Adjustment of conformation change and charge trapping in ion-doped polymers to achieve ternary memory performance

Article abstract of DOI:10.1039/c3tc31759e

Ion-doped poly(4-vinylpyridine) derivatives (P4VPCz), where in the pendant chains the electron-withdrawing pyridine moiety and acceptor carbazole moiety are linked by a flexible alkyl spacer were designed and synthesized. Sandwiched ITO/P4VPCz/Al devices, made through simple spin-coating processes have shown that they could be tuned from a binary to ternary memory performance, by increasing the carbazole content. P4VPCz2 with a 20% mole ratio of the carbazole moiety shows a binary performance according to the charge trapping of the pyridine acceptor in the polymer, while P4VPCz5 which contains a 50% mole ratio of carbazole in the pendant chains, exhibits a ternary data storage ability, probably induced by double mechanisms (i.e. a conformational change and charge trapping). Interestingly, as the carbazole mole ratio reached 80%, the polymer P4VPCz8 has a high conductivity state, with no switching behavior, because of the large amount of doped ions improving the charge transfer mobility. Thus, we hope can offer a guideline to achieve a high-performance multilevel memory material, via combining different mechanisms as well as doping ions.

Full text of DOI:10.1039/c3tc31759e

Journal of
Materials Chemistry C
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Adjustment of conformation change and charge
trapping in ion-doped polymers to achieve ternary
Cite this: J. Mater. Chem. C, 2013, 1,
7883
memory performance†
a
a
a
a
a
*
Dongwei He, Hao Zhuang, Haifeng Liu, Hongzhang Liu, Hua Li
ab
*
and Jianmei Lu
Ion-doped poly(4-vinylpyridine) derivatives (P4VPCz), where in the pendant chains the electron-
withdrawing pyridine moiety and acceptor carbazole moiety are linked by a flexible alkyl spacer were
designed and synthesized. Sandwiched ITO/P4VPCz/Al devices, made through simple spin-coating
processes have shown that they could be tuned from a binary to ternary memory performance, by
increasing the carbazole content. P4VPCz2 with a 20% mole ratio of the carbazole moiety shows a
binary performance according to the charge trapping of the pyridine acceptor in the polymer, while
P4VPCz5 which contains a 50% mole ratio of carbazole in the pendant chains, exhibits a ternary data
storage ability, probably induced by double mechanisms (i.e. a conformational change and charge
trapping). Interestingly, as the carbazole mole ratio reached 80%, the polymer P4VPCz8 has a high
conductivity state, with no switching behavior, because of the large amount of doped ions improving
the charge transfer mobility. Thus, we hope can offer a guideline to achieve a high-performance
multilevel memory material, via combining different mechanisms as well as doping ions.
Received 7th September 2013
Accepted 3rd October 2013
DOI: 10.1039/c3tc31759e
www.rsc.org/MaterialsC
process of the ternary device is still unclear and thus more
investigation should be focused on this perspective to help
Introduction
Polymer-based electrically bistable devices are rather attractive
nowadays for their potential applications in organic memory,
due to their advantages of low operating voltage, rich structural
exibility, solution processability and three-dimension stacking
capability.1 Confronted with the ever-increasing requirements
of super high density data storage in the coming message
explosion era,2 recently, several ternary memory devices based
on small-molecule materials emerged through incorporating
two different charge traps in one molecular backbone, leading
to an exponential increase in data storage capacity relative to
the binary memory systems.3 However, such devices based on
polymeric materials were scarcely reported, probably because of
the lack of appropriate design principles.4 The strategy of
combining more than one mechanism in one device (e.g. charge
transfer and metal lament formation,4a,b conformational
change and charge transfer4c) has been effectively demonstrated
to achieve ternary memory performances. To date, the operating
researchers gain more insight.
Herein, we designed and synthesized ion-doped poly(4-
vinylpyridine) derivatives (P4VPCz) containing pyridine and
carbazole moieties in the lateral chain. In poly(vinylpyridine)
(P4VP), the electron-withdrawing pyridine moieties can not
only act as charge trapping sites,5 but also provide active
points to gra bromoethyl-carbazole moieties with ionic
bonds. So, it is easy for charge transfer and charge trapping to
occur in P4VPCz.6 Besides, a exible alkyl spacer was inten-
tionally introduced between the pendent carbazole moiety and
the pyridine ring to hopefully induce the conformational
change of the planar carbazole moieties when applying an
external electric eld.7 The result shows that when a 50% mole
ratio of bromoethyl-carbazole was connected to the pyridine
moieties, the sandwiched ITO/P4VPCz5/Al device exhibited
ternary data storage characteristics, which was shown to be
under a double mechanism, synergistic effect (i.e., conforma-
tional change and charge trapping). Besides, the ions
produced by bromoethyl-carbazole reacting with pyridine are
helpful to achieve a balanced charge injection and transport
throughout the active layer, thereby enhancing the carrier
mobility and improving the device stability.8,9 However, once
the doped-ions reached a certain amount, the memory device
only showed a high conductivity state with no switching
phenomenon, owning to the improved charge transfer
aCollege of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, P. R. China. E-mail: lujm@suda.edu.cn; Fax: +86 512 65880367;
Tel: +86 512 65880368
bState Key Laboratory of Treatments and Recycling for Organic Effluents by Adsorption
in Petroleum and Chemical Industry, Suzhou 215123, P. R. China
† Electronic supplementary information (ESI) available: The compositions of the
copolymers, SEM image, AFM, stability test of P4VPCz2 and I–V characteristics,
theoretical conduction models. See DOI: 10.1039/c3tc31759e
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mobility. Therefore, the contribution of conformational
change and charge transfer between the pyridine acceptor and
carbazole donor in the P4VPCz tuning the device from binary
to ternary and the doped ions enhancing the devices’ stability
may a guide the design and synthesis of stable multilevel
memory materials in the future.
Results and discussion
The synthetic route of P4VPCz is shown in Scheme 1. The
chemical structures were conrmed by 1H NMR spectroscopy
(see Fig. 1). The aromatic peaks of the carbazole rings could
be noted for P4VPCz, compared to the 1H NMR spectra of
P4VP. The compositions of the copolymers (i.e., the content
of each moiety) were determined from the UV-vis absorption
according to the standard curve of carbazole in DMF (Fig. S1,
see the ESI†). The thermal properties of P4VPCz were inves-
tigated by TGA measurements. All the polymers exhibited
good thermal stability with thermal decomposition temper-
atures (5% weight loss) over 270 ꢀC, implying that the
copolymers are able to endure heat deterioration in the
memory devices.
The sandwich structure of the memory device is illustrated in
Fig. 2a. High-quality polymer thin lms could be prepared
through spin-coating. Fig. 2b shows the AFM image of P4VPCz2
on an ITO substrate, which is quite smooth with a root-mean
square (RMS) roughness of ca. 2.0 nm (Fig. 2c). The observed
smooth morphology of the thin lms is benecial for the hole
injection from the ITO into the HOMO of the polymers.10
The effects of various compositions of poly(4-vinylpyridine)
derivatives (P4VPCz) on the device memory performance were
systematically investigated. The typical I–V characteristics of the
ITO/P4VPCz2/Al device are shown in Fig. 3a. During the rst
sweep, from 0 to ꢁ6.0 V, the device was initially in the low-
conductivity state (OFF state) with a slowly increasing current;
then an abrupt leap in the current was observed at a threshold
voltage (Vths) around ꢁ3.3 V, indicating the transition from the
low-conductivity state (OFF state) to the high-conductivity state
(ON state). This OFF–ON transition response to an external
voltage can be utilized to store data in those electrical memory
devices. Aer a subsequent sweep from 0 to ꢁ6.0 V (or 6.0 V),
the memory device remained in the ON state and could not
return to the original OFF state, whether by removing the
electric eld or applying a reverse voltage scan (0 to 6 V). These
results demonstrate that the memory device fabricated with
Fig. 1 1H NMR spectra of P4VP and P4VPCz in DMSO.
(WORM) memory behavior with an ON/OFF current ratio of
approximately 107. Compared with P4VPCz2, the electric
behavior of the P4VPCz5-based device is rather different,
exhibiting three distinct conductance states (Fig. 3b). Similarly,
the bias is applied in a sequence of 0 to ꢁ6.0 V, two abrupt
increases in the current could be observed around ꢁ2.5 V and
ꢁ3.5 V, respectively, indicating that the device underwent two
electrical transitions, the OFF state to an intermediate state
(ON1 state) and then to a high-conductivity state (ON2 state).
Aer the two transitions were achieved, the device was found to
maintain the ON2 state, even aer the power was turned off or
by applying a reverse bias. These results suggest that the
P4VPCz5-based device could be used as a nonvolatile ternary
WORM device. It should be worthy of note that the Vths is very
uniform and stable aer testing a large number of memory cells
(Fig. S8, ESI†). In addition, segmented sweeps were also carried
out from 0 to ꢁ3.0 V and 0 to ꢁ6.0 V, to study whether the
programmed ON1 and ON2 states were erasable. The obtained
ON1 and ON2 states were unable to be erased by reverse volt-
ages. With a further increase in the content of carbazole
moieties in the pendant chains, the P4VPCz8 thin lm showed
only a single high-conductivity state, with no switching
phenomenon, which indicates that when the carbazole moiety
mole ratio reaches 80%, the device has no data storage ability.
In summary, the electrical properties of P4VPCz devices were
found to vary from binary to ternary and further to a conductor
behavior with the content of carbazole and doped ions
increasing.
P4VPCz2 exhibited
a
binary write-once-read-many-times
Scheme 1 Synthetic route and molecular structure of P4VPCz.
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Fig. 2 (a) A schematic diagram of the ITO/P4VPCz/Al device; (b) the tapping-
mode (5 mm ꢂ 5 mm) AFM topography image of P4VPCz2 on an ITO substrate; (c)
roughness of the (b) thin film.
The long-term stability and endurance performance were
also evaluated from the retention time and stimulus effect test.
Fig. 4 shows that the P4VPCz5 device could endure over 108
continuous read pluses of ꢁ1.0 V and no signicant degrada-
tion in the current for any state was observed for 100 min. The
retention time and pulse endurance test of P4VPCz2 at ꢁ1.0 V
also showed good stability (Fig. S4, ESI†). It is noteworthy that
the writing voltages are all smaller than ꢁ4.0 V and the reading
voltage is as low as ꢁ1.0 V, which imply a low power
consumption of the P4VPCz device during practical data storage
processes.
The P4VPCz containing different carbazole content, showing
different memory performance was precisely discussed. The
P4VPCz2 which only contains a 20% mole ratio of carbazole
moieties shows a WORM memory effect, which we deduced was
arising from the charge trapping process. As is well known,
pyridine is an electron acceptor group and could act as a charge
trapping site when the carriers were injected, under an electric
eld. Aer the charge traps were lled to full, the device
exhibited an OFF to ON transition. In order to exclude that the
OFF to ON transition of the P4VPCz2 based device was not
contributed to by the charge transfer between the pyridine
acceptors and the carbazole donors, the UV-visible spectra and
the cyclic voltammetry (CV) curves of the P4VPCz thin lms were
examined. As can be seen from Fig. 5a, the P4VPCz lm shows
three strong absorption bands centered at 298, 331 and 344 nm,
which is consistent with the spectral features of carbazole. The
absorption of the copolymers with different compositions
exhibited no signicant change, indicating the weak intra-
molecular charge transfer between the carbazole donor and the
pyridine acceptor within the lm.11 Furthermore, Fig. 5b shows
that in a solution of dichloromethane with tetrabutylammo-
Fig. 3 I–V characteristics of (a) ITO/P4VPCz2/Al, (b) ITO/P4VPCz5/Al and (c)
ITO/P4VPCz8/Al devices.
1.28 eV. The HOMO and LUMO energy levels of the polymer
were estimated from the CV results with reference to ferrocene
(4.8 eV) by the following equations:
(onset)
ox
EHOMO ¼ ꢁ[E
+ 4.8 ꢁ EFerrocence
]
ELUMO ¼ EHOMO + Eg
Eg ¼ hc/lEdge
The HOMO and LUMO energy levels are summarized in
nium perchlorate (TBAP) (0.1 M) as the electrolyte at a scan rate Table 1. The slight uctuation also claried that the charge
of 100 mV sꢁ1, all the polymers exhibit an irreversible oxidation transfer between the carbazole donor and pyridine acceptor
behavior with the rst oxidation peaks in the range 1.38– is relatively weak, suggesting that the observed memory
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Fig. 4 Stability test of the ITO/P4VPCz5/Al device. (a) Retention time of the OFF,
ON1 and ON2 states under a continuous readout voltage (ꢁ1.0 V); (b) stimulus
effect of a read pulse of ꢁ1.0 V on the three distinct states. The inset shows the
pulse shapes adopted in the measurement.
Fig. 5 (a) UV-visible absorption spectra of the P4VPCz films on a quartz plate; (b)
cyclic voltammogram of P4VPCz thin films spin-coated on an ITO substrate in
dichloromethane containing 0.1 M TBAP at a scan rate of 100 mV sꢁ1
.
performance (for P4VPCz2) originated from the pyridine trap, lled up, consequently forming another charge-transporting
instead of the charge transfer effect.12
channel, resulting in the observed high-conductivity state
Kang et al. have demonstrated that the pendant polymers (Fig. 6a and c). The result is also veried by experimental data.
containing an electroactive chromophore in the side-chains, The conformational effect is supported by the UV-vis spectra
such as the carbazole (Cz), naphthalene (Na) and anthracene measurements of the P4VPCz5 thin lm in the before-
(An) moieties or p-conjugated moieties, exhibited electrical bi- conductivity (OFF state) and aer-conductivity (ON1 state)
stability controlled by the electric-eld-induced conformational states. As shown in Fig. 6b, aer a voltage sweep of 0 to ꢁ3.0 V
change, and the different memory characteristics are decided by with a liquid Hg droplet instead of the Al electrode, a slight but
the exible spacer between the polymer backbone and the evident red shi could be seen, arising from the ordered p–p
functional chromophore.13 Herein, the designed P4VPCz stacking of the carbazole groups.7,14,15 Besides, the second
contains a exible spacer and as long as the carbazole moieties switching may be explained by the formation of a trapping level
reach a certain number, conformational change will certainly in the device.5,6 It can be rationalized by evaluating the HOMO
take place under an electric eld. As for P4VPCz5 with a rela- and LUMO energy levels of the P4VPCz combined with the work
tively high carbazole ratio, under a low voltage bias (0 to ꢁ2.5 V), function of the ITO and Al electrode, as shown in Fig. 7. It can be
holes started to inject from the ITO to the carbazole segment conrmed that the charge transfer (CT) between the carbazole
near the interface, forming active species. The neighboring and pyridine moieties is relatively weak because no obvious
neutral carbazole groups tended to distort and form a partial or change was observed with different D/A ratios from the UV-vis
full face-to-face conformation attracted by the active carbazole and CV results. For a p-type semiconductor material, it is easier
species. Then, the charge was delocalized to the neighboring for the fabricated device to achieve current ow when the ITO is
carbazole groups through a more ordered conformation. It is enriched with enough holes. Herein, the electron-withdrawing
better for charge carrier transportation to form free channels pyridines are likely to serve as the hole-blocking moieties in
under an electrical eld, but the carriers do not have sufficient theactive layer and would be lled with holes to full, under an
energy to overcome the barrier of the pyridine trap and are electric eld.16 The Vths of P4VPCz5 (around ꢁ3.5 V) and
blocked by the electron-accepting pyridine moieties in the lm, P4VPCz2 (around ꢁ3.3 V) indicated their fabricated devices have
thus only exhibiting the intermediate state. With the voltage almost the same switching voltage, which is due to the same
further increasing, traps of the pyridine moieties were gradually energy gaps lying in the ITO and their own HOMO orbits.
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Table 1 Thermal, optical and electrochemical properties of the P4VPCz
TGA (ꢀC)
lEdge (nm)
Eg (eV)
Eox (eV)
HOMO (eV)
LUMO (eV)
P4VPCz2
P4VPCz5
P4VPCz8
270
272
275
395.1
395.5
395.3
3.14
3.14
3.14
1.38
1.36
1.28
ꢁ5.56
ꢁ5.58
ꢁ5.66
ꢁ2.42
ꢁ2.44
ꢁ2.52
Fig. 8 Analysis of the fitted I–V characteristics of the ITO/P4VPCz8/Al device in
the ON state, the inset is the P4VPCz resistance (R) analysis in the OFF state.
Fig. 6 (a and c) Schematic diagram of the charge carrier transport process in the
P4VPCz5-based memory device. (b) UV-visible spectra of the P4VPCz5 thin films
in the OFF and ON1 state on an ITO substrate; the ON1 state was programmed by
using a removable liquid Hg droplet as the top electrode.
concentration of carbazole moieties, as well as the doped
ions, which made it easier to form pathways for charge
hopping between neighboring carbazole units, leading to
initially exhibit the ON state. The I–V curves of the P4VPCz2
and P4VPCz5 devices were also analyzed with appropriate
theoretical conduction models to clarify the carrier transport
and mechanism using thermionic emission limited and
ohmic current model (Fig. S9 and S10, ESI†). The OFF state
(or ON1 state) current, below the threshold voltage might be
attributed to the thermally generated charge carriers
through the straight line tting on the plot of ln(I) versus V1/2
,
while the ON state (or ON2 state) current shows a linear
tting plot of ln(I) versus ln(V), based on the ohmic current
model. From the systematic electrical characterization and
analysis, it is better to understand the different switching
behavior of the P4VPCz devices based on copolymers with
different compositions.
Fig. 7 Molecular simulation of the HOMO and LUMO energy levels for P4VPCz
along with the work function of the electrodes.
Therefore, the double switching process in P4VPCz5 is well
explained.
Conclusions
The doped ions could help to balance charge injection and We have demonstrated ternary memory properties controlled by
transport within the active layer to improve the device stability a dual mechanism, achieved with P4VPCz copolymers. Through
performance, so the devices based on P4VPCz2 (or P4VPCz5) tuning the content of exible lateral carbazole moieties and the
show a very uniform and stable Vths (Fig. S7 and S8, ESI†). doped ions, the devices based on P4VPCz were able to show
Besides, the OFF state current of P4VPCz5 is increased with the various electrical behaviors such as bistable switching, tristable
increasing of the doped ions. Yet, in further increasing the switching and a conductor behavior. The P4VPCz2 only
content of the carbazole moieties and doped ions, the P4VPCz8 exhibited bistable states with a nonvolatile WORM character-
thin lm based devices show only a single high-conductivity istic due to charge trapping under the electrical eld. However,
state, with no switching phenomenon. The characteristics of the P4VPCz5 showed a double switching behavior (tristable
the high conductance state of the P4VPCz8 device completely states) that could mainly be attributed to the conformational
showed an ohmic trend (Fig. 8).17 The P4VPCz8 has a high change and charge trapping, successively. It is believed that
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our study may offer a guideline for the design of practical
polymer-based multilevel devices via introducing more than
one mechanism in the active materials and the double switch-
ing process in the ternary device provides clarication of this
proposal.
Instruments
1
The H NMR spectra were obtained on an Inova 400 MHz FT-
NMR spectrometer. The UV-vis absorption spectra were recor-
ded on a Perkin Elmer Lambda-17 spectrophotometer at room
temperature. Thermogravimetric analysis (TGA) was conducted
on a TA instrument Dynamic TGA 2950 at a heating rate of 20 ꢀC
minꢁ1 under a nitrogen ow rate of 100 mL minꢁ1. Cyclic vol-
tammetry was performed at room temperature using an ITO
working electrode, a reference electrode of Ag/AgCl, and a
counter electrode (Pt wire) at a scanning rate of 100 mV sꢁ1
(CorrTest CS Electrochemical Workstation analyzer) in a solu-
tion of tetrabutylammonium perchlorate (TBAP) in dichloro-
methane (0.1 M). The SEM images were taken on a Hitachi
S-4700 scanning electron microscope. The atomic force
microscopy (AFM) measurements were performed by using a
MFP-3DTM (Digital Instruments/Asylum Research) AFM
instrument in the tapping mode.
Experimental section
Materials
Carbazole (99%), 2-chloroethanol (99%), phosphorus tri-
bromide (PBr3) (99%), poly(4-vinylpyridine) (Mw ca. 60 000),
potassium hydroxide (KOH) (99%), and the solvents were all
purchased from the Shanghai Chemical Reagent Co. Ltd.
N,N-dimethylformamide (DMF) was puried by reduced pres-
sure distillation. The other reagents were used as received
without any further purication.
Synthesis of 2-(9H-carbazol-9-yl)ethanol
A mixture of carbazole (6.68 g, 40 mmol), 2-chloroethanol (7.2
mL, 52 mmol), KOH (14.0 g, 0.25 mol), and DMF (80 mL) was
stirred at 50 ꢀC for 12 hours. Aer the reaction was completed, the
mixture was poured into a large excess of water, and suspended
for 6 hours. The precipitated product was ltered and further
puried via a silica gel column with chloroform and petroleum
Device fabrication
The indium tin oxide (ITO) glass was pre-cleaned sequentially
with deionized water, acetone and ethanol in an ultrasonic
bath, each for 20 min. The polymer was dissolved in N,N-
dimethylformamide (15 mg mLꢁ1) and ltered through micro-
lters with a pinhole size of 0.22 mm. Thereaer, the solutions
were spin-coated onto ITO at 2000 rpm and the solvent was
1
ether (volume ratio ¼ 2 : 1), yield 80%. H NMR (DMSO-d6, 400
MHz), d (ppm): 3.74–3.85 (m, 2H), 4.45 (dd, J ¼ 4.41, 7.07 Hz, 2H),
5.51 (t, J ¼ 5.37 Hz, 1H), 7.15–7.26 (m, 2H), 7.40–7.50 (m, 2H), 7.65
(dd, J ¼ 4.70, 10.23 Hz, 2H), 8.10–8.21 (m, 2H).
removed in a vacuum chamber at 10ꢁ1 Pa and 50 C for 12 h.
ꢀ
The thickness of the polymer layer was about 80 nm. Finally, the
top Al electrode, 100 nm thick was thermally evaporated
through a shadow mask under a pressure of 5 ꢂ 10ꢁ6 Torr. The
active area of each cell was 0.126 mm2 (a nummular point with a
radius of 0.2 mm). All the electrical measurements of the
devices were characterized under a nitrogen atmosphere, using
an HP 4145B semiconductor parameter analyzer.
Synthesis of 9-(2-bromoethyl)-9H-carbazole (BCz)
The DMF (puried) solution containing 2-(9H-carbazol-9-yl)
ethanol (2.07 g, 20 mmol) in a 50 mL ask was purged with
ꢀ
argon, and then stirred at 0 C. Then PBr3 (2.8 mL)–DMF (5
mL) was slowly added. The mixture was stirred at room
temperature for about 12 hours. Aer the reaction, a satu-
rated Na2CO3 solution was slowly added to remove excess
PBr3. The mixture was extracted with CH2Cl2, dried over
Na2SO4 and then the solvent was evaporated. The nal
product was obtained through a silica gel column (ethyl
acetate: petroleum ether ¼ 1 : 2) to give a colorless liquid.
Acknowledgements
This work was nancially supported by the NSF of China
(21076134, 21176164, 21206102 and 21336005), the NSF of
Jiangsu Province (BK2010208 and BE2013052), a project of the
Department of Education of Jiangsu Province (12KJB430011),
Suzhou Nano-project (ZXG2012023) and Project supported by
the Specialized Research Fund for the Doctoral Program of
Higher Education of China (Grant no. 20113201130003 and
20123201120005). This is also a project funded by the Priority
Academic Program Development of the Jiangsu Higher Educa-
tion Institutions (PAPD).
1
Yield: 85%. H NMR (DMSO-d6, 400 MHz), d (ppm): 3.92 (t,
J ¼ 6.31 Hz, 2H), 4.85 (t, J ¼ 6.31 Hz, 2H), 7.22 (t, J ¼ 7.46 Hz,
2H), 7.46 (t, J ¼ 7.68 Hz, 2H), 7.66 (d, J ¼ 8.20 Hz, 2H), 8.16 (d,
J ¼ 7.73 Hz, 2H).
Synthesis of P4VPCz
A mixture of poly(4-vinylpyridine) (200 mg, 3.3 mmol) (Mw ca.
60 000) and 9-(2-bromobutyl)-9H-carbazole (540–1620 mg, 2.0–
6.0 mmol) in 4.0 mL of DMF–CH3OH (1 : 1) was stirred at 50 ꢀC
in a round-bottom ask for one week. The mixture became clear
aer 3 h and then turned red 3 days later. Toluene was used to
precipitate the brownish solid product, followed by a Soxhlet
extraction for 3 days, giving the product aer vacuum drying. 1H
NMR (DMSO-d6, 400 MHz) d (ppm): 1.04–1.92, 4.65–5.28, 6.32–
6.92, 6.98–7.55, 7.90–8.62. This polymer was soluble in DMSO
and DMF.
Notes and references
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