Unusual cyclic polymerization through Suzuki-Miyaura coupling of polyphenylene bearing diboronate at both ends with excess dibromophenylene

Article abstract of DOI:10.1039/C6CC08333A

The Suzuki-Miyaura coupling polymerization of p-dibromophenylene and m-phenylenediboronic acid ester, as well as m-dibromophenylene and p-phenylenediboronic acid ester, and the combination of two meta-phenylene monomers in the presence of the t-Bu₃

Full text of DOI:10.1039/C6CC08333A

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DOI: 10.1039/C6CC08333A
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Unusual cyclic polymerization through Suzuki‐Miyaura coupling of
polyphenylene bearing diboronate at both ends with excess
dibromophenylene
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
Hajime Sugita, Masataka Nojima, Yoshihiro Ohta and Tsutomu Yokozawa
DOI: 10.1039/x0xx00000x
www.rsc.org/
Suzuki‐Miyaura coupling polymerization of p‐dibromophenylene
and m‐phenylenediboronic acid ester, as well as m‐
dibromophenylene and p‐phenylenediboronic acid ester, and the
combination of two meta‐phenylene monomers, in the presence of
t‐Bu PPd(0) catalyst selectively afforded cyclic polyphenylenes
3
through polyphenylene bearing boronate at both ends when excess
dibromophenylene was used.
1
Cyclic oligophenylenes were initially synthesized because of
2
their structural interest, but have recently attracted attention as
bipolar charge carrier transport materials for organic light-
3
emitting diodes (OLEDs). They can be synthesized stepwise, or
by one-step reaction followed by separation with HPLC or
column chromatography. In polymer synthesis, cyclic
oligophenylenes are obtained as by-products in the synthesis of
high-molecular-weight linear polyphenylenes. For example,
Sakamoto and coworkers conducted Suzuki-Miyaura
polycondensation of
m-bromophenylboronic acid ester by slow
4
Scheme 1. Cyclic polyphenylene obtained by unstoichiometric Suzuki‐Miyaura
coupling polymerization of 1.3 equiv. of 1a and 1.0 equiv. of 2a in the presence of
Pd catalyst 3 and CsF/18‐crown‐6.
monomer addition to suppress cyclization. From the opposite
point of view, however, selective synthesis of cyclic
polyphenylenes without formation of linear polymer has not
been explored.
We recently reported that
Miyaura condensation polymerization of dibromoarene and
arylenediboronic acid ester affords high-molecular-weight
conjugated polymers with boronate moieties at both ends, even
if excess dibromoarene is used.5 This unstoichiometric
polycondensation behavior was attributed to intramolecular
transfer of the Pd catalyst on dibromoarene. We subsequently
phenylenediboronic acid ester 2a; however, we found that the
3
t-Bu PPd-catalyzed Suzuki-
reaction afforded not polyphenylene with boronate chain ends,
7
but cyclic polyphenylene (Scheme 1). Here, we present details

-
of this selective cyclic polymerization of dibromophenylene
and phenylenediboronic acid ester in the presence of
Bu PPd(0) catalyst under unstoichiometric conditions. This
1
t-
2
3
unstoichimetric cyclic polymerization is intriguing, because
Kricheldorf and coworkers published a general overview of
-phenylene)]6
attempted to synthesize poly[(
by polymerization of excess
p-phenylene)-alt-(m
8
unstoichiometric polycondensation, in which they claimed that
p
-dibromophenylene 1a with m-
telechelic oligomers or extremely high-molecular-weight
polymers are obtained due to suppression of cyclization, in
contrast to the case of stoichiometric polycondensation, which
gave cyclic polymers in the final stage. However, we show here
that cyclic polymer is predominantly formed under
unstoichiometric conditions, rather than stoichiometric
a.
Department of Materials and Life Chemistry, Kanagawa University,
Rokkakubashi, Kanagawa‐ku, Yokohama 221‐8686 (Japan), E‐mail:
yokozt01@kanagawa‐u,ac.jp.
9
Electronic Supplementary Information (ESI) available: Detailed experiments, effect
of excess amount of 1a, polymerization with other catalysts, and attempts to
synthesize cyclic polyphenylenes by using ortho‐monomers (MALDI‐TOF mass conditions, contrary to Kricheldorf’s view.
spectra). See DOI: 10.1039/x0xx00000x
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Suzuki-Miyaura coupling polymerization of 1.3 equiv. of 1a
and 1.0 equiv. of 2a was first carried out in the presence of 5.0
View Article Online
DOI: 10.1039/C6CC08333A
3
t-Bu P-ligated Pd precatalyst 3 t-
,10 which generates
mol% of
Bu PPd(0) with the assistance of base, and CsF/18-crown-6 as a
3
base at room temperature for 24 h. The matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass
spectrum of the products (
n w n
M = 5400, M /M = 1.69) surprisingly
showed essentially one series of peaks due to cyclic poly[(
p
-
phenylene)-alt-(m
-phenylene)] (Fig. 1a).11 The H and 13C NMR
1
spectra showed only signals of repeat units (ESI). When
equimolar 1a and 2a were polymerized under the same
conditions, linear polyphenylenes with boronate at both ends
(
designated as PinBPh/BPin; PinB = pinacol boronate) and with
Fig. 2 M
n
and M
w
/M
n
values of products as a function of monomer concentration
(
[2a] ) in polymerization of 1.3 equiv. of 1a and 1.0 equiv. of 2a with 5.0 mol % of 3,
0
a hydrogen at one end and BPin at the other (H/BPin) were
formed, as well as cyclic polyphenylene (
1
CsF (4 equiv.), and 18‐crown‐6 (8 equiv.) in THF and water (THF/water = 3.0/0.1, v/v)
at rt for 24 h.
M
n
= 4850,
M
w
/
M
n
n
=
=
.51; Fig. 1b). On the other hand, when 1.4 equiv. of boronate
monomer 2a was used, boronate-terminal polyphenylene (
090,
M
kinds of polymer ends (Fig. S2 and S3). Consequently, it turned
-Bu PPd(0), which has high intramolecular transfer
2
M
w
/
M
n
= 1.14) was selectively obtained (Fig. 1c). It should out that
t
3
be noted that this polymerization protocol can selectively ability on aromatics, was the most appropriate catalyst for this
generate either linear polymer or cyclic polymer simply by unstoichiometric cyclic polymerization.
changing the feed ratio of the two monomers. In general, cyclic
In order to investigate the cyclic polymerization mechanism,
polymer12 is synthesized by using a cyclic initiator in chain- we followed the conversion of 2a and changes of products with
growth polymerization13 or by cyclization of end-functionalized polymerization time (Fig. 3). The MALDI-TOF mass spectra
polymers at high dilution.14
indicated that linear polyphenylene with PinBPh/BPin ends was
Polymerization of 1a and 2a resulted in cyclic polymer even predominantly formed until the conversion reached 89% (1 h).
at high monomer concentration, and the molecular weight of the This behavior was the same as in the case of unstoichiometric
cyclic polymer increased up to
concentration. The polydispersity approached 2.0 (Fig. 2).
M
n
= 8540 with increasing polymerization of excess
p
-dibromophenylene and 1.0 equiv. of
.5 Cyclic polymer and
p
-phenylenediboronoc acid ester with
3
Therefore, the molecular weight of the cyclic polymer can be polymer with H/BPin ends were observed at 97% conversion (3
controlled to some extent by changing the monomer h), and both peaks then increased until 100% conversion (5 h).
concentration. As for other Pd catalysts often used in Suzuki- After 24 h, the products converged to cyclic polymer. Since the
3 4
Miyaura coupling polymerization, (PPh ) Pd afforded mainly H/BPin-ended polymer was converted to cyclic polymer, the
linear oligomer. XPhos Pd(0),15 generated from the hydrogen end was presumably formed by hydrolysis of a
corresponding precatalyst having the biphenylamino moiety with polymer-Pd-Br end during quenching with hydrochloric acid.
base, gave both cyclic polymer and linear polymers with several
Fig. 1 MALDI‐TOF mass spectra of the products obtained by the polymerization of
Fig. 3 Changes of products with polymerization time and conversion of 2a in the
1
=
a and 2a with 5.0 mol % of 3, CsF (4 equiv.), and 18‐crown‐6 (8 equiv.) in THF ([2a]
0
polymerization of 1.3 equiv. of. 1a and 1.0 equiv. of 2a with 5.0 mol % of 3, CsF (4
equiv.), and 18‐crown‐6 (8 equiv.) in THF ([2a]0 = 16.6×10‐3 M) and water
16.6×10‐3 M) and water (THF/water = 3.0/0.1, v/v) at rt for 24 h, followed by
quenching with 1 M hydrochloric acid: monomer feed ratio of 1a/2a is (a) 1.3/1.0,
b) 1.0/1.0, and (c) 1.0/1.4
(THF/water = 3.0/0.1, v/v) at rt: time, conversion of 2a, and M
1640; (b) 30 min, 83%, 1720; (c) 45 mim, 86%, 1810; (d) 1 h, 89%, 1940; (e) 3 h, 97%,
770; (f) 5 h, 100%, 3790; (g) 24 h, 100%, 4920.
n
are (a) 15 min, 72%,
(
2
2
| J. Name., 2012, 00, 1‐3
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On the basis of the changes of products with polymerization
time, we propose the following mechanism (Scheme 2). The
coupling polymerization of 1a and 2a proceeded via
intramolecular catalyst transfer on dibromo monomer 1a to
View Article Online
DOI: 10.1039/C6CC08333A
Table 1. Cyclic polymerization of 1.3 equiv. of 1 and 1.0 equiv. of 2 with 3
and CsF/18-crown-6.
a
Entry
1
1 b
2
Yield (%) c
83
M
n
(
M
w
/Mn) d
afford polyphenylene
A with PinBPh/BPin ends until diboronate
monomer 2a was almost completely consumed. Excess 1a
5610
reacted with the boronate chain end, followed by intramolecular
(1.86)
oxidative addition to generate Br-Pd/BPin-ended polymer
B. If
polymer has a favorable conformation for cyclization, the two
B
chain ends would react to yield cyclic polymer. Cyclic polymer
could be also formed by intramolecular reaction of the two chain
2
3
93
90
5030
(
1.72)
ends of polymer
between polymer
C
B
formed by intermolecular coupling reaction
s. However, the former cyclization would
mainly take place, because the molecular weight of polymer was
increased only slightly in the final stage of polymerization. Since
this polymerization proceeds through the formation of boronate-
2c
5320
terminal polymer
cyclization precursor polymer
A
, excess 1a should promote generation of
in the final stage. In addition,
(1.65)
B
kink structure of meta-monomer is crucial for cyclic
polymerization because the combination of para-monomers
gave linear polymer with BPin at both ends.5
Unstoichiometric cyclic polymerization of other phenylene
monomers was also examined under similar conditions (Table 1).
4
5
88
94
5600
2b
2a
(
1.64)
The reverse combination of monomers, i.e., 1.3 equiv. of
m-
dibromophenylene 1b and 1.0 equiv. of p-phenylenediboronic
acid ester 2b,10 resulted in cyclic polymer with similar molecular
2580
weight (Entry 1). Cyclic polymerization through intramolecular
(1.42)
catalyst transfer on dibromoarene
1 was not influenced by an
electron-donating substituent (1c, Entry 2) or an electron-
a
Polymerization of 1.3 equiv of 1 and 1.0 equiv .of 2 was carried out with 5.0
‐3
0
mol % of 3, CsF (4 equiv), and 18‐crown‐6 (8 equiv) in THF ([2] = 16.1×10
M) and water (THF/water = 3/0.1, v/v) at rt for 24 h, followed by quenching
RO
RO
RO
BPin
BPin
b
c
n
1a/3
n
with 1 M hydrochloric acid. 1.3 equiv. Isolated yield after preparative HPLC.
d
OR
OR
OR
Estimated by GPC based on polystyrene standards (eluent: THF).
A
BPin
withdrawing substituent (1d, Entry 3) on 1. Furthermore,
R = C6H13
L = t-Bu3P
Br
polymerization of acceptor dibromophenylene 1e and donor
phenylenediboronic acid ester 2b similarly afforded cyclic
polymer (Entry 4), although the combination of acceptor
dibromoarene and donor arylenediboronic acid ester resulted in
low-molecular-weight polymer in the unstoichiometric Suzuki-
Miyaura coupling polymerization that we previously reported.5
Pd
L
RO
RO
RO
BPin
n
n
OR
OR
OR
OR
Furthermore,
we
examined
polymerization
of
m-
dibromophenylene and
m
-phenylenedioronic acid ester. Cyclic
Pd Br
L
RO
B
polymer was not clearly detected by MALDI-TOF mass
spectrometry in the polymerization of 1.3 equiv. of 1c and 1.0
equiv. of 2a, but the use of dialkoxy-substituted 1f instead of 1c
OR
PinB
Pd Br
L
m
RO
resulted in cyclic polymer formation (Entry 5). When
o-
dibromophenylene and -phenylenediboronic acid ester were
o
RO
RO
RO
RO
BPin
n+1
used in any combination of monomers, linear polymer was
mainly obtained, along with some cyclic oligomers (Fig. S9-
S12).
n+1
OR
OR
OR
OR
In conclusion, we have demonstrated that Suzuki-Miyaura
Pd Br
L
coupling polymerization of meta
combinations of monomers in the presence of
catalyst, which has a high propensity for intramolecular
/
para
-
and meta
-Bu PPd(0)
-face
/meta
m
m
C
t
3
Scheme 2. Proposed mechanisms for cyclic polymerization of excess 1a and 1.0
equiv. of 2a with Pd catalyst

transfer, selectively affords cyclic polyphenylenes when excess
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dibromophenylene is used, in contrast to similar Suzuki-Miyaura 12 H. R. Kricheldorf, J. Polym. Sci., Part A: Polym.ViCewhAermtic.l,e2O0n1lin0e,
8, 251.
H. R. Kricheldorf, S.-R. Lee and N. Schittenhelm, Macromol.
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4
DOI: 10.1039/C6CC08333A
coupling polymerization of the para/para combination of
monomers, which results in linear high-molecular-weight
polyphenylene with boronate moieties at both ends. This cyclic
1
3
Busch, J. Org. Chem., 1998, 63, 5746; H. R. Kricheldorf, S.-
R. Lee, S. Eggerstedt, D. Langanke and K. Hauser, ACS Symp.
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proceeded
irrespective
of
monomer
concentration, and the molecular weight of the cyclic polymer
increased with increasing monomer concentration. MALDI-TOF
mass spectra of the products obtained during polymerization of
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5964;
H. Kudo, M. Sato, R. Wakai, T. Iwamoto and T.
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polyphenylene with boronate ends, formed until 2a was almost
completely consumed, would undergo cyclization by reaction of
the boronate polymer ends with excess 1a in the final stage.
Since this cyclic polymerization is hardly influenced by
electronic substituent effects on the monomers, it should be
useful in the design and synthesis of many kinds of cyclic
polyphenylenes for application to OLEDs. Extension of this
unstoichiometric cyclic polymerization to other aromatics and
applications to OLEDs are under investigation.
14
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Acknowledgements
15
This study was supported by a Grant in Aid (No. 15H03819) for
Scientific Research from the Japan Society for the Promotion of
Science (JSPS) and by the MEXT-Supported Program for the
Strategic Research Foundation at Private Universities (No.
S1311032), 2013-2018.
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The MALDI-TOF mass spectra of products obtained by other
excess amounts of 1a are shown in Figure S1 in Electronic
Supplementary Information.
4
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