Helicity induction on poly(phenylacetylene)s bearing phosphonic acid pendants with chiral amines and memory of the macromolecular helicity assisted by interaction with achiral amines in dimethyl sulfoxide

Article abstract of DOI:10.1021/ma049066v

Two novel stereoregular poly(phenylacetylene)s bearing a phosphonic acid residue (poly-1) and its monoethyl ester (poly-2) as pendants were prepared by the polymerization of diethyl (4-ethynylphenyl)phosphonate followed by hydrolysis of the diethyl ester groups and polymerization of ethyl (4-ethynylphenyl)phosphonate, respectively. The polymers were found to form a predominantly one-handed helical conformation upon complexation with various chiral amines through noncovalent acid-base interactions in dimethyl sulfoxide (DMSO). The complexes exhibited an induced circular dichroism (ICD) in the UV-visible region of the polymer backbones. In particular, poly-2 is an induced helical polymer more sensitive to the chirality of amines than poly-1 and poly((4-carboxyphenyl)acetylene) and yields the same Cotton effect sign when complexed with chiral amines of the same absolute configuration. Moreover, the macromolecular helicity of poly-1 and poly-2 induced by chiral amines was memorized after the chiral amines were completely removed and replaced with various achiral diamines and oligoamines in DMSO. In sharp contrast to the same memory effect on the induced helical poly((4- carboxyphenyDacetylene), the helical structures of poly-1 and poly-2 could not be efficiently maintained by achiral monoamines. The effect of the structure of the achiral diamines and oligoamines on the efficiency of the helicity retention and the stability of the memorized polymers were also investigated.

Full text of DOI:10.1021/ma049066v

Macromolecules 2004, 37, 5495-5503
5495
Helicity Induction on Poly(phenylacetylene)s Bearing Phosphonic Acid
Pendants with Chiral Amines and Memory of the Macromolecular
Helicity Assisted by Interaction with Achiral Amines in Dimethyl
Sulfoxide
Hisa n a r i On ou ch i, Da isu k e Ka sh iw a gi, Kiich ir o Ha ya sh i, Ka tsu h ir o Ma ed a , a n d
Eiji Ya sh im a *
Department of Molecular Design and Engineering, Graduate School of Engineering,
Nagoya University, Chikusa-ku, Nagoya 464-8603, J apan
Received May 12, 2004; Revised Manuscript Received May 23, 2004
ABSTRACT: Two novel stereoregular poly(phenylacetylene)s bearing a phosphonic acid residue (poly-1)
and its monoethyl ester (poly-2) as pendants were prepared by the polymerization of diethyl (4-
ethynylphenyl)phosphonate followed by hydrolysis of the diethyl ester groups and polymerization of ethyl
(4-ethynylphenyl)phosphonate, respectively. The polymers were found to form a predominantly one-handed
helical conformation upon complexation with various chiral amines through noncovalent acid-base
interactions in dimethyl sulfoxide (DMSO). The complexes exhibited an induced circular dichroism (ICD)
in the UV-visible region of the polymer backbones. In particular, poly-2 is an induced helical polymer
more sensitive to the chirality of amines than poly-1 and poly((4-carboxyphenyl)acetylene) and yields
the same Cotton effect sign when complexed with chiral amines of the same absolute configuration.
Moreover, the macromolecular helicity of poly-1 and poly-2 induced by chiral amines was “memorized”
after the chiral amines were completely removed and replaced with various achiral diamines and
oligoamines in DMSO. In sharp contrast to the same memory effect on the induced helical poly((4-
carboxyphenyl)acetylene), the helical structures of poly-1 and poly-2 could not be efficiently maintained
by achiral monoamines. The effect of the structure of the achiral diamines and oligoamines on the efficiency
of the helicity retention and the stability of the memorized polymers were also investigated.
Ch a r t 1. Str u ctu r es of P oly-1, P oly-2, P CP A, a n d
Ch ir a l Am in es
In tr od u ction
The detection and amplification of chirality by helical
polymers1 and supramolecular helical assemblies2 have
attracted great interest in recent years, because these
systems can be applied to developing novel chiroptical
devices and chiral materials as enantioselective adsor-
bents and catalysts.3 In a series of our studies, we
reported the helicity induction on optically inactive,
stereoregular cis-transoidal poly(phenylacetylene)s bear-
ing functional groups, which can change their structures
into the prevailing, dynamic one-handed helices upon
complexation with specific chiral guests.1e,4 For instance,
the introduction of carboxy,5 amino,6 and boronate7
groups as the pendant produced poly(phenylacetylene)s
which can respond to the chirality of chiral amines,
acids, and sugars, respectively, and their complexes
show a characteristic induced circular dichroism (ICD)
in the UV-visible region of the polymer backbones. The
Cotton effect signs corresponding to the helical sense
can be used as a novel probe for the chirality assign-
ments of the guest molecules.1,4 The macromolecular
helicity induced on these poly(phenylacetylene)s is
dynamic in nature. However, we recently found that
such a dynamic macromolecular helicity of poly((4-
carboxyphenyl)acetylene) (PCPA) induced by an opti-
cally active amine such as (R)-1-(1-naphthyl)ethylamine
((R)-3) (Chart 1) could be “memorized” even after the
chiral amine was completely replaced by various achiral
amines and amino alcohols in dimethyl sulfoxide
(DMSO).8 The mechanism of the helicity induction and
the memory of the helical chirality of PCPA have been
thoroughly investigated, and the following conclusions
were drawn: (1) the ion pairing of PCPA with chiral
amines is essential for the helicity induction, and (2)
the electrostatic repulsion between the carboxylate
groups of PCPA derived from the dissociation of the ion
pairs plays a central role in the macromolecular helicity
memory of PCPA in DMSO.8b
We expected that a similar macromolecular helicity
memory could be possible for analogous stereoregular
poly(phenylacetylene)s bearing other functional groups
as pendants. We now report that novel stereoregular
poly(phenylacetylene)s bearing a phosphonic acid resi-
due (poly-1) or its monoethyl ester (poly-2) as pendants
(Chart 1) can form a predominantly one-handed helical
conformation in the presence of chiral amines and their
induced helical chirality can be maintained when the
* To whom correspondence should be addressed. E-mail:
yashima@apchem.nagoya-u.ac.jp.
10.1021/ma049066v CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/29/2004

5496 Onouchi et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 2. Titration curves of poly-1 (∆ꢀsecond of 365 nm) and
poly-2 (∆ꢀsecond of 370 nm) with (R)-3 (A) and (R)-4 (B) in DMSO
F igu r e 1. CD spectra of poly-1 and poly-2 with (R)-3 and (R)-4
([3 or 4]/[polymer] ) 2) in DMSO at ambient temperature (ca.
20-22 °C) with a polymer concentration of 1.0 mg/mL. The
absorption spectrum of poly-2 with (R)-3 is also shown.
at ambient temperature (ca. 20-22 °C) with
a polymer
concentration of 1.0 mg/mL.
The complexes exhibited split-type ICDs in the UV-
visible region of the polymer main chains due to the
formation of a predominantly one-handed helical con-
formation of the polymers. The Cotton effect patterns
were similar to each other, but the intensities were
weak for the complexes with (R)-4. The CD titrations
using (R)-3 and (R)-4 were then performed (Figure 2).
The CD intensities increased with an increase in the
concentration of (R)-3 and (R)-4 and reached a constant
value in the presence of an almost equivalent amount
of the chiral amine and amino alcohol. Particularly, the
ICD magnitudes of the complexes dramatically changed
at around the region where the molar ratio of the amine
and amino alcohol to the polymers is from 0.5 to 1. These
sudden onsets and rapid increases in the optical activity
of poly-1 and poly-2 with a sigmoidal fashion suggest
that the polymers form a slight excess of a one-handed
helical structure at around the region ([amine]/[polymer]
) 0-0.4 mol/mol) and further change in the population
of the right- and left-handed helices of the polymer main
chains into a one-handedness cooperatively occurred on
the polymers with a further increase in the concentra-
tion of the amine and amino alcohol.3b,8b,15
chiral amines are removed and replaced with various
achiral diamines and oligoamines in DMSO. The results
are compared with the previously reported helicity
induction and memory effect of PCPA. Poly-1 and poly-2
have a more acidic phosphonic acid group than the
carboxy group of PCPA as the pendant, so that helicity
induction may occur more efficiently in poly-1 and poly-
2, resulting in more intense ICDs in their polymer
backbone regions. Poly-1 was already reported to exhibit
an induced helix upon complexation with various bio-
molecules such as amino acids and aminosugars in
water,9 but the helicity induction on poly-1 and its
memory in organic solvents have not yet been reported
in detail.10
Resu lts a n d Discu ssion
Syn th esis a n d Helicity In d u ction of P oly-1 a n d
P oly-2 w ith Ch ir a l Am in es. Cis-transoidal stereo-
regular poly-1 and poly-2 were prepared by a method
similar to that previously reported as outlined in
Schemes 1 and 2 (see Experimental Section), respec-
tively.9,11,12 The number average molecular weight (Mn)
and its distribution (Mw/Mn) were estimated to be 14.8
× 104 and 1.9 (poly-1) and 1.9 × 104 and 3.5 (poly-2),
respectively, as determined by size exclusion chroma-
tography (SEC) with poly(ethylene oxide) and poly-
(ethylene glycol) standards in N,N-dimethylformamide
(DMF) containing 10 mM lithium chloride as the eluent.
The stereoregularity of the polymers was investigated
The complexes with (R)-3 showed more intense ICDs
than those with (R)-4, and poly-2 is more sensitive to
the chirality of (R)-3 than poly-1, thus exhibiting a more
intense ICD, but for (R)-4, poly-1 is slightly better based
on the CD titration curves in Figure 2B. These results
are in sharp contrast to the previously observed CD
results using PCPA. During the complexation of PCPA
with chiral amines, the magnitude of the ICD corre-
sponding to an excess of a single-handed helix of PCPA
increased with an increase in the bulkiness and basicity
of the chiral amines, whereas chiral amino alcohols
including (R)-4 exhibited a very intense ICD irrespective
of the bulkiness because the hydroxy group can partici-
pate in hydrogen bonding with the carboxy group of a
neighboring monomer unit together with the acid-base
ion-pairing interaction of the amino group.5b Therefore,
the complexes of PCPA with (R)-3 and (R)-4 exhibited
almost the same ICD intensities, although the ICD signs
were opposite from those of the complexes with poly-1
and poly-2; the second Cotton intensities (∆ꢀsecond) of the
complexes of PCPA with (R)-3 and (R)-4 in DMSO were
-9.27 and -9.73 ([amine]/[PCPA] ) 50), respectively.5b
The present results indicate that the hydroxy group of
(R)-4 may not be effective for hydrogen-bonding forma-
tion to poly-1 and poly-2 and that a change in the acidity
of the pendant functional group significantly influences
the acid-base complexation.
1
by H NMR and laser Raman spectroscopies. The 1H
NMR spectra of poly-1 and poly-2 in DMSO-d6 showed
sharp singlets centered at 5.86 and 5.83 ppm, respec-
tively, due to the main chain protons, indicating that
these polymers possess a highly cis-transoidal, stereo-
regular structure (see Figure S-1 in the Supporting
Information).13 The laser Raman spectra of the polymers
also support this conclusion; poly-1 and poly-2 showed
characteristic peaks at 1353 and 974 cm-1 due to the
C-C and C-H bond vibrations in the cis-polyacetylenes,
respectively,14 while those in the trans-polyacetylene
were scarcely observed (see Figure S-2 in the Supporting
Information). The CD spectra of poly-1 and poly-2 in
the presence of various chiral amines were then mea-
sured to investigate if the polymers could respond to
the chirality of the chiral amines, thus showing char-
acteristic ICDs.
The typical CD and absorption spectra of poly-1 and
poly-2 in the presence of (R)-3 and (R)-2-amino-1-
propanol ((R)-4) (2 equiv to monomer units of the
polymers) (Chart 1) in DMSO are shown in Figure 1.
The results of the ICD for the complexation with other
primary chiral amines and amino alcohols (Chart 2) are

Macromolecules, Vol. 37, No. 15, 2004
Helix Induction and Macromolecular Helicity Memory 5497
Ch a r t 2. Str u ctu r es of Ch ir a l Am in es a n d Am in o
Alcoh ols
Cotton effect signs of PCPA and the absolute configu-
ration of the chiral primary amines (Figure 3A).5b On
the basis of this model together with the present ICD
results of poly-1 and poly-2 in DMSO, a possible
mechanism for the helicity induction of the poly-1- and
poly-2-amine complexes in DMSO can be proposed as
follows (Figure 3B). Amines may be favorably complexed
with PCPA to form an ion pair as shown in Figure 3A,
where the conformation of the amine is closer to the
anti-staggered and the predominant helix-sense can be
determined by the steric difference in the bulkiness of
the smaller (S) and the bulky (L) substituents of the
neighboring monomer units. The presumed molecular
model of the PCPA-(R)-amine complexes suggests that
the left-hand side monomer unit in Figure 3A is favor-
ably positioned on the front side, while the right-hand
side monomer unit is on backside, so that the PCPA
backbone will possess a left-handed helix.20 We note that
the pendant groups of the PCPA in Figure 3A appear
to arrange in a right-handed screw-sense from the side-
view, but the main chain folds into a left-handed screw-
sense.
Ta ble 1. Differ en ce in Exciton Coefficien t of th e Secon d
Cotton (∆Esecon d ) for th e Com p lexes of P oly-1, P oly-2, a n d
P CP A w ith Ch ir a l Am in es a n d Am in o Alcoh ols in DMSOa
second Cotton [λ (nm) and ∆ꢀsecond (M-1 cm-1)]
poly-1
∆ꢀ (λ)
poly-2
∆ꢀ (λ)
PCPAb
∆ꢀ (λ)
amine
(R)-3
(S)-3
(R)-4
(R)-5
(R)-6
(S)-7
(S)-8
+11.24 (365)
-11.03 (365)
+3.22 (365)c
+9.55 (365)
+4.21 (365)
-7.48 (365)
+7.18 (366)
-8.30 (366)
+8.52 (366)
+11.09 (366)
+16.92 (370)
-16.79 (370)
+2.31 (370)c
+15.68 (369)
+8.43 (369)
-17.73 (370)
-17.84 (371)
-18.63 (371)
+16.48 (372)
+18.71 (370)
-9.27 (375)
+8.67 (375)
-9.73 (373)
-3.82 (376)
A similar model can be possible for the complexes of
poly-1 and poly-2 with (R)-amines (Figure 3B). However,
the pendant phosphonate ions of the polymers are
tetrahedral, whereas the carboxylate of PCPA is copla-
nar when complexed with amines, and, therefore, the
predominant helix-sense of poly-1 and poly-2 may be
controlled by the steric difference in the bulkiness
between the S and L substituents of the neighboring
monomer units, together with the S and the OH or OEt
group of the neighboring monomer units. Consequently,
the left-hand side monomer unit in Figure 3B could be
favorably located on the backside and the right-hand
side monomer unit on the front side, leading to an
opposite, right-handed helix of poly-1 and poly-2 when
compared with the model of the PCPA with (R)-amines
(Figure 3A).
+0.42 (378)
+10.0 (371)d
+5.27 (374)
-6.58 (374)
-10.33 (374)
(S)-9
(R)-10
(1R,2S)-11
a
The concentration of polymers is 1.0 mg/mL. [amine]/[poly-1
or PCPA] ) 50 and [amine]/[poly-2] ) 10. Cited from ref 5b. c [4]/
[polymer] ) 2. [amine]/[PCPA] ) 10.
b
d
summarized in Table 1. The ICD results for the com-
plexes with PCPA5b are also shown for comparison. It
is noteworthy that all of the primary amines (3, 5-7)
and amino alcohols (4, 8-11) of the same configuration
gave the same Cotton effect signs for poly-2. A similar
tendency was observed for poly-1, but the complex with
(S)-8 exhibited the opposite Cotton effect sign. Moreover,
the ICD intensities of the complexes of poly-2 with the
chiral amines were stronger than those of the poly-1-
amine and PCPA-amine complexes except for 4 even
in the presence of excess chiral amines ([amine]/[poly-
1 or PCPA] ) 50), indicating that poly-2 is the most
sensitive induced helical polymer as a novel probe for
sensing the chirality of amines and amino alcohols
without derivatization, although the Cotton effect signs
of the poly-2 complexed with the chiral amine and amino
alcohols tested are completely opposite to those of the
corresponding PCPA complexes. The pendant phospho-
nic acid and its monoethyl ester are more acidic than
the carboxylic acid of the PCPA, which must efficiently
enhance the ion-pair formation, resulting in an excess
of a single-handed helix, thus showing more intense
ICDs.16 The pKa values of benzoic acid, phenylphospho-
nic acid, and the phenylphosphonic acid monomethyl
ester, model compounds of PCPA, poly-1, and poly-2,
are 4.19, 1.83 (pKa1) and 7.07 (pKa2),17 and 2.97,18
respectively. In other words, the helical conformation
of the PCPA induced by chiral amines may still be
imperfect.8b,12b The bulky phosphonic acid pendants also
contribute to their high sensitivities of the amine
chiralities.19
Ch ir a l Am p lifica tion (Non lin ea r Effect). In a
previous study, we reported that the complex formation
of PCPA with partially resolved amines, such as 3 and
4, displayed a unique, positive nonlinear relationship
(chiral amplification or “majority rule”)3b,5b,21 between
the enantiomeric excess (ee) of amines and the observed
molar ellipticity of the Cotton effects; that is, the ICD
intensities of PCPA, corresponding to the helical sense
excesses, became out of proportion to the ee’s of the
amines, showing a convex deviation from linearity
through a wide range of ee values of the chiral amines
in DMSO.5b The departure from linearity was significant
for the chiral amino alcohol 4, and 60% ee of 4 was
sufficiently enough to induce a full ICD for PCPA.
However, for the chiral bulky amine 3, the nonlinearity
was very weak.
The changes in the ICD intensity of poly-1 and poly-2
against the ee’s of 3 and 4 in DMSO were then
measured (Figure 4). In sharp contrast to the nonlinear
effects observed in PCPA, the CD intensities of poly-1
and poly-2 with 4 showed a linear effect, while strong
positive nonlinear effects were observed for 3 in DMSO
at ambient temperature. For example, when poly-1 and
poly-2 were dissolved in DMSO with an excess of 3 of
40% ee (R rich), each complex exhibited ICDs as intense
as those of 100% ee. The excess enantiomers bound to
the polymers induce an excess of a single-handed helix
despite its proportion, which results in a more intense
ICD than that expected from the ee of 3. As mentioned
Previously, the interaction of PCPA with optically
active amines in DMSO was systematically studied
using NMR and CD spectroscopies, and a rational model
was proposed to explain the relationship between the

5498 Onouchi et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 3. Possible structures for the PCPA-(R)-amine (A) and poly-1 or poly-2-(R)-amine complexes (B).
Ta ble 2. Mem or y Efficien cies of th e Ma cr om olecu la r
Helicity of P oly-1 a n d P oly-2 In d u ced by (R)-3 Usin g a
Ser ies of Ach ir a l Am in es
memory efficiency (%)b
run
achiral aminea (M)
poly-1
poly-2
1
2
3
4
5
6
7
8
9
12 (0.8)
13 (0.08)
14 (0.8)
14 (0.08)
14 (0.008)
15 (0.8)
16 (0.8)
17 (0.8)
18 (0.08)
ca. 0
ca. 0
68.2
59.8
59.7
67.1
70.0
71.0
78.2
4.0
16.2
93.2
83.4
80.3
86.7
86.9
93.4
c
F igu r e 4. Changes in ICD intensity (∆ꢀsecond (M-1 cm-1)) of
poly-1 and poly-2 (1.0 mg/mL) versus the % ee of 3 (R rich)
(A) and 4 (R rich) (B) ([amine]/[poly-1] ) 50 and [amine]/[poly-
2] ) 10) in DMSO at ambient temperature (ca. 20-22 °C).
a
The parentheses indicate the concentration in DMSO as the
mobile phase. Memory efficiencies (%) were estimated on the
b
basis of the ICD values at ∆ꢀsecond just after the SEC fractionation
of the polymer-(R)-3 complex solution ([polymer] ) 1.0 mg/mL,
[(R)-3]/[polymer] ) 2) using various achiral amines in DMSO as
the mobile phase. c Not eluted because of precipitation.
above, poly-1 and poly-2 showed weak ICDs upon
complexation with optically pure 4 (Figure 1 and Table
1), and this weak helicity inducing ability might be
closely correlated with the nonchiral amplification
phenomena of these polymers with 4.
Mem or y of th e Macr om olecu lar Helicity of P oly-1
a n d P oly-2. As described above, the macromolecular
helicity of PCPA induced by (R)-3 can be “memorized”
after complete replacement of the (R)-3 by various
achiral amines in DMSO.8 We then investigated if a
similar macromolecular helicity memory could be pos-
sible for analogous stereoregular poly-1 and poly-2
bearing a phosphonic acid or its ethyl ester as the
pendant instead of the carboxy group.
2-Aminoethanol (12) and n-butylamine (13) were first
selected as achiral amines for the memory experiments,
because these achiral amines were good chaperoning
molecules to assist in the memory of the macromolecular
helicity of PCPA induced by (R)-3.8 The memory experi-
ments were carried out in the same way as was
previously reported.8 Typically, the complexes of poly-1
and poly-2 with 2 equiv of (R)-3 were prepared in
DMSO; the complexes showed an almost full ICD. Each
complex solution was then injected into an SEC system
using a DMSO solution of 12 or 13 (0.8 and 0.08 M,
respectively) as the mobile phase to isolate the poly-1
and poly-2. Rather surprisingly, the fractionated poly-
mer solutions showed no ICD for the isolated poly-1 and
a very weak ICD for the poly-2 after replacement of the
(R)-3 by 12 and 13 during the SEC fractionation (Table
2). A possible explanation for these no and weak
memory effects will be described later. We note that,
under the same conditions, the PCPA fractions contain-
ing an excess of achiral 12 or 13 exhibited intense ICDs
F igu r e 5. Chromatogram for the separation of the poly-2-
(R)-3 complex. SEC fractionation was performed using a UV-
vis detector (300 nm) with 0.8 M 14 in DMSO as the eluent at
a flow rate of 1.0 mL/min.
comparable to those measured before the SEC fraction-
ations.8 Other achiral bulky amines including t-butyl-
amine and 1-aminoadamantane also showed no memory
effect for poly-1 in DMSO. However, we have found that
achiral diamines such as ethylenediamine (14) very
effectively maintain the induced macromolecular helic-
ity of the phosphonic acid-bound poly(phenylacetylene)s
in DMSO.
A typical SEC chromatogram for the separation of the
poly-2-(R)-3 complex using a DMSO solution containing
0.8 M 14 is shown in Figure 5. The poly-2 eluted first,
followed by the (R)-3, and both were completely sepa-
rated. Each fraction was collected and subjected to CD
and absorption measurements. On the basis of the UV

Macromolecules, Vol. 37, No. 15, 2004
Helix Induction and Macromolecular Helicity Memory 5499
nocyclohexane could more efficiently induce a one-
handed helicity on poly-1 and poly-2. However, upon
complexation with 0.1 equiv of the chiral diamine, the
polymers precipitated in DMSO, and further experi-
ments were difficult.
Very recently, we found that PCPA and chiral amines
could be complexed more efficiently through ion pairing
in the presence of the common salts of the chiral amines,
and the memory efficiency significantly improved when
the replacement of the chiral amine with achiral amines
was performed in the presence of the common salts of
the chiral amines.8b To examine if a similar enhance-
ment of the memory efficiency or appearance of memory
effect could be possible for poly-1 and poly-2, we carried
out the same memory experiments using the hydrochlo-
ride or para-toluenesulfonic acid salt of (R)-3 ((R)-3‚HCl
or (R)-3‚Tos). The poly-1-(R)-3 and poly-2-(R)-3 com-
plexes ([(R)-3]/[polymer] ) 2) containing various amount
of the salts in DMSO (0.5-2.0 equiv to monomer units
of polymers) were prepared, all of which showed a full
ICD regardless of the amount of the salts. Each sample
was then injected into the SEC system using a DMSO
solution containing 12 (0.8 M) or 13 (0.08 M) as the
mobile phase to simultaneously remove (R)-3 and its
salt. In the absence of the chiral salts, the helicity of
poly-1 induced by (R)-3 could not be maintained at all
by 12 and 13 as shown in Table 2. However, in the
presence of the salts, the isolated poly-1 showed a weak,
but apparent ICD; the memory efficiency improved from
ca. 0% to 4.7% and 23.5% for 12 and to 5.4% and 10.7%
for 13 when (R)-3‚HCl and (R)-3‚Tos were used as the
common salts during the helicity induction process,
respectively, whereas a significant effect of the chiral
salts was not observed for the memory of an induced
helical poly-2.
F igu r e 6. CD spectra of poly-1 (A) and poly-2 (B) (1 mg/mL)
with (R)-3 (a, c) ([(R)-3]/[poly-1 or poly-2] ) 2) and the isolated
poly-1 and poly-2 (b, d) by SEC fractionation using a DMSO
solution of 14 (0.8 M) as the mobile phase, in DMSO at
ambient temperature (ca. 20-22 °C).
Ch a r t 3. Str u ctu r es of Ach ir a l Am in es
spectrum of the (R)-3 fraction, more than 99% of the
(R)-3 was recovered. On the basis of the ratio of the ICD
intensities of the second Cotton (∆ꢀsecond) of the poly-1
(Figure 6A) and poly-2 (Figure 6B) before (a and c) and
after (b and d) the SEC fractionation, the memory
efficiencies were estimated to be 68.2% and 92.1%,
respectively. Unfortunately, PCPA precipitated upon
complexation with the diamine 14 under the same
conditions, and, therefore, we could not determine
whether the induced helical chirality of PCPA by (R)-3
would be maintained by the diamine.
The macromolecular helicity memory of poly-1 and
poly-2 assisted by the interactions with achiral amines
lasted for a long time of over 3 months as did that of
PCPA.8b Macromolecular helicity amplification (repair)
of the memorized poly-1 and poly-2 was not observed
in these systems, but the memory gradually declined
at ambient temperature (20-25 °C) and the ICD
intensities showed a decrease with time (see Figure S-3
in the Supporting Information). The stability of the
memory was almost independent of the structures of the
achiral amines used, but highly dependent on the
concentration of the achiral amines. The ICD intensities
of the poly-1-14 and poly-2-14 solutions fractionated
by SEC decreased with time, and the decreasing rate
was accelerated with the increasing concentration of 14
(see Figure S-4 in the Supporting Information).
We further investigated the macromolecular helicity
memory of the poly-1 and poly-2 induced by (R)-3 using
a series of achiral diamines (15, 16) and tri- and
tetraamines (17, 18) (Chart 3). The experiments were
performed in the same manner, and the results of the
memory efficiency experiments are summarized in
Table 2. The effect of the concentration of a typical
achiral diamine 14 in the mobile phase (0.8-0.008 M)
on the memory efficiency was also investigated. The
memory efficiency tended to slightly decrease with the
decreasing concentration of 14 (Table 2). The magni-
tudes of the ICDs of the memorized poly-1 and poly-2
are dependent on the structures of the achiral amines
used as well as the pendant functionality of the poly-
mers. All achiral amines exhibited a rather high memory
efficiency for poly-2 (80-93%) except for 18, which
caused precipitation of the polymer upon complexation
in DMSO, while they showed moderate helicity memory
effects on poly-1 (60-78%) (Table 2). Among the achiral
amines tested, a diamine 16 and tri- or tetraamine (17
or 18) worked most efficiently as achiral chaperoning
molecules to assist in the memory of the macromolecular
helicity of poly-1, while 14 and 17 showed a good
memory effect for poly-2. As for a series of achiral
diamines (14-16), the number of methylene groups
influenced the memory efficiency, but the effect was not
significant. On the basis of these observations, one may
think that chiral diamines such as (1R,2R)-1,2-diami-
As for the mechanism of the memory effect, we
recently reported the detailed studies for PCPA with
achiral amines and concluded that the intramolecular
electrostatic repulsions between the carboxylate groups
play a central role in the maintenance of the helical
chirality of PCPA to prevent the atropisomerization of
the PCPA main chain.8b On the basis of the previous
results together with the present ones, a possible
mechanism of the helicity induction and memory of the
helical chirality of poly-1 and poly-2 can be proposed.
The differences in the helicity induction and memory
effect observed between PCPA and poly-1 and poly-2
might be correlated to the differences in the chemical
and physical properties of the pendant groups, because
all of the polymers consisted of the same repeating units
of the π-conjugated diene structure. The phenylphos-

5500 Onouchi et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 7. Schematic illustration of the mechanism of weak (A) and effective (C) macromolecular helicity memory of poly-1 and
poly-2 with achiral monoamines (A) and diamines (C).
phonic acid and its monoesters are more acidic than
benzoic acid, and, therefore, the former can form more
stable ion-paired complexes with chiral amines in polar
DMSO,22 whereas the less acidic benzoic acid residues
of PCPA may not be completely ionized, leading to
weaker ion-paired interactions.23 In fact, the binding
affinities (binding constants) of phenylphosphonic acid
derivatives to primary amines are 1-order greater than
those of the benzoic acid derivatives in polar sol-
vents.8,22,23 This high ionic complexation ability of the
pendant’s phophonic acid residues of poly-1 and poly-2
with amines might result in an efficient induction of a
predominantly one-handed helical conformation of the
polymers upon complexation with chiral amines in
DMSO as well as in water. The steric effect of the bulky
phosphonic acid residues at the para position on the
phenyl group might also contribute to the effective
helicity induction on poly-1 and poly-2,12b,19 because, as
we already reported, copolymers of an optically active
phenylacetylene with an achiral phenylacetylene bear-
ing a bulky substituent at the para position showed an
intense ICD, while a copolymer of the same optically
active phenylacetylene with a less bulky phenylacety-
lene exhibited a weak ICD in the copolymer backbone
regions.19
poly-1 and poly-2 (Figure 7), probably because the
negative charge of the phosphonate groups may be
localized in two of the three P-O bonds upon complex-
ation with the amines (Figure 7), so that the atrop-
isomerization may smoothly occur between the adjacent
monomer units, in which the less charged substituents
(OH or OEt) are favorably positioned close to each other
(Figure 7A). The longer bond lengths between the
aromatic carbon and the phosphorus (1.74-1.79) and
of the three P-O bonds (1.48-1.56) in the phosphonate
group25 as compared with the bond lengths between the
aromatic carbon and the carboxyl carbon (1.48) and of
the two C-O bonds (1.24-1.29) in the benzoate group24
should be also taken into consideration for the ready
inversion of the helicity of poly-1 and poly-2 with achiral
monoamines.
Con clu sion s
In summary, we found that novel poly(phenylacety-
lene)s with the phosphonic acid and its monoethyl ester
as the pendants formed a predominantly one-handed
helical conformation induced by a noncovalent interac-
tion with various chiral amines in DMSO. The com-
plexes exhibited characteristic and more intense ICDs
than PCPA. In particular, poly-2 was highly sensitive
to the chirality of amines and showed the same Cotton
effect signs if the configurations of the chiral amines
were the same. The macromolecular helicity of poly-1
and poly-2 induced by chiral amines could be success-
fully memorized by achiral diamines or oligoamines with
a rather high memory efficiency. Consequently, the
present results demonstrate that analogous helical
polyacetylenes6,7 and other dynamic helical polymers26
bearing rationally designed functional pendants induced
by chiral compounds may be further memorized by
achiral compounds with multiple functional groups.27
Although the induced helical conformation of PCPA
with chiral amines can be memorized by achiral amines
such as 12 and 13, these achiral monoamines were not
effective for the memory of the induced helical struc-
tures of poly-1 and poly-2; achiral diamines or oligoam-
ines are necessary for the memory effect by the poly-
mers. This indicates that the bidentate or multi-ion-
paired electrostatic interactions between the neighboring
phosphonate groups of the polymers and the di- or
multi-ammonium moieties are essential for the memory
(Figure 7C). Moreover, this also suggests that the
intramolecular electrostatic repulsions between the
neighboring phosphonates complexed with a monoamine
were not efficient enough to prevent the atropisomer-
ization of the main chains of poly-1 and poly-2, which
is in contrast to the memory effect of the PCPA (Figure
7A). This might be ascribed to the difference in the
structures of the pendants; the benzoate groups are
almost coplanar,24 while the phenylphosphonate groups
are tetrahedral,25 which may serve to lower the barrier
to the atropisomerization of the polymer backbones of
Exp er im en ta l Section
Ma t er ia ls. Anhydrous DMSO (water content <
0.005%) and methanol (water content < 0.002%) were
purchased from Aldrich and stored under nitrogen. THF
was dried over sodium benzophenone ketyl, distilled
onto LiAlH4 under nitrogen, and distilled under high
vacuum just before use. Sodium methoxide was obtained
from Kishida (Osaka, J apan). Ethylenediamine (14) was

Macromolecules, Vol. 37, No. 15, 2004
Helix Induction and Macromolecular Helicity Memory 5501
Sch em e 1
Sch em e 2
obtained from Tokyo Kasei (TCI, Tokyo, J apan), dried
over potassium hydroxide, and distilled under nitrogen.
2-Aminoethanol (12) was dried over calcium oxide and
distilled under reduced pressure. These amines were
stored under nitrogen. (R)-(+)- and (S)-(-)-1-(1-Naph-
thyl)ethylamine ((R)-3 and (S)-3) and (R)-1-phenylethyl-
amine ((R)-5) were kindly supplied from Yamakawa
Chemical (Tokyo, J apan), distilled under reduced pres-
sure, and stored under nitrogen. Other optically active
amines and achiral amines were available from Aldrich
or TCI.
Cis-transoidal poly-1 was prepared by polymeriza-
tion of diethyl (4-ethynylphenyl)phosphonate with [Rh-
(nbd)Cl]2 (nbd ) norbornadiene), followed by hydrolysis
of the ester groups (86% yield) according to the previ-
ously reported method (Scheme 1).9,11 The stereoregu-
larity of the obtained polymer was examined by mea-
suring the 1H NMR spectrum and was found to be
almost complete cis-transoid.13 The molecular weight
(Mn) and its distribution (Mw/Mn) of poly(diethyl (4-
ethynylphenyl)phosphonate) were estimated to be 1.48
× 105 and 1.9, respectively, by SEC with poly(ethylene
oxide) and poly(ethylene glycol) standards in DMF
containing 10 mM lithium chloride. Poly(ethyl (4-
ethynylphenyl)phosphonate) (poly-2) was prepared by
the polymerization of the corresponding monomer (2)
in water with a water-soluble rhodium catalyst in the
presence of NaOH.12
(νtC-H), 1215 (νPdO). 1H NMR (DMSO-d6): δ 1.13-1.18
(t, CH3, 3H), 3.82-3.91 (m, CH2, 2H), 4.38 (s, tCH, 1H),
7.56-7.60 (m, aromatic, 2H), 7.65-7.72 (m, aromatic,
2H). 13C NMR (DMSO-d6): δ 16.3 (d, J ) 6.2 Hz), 60.8
(d, J ) 5.1 Hz), 82.7 (d, J ) 1.1 Hz), 82.9, 124.6 (d, J )
3.4 Hz), 131.2 (d, J ) 23.9 Hz), 131.3 (d, J ) 28.5 Hz),
131.9 (d, J ) 181.3 Hz). 31P NMR (DMSO-d6): δ 14.0.
Anal. Calcd for C10H11O3P‚1/5H2O: C, 56.19; H, 5.38.
Found: C, 56.09; H, 5.25.
P olym er iza tion of 2. Polymerization of 2 was car-
ried out in water using [Rh(cod)2]BF4 as a catalyst in a
way similar to that reported previously (Scheme 2).12
Monomer 2 (1.0 g, 4.8 mmol) was placed in a dry
ampule, which was then evacuated on a vacuum line
and flushed with dry nitrogen. After this evacuation-
flush procedure was repeated three times, a three-way
stopcock was attached to the ampule, and the appropri-
ate amounts ion-exchanged, distilled water and aqueous
NaOH (1.0 N) ([NaOH]/[2] ) 1.5) were added with a
syringe. To this was added a solution of [Rh(cod)2]BF4
(0.025 M) in water at 30 °C. The color of the mixture
changed instantly to dark red. The concentrations of the
monomer and the rhodium complex were 0.5 and 0.0025
M, respectively. After 24 h, the resulting polymer (the
sodium salt of poly-2) was precipitated into a large
amount of ethanol, collected by filtration, and dried in
vacuo at room temperature overnight (1.1 g, 100% yield).
The sodium salt of poly-2 (1.0 g, 4.5 mmol) was dissolved
in a small amount of distilled water. The aqueous
solution was acidified with aqueous HCl (5 N), and the
precipitated poly(ethyl (4-ethynylphenyl)phosphonate)
(poly-2) was collected by centrifugation and dried in
vacuo at room temperature overnight (0.89 g, 94% yield).
A part of poly-2 was quantitatively converted to its
methyl esters by reaction with CH2N2 in diethyl ether.
The Mn and Mw/Mn were estimated by SEC with poly-
(ethylene oxide) and poly(ethylene glycol) standards
using DMF containing 10 mM lithium chloride as the
eluent (Mn ) 1.9 × 104, Mw/Mn ) 3.5).
Ethyl (4-ethynylphenyl)phosphonate (2) was prepared
according to Scheme 2.
Eth yl (4-Eth yn ylp h en yl)p h osp h on a te (2). To a
solution of diethyl (4-ethynylphenyl)phosphonate (4.1 g,
17 mmol) in ethanol (50 mL) was added aqueous NaOH
(13.6 N, 63 mL), and the solution was stirred at room
temperature. After 5 h, the solution was acidified with
concentrated HCl (80 mL). After filtration, the filtrate
was extracted with chloroform (800 mL) and the organic
layer was dried over MgSO4. After filtration, the solvent
was removed by evaporation to give 3.7 g of 2 as a
slightly brown solid (100% yield); no further purification
was necessary. Mp 62.7-64.6 °C. IR (KBr, cm-1): 3278
Spectroscopic data of poly-2. IR (KBr, cm-1): 1200
(νPdO). 1H NMR (DMSO-d6, 80 °C): δ 1.06-1.10 (m,

5502 Onouchi et al.
Macromolecules, Vol. 37, No. 15, 2004
CH3, 3H), 3.71-3.75 (m, CH2, 2H), 5.83 (s, dCH, 1H),
6.71 (s, aromatic, 2H), 7.41 (s, aromatic, 2H). 31P NMR
(DMSO-d6, 80 °C): δ 15.4. Anal. Calcd for (C10H11O3P‚
1/3H2O)n: C, 55.56; H, 5.44. Found: C, 55.52; H, 5.30.
with a screwcap, and to this was added 30 µL of the
stock solution of (R)-3. The solution was then diluted
with DMSO so as to keep the total volume of the
solution to be 300 µL. The initial CD and absorption
spectra were taken using a 0.01-cm quartz cell. SEC
fractionation was performed using a J asco PU-980 liquid
chromatograph equipped with a UV (300 nm; J asco UV-
970) detector. A Shodex KF-806L SEC column (30 cm)
was connected, and 0.8 M of 14 in DMSO was used as
the mobile phase at a flow rate of 1.0 mL/min. One
hundred microliters of the solution of the poly-1-(R)-3
complex was injected to the SEC system, and the poly-1
and (R)-3 fractions were separately collected. The
recovery of (R)-3 was estimated on the basis of the UV
spectrum of the (R)-3 fraction using the ꢀ value of (R)-3
(ꢀ284 ) 6150 or ꢀ300 ) 2520 M-1 cm-1). The CD and
absorption spectra of the fractionated poly-1 were
measured in a 10-mm quartz cell. The same procedure
was done for the SEC fractionation of helical poly-1 and
poly-2 induced by (R)-3 using DMSO containing other
achiral amines as the mobile phase.
In str u m en ts. The melting point was measured on a
Bu¨chi melting point apparatus and is uncorrected. NMR
spectra were taken on a Varian Mercury 300 (300 MHz
for 1H, 75 MHz for 13C, and 121.5 MHz for 31P) or a
Varian VXR-500S spectrometer (500 MHz for 1H) in
1
CDCl3 or DMSO-d6 using TMS (for CDCl3, H and 13C)
1
or a solvent residual peak (for DMSO-d6, H and 13C)
as the internal standards. H3PO4 (for CDCl3 and DMSO-
d6) was used as the external standard for 31P NMR
measurements. SEC measurements of poly(diethyl (4-
ethynylphenyl)phosphonate) and poly(ethyl methyl (4-
ethynylphenyl)phosphonate) were performed with a
J asco PU-980 liquid chromatograph equipped with an
RI detector (J asco RI-930) using Tosoh TSKgel R-3000
(30 cm) and R-5000 (30 cm) SEC columns in series. DMF
containing 10 mM LiCl was used as the eluent at a flow
rate of 0.5 mL/min. The molecular weight calibration
curves were obtained with poly(ethylene oxide) and poly-
(ethylene glycol) standards (Tosoh). IR spectra were
recorded with a J asco Fourier Transform IR-620 spec-
trophotometer. The absorption and CD spectra were
measured in a 0.1-, 0.5-, 1.0-, or 10-mm quartz cell on a
J asco V-570 spectrophotometer and a J asco J -725 spec-
tropolarimeter, respectively. The concentrations of poly-
mers were calculated on the basis of the monomer units
and were corrected using the ꢀ (molar absorptivity)
values of the polymers: ꢀ400 ) 2253 (poly-1 in DMSO)
and 2623 (poly-2 in DMSO).
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research from
the J apan Society for the Promotion of Science and the
Ministry of Education, Culture, Sports, Science, and
Technology, J apan, and the 21st Century COE Program
“Nature-Guided Materials Processing” of the Ministry
of Education, Culture, Sports, Science, and Technology.
H.O. expresses thanks for a J SPS Research Fellowship
(No. 05704) for Young Scientists.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR and laser
Raman spectra of poly-1 and poly-2 and stability of the
memorized poly-1 and poly-2. This material is available free
of charge via the Internet at http://pubs.acs.org.
CD Mea su r em en ts. A typical experimental proce-
dure is described below. Stock solutions of poly-1 (2 mg/
mL) and (R)-3 (22.0 mM) in DMSO were prepared. A
180 µL aliquot of the stock solution of poly-1 was
transferred to a vessel equipped with a screwcap using
a micropipet (Mettler-Toledo GmbH, Switzerland). To
the vessel was added 180 µL of the stock solution of
(R)-3 ([(R)-3]/[poly-1] ) 2), and the CD and absorption
spectra were measured.
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and poly-2 were investigated in DMSO. A typical
experimental procedure is described below. Stock solu-
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