Well-defined lyotropic liquid crystalline properties of rigid-rod helical polyacetylenes

Article abstract of DOI:10.1021/ma0503312

The synthesis of stereo-regular (cis-transoidal) poly(phenylacetylene)s bearing L-, D-, or racemic DL-alanine residues with a long alkyl chains as pendants (poly-L-1, poly-D-1, and poly-DL-1) was investigated. The polyacetylene was found to form well-defi

Full text of DOI:10.1021/ma0503312

Macromolecules 2005, 38, 4061-4064
4061
Chart 1. Structures of Polymers
Well-Defined Lyotropic Liquid Crystalline
Properties of Rigid-Rod Helical
Polyacetylenes
Kento Okoshi,* Koichi Sakajiri, Jiro Kumaki, and
Eiji Yashima*
Yashima Super-structured Helix Project, Exploratory
Research for Advanced Technology (ERATO), Japan Science
and Technology Agency (JST), 101 Creation Core Nagoya,
2266-22 Anagahora, Shimoshidami, Moriyama-ku,
Nagoya 463-0003, Japan
organic solvents as evidenced by their indisputably clear
fingerprint and Schlieren textures. Although a number
of LC polyacetylenes have been prepared, in most cases,
their liquid crystallinities have originated from the
mesogenic pendants covalently bonded to the main
chain.10 The poly-1’s are the first well-defined lyotropic
LC polyacetylenes in organic solvents based on the
main-chain stiffness with a macromolecular helicity.
All polymers were prepared by the polymerization of
the corresponding optically pure and racemic monomers
(L-1, D-1, and DL-1) with a rhodium catalyst in tetra-
hydrofuran (THF) according to previously reported
methods.11 The molecular weight (Mw) and its distribu-
tion (Mw/Mn) are 498 000 and 2.18 for poly-L-1, 465 000
and 2.30 for poly-D-1, and 774 000 and 1.81 for poly-
DL-1, respectively, as estimated by size exclusion chro-
matography (SEC). All the polymers have a highly cis-
transoidal structure based on their 1H NMR spectra.11,12
The typical CD and absorption spectra of poly-L-1,
poly-D-1, and poly-DL-1 in chloroform are shown in
Figure 1. Poly-L-1 and poly-D-1 exhibited mirror images
of split-type intense induced circular dichroisms (ICDs)
assigned to the π-π* electronic transition of the con-
jugated polyene chromophore regions, indicating that
the polymers have a predominantly one-handed helical
structure, while poly-DL-1 showed no ICD at all.
To investigate the global conformational properties
of these polymers, the relationship between the intrinsic
viscosity [η] and the absolute molecular weight of the
polymers was explored in chloroform by SEC equipped
with refractive index and light scattering detectors and
a viscometer, and the results were analyzed using the
wormlike cylinder theory (Figure 2).
Received February 16, 2005
Revised Manuscript Received April 4, 2005
Liquid crystalline (LC) phases formed by rigid and
semirigid helical polymers have been extensively stud-
ied with much interest since the 1980s. Typical ex-
amples are biological macromolecules, such as DNA,1
polysaccharides,2 polypeptides,3 and some viruses,4
which chiefly form cholesteric LC phases in water due
to their stiff backbones composed of their homochiral
components. Fully synthetic optically active helical
polymers, such as polyisocyanates,5 polysilanes,6 and
polyguanidines,7 also form LC phases in organic sol-
vents or in the melt, when their backbones are stiff
enough with an excess one-handedness to self-assemble
into supramolecular helical arrays in the LC state or
solid state, resulting in a cholesteric or a twist grain
boundary phase.8 Optically active components incorpo-
rated into the main chain or at the pendants control
the helix-sense excess and the handedness in these
dynamic helical polymers, leading to further macro-
scopic chirality in the LC phase. Therefore, a mixture
of equal amounts of helical polymers with the opposite
helical senses results in the achiral nematic LC. How-
ever, an optically inactive, dynamic helical polymer
composed of racemic monomer units may not show such
a clearly identified nematic phase because the polymer
appears to have a number of helical reversals between
the interconverting right- and left-handed helical seg-
ments, which may destabilize the rigidity of the polymer
backbone, so that close parallel packing of the polymer
chains may not be possible.
Recently, we reported the first example of a helical
polyacetylene consisting of a chromophoric polyacet-
ylene backbone and the hydrochloride of the 4-(N,N-
diisopropylaminomethyl)phenyl group as the pendant
that formed a cholesteric LC in aqueous solution based
on the main-chain stiffness with an excess of one helical
sense of the polymer induced by a small amount of a
nonracemic chiral dopant.9 The polyelectrolyte function
accompanied by the hydrophobic pendants of this poly-
mer may be essential for the appearance of the LC phase
in water because the neutral polymer showed no LC
phase in organic solvents.9
In this study, we synthesized novel sets of stereo-
regular (cis-transoidal) poly(phenylacetylene)s bearing
L-, D-, or racemic DL-alanine residues with a long alkyl
chain as the pendants (poly-L-1, poly-D-1, and poly-DL-
1, respectively; Chart 1) and found that these polyacet-
ylenes form well-defined lyotropic cholesteric or nematic
LCs depending on the chiral nature in the pendants in
Figure 1. CD and UV-vis spectra of the polymers (poly-L-1,
poly-D-1, and poly-DL-1) taken at ambient temperature in
dilute chloroform solution (1.5 mg/mL) in a 0.2 mm quartz cell.
* To whom correspondence should be addressed. E-mail:
kokoshi@yp-jst.jp or yashima@apchem.nagoya-u.ac.jp.
10.1021/ma0503312 CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/21/2005

4062 Communications to the Editor
Macromolecules, Vol. 38, No. 10, 2005
these values are significantly longer than that of the
reported value of a helical PCPA induced by chiral
amines (8.6 nm).16 These results indicate that not only
poly-L-1 and poly-D-1 composed of optically pure mono-
mers but also optically inactive poly-DL-1 bearing the
racemic pendants have stiff main-chain conformations.
The origin of the main-chain rigidity of poly-1’s may be
largely ascribed to the intramolecular hydrogen bonding
between the amide groups in the neighboring side
chains as well as the steric effect of the adjacent side
chains. Intramolecular hydrogen bonding has been used
to construct synthetic helical polymers such as polyiso-
cyanopeptides17 and polypropargylamides.18,19 In these
cases, the persistence lengths were found to be 76 ( 617b
and 13.5 nm,18b respectively, and a clear cholesteric LC
phase was observed in the former case.17a,20
Figure 3 shows the polarized optical micrographs of
poly-L-1 (a) and poly-DL-1 (b) in concentrated solutions
of 1,2-dichloroethane. Poly-L-1 forms a cholesteric phase
as evidenced by its indisputably clear fingerprint texture
(Figure 3a), while poly-DL-1 forms a nematic phase
which shows a typical Schlieren texture (Figure 3b). As
far as we know, such a clear Schlieren texture has not
been observed in nematic phases derived from a “race-
mic” helical polymer composed of racemic monomer
units based on the main-chain stiffness. Consequently,
the poly-1’s are the first liquid crystal polyacetylenes
in organic solvents based on the main-chain stiffness.
Similar clear microscopic textures were also observed
in concentrated solutions of nonpolar solvents, such as
benzene, chloroform, 1,1,2,2-tetrachloroethane, and car-
bon tetrachloride, but not observed in polar solvents
such as THF. These results indicate that the main-chain
stiffness of the polymers may be maintained through
intramolecular hydrogen bonds between the neighboring
pendant amide residues because an aprotic solvent like
THF is supposed to hamper the hydrogen bonding.21
Moreover, transmission electron microscopy (TEM)
observations showed that the helicoidal structure of the
cholesteric phase formed in poly-L-1 solution could be
successfully solidified into a cast film by gradual evapo-
ration of the solvent. Figure 4 shows a typical bright-
field TEM image of a cross section of the poly-L-1 cast
film. A clear banded texture was observed due to the
thickness variations of the sample. This sinusoidal
surface topology may be produced due to the preferential
crack propagation along the twist of the director field.8,22
The observed banding repeat distance was ca. 150 nm,
which also depends on the local contact angle between
Figure 2. Double-logarithmic plots of the intrinsic viscosity
vs molecular weight of poly-L-1 (red points), poly-D-1 (blue
points), and poly-DL-1 (green points), taken at 40 °C in
chloroform. Solid curves (black lines) were obtained from the
wormlike cylinder theory and fit well with the experimental
data. The evaluated parameters are q ) 36.9 nm, d ) 1.63
nm, ML ) 1577.4 nm-1, h ) 0.23 nm for poly-L-1, q ) 40.9
nm, d ) 1.49 nm, ML ) 1487.1 nm-1, h ) 0.24 nm for poly-
D-1, and q ) 16.1 nm, d ) 1.24 nm, ML ) 1710.1 nm-1, h )
0.21 nm for poly-DL-1.
The viscosity index (R) can be roughly estimated by
the slope of the plots (log [η]/log Mw) and was 1.18, 1.14,
and 0.96 for poly-L-1, poly-D-1, and poly-DL-1, respec-
tively, indicating that these polymers have an entirely
stiff-rod conformation. According to Bushin et al.13 and
Bohdanecky,14 the intrinsic viscosity [η] in the unper-
turbed wormlike cylinder model of the Yamakawa-
Fujii-Yoshizaki theory15 can be described as an ana-
lytical function of the molecular weight if the persistence
length (q), the diameter of the cylinder (d), and the
molar mass per unit contour length (ML), which eventu-
ally leads to the monomer unit height (h), are given.
The solid curves in the plots were calculated using the
parameters determined from the fits between the theo-
retical and experimental [η] values (Figure 2).
The calculated monomer unit height (h) values almost
coincide with the reported value (0.22 nm) of poly(4-
carboxyphenylacetylene) (PCPA),16 indicating that these
polymers take almost the same helical pitch in their
main-chain conformations irrespective of the pendant
chirality. The calculated q values of poly-L-1, poly-D-1,
and poly-DL-1 are 36.9, 40.9, and 16.1 nm, respectively;
Figure 3. Polarized optical micrographs of poly-L-1 (a) and poly-DL-1 (b) in 15 wt % 1,2-dichloroethane solution taken at ambient
temperature (20-25 °C). Scale bars in the images represent 100 (a) and 200 µm (b). The solution of poly-L-1 was sealed in a glass
capillary tube, while the solution of poly-DL-1 was placed on a glass plate without a cover glass to develop the planar structure
before observation of the Schlieren texture. The cholesteric LC phase of poly-L-1 was phase-separated from an isotropic phase in
equilibrium, and the poly-L-1 maintained its cholesteric pitch for over 4 months.

Macromolecules, Vol. 38, No. 10, 2005
Communications to the Editor 4063
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We expect that related helical polyacetylenes bearing
other amino acid residues and functional groups10,18,20,23
will also form lyotropic and/or thermotropic LC phases,
and such LC polyacetylenes may provide novel chiral
materials for optical switching, sensors, and display
devices.
Acknowledgment. We are deeply grateful to Pro-
fessor T. Sato (Osaka University) for his fruitful dis-
cussions. We also thank Dr. M. Awano and Ms. Y. Hu
at the National Institute of Advanced Industrial Science
and Technology (AIST) for their kind help with the TEM
observations.
Supporting Information Available: Detailed experi-
mental procedures and HPLC chromatograms of the enantio-
separation of the monomers (L-1, D-1, and DL-1). This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
(20) Tang et al. and Masuda et al. independently synthesized
helical polyacetylenes bearing various amino acids as the
pendants, and their chiroptical properties including their

4064 Communications to the Editor
Macromolecules, Vol. 38, No. 10, 2005
helical conformations, inversion of the helicity, and supra-
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(21) Formation of hydrogen bonds was clearly proved by IR
spectra of poly-1’s in these solvents. The detailed IR
measurement results will be reported elsewhere.
MA0503312