Synthesis of poly(m-phenylene) and poly(m-phenylene)-block-poly(3- hexylthiophene) with low polydispersities

Article abstract of DOI:10.1002/pola.24703

Well-defined poly(m-phenylene) (PMP), which is poly(1,3-dibutoxy-m- phenylene), was successfully synthesized via Grignard metathesis polymerization. PMP with a reasonably high number-average molecular weight (M_n) of 25,900 and a very low polydi

Full text of DOI:10.1002/pola.24703

Synthesis of Poly(m-phenylene) and Poly(m-phenylene)-block-
Poly(3-hexylthiophene) with Low Polydispersities
KAORU OHSHIMIZU,1 AYUMI TAKAHASHI,1 TOMOYA HIGASHIHARA,1,2 MITSURU UEDA1
1Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-H-120, O-okayama, Meguro-Ku, Tokyo 152-8552, Japan
2PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
Received 28 February 2011; accepted 1 April 2011
DOI: 10.1002/pola.24703
Published online 27 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Well-defined poly(m-phenylene) (PMP), which is
poly(1,3-dibutoxy-m-phenylene), was successfully synthesized
via Grignard metathesis polymerization. PMP with a reason-
ably high number-average molecular weight (Mn) of 25,900
and a very low polydispersity index of 1.07 was obtained. The
polymerization of a Grignard reagent monomer, 1-bromo-2,4-
benzene ring. PMP showed a good solubility in the common
organic solvents, such as tetrahydrofuran, CH2Cl2, and CHCl3.
Furthermore, a new block copolymer comprised of PMP and
poly(3-hexylthiophene) was also prepared. The tapping mode
atomic force microscopy image of the surface of the block co-
polymer thin film on a mica substrate showed a nanofibril
C
V
dibutoxy-5-chloromagnesiobenzene, proceeded in
a
chain-
morphology with a clear contrast.
2011 Wiley Periodicals,
growth manner, probably due to the meta-substituted design
producing a short distance between the MgCl and Br groups
and thereby making a smooth nickel species (ACANiACA)
transfer to the intramolecular chain end (ACANiABr) over a
Inc. J Polym Sci Part A: Polym Chem 49: 2709–2714, 2011
KEYWORDS: conjugated polymers; diblock copolymers; GRIM
polymerization; molecular weight distribution; poly(m-phenylene)
INTRODUCTION Well-defined architectures of polymers are
quite important to clarify the relationships between struc-
tures and properties. However, the control of the molecular
weights (MWs) and polydispersity indices (PDIs) of conden-
sation polymers is generally very difficult compared to a
controlled system based on the addition or ring-opening liv-
ing polymerization. Since Yokozawa1,2 and McCullough3,4
independently discovered the quasi-living system of Grignard
metathesis (GRIM) polymerization for synthesizing regiore-
gular poly(3-hexylthiophene) (P3HT) with well-controlled
MWs and PDIs, various P3HT-based block copolymers have
been reported.5–9 These materials generally formed nano-
fiber structures as thin film on various substrates.10–14 Other
well-defined p-conjugated polymers, such as poly(p-phenyl-
ene)s (PPPs)15 and polypyrroles,16 have also been success-
fully prepared based on the GRIM polymerization system.
phase-separated behavior of PPP-b-P3HT in 2009 by Wu
et al.14 They reported that PPP-b-P3HT showed phase-sepa-
rated crystalline–crystalline domains and a lamellar mor-
phology by differential scanning calorimetry (DSC) and
atomic force microscopy (AFM), respectively. With regard to
the poly(m-phenylene)s (PMPs) having an amorphous
nature, there have been neither information about a con-
trolled synthetic system for block copolymers nor even for
homopolymers. Therefore, their fundamental properties and
morphologies are still unknown.
In this article, we report for the first time the GRIM polymer-
ization of a meta-substituted monomer, 1-bromo-2,4-dibu-
toxy-5-iodobenzene (2), which yields PMP with very low
PDI. We expect that the meta-substituted design is favorable
for the smooth intramolecular transfer of a nickel species
(ACANiACA) to the chain end (ACANiABr), walking over a
benzene ring, because of a shorter distance between the
MgCl and Br groups than the para-substituted one, which
lead to the facile control of the polymerization system.
Furthermore, new block copolymers with PMP and P3HT
segments can be successfully synthesized, while maintaining
a low PDI by a similar addition sequence of monomers to
PPP-b-P3HT.22 The phase-separated behavior of poly(1,3-
Although some controlled systems for p-conjugated polymers
other than P3HT were discovered,17–21 attention has focused
only on the AB type diblock copolythiophenes, and new
combination of different main chains is very limited. Three
synthetic examples of well-defined block copolymers with
different main chains, poly(2,5-dihexyloxy-1,4-phenyl-
ene)(PPP)-b-P3HT,14,22 PPP-b-poly(N-hexylpyrrole),23 and
P3HT-b-poly(9,9-dioctylfluorene),24 were very recently
reported, based on the sequential quasi-living GRIM poly-
merization. Among them, only one article described the
dibutoxy-m-phenylene)-b-poly(3-hexylthiophene)
(PMP-b-
P3HT) is also observed by DSC and AFM.
Correspondence to: M. Ueda (E-mail: ueda.m.ad@polymer.titech.ac.jp) or T. Higashihara (E-mail: thigashihara@polymer.titech.ac.jp)
C
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2709–2714 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS OF PMP AND PMP-b-P3HT, OHSHIMIZU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ꢀ
MHz, CDCl3, ppm, 25 C): d ¼ 7.16 (s, ArH, 1H), 6.47 (s, ArH,
1H), 3.84 (m, AOCH2A, 4H), 1.56 (m, ACH2A, 4H), 1.27 (m,
ACH2A, 4H), 0.75 (m, ACH3, 6H).
Synthesis of PMP-b-P3HT
A round-bottomed flask equipped with a three-way stopcock
was charged with lithium chloride (0.21 g, 4.95 mmol) and
heated by a heat-gun under reduced pressure. After the flask
was cooled to room temperature under a nitrogen atmos-
phere, 2 (428 mg, 1.00 mmol) and THF (20 mL) were added,
and the mixture was stirred at room temperature. To the
SCHEME 1 Synthesis of compound 2.
EXPERIMENTAL
Materials
i
mixture was added PrMgCl (2.0 M solution in THF, 0.551
Tetrahydrofuran (THF) was dried over sodium benzophe-
none and distilled before use under nitrogen. 4-Bromo-6-
iodoresorcinol25 and 2-bromo-3-hexyl-5-iodothiophene (1)1
were prepared according to the literature. 1-Bromo-2,4-dibu-
toxy-5-iodobenzene (2) was prepared (Scheme 1) by modify-
ing the procedure in a previous report.25 Lithium chloride
was dried by a heat-gun under reduced pressure and used
under nitrogen. All other reagents and solvents were used
without further purification.
mL, 1.10 mmol) via a syringe, and the mixture was stirred
for 30 min at the temperature. Then, a suspension of
Ni(dppp)Cl2 (6.8 mg, 0.0125 mmol) in dry THF (5.0 mL) was
added to the mixture via a syringe. The polymerization con-
tinued for 1 h at room temperature, and then a solution of
the Grignard exchanged compound 1 in dry THF (5 mL) was
added to the mixture. The Grignard exchanged compound 1
solution was prepared as follows.
A round-bottomed flask equipped with a three-way stopcock
was charged with lithium chloride (0.21 g, 4.95 mmol), and
heated by a heat-gun under reduced pressure. The flask was
cooled to room temperature under a nitrogen atmosphere.
Then, compound 1 (0.365 mg, 0.978 mmol) and THF (5 mL)
Synthesis of 1-Bromo-2,4-dibutoxy-5-iodobenzene (2)
A THF (100 mL) solution of 4-bromo-6-iodoresorcinol (3.67
g, 11.6 mmol), PPh3 (6.72 g, 25.6 mmol), and 1-butanol
ꢀ
(1.90 g, 25.6 mmol) were cooled to 0 C. To the solution was
added diethyl azodicarboxylate in toluene solution (12.0 mL,
26.3 mmol, 40% w/v), and the reaction mixture was
warmed to room temperature and stirred for 1 h at the tem-
perature. It was then poured into hexane and filtered. The
filtrate was evaporated, and the residue was recrystallized
from methanol to give white needles (3.5 g, 70% yield); m.p.
ꢀ
were added, and the mixture was stirred at 0 C for 30 min.
i
To the mixture was added PrMgCl (2.0 M solution in THF,
0.538 mL, 1.076 mmol) via a syringe, and the mixture was
ꢀ
stirred for 30 min at 0 C.
The polymerization of the second monomer was continued
for 1 h at room temperature and quenched by addition of
5M HCl solution. Then, the polymer solution was poured
into a mixture solution of methanol (200 mL) and water
(200 mL), and the residue was filtered and purified via a
Soxhlet extraction with methanol. The residue was extracted
again with chloroform using a Soxhlet apparatus. Chloroform
was removed by evaporation under reduced pressure, and
the polymer was dried overnight under reduced pressure to
give PMP-b-P3HT (Mn ¼ 16,400, PDI ¼ 1.15) as a purple
solid (0.185 g, 52%).
ꢀ
¼ 73.8–74.5 C.
1H NMR (300 MHz, CDCl3, ppm, 25 ꢀC): d ¼ 7.83 (s, ArH,
1H), 6.39 (s, ArH, 1H), 3.99 (q, AOCH2A, 4H), 1.81 (m,
ACH2A, 4H), 1.55 (m, ACH2A, 4H), 0.99 (m, ACH3, 6H). 13C
ꢀ
NMR (300 MHz, CDCl3, ppm, 25 C): d ¼ 158.1, 156.8, 141.3,
103.7, 98.8, 75.2, 69.5, 69.3, 31.3, 31.2, 19.5, 19.4, 14.0. Anal.
Calcd. for C14H20BrIO2; C, 39.37; H, 4.72; Br, 18.71; I, 29.71;
O, 7.49. Found: C, 39.46; H, 4.62.
Synthesis of Poly(1,3-dibutoxy-m-phenylene)
1H NMR (300 MHz, CDCl3, ppm, 25 ꢀC): d ¼ 7.18 (s, Ar H,
1H), 6.99 (s, thiophene, 1H), 6.51 (s, ArH, 1H), 3.86 (t,
AOCH2A, 4H), 2.83 (t, ACH2 (P3HT), 2H), 1.80–0.75 (m,
alkyl, 7H þ 11H(P3HT)).
A round-bottomed flask equipped with a three-way stopcock
was charged with lithium chloride (0.10 g, 2.36 mmol) and
heated by a heat-gun under reduced pressure. After the flask
was cooled to room temperature under a nitrogen atmos-
phere, 2 (214 mg, 0.501 mmol) and THF (10 mL) were
added, and the mixture was stirred at room temperature. To
the mixture was added iPrMgCl (2.0 M solution in THF,
0.287 mL, 0.574 mmol) via a syringe, and the mixture was
stirred for 30 min. Next, a suspension of Ni(dppp)Cl2 (3.0
mg, 0.0055 mmol) in dry THF (3.0 mL) was added to the
mixture via a syringe, and then the mixture was stirred for
24 h. The polymerization was quenched by the addition of
5M HCl solution. The polymer solution was poured into a
mixture solution of methanol (200 mL) and water (200 mL),
and the residue was filtered, washed with methanol, and
dried under reduced pressure to give PMP (Mn ¼ 25,900,
PDI ¼ 1.07) as a yellow solid (64.0 mg, 62%).1H NMR (300
Measurement
The 1H and 13C NMR spectra were recorded with a Bruker
DPX300S spectrometer. Size exclusion chromatography (SEC)
was performed on an Asahi Techneion AT-2002 equipped
with a Viscotek TDA model 305 triple detector array using
THF as a carrier solvent at a flow rate of 1.0 mLmin–1 at 30
ꢀC. Three polystyrene gel columns of pore sizes (650, 200,
and 75 Å) (bead size 9 lm) were used. The absolute Mn val-
ues were calculated by collecting data from right-angle laser
light scattering (RALLS). Thermal analysis was performed on
a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a
heating rate of 10 ꢀC min–1 for thermogravimetry (TG) and
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ARTICLE
SCHEME 2 Synthesis of PMP.
on a DSC 6200 at a heating rate of 20 ꢀC min–1 for DSC
under nitrogen. UV–vis spectra of polymer thin films and sol-
utions in CHCl3 at 10–5 mol L–1 were taken on a Jasco V-560
UV–vis spectrophotometer over a wavelength range of 250–
750 nm. AFM images were taken with a SII-NT SPA 400
operating in a tapping mode.
monomer conversions of the Grignard exchanged products of
1,4-dibromo-2,5-dihexyloxybenzene and 1-bromo-2,5-dihexy-
loxy-4-iodobenzene were also around 70% for the synthesis
of the PPPs.
PMP well dissolves in the common organic solvents, such as
THF, CH2Cl2, and CHCl3. This good solubility is attributed to
the twisted structure of the meta-linked main chain in addi-
tion to the dibutoxy side chains. The obtained polymer, PMP,
RESULTS AND DISCUSSION
1
was characterized by a H NMR spectrum and size exclusion
Synthesis of Monomer 2
chromatography - right-angle laser light scattering (SEC-
RALLS) profile. Figure 1 shows the 1H NMR spectrum of
PMP, in which the characteristic signal assignable to the oxy-
methylene proton appears at 3.84 ppm. The SEC-RALLS pro-
file of PMP is shown in Figure 2, which displays a unimodal
peak with the very low PDI of 1.07. This low PDI should be
due to the chain-growth polymerization mechanism occur-
ring by a catalyst transfer condensation polymerization.1–4
For the synthesis of P3HT using the catalyst transfer poly-
condensation system, Ni(dppp)Cl2 reacts with 2 equiv. of 1,
and the coupling reaction occurs with the concomitant gen-
eration of a zero-valent Ni complex. In this case, the Ni(0)
species does not diffuse to the reaction solution media but is
selectively transferred to the intramolecular CABr bond.
Another 1 reacts with this Ni inserted dimer, followed by the
cross-coupling reaction and transfer of the Ni catalyst to the
next CABr bond. The chain-growth reactions continue in
such a way that the Ni catalyst transfers to the polymer end
group over a thiophene ring without diffusion. This selective
Ni transfer system should be due to the proper distance
between the CAMgCl and CABr groups. Because, the poly-
merization of dihalogenated monomers based on the
We have already reported the synthesis of PMP by oxidation
coupling polymerization of 1,3-di-n-butoxybenzene.26 How-
ever, their molecular weight distributions were broad, based
on a conventional polycondensation system. To obtain PMP
with a high MW and a low PDI, the monomer 2 was
designed and successfully prepared by Scheme 1, as identi-
fied by 1H NMR and 13C NMR spectra and elemental
analysis.
Synthesis of Poly(1,3-dibutoxy-m-phenylene)
We first examined the Grignard exchange reaction of mono-
i
mer 2 with an equivalent molar ratio of PrMgCl in the pres-
ence of LiCl in THF at room temperature for 30 min, fol-
lowed by quenching with methanol under the same
conditions reported in a previous study.10 The quantitative
and selective Grignard exchange reaction at the iodine-sub-
stituted 5-position proceeded as confirmed by the 1H NMR
spectra.
Based on this result, the nickel-catalyzed cross-coupling po-
lymerization of the Grignard exchanged monomer 2 was car-
ried out at room temperature in THF in the presence of LiCl
(Scheme 2). The polymerization initiated with Ni(dppp)Cl2
(dppp
¼
1,3-bis(diphenylphosphino)propane) successfully
proceeded to give PMP with a 67% conversion and the Mn
and PDI values determined by SEC-RALLS were 25,900 and
1.07, respectively. The incomplete monomer conversion
might be derived from the deactivation process of the Gri-
gnard exchanged monomer 2 during the long polymerization
time of 24 h. As Yokozawa and coworkers15,22 reported, the
FIGURE 1 1H NMR spectrum of PMP in CDCl3.
FIGURE 2 SEC-RALLS profile of PMP.
SYNTHESIS OF PMP AND PMP-b-P3HT, OHSHIMIZU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 1H NMR spectrum of P3HT-b-PMP.
FIGURE 4 1H NMR spectrum of PMP-b-P3HT.
fluorene,27 carbazole,28 and bithiophene29 structures, having
longer distances than the thiophene monomers, could not
maintain the low PDI, probably due to the diffusion of Ni(0)
species to the reaction solution media, which leads to the
general condensation polymerization. Similarly, in the case of
the meta-substituted monomer 2, the shorter distance
between the CAMgCl and CABr groups of PMP than that of
PPP leads to the very low PDI.
16,400 and low PDI (1.15) was obtained. The obtained block
copolymer, PMP-b-P3HT, was characterized by H NMR spec-
1
troscopy and SEC-RALLS. Figure 4 shows the 1H NMR spec-
trum of PMP-b-P3HT, in which a triplet signal due to the
oxymethylene protons (a) next to the phenylene ring of PMP
segment and a triplet signal assignable to the methylene pro-
tons (b) of P3HT segment are observed at 3.86 and 2.83
ppm, respectively. The molar ratio of the segment units
(n:m) for PMP:P3HT was 21:79 determined by the integra-
tion of signals (a) and (b); however, the feed ratio of the
monomers was 50:50. This difference is due to the incom-
plete conversion of monomer 2 due to the deactivation; how-
ever, the SEC profile of PMP-b-P3HT shows a unimodal peak
with a low PDI and without shoulders (Fig. 5), which indi-
cates the formation of the expected block copolymer.
The thermal property of PMP was evaluated by thermogravi-
metric analyzer (TGA) and DSC. PMP exhibited the relatively
high-degradation temperature (10% weight loss) of 390 ꢀC and
ꢀ
the glass transition temperature (Tg) of 24 C (Fig. 6) under
nitrogen, which are values similar to those found in a ref. 26.
Synthesis of PMP-b-P3HT
The phase transition temperatures of PMP-b-P3HT were
measured by DSC in a nitrogen atmosphere (Fig. 6). The
DSC profile of the PMP homopolymer is also included for
comparison. The block copolymer exhibits two phase transi-
tion temperatures at 16 and 220–240 ꢀC, which can be
ascribed to the glass transition temperature (Tg) of PMP
and the melting points (Tm) of the P3HT segments, respec-
tively, by comparing the DSC scans of PMP-b-P3HT and the
PMP homopolymer. The double signals present in the Tm of
the P3HT segment are attributed to a combination of melt-
For the synthesis of the block copolymers of P3HT and PMP,
we first conducted the polymerization of 1 in THF initiated
by Ni(dppp)Cl2 in the presence of LiCl, followed by the post-
polymerization of 2. The 1H NMR spectrum (Fig. 3) of the
product showed that a very small amount of the PMP mono-
mer units were contained. The low conversion of 2 may be
due to the wrong order (1 to 2) of monomer addition as
previously reported.22 Thus, the block copolymerization was
then carried out in the reversed order (2 to 1) (Scheme 3).
This order would be preferable due to the better p-donor
ability of P3HT than that of PMP, where p-electrons of the
polymers are considered to assist the transfer of the Ni(0)
species during the catalyst-transfer polymerization. In prac-
tice, the block copolymer, PMP-b-P3HT, with the Mn of
ing processes, which is consistent with
a
previous
report.30,31 This result clearly indicates the emergence of a
microphase separation between the PMP and P3HT seg-
ments in the PMP-b-P3HT. To further confirm the
SCHEME 3 Synthesis of PMP-b-P3HT.
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ARTICLE
FIGURE 8 UV–vis spectra of PMP, P3HT, and PMP-b-P3HT in
film state, respectively. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 5 SEC-RALLS profile of PMP-b-P3HT.
microphase separation of PMP-b-P3HT, its thin film was
prepared by drop casting from a 1 mg mLꢁ1 toluene solu-
tion onto a mica substrate, and the solution was slowly
evaporated at room temperature for 2 days to promote the
phase separation. The surface morphology of the thin film
was characterized by AFM. As shown in Figure 7, the film
clearly exhibited a lamellar-like phase-separated morphol-
ogy induced by the different nature of the crystalline P3HT
and amorphous PMP segments.
The optical properties of PMP and PMP-b-P3HT were further
investigated by UV–vis spectroscopy. The UV–vis spectra of
P3HT, PMP, and PMP-b-P3HT in the film state were investi-
gated (Fig. 8). The shoulder at 610 nm of PMP-b-P3HT is
related to the vibronic absorption of the P3HT segment and
is an indicator of the high degree of ordering of the intermo-
lecular P3HT chains, despite the presence of the covalently
bonded amorphous PMP segment.
FIGURE 6 DSC profile of PMP and PMP-b-P3HT. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
CONCLUSIONS
Poly(1,3-dibutoxy-m-phenylene) (PMP) with a high MW and
a very low PDI of 1.07 was successfully synthesized via the
GRIM polymerization. PMP showed a relatively high thermal
stability and good solubility in the common organic solvents.
Furthermore, the PMP-b-P3HT diblock copolymer was syn-
thesized by sequential GRIM polymerization. It is quite im-
portant to set the addition order of the monomers for the ef-
ficient crossover reactions and postpolymerization of the
second monomer. The diblock copolymer, PMP-b-P3HT,
exhibited two endothermal transitions corresponding to the
Tg of the PMP blocks and the Tm of the P3HT blocks, respec-
tively, which is indicative of an amorphous–crystalline nature
and the existence of microphase-separated domains. The for-
mation of the microphase-separated nanostructures was also
supported by AFM observations.
This work was partially supported by the Japan Science and
Technology Agency (JST) PRESTO program. This work was
also supported in part by Japan and Research Fellowships of
the Japan Society for the Promotion of Science for Young
Scientists.
FIGURE 7 AFM image of PMP-b-P3HT on mica substrate cast
from toluene.
SYNTHESIS OF PMP AND PMP-b-P3HT, OHSHIMIZU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
17 Holder, E.; Tessler, N.; Rogach, A. L. J Mater Chem 2008,
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