 3 days at room temperature [14]. However, CD and UV-vis

absorption spectra of the optically active poly(HGPA) exhibited both thermo- and

solvatochromism in the solution examined immediately after preparation [15]. The

observation of chiroptical thermo- or solvatochromism is novel for one-handed

helical polymers that possess no chiral moieties in the polymer chains because the

helical polymers require both a high activation energy for the conformational

rotations to the conformational enantiomer and the availability of intermediate

conformational states with lower activation energies, whose CD signals become

weakened in intensity or have an opposite sign compared with those of the helical

conformation of the ground state. The [Rh(cod)Cl]2 (cod.1,5-cyclooctadiene)

catalyst system for the HSSP of HGPA also yielded a red polymer, whose CD

spectrum showed a larger Cotton effect, although the yield and molecular weight

were lower than those of the polymer obtained by the [Rh(nbd)Cl]2 catalyst

system [16]. Moreover, the CD signals of the polymers obtained by polymerization

using [Rh(cod)Cl]2 and [Rh(nbd)Cl]2 were nearly mirror image of each other,

except for the magnitudes of the signals, despite using the same chiral environment

(i.e., in the presence of (R)-PEA [Scheme 15.22]). This result indicates that, for

polymerization in the presence of (R)-PEA, poly(HGPA) from [Rh(nbd)Cl]2 and

[Rh(cod)Cl]2 were assigned to P-helix and M-helix, respectively. The plausible

polymerization mechanism related to the helix-sense-selectivity was proposed

based on the aforementioned results and semiempirical molecular orbital calcula-

tion, in which each phenyl group of PEAwas placed on the near side of the smaller

nbd and the opposite side of the larger cod, respectively. This finding indicates that

the helix sense in the HSSP was controlled by the bulkiness of achiral diene ligands

in combination with chiral PEA as the chiral bias.

The monomer HGDHPA was polymerized in the presence of [Rh(nbd)Cl]2 or

Rhþ(nbd)[(h6-C6H5)B–(C6H5)3] catalysts, chiral PEA cocatalyst, and an achiral

solvent. The red solid polymer poly(HGDHPA) was obtained by precipitation from

the polymerization mixtures into methanol (Mw� 105). The addition of copper (I)

iodide (CuI) into the catalytic system improved the polymer yield. The effect of a CuI

addition in the polymerization of HGDHPAwas estimated as the transmetalation of

copper acetylide to Rh accelerated complexation to polymerization active species.

Moreover, the replacement of [Rh(nbd)Cl]2 by Rhþ(nbd)[(h6-C6H5)B–(C6H5)3] in

the polymerization catalytic system seemed to enhance the reactivity remarkably

probably because the elimination of tetraphenyl borate anion proceeded more readily

compared with the chloride anion. Split-type–induced CD signals were observed for

the THF solution of poly(HGDHPA) at 300 nm, indicating an excess of one-handed

helical polyacetylene backbone. It is clear that HSSP occurred in the presence of

chiral PEA because monomer HGDHPA has no chiral moieties and the CD signal at

300 nm was similar to that of the previously reported poly(DoDHPA). The CD

intensity of poly(HGDHPA) was nearly constant even when the solution was heated

to 60 �C. But the CD signals decreased and disappeared in CD spectra of poly

(HGDHPA) when adding DMF to the THF solution, which supported the notion that

the rigid and one-handed helical conformation was stabilized by intramolecular


O

OH

[R

h(cod)C

l]2

(S

)-P

EA

HG

PA

(R

)-P

EA

H2N H

(S

)-P

EA

right-handed h

elical

poly(H

GP

A)

left-handed h

elical

poly(H

GP

A)

[R

h(nbd)C

l]2

NH

2

H

O

HO

O

OH

RhC

l

ClR

h

RhC

l

ClR

h(R

)-P

EA

NH

2

H

H2N H

SCHEME15.22.Achiral

diene-ligandscontrolthehelix

sense

intheHSSPofHGPA.

453

hydrogen bonds. This behavior is in contrast to the results for poly(HGPA) described

earlier, whose CD signal intensity was dependent on temperature over the experi-

mental range and decreased reversibly with an increasing temperature for the CD

spectra of poly(HGPA) in the solution used immediately after preparation. The

hydrogalvinoxyl units of poly(HGDHPA) were converted to the corresponding

galvinoxyl radicals after treatment with fresh PbO2. The absorption maximum at

420 nm resulting from the hydrogalvinoxyl chromophore decreased, and a new

absorption peak resulting from the galvinoxyl radical chromophore appeared at

470 nm. The polyradical poly(GDHPA) was stable enough to maintain the initial spin

concentration under electron spin resonance (ESR) and CDmeasurement conditions.

In the CD spectra of the polyradical, the Cotton effect was still observed at 300 nm,

which indicates that maintenance of the excess of the one-handed helix and the

successful synthesis of an optically active polyradical with a high spin concentration

was achieved even after the oxidation reaction compared with the optically active

helical copolymer–polyradicals described previously.

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