Synthesis of poly(phenylene oxide) containing trifluoromethyl groups via selective and sequential nucleophilic aromatic substitution reaction

Article abstract of DOI:10.1021/ma034292a

A new poly(phenylene oxide) containing pendent trifluoromethyl groups (CF₃-PPO) was synthesized by the S_NAr reaction of 5-fluoro-2-nitrobenzotrifluoride with hydroquinone. The polymerization was carried out in a sequential and select

Full text of DOI:10.1021/ma034292a

Macromolecules 2003, 36, 3809-3811
3809
Sch em e 1. Selective a n d Sequ en tia l Disp la cem en t of
th e F lu or in e a n d th e Nitr o Gr ou p of
Syn th esis of P oly(p h en ylen e oxid e)
Con ta in in g Tr iflu or om eth yl Gr ou p s via
Selective a n d Sequ en tia l Nu cleop h ilic
Ar om a tic Su bstitu tion Rea ction
5-F lu or o-2-n itr oben zotr iflu or id e
Yu n J u n Kim , Im Sik Ch u n g, a n d Sa n g You l Kim *
Center for Advanced Functional Polymers, Department of
Chemistry and School of Molecular Science (BK21), Korea
Advanced Institute of Science and Technology (KAIST),
373-1, Guseong-Dong, Yuseong-Gu, Daejeon 305-701, Korea
Received March 7, 2003
The nucleophilic aromatic substitution reaction (SNAr)
has been known as a very effective one for the formation
of aromatic ether groups and utilized in the synthesis
of many organic and polymeric compounds.1 The SNAr
reaction generally requires a leaving group activated
with an electron-withdrawing group at the ortho or para
position. Typical leaving groups are fluorine, chlorine,
and nitro groups. The fluorine leaving group shows good
reactivity due to its small size and high electronegativ-
ity. A properly activated nitro group is also displaced
readily, but it produces a reactive nitrite ion as a
byproduct which causes side reactions at elevated
temperature.2 However, trifluoromethyl groups act as
an effective activating group for the nucleophilic nitro
displacement reaction at relatively high temperature
(190 °C) due to the inertness of perfluoroalkyl groups
to the nitrite ions.3 Interestingly, the meta-activated
nitro leaving group with CF3 group undergoes the SN-
Ar reaction.4 The above structural requirements for the
SNAr reaction put a limitation of this reaction in the
synthesis of the compounds with two ether linkages on
the same aromatic ring. For example, poly(phenylene
oxide) (PPO) consisting of benzene rings and para-linked
ether groups5 requires a monomer with two electron-
withdrawing groups6a or a reactive initiator with an AB
type monomer.6b Oxidative coupling7 or the Ullman
coupling reaction8 is another feasible way to obtain
para-linked PPO, but the structural variation through
oxidative coupling reaction is limited. 2,6-Disubstituted
phenol must be used to obtain a linear polymer, and
the incorporation of electron-withdrawing substituents
into the polymer main chain is difficult due to the high
oxidation potential of the corresponding phenolic mono-
mers, even though the regioselective oxidative polym-
erization of phenoxyphenol and 2,6-difluorophenol with
the peroxo dicopper complex has been reported re-
cently.9
To investigate the feasibility of selective and sequen-
tial displacement of the fluorine and the nitro leaving
group of 5-fluoro-2-nitrobenzotrifluoride, model reac-
tions were carried out at various reaction conditions as
shown in Scheme 1. The reaction with phenol in the
presence of K2CO3 in dimethyl sulfoxide (DMSO) quan-
titatively produced 4-nitro-3-trifluoromethyldiphenyl
ether (1) without any side reactions. The reaction was
complete within 12 h at 40 °C and 4 h at 80 °C. The
model reaction indicates that the nitro group is not
displaced with phenol below 100 °C, which is essential
for polycondensation because the fluorine group cannot
be displaced after the displacement of the nitro group.
The nitro group acts not only as a leaving group but
also as an activating group. The nitro displacement
reaction of 1 with phenol was conducted in the presence
of K2CO3 in DMSO/toluene at 150 or 170 °C. Despite
the electron-donating phenoxy group at the para posi-
tion to the nitro leaving group, conversion was complete
within 4 h at 150 °C and 1 h at 170 °C. Steric congestion
between the bulky trifluoromethyl group and the nitro
leaving group facilitates the formation of a stable
Meisenheimer complex with release of steric strain.11
The above model reaction results reveal that the selec-
tive and sequential displacement of the fluorine and the
nitro group of 5-fluoro-2-nitrobenzotrifluoride is feasible
at proper reaction conditions.
However, during the displacement of the nitro group
of 1, transetherification is expected to occur. The model
reaction of 1 with m-cresol at 150 and 170 °C produced
a mixture of four compounds (3a -d : R ) R′ ) H; R )
R′ ) CH3; R ) H, R′ ) CH3; R ) CH3, R′ ) H) despite
complete conversion. It seems that the electron-with-
drawing nitro group is strong enough to make the ether
linkage susceptible for the nucleophile. The results of
model reactions were confirmed by spectroscopic data.12
On the basis of the results of the model reactions,
synthesis of a new poly(phenylene oxide) containing
pendent trifluoromethyl groups (CF3-PPO) was at-
tempted by nucleophilic aromatic substitution of 5-fluoro-
2-nitrobenzotrifluoride with hydroquinone, as shown in
Scheme 2. The polymerization was carried out via the
one-pot method, but in a sequential mannersthe selec-
tive fluorine displacement followed by the nitro dis-
placement. Fluorine displacement of 5-fluoro-2-nitroben-
zotrifluoride with hydroquinone in the presence of
K2CO3 in DMSO at 80 °C for 4 h gave three kinds of
intermediates13 that were subjected to the nitro dis-
placement reaction at 170 °C for 18 h to give the off-
In this communication, we report the selective and
sequential nucleophilic aromatic substitution reaction
and its utilization in the synthesis of poly(phenylene
oxide) containing pendent trifluoromethyl groups from
AA and BC type monomers. The BC type monomer has
the fluorine leaving group activated by the nitro group
at the para position and the nitro leaving group acti-
vated by the trifluoromethyl group at the ortho position.
Another advantage of the CF3 activating group is that
the incorporation of fluorines into the polymer chain
provides some desirable properties such as low dielectric
constant, low moisture absorption, and good solubility.10
* To whom correspondence should be addressed: Tel 82-42-869-
2834; Fax 82-42-869-2810; e-mail kimsy@mail.kaist.ac.kr.
10.1021/ma034292a CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/08/2003

3810 Communications to the Editor
Macromolecules, Vol. 36, No. 11, 2003
1
Sch em e 2. Syn th esis of High Molecu la r Weigh t
P oly(p h en ylen e oxid e) (CF 3-P P O) via Selective a n d
Sequ en tia l SNAr Rea ction
While the H NMR spectrum of CF3-PPO did not show
any peaks corresponding to the proton of nitro-substi-
tuted benzene ring, it showed the peaks around 6.9 ppm
corresponding to the proton of terminal hydroquinone,
indicating the existence of hydroxyl end group in both
chain ends. The 13C NMR spectrum of CF3-PPO showed
all expected 13 carbon peaks. It has the structure with
randomly arranged trifluoromethyl groups because of
the formation of three kinds of intermediates during the
fluorine displacement reaction and transetherification
in the nitro displacement reaction. As a result, three
regioisomeric units, head-to-head, tail-to-tail, and head-
to-tail placements, exist in the polymer backbone, and
these local environments were reflected in the 13C NMR
resonance of the hydroquinone moiety.13
CF3-PPO, having inherent viscosity of 0.32 dL/g,
showed good solubility in common organic solvents, such
as acetone, toluene, THF, chlorobenzene, and CHCl3 as
well as polar aprotic solvents (NMP, DMAc, and DMSO).
The thermal stability of the polymer is similar to those
of fluoroalkyl-containing poly(arylene ether)s. The glass
transition temperature (Tg) of CF3-PPO measured by
DSC was 108 °C, which was slightly higher than the
literature value (ca. 95 °C) of poly(1,4-phenylene oxide).
The 5% weight loss temperature of CF3-PPO measured
by TGA was 529 °C in nitrogen and 499 °C in air.
Sch em e 3. Tr a n seth er ifica tion d u r in g th e Nitr o
Disp la cem en t Rea ction
In summary, a new poly(phenylene oxide) containing
pendent trifluoromethyl groups (CF3-PPO) was synthe-
sized by the SNAr reaction of 5-fluoro-2-nitrobenzotrif-
luoride with hydroquinone. The polymerization was
carried out in a sequential and selective manner based
on the preferential fluorine displacement reaction fol-
lowed by the nitro displacement reaction.
Ack n ow led gm en t. This work was supported by the
Center for Advanced Functional Polymers (CAFPoly) at
KAIST and Brain Korea 21 (BK21) project.
Su p p or tin g In for m a tion Ava ila ble: Details of the ex-
perimental procedures, the spectral data for all of the new
compounds, FTIR spectra, NMR spectra, TGA, and DSC
thermograms of the synthesized compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
white polymer. The weight-average molecular weight
of the polymer determined by GPC with polystyrene
standards was 32.1K, and the polydispersity index was
2.01. To our knowledge, this is the first example of
successful synthesis of high molecular weight poly-
(phenylene oxide) via the selective and sequential
nucleophilic aromatic substitution reaction.
Refer en ces a n d Notes
It seems that the cleavage of aryl ether bonds by the
phenoxide ions (transetherification) occurs mainly at the
polymer chain ends, where the ether linkages are
activated by the strong electron-withdrawing nitro
group at the para position. Therefore, the transetheri-
fication and the nitro displacement are competitive at
the polymer chain end, as shown in Scheme 3. Nitro
displacement at the chain end leads to the growth of
polymer chain, while transetherification at the same
position leads to the ether exchange, which generates
another reactive oligomeric phenoxide. However, the
equimolar ratio of two kinds of functional groups is
maintained, and formation of high molecular weight
polymer is feasible.
The structure of the synthesized polymer was con-
firmed by spectroscopic data.12 The FTIR spectrum of
CF3-PPO showed characteristic absorption bands of
aromatic ether at 1200 cm-1 and C-F stretching at 1134
cm-1. However, it did not show any absorption band
corresponding to nitro stretching. The 1H NMR spec-
trum of CF3-PPO was consistent with its structure.
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Macromolecules, Vol. 36, No. 11, 2003
Communications to the Editor 3811
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