Vol.2, No.8, 803-808 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Oxidative coupling polymerization of p-alkoxyphenols
with Mn(acac)2-ethylenediamine catalysts
Soichiro Murakami1, Yuuta Akutsu2, Shigeki Habaue3*, Osamu Haba2, Hideyuki Higashimura4
1Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, Yonezawa,
2Graduate Program of Human Sensing and Functional Sensor Engineering, Graduate School of Science and Engineering, Yamagata
University, Yonezawa, Japan
3Faculty of Health and Nutrition, Shubun University, Ichinomiya, Japan; *Corresponding Author: habaue@shubun-ac.jp
4Tsukuba Laboratory, Sumitomo Chemical Co. Ltd., Tsukuba, Japan
Received 24 May 2010; revised 28 June 2010; accepted 5 July 2010.
The oxidative coupling polymerization of p-
alkoxyphenols with Mn(acac)2-ethylenediamine
catalysts was carried out. The polymerization of
p-methoxyphenol with the manganese(II) ace-
tylacetonate [Mn-(acac)2]-N,N’-diethylethylene-
diamine catalyst in CH2Cl2 at room temperature
under an O2 atmosphere afforded a polymer,
which mainly consists of the m-phenylene unit,
whereas the polymer obtained with Mn(acac)2
was rich in the oxyphenylene structure. The
polymer yield and regioselectivity were signifi-
cantly affected by the monomer and catalyst
structures. The former catalyst system was also
used for the coupling reaction of 2-methoxy-
4-methylphenol. The corresponding carbon-car-
bon coupling product was isolated with a re-
gioselectivity of 95%.
Keywords: Oxidative Coupling Polymerization;
Phenol; Polyphenylene; Manganese Catalyst;
Phenolic polymers bearing a polyphenylene main chain
structure have been mainly synthesized by the transi-
tion-metal-catalyzed coupling reactions of aryl halides,
such as the Wurtz coupling, Ullmann reaction, Ku-
mada-Tamao-Corriu coupling, etc [1-4]. These reactions
are suitable for the regio and/or coupling selective for-
mation of carbon-carbon bonds between aromatics.
However, from the viewpoint of a convenient and green
chemical method, they have some problems, such as the
synthesis of aryl halides, protection of the hydroxyl
group, and disposing of a large amount of the metal hal-
ide from the reaction system.
On the other hand, the catalytic oxidative coupling
polymerization (OCP) of 2,6-dimethylphenol is industri-
ally utilized for the synthesis of poly(2,6-dimethyl-1,4-
phenylene ether) (PPE), which is one of the most com-
mon engineering plastics [5-7]. The OCP is known as the
environmentally conscious method, due to the fact that
the reaction proceeds under mild conditions producing
only water as the by-product. Meanwhile, the OCP me-
diates a free radical coupling process; therefore, it is
generally very difficult to control the coupling regiose-
lectivity of the phenoxy radicals without producing a
branched chain. For example, the OCP of p-substituted
phenols 1 generally affords a polymer composed of a
mixture of the phenylene (CC
) and oxyphenylene (CO)
units (Scheme 1).
The highly regiocontrolled polymers having a poly(m-
phenylene) skeleton should have a conjugated higher-
order structure which is applicable as novel electronic
and electrochemical materials [8-11]. For instance, a
unique conformation change caused by the cisoid and
transoid main chain structures was reported [1], and a
helix-induction in a chiral environment was also achieved
[2]. The precise coupling regiocontrol of the phenoxy
radicals during the OCP will significantly contribute to
the facile synthesis of novel phenolic polymer materials.
Scheme 1. OCP of 1.
S. Murakami et al. / Natural Science 2 (2010) 803-808
Copyright © 2010 SciRes. OPEN ACCESS
Studies of the catalyst systems for the regioselective
OCP leading to a poly(phenylene ether) or poly(pheny-
lene) derivative, such as the enzymatic and enzyme-
model metal ones, and the copper-amine immobilized on
mesopores, have been reported [12-16]. We also reported
that the OCP of the bifunctional p-alkoxyphenol mono-
mer 2 (Scheme 2) using the commercially available and
typical copper catalyst, di-
-hydrox o-bis[(N,N,N’,N’-
tetramethylethylenediamine)copper(II)] chloride [CuCl
(OH)-TMEDA] proceeds in a regioselective manner to
afford a polymer with a CC-unit selectivity of up to 88%
[17]. However, the reigoselectivity has still not been
sufficiently controlled.
p-Alkoxyphenol is one of the attractive phenolic
monomers, because its polymer possesses the poly(hy-
droquinone) structure, which is a typical redox-active
polymer. In this study, the OCP of the p-alkoxyphenols 1
with various metal catalysts was investigated, and novel
manganese (II) acetylacetonate [Mn(acac)2]-ethylenedia-
mine catalysts for the CC-selective coupling formation
were found.
2.1. Materials
The monomers, p-methoxyphenol (1-OMe, Kanto), 4-
tert-butoxyphenol (1-OtBu, TCI), p-hydroxybenzoic
acid methyl ester (1-CO2Me, TCI) (Scheme 2), and 2,
were purchased or synthesized as previously reported
[17]. Mn(acac)2, Mn(acac)3 (Wako), manganese(II) ace-
tate [Mn(OAc)2] (Kanto), VO(acac)2, and Co(acac)2
(TCI) were used as received. The dry solvents, CH2Cl2,
tetrahydrofuran (THF), MeOH, and N,N-dimethylfor-
mamide (DMF) (Kanto), were employed for the oxida-
tive coupling. The diamines (Scheme 2) were used
without further purification.
2.2. Polymerization
A monomer was added to a mixture of Mn(acac)2 and
ethylenediamine ([monomer]/[Mn(acac)2]/[ethylenedia-
mine] = 1/0.08/0.08) in a solvent (0.6 M), and the mix-
ture was stirred at room temperature under an O2 at-
mosphere. The product was isolated as the MeOH-1N
HCl (10/1 (v/v))-insoluble part by centrifugation and
drying under reduced pressure at 50. The regioselec-
tivity (CC/CO) of the obtained polymers was estimated
from H2 volume generated by adding a polymer solution
in THF to a mixture of an excess amount of lithium alu-
minum hydride (LiAlH4) in THF [10,12-14,17].
2.3. Measurements
The 1H NMR spectra were measured using a Varian
Unity-Inova (500 Mz) or Mercury 200 (200 MHz) spec-
Scheme 2. Monomers and ligands.
trometer in CDCl3. The infrared (IR) spectra were re-
corded using a HORIBA FT-720 spectrometer. The ul-
traviolet (UV) absorption spectra were taken by a
JASCO V-630 spectrophotometer. The size exclusion
chromatographic (SEC) analyses were performed using a
JASCO PU-2080-Plus equipped with a JASCO UV-
2075-Plus detector and Shodex AC-8025 and TSK-GEL
columns connected in series [eluent: CHCl3, flow rate =
1.0 mL/min]. Calibration was carried out using standard
3.1. OCP with Manganese Catalyst
The OCP of 1-OMe with various catalysts was carried
out. The results are summarized in Table 1. The re-
gioselectivity of CC/CO could not be estimated from the
1H NMR spectra, because the peaks of the aromatic rings
and hydroxyl group were broad and overlapped, as re-
ported in previous studies [10,12-14,17]. Therefore, it
was evaluated by titration of the hydroxyl group of the
The polymerization with VO(acac)2 or Co(acac)2, the
former of which was effective as a catalyst for the OCP
of 2,3-dihydroxynaphthalene [18-20], in CH2Cl2-MeOH
[7/1 (v/v)] at room temperature under an O2 atmosphere
did not proceed (entries 1 and 2), whereas Mn(acac)2
showed a catalytic activity to afford a polymer as a
methanol-1N HCl [10/1 (v/v)]-insoluble fraction in 69%
yield with a regioselectivity (CC/CO) of 24/76 (entry 3).
The polymerization in CH2Cl2 also resulted in good to
high yields, and the selectivity was slightly affected by
the solvent (entries 4 and 5). The polar solvents, such as
THF and DMF, however, prevented the production of a
polymer (entries 6 and 7). Therefore, the polymerization
solvent significantly influenced the catalytic activity
during the polymerization with Mn(acac)2. The polym-
erization with Mn(acac)3 also gave a polymer, although
the catalytic activity was lower than that of Mn(acac)2
(entry 8). This result suggests that the polymerization
S. Murakami et al. / Natural Science 2 (2010) 803-808
Copyright © 2010 SciRes. OPEN ACCESS
proceeds through the Mn(III) species generated by the
one-electron oxidation of the Mn(II) ones as mentioned
later. The counter anion also affected the catalyst per-
formance (entry 9).
3.2. OCP with Mn(acac)2-Diamine Catalyst
The OCP of 1-OMe with Mn(acac)2 in the presence of
various ethylenediamines ([Mn(acac)2]/[ethylenediamine]
= 1) in CH2Cl2 was then examined (Table 2). The OCP
with TMEDA resulted in a poor yield (entry 1). However,
the polymerizations with pyridine (2 equiv.) and the
ethylenediamines having primary and secondary amino
groups, such as N,N-diethylethylenediamine [NNDEEDA],
N,N’-diethylethylenediamine [DEEDA], N,N’-di-n-bu-
tylethylenediamine [DBEDA], and N,N’-diphenylethy-
lenediamine [DPhEDA] (Scheme 2), gave a polymer in
moderate to good yields, whose regioselectivity was
quite different from that of the polymer obtained without
the amine ligand, especially, the polymers obtained with
the ethylenediamine were rich in the CC-unit (entries
3-6). For example, the polymerization with DEEDA for
96 h gave a polymer in 47% yield with the regioselectiv-
ity (CC/CO) of 87/13.
The effects of the Mn(acac)2-DEEDA catalyst system
during the polymerization under various conditions were
further investigated (Table 3). Although the 24 h-po-
Table 1. OCP of 1-OMe with Various Catalystsa.
Entry Catalysta Solvent Time (h) Yield (%)b Mw × 10 –3 (Mw/Mn)c Selectivityd (CC/CO)
1 VO(acac)2 CH2Cl2-MeOHe 72 0 — —
2 Co(acac)2 CH2Cl2-MeOHe 72 0 — —
3 Mn(acac)2 CH2Cl2-MeOHe 48 69 5.8 (1.1) 24/76
4 Mn(acac)2 CH2Cl2 48 72 5.1 (1.2) 33/67
5 Mn(acac)2 CH2Cl2 96 91 5.6 (1.2) 28/72
6 Mn(acac)2 THF 48 0 — —
7 Mn(acac)2 DMF 48 0 — —
8 Mn(acac)3 CH2Cl2 48 40 4.6 (1.2) 31/69
9 Mn(OAc)2 CH2Cl2 48 0
aConditions: [catalyst]/[1-OMe] = 0.08, [1-OMe] = 0.6 M, temp. = room temperature, O2 atmosphere. bMeOH-1N HCl (10/1 (v/v))-insoluble part.
cDetermined by SEC in CHCl3 (polystyrene standard). dEstimated from the generated H2 volume by the reaction of the obtained polymer with LiAlH4.
eCH2Cl2/MeOH = 7/1 (v/v).
Table 2. OCP of 1-OMe with Mn(acac)2-Ethylenediamine Catalystsa.
Entry Catalysta Yield (%)b Mw × 10 –3 (Mw/Mn)c Selectivityd (CC/CO)
1 Mn(acac)2-TMEDA 7 — —
2 Mn(acac)2-2Pyridine 60 7.4 (1.3) 50/50
3 Mn(acac)2-NNDEEDA 77 4.4 (1.3) 56/44
4 Mn(acac)2-DEEDA 47 5.2 (1.2) 87/13
5 Mn(acac)2-DEEDA 37 5.7 (1.2) 68/32
6 Mn(acac)2-DPhEDA 62 5.6 (1.2) 79/21
aConditions: [catalyst]/[1-OMe] = 0.08, [1-OMe] = 0.6 M, solvent = CH2Cl2, temp. = room temperature, time = 96 h, O2 atmosphere. bMeOH-1N HCl
(10/1 (v/v))-insoluble part. cDetermined by SEC in CHCl3 (polystyrene standard). dEstimated from the generated H2 volume by the reaction of the
obtained polymer with LiAlH4.
Table 3. OCP of 1-OMe with Mn(acac)2-DEEDA under Various Conditionsa.
Entry Catalysta Time (h) Yield (%)b Mw × 10 –3 (Mw/Mn)c Selectivityd (CC/CO)
1 Mn(acac)2-DEEDA 24 14 4.4 (1.2) 92/8
2 Mn(acac)2-DEEDAe 96 0 — —
3 Mn(acac)2-DEEDAf 96 61 5.6 (1.6) 67/33
4 Mn(acac)2-DEEDAg 96 86 7.3 (1.4) 64/36
5 Mn(acac)2-2DEEDA 96 30 6.1 (1.4) 61/39
aConditions: [catalyst]/[1-OMe] = 0.08, [1-OMe] = 0.6 M, solvent = CH2Cl2, temp. = room temperature, O2 atmosphere. bMeOH-1N HCl (10/1 (v/v))-in-
soluble part. cDetermined by SEC in CHCl3 (polystyrene standard). dEstimated from the generated H2 volume by the reaction of the obtained polymer
with LiAlH4. eTemperature = 50. f[Catalyst]/[1-OMe] = 0.16. gSolvent = CH2Cl2/MeOH [7/1 (v/v)].
S. Murakami et al. / Natural Science 2 (2010) 803-808
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lymerization produced a polymer in a low yield, the re-
gioselectivity of the obtained polymer was CC/CO =
92/8 (entry 1). Accordingly, during the first stage of the
polymerization, a highly regioselective coupling reaction
should occur. The polymerization at 50 did not give a
polymer. The other polymerization conditions, such as
catalyst ratio and solvent, also affected the regioselectiv-
ity and catalytic activity (entries 2-4). When two equiva-
lents of DEEDA to Mn(acac)2 was used, both the poly-
mer yield and CC-unit ratio significantly decreased (en-
try 5), indicating that the 1:1 complex of Mn(acac)2 and
DEEDA should be an active species. Although this cata-
lyst system showed a lower catalytic activity than that of
the polymerization without the diamine, the CC-unit
selectivity was much higher.
The OCP of various monomers, such as 1-OtBu, 1-
CO2Me, 2, with the Mn(acac)2-DEEDA catalyst in
CH2Cl2 at room temperature for 96 h under an O2 at-
mosphere was also performed. The polymerizations of
1-OtBu and 1-COOMe did not afford a MeOH-1N
HCl (10/1 (v/v))-insoluble fraction, and the polym-
erization of 2 resulted in a trace yield. These results
suggest that the steric and/or electronic effects of the
p-substituent significantly influence the polymeriza-
Figure 1 shows the FT-IR spectra of poly(1-OMe)
with a unit ratio of (A) CC/CO = 28/72 and (B) 92/8. In
each spectrum, the absorptions due to the vibrations of
the O-H and C-O-C linkages were observed, indicating
that the polymers are composed of a mixture of CC- and
CO-units. The latter spectrum showed a much larger
phenolic O-H peak due to the fact that this polymer is
rich in the CC-unit [12].
Both polymers with CC/CO = 28/72 and 92/8 were
completely soluble in chloroform, acetone, THF, DMF,
and dimethyl sulfoxide, whereas insoluble in hexane and
methanol. However, the latter polymer was almost solu-
Figure 1. IR spectra of poly(1-OMe) with (A) CC/CO
= 28/72, and (B) CC/CO = 92/8.
ble in an equivolume mixture of methanol and a 2N
NaOH aqueous solution, while the former was insoluble.
In order to clarify the applicability of this Mn(acac)2-
DEEDA catalyst, the oxidative coupling reaction of 2-
methoxy-4-methylphenol 3, which can afford two cou-
pling dimer products, the CC-dimer and CO-dimer [21-23],
as well as the polymeric compounds, was investigated
(Scheme 3). The reaction was conducted under the same
reaction conditions as the polymerization in CH2Cl2 for
96 h at room temperature under an O2 atmosphere, and
the dimers were isolated by silica gel column chroma-
tography (hexane/AcOEt = 10/1). The coupling reaction
with only Mn(acac)2 afforded a 31% yield of the
CC-dimer and 10% yield of the CO-dimer, that is, the
regioselectivity (CC/CO) was 76/24. In the case of the
reaction using Mn(acac)2-DEEDA, the isolated yields
were 57% and 3%, respectively, giving a selectivity of
CC/CO = 95/5. The Mn(acac)2-DEEDA catalyst was
quite effective for the regioselective oxidative coupling.
The UV-Vis spectra of poly(1-OMe) and the obtained
dimeric products of 3 are shown in Figure 2. The maxi-
Scheme 3. Oxidative coupling reaction of 3.
Figure 2. UV-Vis spectra of poly(1-OMe) with (A)
CC/CO = 28/72, (B) CC/CO = 92/8, (C) CC-dimer, (D)
CO-dimer (in CHCl3).
S. Murakami et al. / Natural Science 2 (2010) 803-808
Copyright © 2010 SciRes. OPEN ACCESS
mum absorption wavelength was observed at 287 nm for
the polymer with CC/CO = 28/72, whereas the red-
shifted absorption with the maximum of 299 nm was
observed for the polymer with CC/CO = 92/8. This
should be explained by the extension of the -conjuga-
tion length due to the phenylene main chain structure for
the latter polymer. Actually, the CC-dimer of 3 also
showed a maximum absorption at 290 nm, which is
greater than that of the CO-dimer of 282 nm.
The plausible OCP mechanism with Mn(acac)2-
DEEDA was proposed as follows (Figure 3): The com-
plex of an in situ generated Mn(III) species and 1-OMe
causes the one-electron oxidation of the phenol to form a
species of Mn(II) and the phenoxy radical, which con-
certedly induces the regioselective intermolecular radi-
cal-radical coupling to produce the corresponding car-
bon-carbon coupling product [15]. The dissociated
manganese species are oxidized by dioxygen to regener-
ate the active Mn(III) species.
The OCP with Mn(acac)2-ethylenediamine catalyst sys-
tems, that regioselectively produces a polymer with the
poly(m-phenylene) backbone, was developed. The cata-
lytic activity and regioselectivity during the polymeriza-
tion were significantly affected by the monomer and
catalyst structures, and polymerization conditions. Espe-
cially, the Mn(acac)2-DEEDA catalyst showed a high
regiocontrol ability. The catalyst can be readily and sim-
ply prepared by mixing of the commercially available
Mn(acac)2 and DEEDA.
Figure 3. Plausible mechanism for OCP of 1-OMe with
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