Solid-state polymerization of monophenylbutadiyne derivatives

Article abstract of DOI:10.1080/15421400902950030

Solid-state polymerization of phenylbutadiyne derivatives with o-nitro, m-nitro or p-nitro group was investigated. All monomers in bulk crystals could be thermally polymerized to become black suggesting formation of extended pi-conjugation system. However, no regular polydiacetylene structures were confirmed by the structural analysis. Nanocrystals of the m-nitro derivative were prepared, and the different polymerization behavior compared with in the bulk crystals was observed. Copyright Taylor & Francis Group, LLC.

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Solid-State Polymerization
of Monophenylbutadiyne
Derivatives
Jun-ichi Watanabe a , Harumi Yokoyama a , Shuji
Okada a , Shigeru Takaragi b , Hiro Matsuda c &
Hachiro Nakanishi d
a Department of Polymer Science and Engineering,
Graduate School of Science and Engineering,
Yamagata University, Yonezawa, Japan
b Toda Kogyo Corp., Otake, Hiroshima, Japan
c National Institute of Advanced Industrial Science
and Technology, Tsukuba, Japan
d Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, Aoba,
Sendai, Japan
Published online: 26 May 2010.
To cite this article: Jun-ichi Watanabe , Harumi Yokoyama , Shuji Okada , Shigeru
Takaragi , Hiro Matsuda & Hachiro Nakanishi (2009) Solid-State Polymerization of
Monophenylbutadiyne Derivatives, Molecular Crystals and Liquid Crystals, 505:1, 203/
[441]-209/[447], DOI: 10.1080/15421400902950030
To link to this article: http://dx.doi.org/10.1080/15421400902950030
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Mol. Cryst. Liq. Cryst., Vol. 505, pp. 203=[441]–209=[447], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902950030
Solid-State Polymerization of Monophenylbutadiyne
Derivatives
Jun-ichi Watanabe1, Harumi Yokoyama1, Shuji Okada1,
Shigeru Takaragi2, Hiro Matsuda3, and
Hachiro Nakanishi4
1Department of Polymer Science and Engineering, Graduate School
of Science and Engineering, Yamagata University, Yonezawa, Japan
2Toda Kogyo Corp., Otake, Hiroshima, Japan
3National Institute of Advanced Industrial Science and Technology,
Tsukuba, Japan
4Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Aoba, Sendai, Japan
Solid-state polymerization of phenylbutadiyne derivatives with o-nitro, m-nitro or
p-nitro group was investigated. All monomers in bulk crystals could be thermally
polymerized to become black suggesting formation of extended p-conjugation
system. However, no regular polydiacetylene structures were confirmed by the
structural analysis. Nanocrystals of the m-nitro derivative were prepared, and
the different polymerization behavior compared with in the bulk crystals was
observed.
Keywords: nanocrystal; polydiacetylene; solid-state polymerization
INTRODUCTION
Polydiacetylenes (PDAs) are quite unique p-conjugated polymers
prepared by solid-state polymerization of butadiyne monomers [1]
and they are promising materials for photonics such as third-order
nonlinear optics [2]. In order to improve their optical properties, exten-
sion of the p-conjugation system is an interesting challenge. PDAs
with aromatic rings directly bound to the main chain are the good
examples, and higher third-order nonlinear optical properties have
Address correspondence to Shuji Okada, Department of Polymer Science and
Engineering, Graduate School of Science and Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa 992-8510, Japan. E-mail: okadas@yz.yamagata-u.ac.jp
203=[441]

204=[442]
J.-I. Watanabe et al.
FIGURE 1 Synthesis scheme of (nitrophenyl)butadiyne derivatives
through 3.
1
already been reported in some compounds [3]. However, the corre-
sponding monomers, e.g., butadiyne derivatives with two aromatic
substituents, are often not polymerizable in crystals. Generally, mole-
cular engineering, such as introduction of intermolecular hydrogen
bonding, fluorinated substituents with effective electrostatic interac-
tion, etc., is required to achieve their solid-state polymerization [4].
In this connection, solid-state polymerization of monomers with sim-
pler structures, i.e., monophenylbutadiyne derivatives 1, 2 and 3 in
Figure 1, was investigated in both bulk crystals and nanocrystals in
the present study. Although polymerizability of 1-3 has already been
confirmed in our preliminary experiments [5], details of the polymer
structures and nanocrystallization have not been reported.
EXPERIMENTAL
Monomers 1-3 were synthesized according to Figure 1. The structures
1
were confirmed by IR spectra and H- and 13C-NMR spectra. Melting
points of 1, 2 and 3 are 123–124ꢀC, 97–98ꢀC and 142–143ꢀC, respec-
tively. For photopolymerization, UV at 254 nm was irradiated to the
monomer bulk crystals or the monomer nanocrystal dispersions using
a 4-W UV lamp (UVP UVG-254). For thermal polymerization, samples
were kept at 85ꢀC. IR spectra of the bulk crystals were recorded for
KBr-pelletized samples using a Horiba FT-210 spectrometer. Mono-
mer conversions to the corresponding polymers were calculated from
the decreasing ratio of the relative intensity of the peak assigned to
n
after UV irradiation. UV-visible spectra were obtained using a
ꢁC-H
Jasco V-560 spectrophotometer. For UV-visible spectral measurement

Solid-State Polymerization
205/[443]
of bulk crystals, the KBr-pelletized samples were also used. Solid-state
13C-NMR spectra were measured using a Bruker MSL-300 spectro-
meter. Powder X-ray diffraction patterns were measured by using a
Mac Science M18XHF22-SRA diffractometer with a Cu Ka source.
By using the reprecipitation method [6], nanocrystals of 2 were
prepared as water dispersion. THF solution of 2 (5 mmol=L, 200 mL)
was injected into 10 ml of stirred pure water. The microwave-assisted
method [7] was also applied. Average crystal sizes in dispersion were
evaluated by dynamic light scattering (DLS) using an Otsuka
DLS-6000 photometer.
RESULTS AND DISCUSSION
Solid-state polymerization of 1-3 could be monitored by decrease of an
absorption peak corresponding to n
in IR spectra. Figure 2 shows
ꢁC-H
IR spectral change of 2 in the course of thermal annealing as a typical
example. All monomers showed thermal reactivity and time to com-
plete the thermal polymerization at 85ꢀC was 1 h for 2, 2 h for 1 and
FIGURE 2 IR spectral change of 2 in the course of solid-state polymerization
at 85ꢀC. Spectrum a is for the monomer, and spectra b and c were obtained
after heating for 20 min and 1 h, respectively. The arrows indicate the position
of the decreased peak for n
.
ꢁC-H

206/[444]
J.-I. Watanabe et al.
12 h for 3. By UV irradiation, 1 and 2 could be polymerized although p-
substituted compound 3 was not reacted. From these results, align-
ment of monomer 3 in the bulk crystals is slightly deviated from the
appropriate condition for solid-state polymerization, and molecular
motions activated by annealing seem to stimulate the polymerization.
The order of difference between melting point and thermal polymeri-
zation temperature was 3 > 1 > 2, suggesting that the order of relative
magnitude of molecular motion at 85ꢀC was 3 < 1 < 2. This relation
seemed to be reflected to the time to complete thermal polymerization.
Diffuse reflectance spectra of 2 during UV irradiation are shown in
Figure 3. Other compounds also showed similar spectral change in
the course of polymerization whether UV irradiation or thermal treat-
ment. Although all monomers changed to black after polymerization,
typical excitonic absorption peaks often observed for regularly poly-
merized PDAs were not observed. In the solid-state 13C-NMR spectra,
acetylenic carbon peaks decreased and all aromatic carbon peaks
were broadened after polymerization (Fig. 4). In addition, polymers
from 1–3 were confirmed to be amorphous by powder X-ray diffraction
experiments (Fig. 5). Thus, crystal structures of the monomers in this
study are disordered during solid-state polymerization and PDAs with
FIGURE 3 Variation in visible absorption spectra of 2 with UV irradiation
time. Spectrum a is for the monomer, and spectra b and c were obtained after
UV irradiation for 1 h and 2 h, respectively.

Solid-State Polymerization
207/[445]
FIGURE 4 Solid-state 13C-NMR spectra (CP=MAS) of 2. Spectra a and b
are for before heating and after heating for 20 min, respectively. The arrow
indicates the decreased peaks for monomer acetylenic carbons. Spinning
sidebands are marked with asterisks.
regular structures were not obtained although extended p-conjugation
system was produced after polymerization.
Nanocrystals of 2 could be prepared using its THF solution.
Although crystal size just after reprecipitation could not be measured
by DLS, crystal size gradually increased and it became about
250–300 nm after one day. Microwave irradiation just after reprecipi-
tation was found to accelerate the nucleation and rapid crystal-size
increase up to 70 nm was observed at the initial stage. However, the
size still increased depending on time even in this case, and the final
size reached to about 250 nm. Its UV polymerization was confirmed by
the UV-visible absorption change as shown in Figure 6. Large absorp-
tion bands in the wavelength range shorter than 300 nm decreased
and absorption cutoff was extended to around 450 nm. However,
absorption increase in visible region was limited in the
shorter-wavelength range compared with the bulk crystals (cutoff
ꢂ900 nm) and the effective conjugation length of the polymers in the
nanocrystals was suggested to be shorter than that of bulk crystals.

208/[446]
J.-I. Watanabe et al.
FIGURE 5 Powder X-ray diffractograms of monomer 2 (a) and 2 after heating
for 1 h (b) and 2 h (c).
FIGURE 6 Variation in UV-visible absorption spectra of 2 as nanocrystals in
water with UV irradiation time.

Solid-State Polymerization
209/[447]
UV irradiation at 302 nm also showed the similar spectral change.
Although bulk crystals of 2 are thermally polymerizable as mentioned
above, thermal polymerization was not observed for the nanocrystals
even at 90ꢀC. From these results, crystal structures of 2 seem to be dif-
ferent between bulk crystals and nanocrystals, resulting in different
solid-state polymerization behaviors. In the previous study, we found
that 1,4-di(3-quinolyl)butadiyne (DQ) also showed different polymeri-
zation behavior between bulk crystals and nanocrystals [8]. Common
feature between 2 and DQ is their plane molecular shapes, and there
may be several metastable molecular stacking structures in crystals
by slipping adjacent molecules on the molecular p-electron surfaces.
In conclusion, monophenylbutadiyne derivatives 1-3 was prepared
and solid-state polymerization behaviors were studied. In bulk
crystals, all monomers showed thermal polymerizability and black
polymers with extended p-conjugation system were obtained. How-
ever, their structures are not regular PDAs and p-conjugation struc-
tures are disordered. Nanocrystals of 2 with size of around 250 nm
were successfully prepared by the reprecipitation method, and their
polymerization behavior was found to be different from that of bulk
crystals. Although the regular PDA structures were not obtained in
this study, PDAs from monophenylbutadiynes are interesting because
electronic perturbation of polymer backbone may be caused by not
only p-conjugation effect from the attached aromatic ring but also
intermolecular interaction from the non-substituted side. Thus,
further extension on this series of compounds is worth investigating
as future works.
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