Investigation of Mizoroki-Heck coupling polymerization as a catalyst-transfer condensation polymerization for synthesis of poly(p-phenylenevinylene)

Article abstract of DOI:10.1002/pola.27472

Mizoroki-Heck coupling polymerization of 1,4-bis[(2-ethylhexyl)oxy]-2-iodo-5-vinylbenzene (1) and its bromo counterpart 2 with a Pd initiator for the synthesis of poly(phenylenevinylene) (PPV) was investigated to see whether the polymerization proceeds in

Full text of DOI:10.1002/pola.27472

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Investigation of Mizoroki-Heck Coupling Polymerization as a
Catalyst-Transfer Condensation Polymerization for Synthesis of
Poly(p-phenylenevinylene)
Masataka Nojima, Ryosuke Saito, Yoshihiro Ohta, Tsutomu Yokozawa
Department of Material and Life Chemistry, Kanagawa University, 3–27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686,
Japan
Correspondence to: T. Yokozawa; (E-mail: yokozt01@kanagawa-u.ac.jp)
Received 16 August 2014; accepted 1 November 2014; published online 2 December 2014
DOI: 10.1002/pola.27472
ABSTRACT: Mizoroki-Heck coupling polymerization of 1,4-bis[(2-
ethylhexyl)oxy]-2-iodo-5-vinylbenzene (1) and its bromo coun-
terpart 2 with a Pd initiator for the synthesis of poly(phenylene-
vinylene) (PPV) was investigated to see whether the
sharply increased after that, indicating conventional step-
growth polymerization. The occurrence of step-growth poly-
merization, not catalyst-transfer chain-growth polymerization,
may be interpreted in terms of low coordination ability of H-
Pd(II)-X(tBu3P) (X 5 Br or I), formed in the catalytic cycle of the
Mizoroki-Heck coupling reaction, to p-electrons of the PPV
backbone; reductive elimination of H-X from this Pd species
polymerization proceeds in
a chain-growth polymerization
manner. The polymerization of 1 with tBu3PPd(Tolyl)Br (10)
proceeded even at room temperature when 5.5 equiv of
Cy2NMe (Cy 5 cyclohexyl) was used as a base, but the molecu-
lar weight distribution of PPV was broad. The polymerization
of 2 hardly proceeded at room temperature under the same
conditions. In the polymerization of 1, PPV with H at one end
and I at the other was formed until the middle stage, and the
polymer end groups were converted into tolyl and H in the
final stage. The number-average molecular weight (Mn) did not
increase until about 90% monomer conversion and then
with base would take place after diffusion into the reaction
C
V
mixture.
2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2015, 53, 543–551
KEYWORDS: catalyst-transfer condensation polymerization;
conjugated polymers; catalysts; MALDI-TOF mass spectra;
Mizoroki-Heck coupling reaction; poly(phenylenevinylene)
Turner and co-workers succeeded in controlled synthesis of
PPV by ring-opening metathesis polymerization of cyclo-
phane monomers.5 Junkers and co-workers carried out con-
trolled anionic and radical polymerization of quinodimethane
monomers containing a sulfinyl group, followed by thermal
elimination to obtain well-defined PPV.6,7 However, these
polymerization methods can afford only block copolymers of
PPV with different substituents.
INTRODUCTION p-Conjugated polymers have received much
attention due to potential applications in thin film transistors
(TFTs),1 organic light-emitting diodes (OLEDs),2 and photo-
voltaic cells.3 Among p-conjugated polymers, poly(p-phenyl-
enevinylene) (PPV) has been intensively investigated,
especially for application to OLEDs.2 General synthetic pro-
cedures for PPV include the Gilch route, Witting reaction,
and transition metal-catalyzed cross-coupling reactions.3
These polymerizations proceed through
a step-growth
On the other hand, development of catalyst-transfer conden-
sation polymerization (CTCP) with a transition metal catalyst
has made it possible to synthesize well-defined p-conjugated
polymers.8 We have proposed that this polymerization
involves intramolecular catalyst transfer on the polymer
backbone. Furthermore, block copolymers consisting of dif-
ferent p-conjugated polymers can also be obtained in one
pot.8(a,b) Therefore, if PPV could be synthesized by means of
CTCP, it would be easy to obtain well-defined p-conjugated
polymerization mechanism, and it is difficult to control
molecular weight and to obtain polymers with low poly-
dispersity. However, Galvin and co-workers prepared well-
defined PPV by means of stepwise synthesis, and they
found that OLED devices made from PPV with narrow pol-
ydispersity showed better external quantum efficiency as
compared to those made from PPV with broad polydisper-
sity.4 Therefore, precision synthesis of PPV is an important
research goal.
Additional Supporting Information may be found in the online version of this article.
C
V
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SCHEME 1 Proposed Pd-CTCP mechanism of (a) Suzuki-Miyaura coupling polymerization and (b) Mizoroki-Heck coupling
polymerization.
block copolymers of PPV and other p-conjugated polymers
such as poly(3-hexylthiphene), opening an entry into a new
class of p-conjugated materials.
In the previous report, we investigated Ni-catalyzed Kumada-
Tamao coupling polymerization and Pd-catalyzed Suzuki-
Miyaura coupling polymerization of phenylenevinylene
SCHEME 2 Synthesis of Monomers 1 and 2.
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monomers to see whether these polymerizations would pro-
ceed in the CTCP manner to afford well-defined PPV. However,
Ni-catalyzed Kumada-Tamao coupling polymerization was not
suitable for the synthesis of PPV. On the other hand, Pd-
catalyzed Suzuki-Miyaura coupling polymerization afforded
the corresponding PPV with high molecular weight within a
few minutes, but the molecular weight distribution was broad,
probably due to dehalogenation and disproportionation side
reactions.9
(Scheme 1). The present article reports an investigation of
Mizoroki-Heck coupling polymerization with Bu3PPd(Ar)Br as
a new candidate for CTCP leading to PPV.
t
EXPERIMENTAL
General
1H and 13C NMR spectra were obtained on JEOL ECA-600
spectrometers. The internal standard for 1H NMR spectra in
CDCl3 was tetramethylsilane (0.00 ppm) and the internal
standard for 13C NMR spectra in CDCl3 was the midpoint of
CDCl3 (77.0 ppm). Column chromatography was performed
on silica gel (Kieselgel 60, 230–400 mesh, Merck) with a
specified solvent. Commercially available dehydrated tetrahy-
drofuran (THF, stabilizer-free, Kanto), dehydrated diethyl
ether (Kanto), dehydrated dimethyl sulfoxide (DMSO)
(Kanto) and dehydrated N,N-dimethylformamide (DMF)
(Wako) were used as dry solvents. n-Butyllithium (1.6 M
In the present article, we focused on Mizoroki-Heck coupling
reaction for CTCP leading to PPV. This reaction does not
require metalation of monomers, unlike other cross-coupling
reactions, and therefore it is easy to prepare monomers.3
Furthermore, Mizoroki-Heck coupling polymerization has
recently been reassessed as an atom-efficient, greener syn-
thetic method involving direct arylation.10 Although direct
arylation has been employed for CTCP,11 the feasibility of
Mizoroki-Heck CTCP has not been investigated. Mastrorilli
and co-workers conducted Suzuki/Heck copolymerization of
dibromofluorene and potassium vinyl trifluoroborate for the
synthesis of poly(fluorenylene vinylene), and they observed
that the polymerization proceeded in a chain-growth poly-
merization manner until the middle stage, while the formed
oligomers were coupled in a step-growth polymerization
manner in the final stage.12 They proposed that the polymer-
ization of fluorenylene vinylene monomer generated in situ
might proceed in the CTCP manner, but the details of poly-
merization mechanism remained unclear.
t
solution in hexane, Kanto) was used as received. Bu3PPd(o-
tolyl)Br (10) was prepared as described.16 The Mn and Mw/
Mn values of polymers were measured on a Tosoh HLC-8020
gel permeation chromatography (GPC) unit (eluent, THF; cal-
ibration, polystyrene standards) with two TSK-gel columns
(2 3 Multipore HXL-M). Matrix-assisted laser desorption ioni-
zation time-of-flight (MALDI-TOF) mass spectra were
recorded on a Shimadzu/Kratos AXIMA-CFR plus in the
reflectron ion mode by use of a laser (k 5 337 nm). Dithra-
nol (1,8-dihydroxy-9[10H]anthracenone) was used as the
matrix for the MALDI-TOF mass measurements. Synthetic
procedures for Monomers 1 and 2 were described in Sup-
porting Information.
Mizoroki-Heck coupling reaction is generally carried out
under reflux condition,3 though Fu and co-workers reported
an exceptionally mild Mizoroki-Heck coupling reaction in the
presence of Cy2NMe (Cy 5 cyclohexyl) and Pd/tBu3P, which
proceeds even at room temperature.13 They proposed that
the catalytically active species might be PdPtBu3 (A). On the
other hand, it turned out that A has an intramolecular trans-
fer property in Suzuki-Miyaura coupling reactions,14 and we
General Procedure for Mizoroki-Heck Coupling
Polymerization
All glass apparatus was dried prior to use. Addition of
reagents to the reaction flask and withdrawal of small ali-
quots of the reaction mixture for analysis were carried out
via a syringe from a three-way stopcock under a stream of
nitrogen. A round-bottomed flask equipped with a three-way
stopcock was heated under reduced pressure, and then
cooled to room temperature under an argon atmosphere.
Monomer 1 or 2 (0.1 mM) and N,N-dicyclohexylmethylamine
(0.55 mM) were placed in the flask, and the atmosphere in
the flask was replaced with argon. Dry THF (0.1 mL) was
added to the flask via a syringe, and the mixture was
degassed with argon. A solution of 10 (0.005 mM, 5.0 mol
%) in dry THF (0.3 mL, degassed with argon) was added via
a syringe, and the reaction mixture was stirred at the
t
succeeded in Suzuki-Miyaura CTCP by using Bu3PPd(Ar)Br
as an external initiator.8(o–q) Therefore, it occurred to us that
Mizoroki-Heck polymerization with this Pd initiator might
proceed at room temperature in a CTCP manner.
However, the reaction mechanism of Mizoroki-Heck coupling
reaction differs from that of other cross coupling reactions,
such as Suzuki-Miyaura coupling reaction. In the case of Pd-
catalyzed Suzuki-Miyaura CTCP, palladium(0) complex A, which
has three unshared coordination sites, would be generated
after reductive elimination and would move to the terminal C-
X bond of the elongated monomer unit through coordination
of p orbitals of the polymer main chain. By contrast, in the
case of Mizoroki-Heck coupling polymerization, threefold coor-
dinated palladium(II) complex B is generated after carbopalla-
dation and b-hydrogen elimination.15 If reductive elimination
of HX from B occurs with coordination of the polymer main
chain, CTCP can proceed. On the other hand, if reductive elimi-
nation occurs after the diffusion of B into the reaction mixture,
because of low coordination ability of B, intermolecular trans-
fer of A will occur, resulting in step-growth polymerization
SCHEME 3 Polymerization of 1 with 10 for the synthesis of
PPV.
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TABLE 1 Polymerization of 1 with 5 mol % of 10 and Cy2NMe
Mizoroki-Heck Coupling Polymerization of Iodo
Monomer 1
Since Pd-CTCP is well-controlled at lower temperature,8(q)
polymerization of 1 with palladium initiator 10 ([1]0/
[10]0 5 20) was first carried out in the presence of 1.1 equiv
of Cy2NMe in THF at room temperature according to Fu’s
procedure (Scheme 3).13 However, the polymerization was
slow, and 1 remained even after 54 hours, resulting in
oligomer formation (Table 1, Entry 1). A higher monomer
concentration (increased from 0.025 M to 0.25 M) gave a
similar result (Entry 2). We next examined the polymeriza-
tion temperature. Following a report of Mizoroki-Heck cou-
pling polymerization that employed the same catalyst ligand
Equivalent
of base
Temperature
(oC)
Time
(h)
Mn
Entry
(Mw/Mn)a
1b
2c
3c
4c
5c
6c
1.1
1.1
1.1
1.1
5.5
5.5
rt
rt
54
81
1,630 (1.08)d
2,450 (1.33)d
11,260 (2.04)
4,850 (1.73)d
10,130 (2.16)
8,280 (2.07)
50
30
50
rt
125
288
1
3.5
a
Estimated by GPC based on polystyrene standards (eluent: THF).
b
c
[1]0 5 0.025 M.
[1]0 5 0.25 M.
Monomer remained.
and base,19 the polymerization of 1 was carried out at 50 C
ꢀ
d
for 5 days to afford high-molecular-weight PPV (Mn > 10000)
with a broad molecular weight distribution (Entry 3). To
obtain PPV with a narrower molecular weight distribution
through CTCP, the polymerization of 1 was then carried out
appropriate temperature (see Tables). After 1–312 h, 1 M
hydrochloric acid was added. The mixture was extracted
with chloroform, and the organic layer was washed with
water, dried over anhydrous MgSO4, and concentrated under
reduced pressure to give PPV.
ꢀ
at 30 C, resulting in a lower molecular weight and a slightly
narrower molecular weight distribution (Entry 4). Thus, it
turned out that the polymerization of 1 with 10 was very
sensitive to polymerization temperature.
RESULTS AND DISCUSSION
We next increased the amount of Cy2NMe from 1.1 to 5.5
equiv. This dramatically increased the rate of polymerization,
and the reaction was completed within 1 h at 50 ꢀC (Entry
5), implying that the rate-determining step of polymerization
is reductive elimination of HI from the Pd catalyst. Further-
more, the polymerization proceeded even at room tempera-
ture, and 1 was consumed within 3.5 h (Entry 6). To our
knowledge, there is no previous report of Mizoroki-Heck
coupling polymerization occurring at room temperature.
Unfortunately, however, the molecular weight distribution
was broad, contrary to our expectation.
Synthesis of Monomers
In general, Mizoroki-Heck coupling polymerization employs A2
and B2 type monomers, but investigation of CTCP requires AB
type monomers. We prepared iodophenylenevinylene Mono-
mer 1 and its bromo counterpart 2 to compare their reactiv-
ity (Scheme 2). To increase the solubility of PPV, 2-
ethylhexyloxy groups were introduced into both monomers.
Etherification of hydroquinone with 2-ethylhexyl bromide was
carried out according to the literature,17 and then two iodine
atoms were introduced. One of the iodines of 5 was reacted
with n-butyllithium, followed by DMF, resulting in conversion
to a formyl group. Finally, the formyl group of the obtained 6
was subjected to Wittig reaction to yield the iodine Monomer
1.18 The bromo Monomer 2 was similarly prepared from com-
mercially available 2,5-dibromohydroquinone 7.
The MALDI-TOF mass spectrum of PPV (Entry 6) obtained at
15 min showed several series of peaks, but the major peaks
can be assigned to PPV in which one end is a hydrogen atom
and the other is an iodine atom (designated as H/I); the tolyl
group of Pd initiator 10 was not introduced into the polymer
FIGURE 1 MALDI-TOF mass spectra of products obtained by polymerization of 1 in the presence of 5.5 equiv of Cy2NMe and 5.0 mol
% of 10 at room temperature in THF ([1]0 5 0.25 M) for (a) 15 min, (Mn 5 1570, Mw/Mn 5 1.25) and (b) 240 h, (Mn 5 9700, Mw/Mn 5 2.51).
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TABLE 2 Polymerization of 2 with 5 mol % of 10 and 5.5 equiv-
Accordingly, the Pd catalyst might be inactivated during pro-
longed reaction, resulting in lower molecular weight of PPV.
Therefore, it turned out that the iodo Monomer 1 is more
appropriate than the bromo Monomer 2, especially in
Mizoroki-Heck coupling polymerization at room temperature.
alent of Cy2NMe
Temperature
(oC)
Time
(h)
Mn
(Mw/Mn)a
Conv. of
2b (%)
Entry
1
2
rt
168
312
1,060 (1.24)
4,230 (1.99)
40
50
100
Polymerization of 1 in the Presence of Active Aryl Halide
We next conducted polymerization of 1 with Pd(0) complex
in the presence of active aryl halide 11. Because of the
higher reactivity of C-I of 11 than C-I of 1, Pd(0) would be
rapidly inserted into 11 to generate 11-Pd, followed by poly-
merization of 1, resulting in CF3C6H4-ended PPV (Scheme 4).
We considered that this might result in decreased polydis-
persity, even though intermolecular transfer of Pd(0) occurs
in the Mizoroki-Heck coupling polymerization.
a
Estimated by GPC based on polystyrene standards (eluent: THF).
Estimated by GC using naphthalene as an internal standard
b
substance.
end [Fig. 1(a)]. This result indicated that the polymerization
of 1 proceeded mainly by conventional step-growth polymer-
ization with intermolecular transfer of the Pd catalyst (chain
transfer). Surprisingly, however, in the polymerization for 10
days the polymer ends became almost uniformly Tolyl/H
[Fig. 1(b)]. Accordingly, the polymerization of 1 presumably
proceeded by step-growth polymerization, and then the H
atom of the vinyl end group of PPV was converted into the
tolyl group derived from 10 in the final stage. The other
hydrogen end might be formed by hydrolysis of the C-Pd-I
end or by deiodination of the polymer end as a side reaction
(see “Proposed Mechanism of Polymerization” below).
Polymerization of 1 was carried out with 0.05 equiv of
bis[tri(tert-butyl)phosphine]palladium(0) and 11 instead of
10 in the presence of 5.5 equiv of Cy2NMe. Contrary to our
expectation, the polymerization was slower than that using
10; 1 remained even after 19 days, and low-molecular-
weight PPV was formed.20 The reason for the slow polymer-
ization is not clear at present, but might be interpreted in
terms of low activity of this Pd(0) complex having two
(tert-Bu)3P ligands toward aryl halides. The MALDI-TOF
mass spectra of PPV obtained at 2 days showed mainly H/I
and an unidentified series of peaks [Fig. 2(a)]. Since the
latter peaks were also observed in the polymerization of
Mizoroki-Heck Coupling Polymerization of Bromo
Monomer 2
We next examined polymerization of bromo Monomer 2 to
examine the effect of halogen in the monomer. The polymer-
ization of 2 was first carried out at room temperature, but 2
remained even after 7 days (Table 2, Entry 1). At 50 ꢀC,
Monomer 2 was consumed after 13 days to afford PPV, the
molecular weight of which was less than half that in the
case of the polymerization of 1 at 50 ꢀC. The molecular
weight distribution was similarly broad (Entry 2). The
MALDI-TOF mass spectrum of the obtained PPV (Entry 2)
showed major H/Br peaks, indicating that polymerization of
2 was not initiated by the Pd initiator 10 and that conven-
tional step-growth polymerization took place (Supporting
Information Fig. S1).
1
with bis[tri(tert-butyl)phosphine]palladium(0) without
addition of 11, they could be due to end groups derived
from excess tri(tert-butyl)phosphine (Fig. S2). The PPV
obtained at 9 days showed only H/I peaks [Fig. 2(b)], while
that obtained at 19 days showed not only H/I peaks, but
also H/H and CF3C6H4/I as minor peaks.
Accordingly, it turned out that selective coupling reaction of
1 occurred even in the presence of reactive 11 and that
introduction of 11 at the polymer end required a long reac-
tion time. Van der Boom and coworkers demonstrated that
the reaction of Ni(PEt3)4 with a bromostilbene derivative
resulted in selective g2-C5C coordination, which was kineti-
cally preferable at low temperature, followed by intramolecu-
lar “ring walking” of the metal center and intramolecular
As compared with 1, slower polymerization is accounted for
by the lower oxidative addition reactivity of C-Br for Pd(0).
SCHEME 4 Proposed initiation of polymerization of 1 with 11 and Pd(0) catalyst.
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Proposed Mechanism of Polymerization
To investigate the polymerization mechanism, we followed
the monomer conversion, Mn, and changes of polymer ends
with polymerization time. The Mn did not increase until the
conversion of 1 reached 90%, but sharply increased after
that, indicating a conventional step-growth polymerization
mechanism (Fig. 3). The MALDI-TOF mass spectrum of PPV
obtained at 5.5
h (conversion of 1 5 90%, Mn 5 1380,
Mw/Mn 5 1.43) contained one major series of H/I peaks, as
well as a minor series of Tolyl/I peaks [Fig. 4(a,b)]. After
6 days (conversion of 1 5 100%, Mn 5 3730, Mw/Mn 5 1.95),
all of 1 was consumed, and the intensity of the Tolyl/I peaks
was increased [Fig. 4(c,d)]. The spectrum at 13 days (conver-
sion of 1 5 100%, Mn 5 7690, Mw/Mn 5 2.16) showed a
major series of Tolyl/I peaks and a minor series of Tolyl/H
peaks [Fig. 4(e,f)].
On the basis of the above results, we propose the following
reaction mechanism for the polymerization of 1 with 10
(Scheme 6). Polymerization is initiated by carbopalladation
of 1 with 10, followed by b-hydrogen elimination with con-
comitant generation of Pd(II) complex B. Since the coordina-
tion ability of
B toward C5C bond is low, reductive
elimination of HI takes place after the diffusion of B into the
reaction mixture to afford unimer and Pd(0) complex A
(Scheme 6, Step 1). The C5C bond of 1 coordinates to A and
induces intramolecular oxidative addition at the C-I bond,
followed by coupling reaction with another 1 to afford dimer.
After that, H/I-ended oligomers are formed in a step-growth
polymerization manner (Step 2). After consumption of 1, H/
I-ended oligomers react with Tolyl/I-ended oligomers to
afford Toly/I-ended PPV (Step 3). Finally, deiodination of
polymer partially occurs by hydrolysis of the C-Pd-I end of
PPV, which is generated by oxidative addition of the C-I end
of PPV with A (Step 4).
FIGURE 2 MALDI-TOF mass spectra of products obtained by
polymerization of 1 in the presence of 5.5 equiv of Cy2NMe
and 5.0 mol % of 11 and bis[tri(tert-butyl)phosphine]palladium
in THF ([1]0 5 0.25 M) at room temperature for (a) 49 h (conver-
sion of 1 5 53%, Mn 5 990, Mw/Mn 5 1.22), (b) 216 h (conversion
of 1 5 82%, Mn 5 1370, Mw/Mn 5 1.22), and (c) 456 h (conversion
of 1 5 96%, Mn 5 1370, Mw/Mn 5 1.40).
oxidative addition of aryl bromide, which was thermody-
namically preferable when the temperature was raised.21
This report, even though the central metal is different from
palladium, indicates that Pd(0) A might preferentially coordi-
nate to the C5C of 1 rather than be inserted into the C-I
bond of reactive 11, followed by intramolecular transfer and
insertion into the C-I bond of 1 (Scheme 5).
Thus, there appear to be two reasons why Mizoroki-Heck
coupling polymerization did not proceed in a CTCP manner.
First, Pd(II) complex B, formed in the catalytic cycle of
Mizoroki-Heck coupling reaction, does not have strong coor-
dination ability to the polymer backbone, and reductive elim-
ination of H-I from B presumably occurs after diffusion of B
into the reaction mixture. Second, A diffuses into the
SCHEME 5 Proposed mechanism of Mizoroki-Heck coupling polymerization of 1 with 11 and Pd(0) catalyst.
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reaction mixture, where it preferentially coordinates to the
C5C bond of 1, followed by intramolecular oxidative addi-
tion of C-I of 1, which would induce step-growth
polymerization.
CONCLUSION
The feasibility of CTCP by means of Mizoroki-Heck coupling
reaction for the synthesis of well-defined PPV was investi-
gated. The polymerization of 1-iodo-4-vinylbenzene Mono-
t
mer 1 with Bu3PPd(Tolyl)Br (10) proceeded even at room
temperature when 5.5 equiv of Cy2NMe was used. However,
the molecular weight distribution of the obtained PPV was
broad, and MALDI-TOF mass spectra indicated that PPV mol-
ecules with H/I end groups, which would not have been ini-
tiated by 10, were formed at the early stage, implying that
the polymerization proceeds via chain transfer of the
FIGURE 3 Mn values of products as a function of monomer
conversion in polymerization of 1 in the presence of 5.5 equiv
of Cy2NMe and 5.0 mol % of 10 in THF ([1]0 5 0.25 M) at room
temperature.
FIGURE 4 MALDI-TOF mass spectra of products obtained by polymerization of 1 in the presence of 5.5 equiv of Cy2NMe and
5.0 mol % of 10 in THF ([1]0 5 0.25 M) at room temperature for (a, b) 5.5 h (conversion of 1 5 90%, Mn 5 1380, Mw/Mn 5 1.43), (c, d)
6 days (conversion of 1 5 100%, Mn 5 3730, Mw/Mn 5 1.95), and (e, d) 13 days (conversion of 1 5 100%, Mn 5 7690, Mw/Mn 5 2.16).
(b), (d), and (f) are expanded (a), (c), and (e), respectively.
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SCHEME 6 Proposed mechanism of Mizoroki-Heck coupling polymerization of 1 with 10.
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catalyst. Furthermore, it turned out that selective coupling
reaction between the monomers occurred even in the pres-
ence of active 4-iodobenzotrifluoride. The relationship
between monomer conversion and Mn also supported a con-
ventional step-growth polymerization mechanism. The
observed lack of CTCP behavior in Mizoroki-Heck coupling
polymerization is presumably due to low coordination ability
of H-Pd-I complex B to the polymer backbone; Pd(0) com-
plex A, which enables Suzuki-Miyaura CTCP, is generated by
reductive elimination of H-I from B after diffusion into the
reaction mixture. Although the molecular weight distribution
of PPV obtained by Mizoroki-Heck coupling polymerization
of 1 was broad, the polymer end groups were controlled to
some extent.
ACKNOWLEDGMENTS
This study was supported by a Grant in Aid (No. 24550141) for
Scientific Research from the Japan Society for the Promotion of
Science (JSPS) and a Scientific Frontier Research Project Grant
from the Ministry of Education, Science, Sport and Culture,
Japan (MEXT).
9 M. Nojima, Y. Ohta, T. Yokozawa, J. Polym. Sci., Part A:
Polym. Chem. 2014, 52, 2643–2653.
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