Helical conformation stability of poly[3,5-bis(hydroxymethyl)phenylacetylene]s depending on the length of their rigid and linear π-conjugated side groups

Article abstract of DOI:10.1246/cl.150616

We synthesized new 3,5-bis(hydroxymethyl)phenylacetylene (HPA) monomers connected with a rigid and linear π- conjugated oligomer. The monomers were successfully polymerized with a rhodium catalyst [Rh(nbd)Cl]₂ in the presence of chiral PEA to give the corresponding polymers. Helix-senseselective polymerization proceeded for DBHPA. CD, UV, and WAXS spectroscopic studies revealed that the intramolecular hydrogen bonds of the polymers contributed to the stabilization of their helical conformation but the stability depended on the length of the rigid and linear π-conjugated side group.

Full text of DOI:10.1246/cl.150616

Received: June 25, 2015 | Accepted: July 18, 2015 | Web Released: August 1, 2015
CL-150616
Helical Conformation Stability of Poly[3,5-bis(hydroxymethyl)phenylacetylene]s Depending
on the Length of Their Rigid and Linear π-Conjugated Side Groups
Zhichun Shi, Masahiro Teraguchi, Toshiki Aoki, and Takashi Kaneko*
Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata 950-2181
(E-mail: kanetaka@gs.niigata-u.ac.jp)
We synthesized new 3,5-bis(hydroxymethyl)phenylacetyl-
ene (HPA) monomers connected with a rigid and linear π-
conjugated oligomer. The monomers were successfully poly-
merized with a rhodium catalyst [Rh(nbd)Cl]2 in the presence
of chiral PEA to give the corresponding polymers. Helix-sense-
selective polymerization proceeded for DBHPA. CD, UV, and
WAXS spectroscopic studies revealed that the intramolecular
hydrogen bonds of the polymers contributed to the stabilization
of their helical conformation but the stability depended on the
length of the rigid and linear π-conjugated side group.
Helical polymers are widely distributed in nature as DNA,
Scheme 1.
proteins, and so on. Their static helical structure is essential
to form specific hierarchical structure which exhibits smart
property depending on the molecular structure, and the helical
structures are often stabilized by intramolecular hydrogen bonds.
Artificial helical polymers, in particular π-conjugated functional
polymers, have attracted much more attention to give unique
prospects to display broad applications such as chiral sensors,
chiroptics, microelectronics, chiral magnets, chiral recognition,
asymmetric catalysis, data storage, and so on.1-11 We have
already succeeded in synthesizing poly(phenylacetylene)s with-
out chiral side groups, but with one-handed helical conformation
in solution by helix-sense-selective polymerization (HSSP) of
the corresponding achiral monomer using a chiral catalyst
system, i.e., polymerization using the catalyst [Rh(nbd)Cl]2
(nbd: 2,5-norbornadiene) in the presence of (R)-1-phenylethyl-
amine ((R)-PEA) or (S)-1-phenylethylamine ((S)-PEA),12-28 and
3,5-bis(hydroxymethyl)phenylacetylenes (HPAs) such as 3,5-
bis(hydroxymethyl)-4-dodecyloxyphenylacetylene (DHPA) and
3,5-bis(hydroxymethyl)-4-(4-dodecyloxyphenyl)phenylacetylene
(DPHPA) gave the corresponding polymer poly(HPA)s whose
rigid and one-handed helical conformation was kinetically
stabilized by intramolecular hydrogen bonds.12,28 In this study,
we have synthesized new HPA monomers DBHPA, DTHPA,
and DPETHPA connected with a rigid and linear π-conjugated
substituent and investigated the helical conformation stability
related to the side chain structure for the corresponding polymers
(Scheme 1).
DBHPA, DTHPA, and DPETHPA were successfully syn-
thesized via Suzuki-Miyaura coupling as described in the
previous work.28 Polymerization of DBHPA, DTHPA, and
DPETHPA using [Rh(nbd)Cl]2 catalyst in the presence of (R)-
PEA or (S)-PEA proceeded to yield the corresponding polymers
by the same procedures as used in the previous work.12 Yellow
solid polymers were obtained by precipitation from the polymer-
ization mixtures into methanol. The polymerization data for
these resultant polymers are summarized in Table 1. The yield
and molecular weight of the polymers were comparable to pre-
viously reported poly(HPA)s.12,28 However, no optical activity
Table 1. Polymerization of HPAs using [Rh(nbd)Cl]2 in the
presence of (R)- or (S)-1-phenylethylamine (PEA)a
b
c
Yield MW
/%
[ª]max
b
No. Monomers PEA
Mw/Mn
/105
/104 deg cm2 dmol
¹1
1
2
3
4
5
6
7
DBHPA
DTHPA
(R)
(S)
(R)
(S)
32
33
86
82
47
36
45
6.9
6.6
17
4.5
5.6
7.1
7.3
4.9
6.4
2.7
4.5
3.1
7.6
8.4
¹6.5
9.4
0
0
0
DPETHPA (R)
DPHPA
(R)
(S)
¹1.1
2.0
aIn toluene, [M]0 = 0.1 M, [M]0/[Cat]0 = 100, [PEA]0/[Cat]0 = 800
for Nos. 1 and 2; in THF, [M]0 = 0.1 M, [M]0/[Cat]0 = 100, [PEA]0/
[Cat]0 = 400 for Nos. 3-7. 25 °C, 3 h, precipitated with MeOH.
bDetermined from GPC calibrated by polystyrene standard (eluent:
c
THF). Molar ellipticity in THF.
was observed for poly(DTHPA) and poly(DPETHPA), which
were prepared under the same polymerization conditions as
poly(DPHPA), but poly(DBHPA) shows stronger CD signals
compared with poly(DPHPA). The split induced CD signals at
300 nm and broad signals at 350-500 nm were observed for the
THF solution of poly(DBHPA) as shown in Figure 1. The CD
intensity was nearly constant over ¹10-50 °C (Figures S1 and
S2). This behavior indicates that the helical conformation of
poly(DBHPA) was stabilized by intramolecular hydrogen bonds
as the previously reported poly(HPA)s.12,28 However, the sensi-
tivity by polar solvent was higher than that of poly(DPHPA),
because the CD signal disappeared in 1% DMSO-THF
(Figures S3 and S4).
The optically inactive poly(DTHPA) and poly(DPETHPA)
were dissolved in enantiopure (R)-PEA or (S)-PEA for 24 h at
room temperature, then yellow solid polymers were obtained
by precipitation from the polymer mixtures into methanol. For
poly(DTHPA) thus obtained, the induced CD signals appeared
in the absorption region (300 nm) of the side group chromophore
Chem. Lett. 2015, 44, 1413–1415 | doi:10.1246/cl.150616
© 2015 The Chemical Society of Japan | 1413

Table 2. Column structure of poly(HPA)s
Column diameter
π-Conjugated
side chain length
/¡
2ªa d spacingb
WAXSc
Calcdd
Polymer
/degree
/¡
/¡
/¡
poly(DTHPA)
poly(DBHPA)
poly(DPHPA)
poly(DHPA)
9.8
7.1
2.8
0
2.50
3.40
3.44f
4.10g
35.5
26.0
25.7f
21.5g
41.1 63.8 (35.4)e
30.0 58.9 (30.4)e
29.6f 50.8 (22.1)e
24.8g 42.6 (13.6)e
aCrystalline peak of the wide-angle X-ray scattering (WAXS) of the
polymers. bEstimated from 2ª (2d sin ª = n-, where n = 1 and - =
1.54 ¡). cEstimated from d (Column diameter = d/sin 60°). dEstimated
from the conformation parameters described in ref 31. eOmitted
f
g
dodecyl group. From ref 28. From ref 12.
Figure 1. CD and UV-vis absorption spectra of poly(DBHPA)
(red line: No. 1 in Table 1; green line: No. 2 in Table 1) at 20 °C
in THF (1 mM).
Figure 3. Top view of the polymer (A: poly(DHPA); B:
poly(DPHPA); C: poly(DBHPA); D: poly(DTHPA)) models
along the polymer chain (dodecyl groups are omitted for clarity).
smaller than that estimated from molecular modeling. The
polymer structures estimated from molecular modeling
(Figure 3) showed that the rigid and linear π-conjugated side
groups were placed more sparsely at the periphery of the
polymer column with the length of side groups.31 The under-
estimation of the column diameter from WAXS would be caused
by the column overlapping each other in the sparse periphery.
We have already reported that HPA analogues bearing flexible
ether groups gave the statically stable and one-handed helix of
the corresponding polymers by HSSP.17,20,21 On the other hand,
for poly(HPA)s connected with rigid and linear π-conjugated
substituents, it seems that the stability of helical conformation
decreased with the length of the π-conjugated side groups. The
former polymers would form reasonable packing of side groups
by flexible rotation of side group if the main-chain conformation
was fixed by the intramolecular hydrogen bonds. However, for
the later polymers the position of the side groups was restricted
by the main-chain conformation. Therefore, the average distance
between side groups in close proximity would increase with
the length of π-conjugated side groups as shown in Figure 3,
because the rigid and linear π-conjugated side groups were
oriented radially to the main chain axis. These models suggest
the reason why the stability of helical conformation decreased
with the length of the π-conjugated side groups, because the
interaction between side groups decreased with the distance of
each other.
Figure 2. CD and UV-vis absorption spectra of poly(DTHPA)
(red line: THF solution of reprecipitated poly(DTHPA) after
dissolution in (R)-PEA; green line: THF solution of repreci-
pitated poly(DTHPA) after dissolution in (S)-PEA) at 20 °C in
THF (1 mM).
(Figure 2), while no optical activity was observed for the
poly(DPETHPA). The CD signal intensity of poly(DTHPA)
was more sensitive to temperature than that of poly(DBHPA)
(Figures S5 and S6). The CD signal intensity of poly(DTHPA)
decreased with increasing temperature from room temperature,
but moderately recovered with decreasing temperature
(Figure S7). This behavior indicates that transformation to the
random coil conformation was prevented by intramolecular
hydrogen bonds, because the CD signal disappeared by addition
of polar solvents (Figure S8).
The wide-angle X-ray scattering (WAXS) of the polymers
was taken as a film state prepared by solvent-casting from THF
solution (Figure S9). Relatively sharp crystalline peaks were
observed for poly(HPA)s at 2ª = 2.5-4.1 degree with broad
halo peak at 2ª = 10-30 degree. These sharp peaks would be
correlated to the (100) reflection of the columnar crystal.12,29,30
The interplanar d spacing of the polymers increase with π-
conjugated side chain length (Table 2). However, the column
diameter of polymers estimated from the d spacing was much
In summary, we synthesized new HPA monomers connected
with a rigid and linear π-conjugated oligomer. The monomers
were successfully polymerized with
a rhodium catalyst,
1414 | Chem. Lett. 2015, 44, 1413–1415 | doi:10.1246/cl.150616
© 2015 The Chemical Society of Japan

[Rh(nbd)Cl]2, in the presence of chiral PEA to give the
corresponding polymers, and it was confirmed that HSSP
proceeded for DBHPA. It was found that the intramolecular
hydrogen bonds of the polymers contributed to the stabilization
of their helical conformation but the stability depended on
the length of the rigid and linear π-conjugated side group.
These results suggested that the molecular design of reasonable
packing of side groups to give adequate interaction between side
groups would be required for the static helical conformation
of poly(HPA)s connected with a rigid π-conjugated substituent.
Macromolecules 2005, 38, 9420.
16 T. Kaneko, Y. Umeda, H. Jia, S. Hadano, M. Teraguchi, T.
Aoki, Macromolecules 2007, 40, 7098.
17 H. Katagiri, T. Kaneko, M. Teraguchi, T. Aoki, Chem. Lett.
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18 H. Jia, M. Teraguchi, T. Aoki, Y. Abe, T. Kaneko, S.
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This work was partially supported by a Grant-in-Aid for
Scientific Research (B) (No. 24310081) from JSPS, and by a
Grant for the promotion of Niigata University Research Projects.
21 L. Liu, Y. Zang, S. Hadano, T. Aoki, M. Teraguchi, T.
Kaneko, T. Namikoshi, Macromolecules 2010, 43, 9268.
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Supporting Information is available electronically on J-STAGE.
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Chem. Lett. 2015, 44, 1413–1415 | doi:10.1246/cl.150616
© 2015 The Chemical Society of Japan | 1415