New evidence supporting the mechanism of Gilch polymerization from an extremely twisted biphenyl monomer

Article abstract of DOI:10.1039/c2cc33008c

A p-biquinodimethane species formed during the Gilch polymerization of an extremely twisted biphenyl monomer has been identified by in situ ¹H-NMR spectroscopy for the first time. From the identification of chain ends, we conclude that the chain growth during Gilch polymerization of biphenyl type monomers is both radical and anionic in nature.

Full text of DOI:10.1039/c2cc33008c

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The existence of p-biquinodimethane species was confirmed by in-situ H NMR
experiment. It suggests that the mechanism of Gilch polymerization is also valid for
an extremely twisted biphenyl monomer.

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ARTICLE T
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P
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New Evidence Supporting the Mechanism of Gilch Polymerization from
an Extremely Twisted Biphenyl Monomer
Jyh-Chien Chen,* Chi-Jui Chiang, Jen-Chun Chiu and Jyh-Jong Ju
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A
p-biquinodimethane species formed during Gilch
or reported.
polymerization of an extremely twisted biphenyl monomer
1
has been identified by in-situ H-NMR spectroscopy for the
1
0
first time. From the identification of structure defects, we
conclude that the chain growth reactions during Gilch
polymerization of biphenyl type monomers are both free-
radical and anionic in nature.
50
Since the discovery of electrical conductivity of doped
polyacetylene in the last century, much effort has been focused
on the synthesis of new π-conjugated polymers and investigations
on their opto-electronic properties associated with versatile
Scheme 1 Formation of p-quinodimethane and motive of this research.
1
2
2
3
3
4
4
5
0
5
0
5
0
5
1
In this communication, we report the in-situ H-NMR evidence
of p-biquinodimethane structure that formed during Gilch
polymerization of an extremely twisted biphenyl monomer, 2,2’-
1
applications in organic electronics. Among these π-conjugated
polymers, poly(p-phenylenvinylene) (PPV) is the first polymer 55 bis(trifluoromethyl)-4,4’-bis(bromomethyl)biphenyl (BTBB). We
2
reported to exhibit electroluminescence. Therefore, its synthetic
procedures have been studies in detail. Many organic reactions
also discuss the characteristics (anionic or radical) and defects
that formed during chain growth reaction.
This monomer (BTBB) was designed to be extremely twisted
by the intense steric hindrance resulted from the bulky
3
4
5
6
7
8
such as sulfinyl, sulfonyl, Wessling, Gilch, Heck, Suzuki,
1
0
11
12
ring-opening metathesis,
Wittig,
and Horner-Emmons
approaches have been exploited to prepare PPVs. Gilch 60 trifluoromethyl groups at the 2 and 2’ positions of 4,4’-biphenyl.
polymerization is one of the most widely used methods because
of the relatively simple treatment on easily accessible monomers
It was synthesized by a seven-step synthetic route as shown in
ESI† Scheme S2. Compounds (2) ~ (6) as shown in ESI†Scheme
1
4
(1,4-bis(halomethyl)benzene) with inexpensive t-butoxide as base.
S2 were prepared according to our previous publication. The
chemical structures and properties were well characterized (Fig. 1
No expensive catalysts are required.
Although the issue regarding the anionic or radical chain- 65 and ESI†). Gilch polymerization was carried out through the
growth mechanism of Gilch route is still a central debate, it is
generally accepted that the first step in Gilch polymerization is
general process (ESI†) to obtain PBPV-1 (yield: 60%, Mn:
149,000, PDI: 2.19 (ESI† Table S1)). The solubility and thermal
properties of PBPV-1 are summarized in ESI† Table S2 and S3.
It is generally believed that the first and the vital step in the
1
,6-dehydrohalogenation of 1,4-bis(halomethyl)benzene-type
monomer, leading to the formation of p-quinodimethane species
6
as shown in Scheme 1 and ESI† Scheme S1. Consequently, the 70 Gilch polymerization is 1,6-dehydrohalogenation of monomer,
feasibility to form p-quinodimethane species is obviously vital in
Gilch polymerization. The existence of the p-quinodimethane
species has been reported by Rehahn and co-workers using in-situ
leading to the formation of p-quinodimethane species. And the
diradical-type initiator is formed by the spontaneous self-
dimerization of 2 moles of p-quinodimethanes. This active
species then polymerized immediately to generate the non-
12
NMR spectroscopy.
A few articles regarding Gilch- and Gilch-like-polymerized 75 eliminated prepolymer. p-Quinodimethane can also be initiated
PPVs with biaryl and hetero-biaryl moieties have also been
by t-BuOK present in the reaction mixture to form the
prepolymer through anioic polymerization. The prepolymers
were finally converted into PPV by macromolecular elimination
13
published. It is reported that a dihedral angle is generated by the
steric hindrance between the two neighboring rings. If the
formation of p-biquinodimethane species is the necessity of Gilch
polymerization as suggested by the mechanism, the non-coplanar
biaryls and hetero-biaryls would be forced to form the coplanar p-
biquinodimethane species by destroying the aromaticity and
overcomimg the large steric hindrance as shown in Scheme 1.
However, p-biquinodimethane species has never been observed
6
cascade (ESI† Scheme S1).
80
As far as we know, all the mechanistic studies of Gilch
6
,12
polymerization were based on the phenyl-type monomers. In
the case of BTBB, the biphenyl is extremely twisted. We are
curious about the possibility of forming the coplanar p-
biquinodimethane species by defying aromaticity and extremely
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large steric hindrance. However, the successful preparation of the
the existence of p-biquinodimethane species as suggested by the
corresponding twisted PBPV-1 by Gilch polymerization implied
mechanism (Scheme 1 and ESI† Scheme S3).
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5
1
Fig. 1 In-situ H-NMR spectra at -50 ℃ with different reaction time in THF-d
8
.
In order to prove the existence of p-biquinodimethane species
p-biquinodimethane species might have been converted to the
during Gilch polymerization of BTBB, we monitored the
polymerization with in-situ H-NMR experiments at -50 ℃. 45 restore aromaticity and alleviate the steric hindrance by forming a
polymer before measurement started. The large driving force to
1
1
1
2
2
3
3
4
0
5
0
5
0
5
0
Monomer BTBB (20 mg, 0.04 mmol) in THF-d (0.4 mL) was
added into an NMR tube and pre-cooled at -50 ℃. The pre-
cooled base solution (t-BuOK, 18 mg, 0.16 mmol in 0.8 mL of
non-coplanar polymer conformation might be responsible for the
short life time.
8
THF-d , at -50 ℃) was injected slowly into the tube, which was
8
1
then transferred into the pre-cooled NMR spectrometer. H-NMR
spectra were recorded at different time intervals. Fig. 1 shows
some of the spectra. Spectrum at the start (0 min) demonstrates
the chemical structure of monomer BTBB. After 7 min, some
new peaks are detected between δ= 5.0 and 7.5 ppm. These new
peaks become more prominent as absorption peaks attributed to
BTBB monomer diminish with time. At 14 min, except the new
peaks, all the peaks attributed to BTBB monomer disappear. The
newly formed peaks were gradually replaced by a set of peaks
attributed to PBPV-1 from 21 min to 30 min.
Closer examination on the chemical shifts, splitting, coupling
constants and integral area of peaks appearing in spectrum at 14
min confirms the chemical structure of p-biquinodimethane
species as α-bromo-2,2’-bis(trifluoromethyl)-p-biquinodimethane
Fig. 2 In-situ UV-vis spectra with different reaction time in THF at -20 ℃.
In order to reconfirm the existence of p-biquinodimethane,
UV-vis spectrometer was also used to monitor the Gilch
polymerization of BTBB in THF at -20 ℃. Fig. 2 shows the UV-
5
0
-
4
vis spectra of monomer BTBB (10 M in THF) in the presence of
t-BuOK (4 equivalents) recorded at different reaction time.
(
BTBQ). This is the first time that p-biquinodimethane species is
1
55 Immediately after the addition of t-BuOK solution to the BTBB
in anhydrous THF, a new absorption peak around 473 nm was
observed. This peak quickly diminished, followed by the
simultaneous growth of absorption peak resulted from the formed
polymer PBPV at 321 nm with the reaction time. After 8 min,
observed and identified by H-NMR spectrum. It proves that p-
biquinodimethane species can be formed regardless the
aromaticity and steric hindrance. It further confirms the validity
of the mechanism of Gilch polymerization, not only for phenyl-
type monomers but also for biphenyl-type monomers even with
large steric hindrance. On the contrast to Gilch polymerization of
phenyl-type monomers, there are no peaks that could correspond
60
only absorption peak of PBPV was observed. Thus, the
absorption peak at 473 nm can be assigned to the p-
biquinodimethane species BTBQ due to longer conjugation
length. The results of UV-vis experiment combined with in-situ
15
to the cyclic byproducts were observed in Fig. 1.(ESI†)
An unsuccessful attempt to observe the p-biquinodimethane
1
1
H-NMR examination are the direct evidence to support the
species in temperature-dependent H-NMR experiments (from -
65
proposed mechanism of Gilch polymerization, even for monomer
with extremely twisted biphenyl structure.
The nature of chain propagation in Gilch polymerization
remains to be controversial until today. Some authors are in
7
0 ℃ to 0 ℃ with an interval of 10 ℃) might be due to the
relatively short life-time of p-biquinodimethane species. The life
time might be shorter than the time required for NMR
spectrometer to reach thermal equilibrium. Therefore, the formed
2
| Journal Name, [year], [vol], 00–00
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favour of the free-radical mechanism while others anionic one.
reactions of Gilch polymerization for an extremely twisted
Therefore, we added free-radical scavenger 2,2,6,6- 40 biphenyl monomer should be free-radical and anionic as well.
1
tetramethylpiperidine-N-oxyl (TEMPO) during the Gilch
polymerization of BTBB to corroborate the free-radical pathway.
As shown in ESI† Table S1, the molecular weight and yield of
PBPV-2 (Mn=15,000 g/mol, yield=42%) decrease drastically
In conclusion, we successfully obtain the direct in-situ H-
NMR evidence of the twisted biphenyl-based p-biquinodimethane
View Online
5
species at -50 ℃. To the best of our knowledge, it is the first
article to prove the feasibilities to form coplanar p-
when 0.5 equivalent of TEMPO was added. Furthermore, when a 45 biquinodimethane BTBQ. It proves that p-biquinodimethane
large excess of TEMPO (1.5 equivalents) was added, only oil-like
oligomer with molecular weight less than 1,000 g/mole can be
obtained. Their SEC chromatograms are shown in ESI† Fig. S1.
species can be formed regardless the aromaticity and steric
hindrance. It further confirms the validity of the mechanism of
Gilch polymerization, not only for phenyl-type monomers but
also for biphenyl-type monomers even with large steric hindrance.
Moreover, from the identification of aldehyde, t-butoxyl, and
benzyl chain ends combined with TEMPO-equivalent dependent
molecular weight, it is concluded that the Gilch polymerization of
twisted BTBB monomer proceeds simultaneously by free-radical
and anionic chain growth.
1
0
50
55
Notes and references
Department of Materials Science and Engineering, National Taiwan
University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei
1
0607, Taiwan. E-mail: jcchen@mail.ntust.edu.tw;
60 Fax: + 886 2 27376544 Tel: + 886 2 27373141 #6526
Electronic Supplementary Information (ESI) available: Experimental
1
Fig. 3 H-NMR spectrum of PBPV with 1.5 equivalent of TEMPO in
†
7
DMF-d .
details, synthesis, polymerization, spectroscospic characterization,
solubility and thermal properties. See DOI: 10.1039/b000000x/
1
Fig. 3 shows the H-NMR spectrum of this oil-like oligomer.
A peak at δ=10.18 ppm, corresponding to aldehyde group, was
observed. The radical chain end generated by Gilch
polymerization can react with TEMPO as shown in Fig. 3. The
formed nitroxide is not stable and will lead to the formation of
aldehyde chain end. The TEMPO equivalent-dependent
molecular weights and the formation of aldehyde chain end
provide us the strong evidence of free-radical chain growth in
Gilch polymerization.
1
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7
.
0
1
2
3
3
5
0
5
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1
1
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have never been reported before. By investigating their chemical
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4
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This journal is © The Royal Society of Chemistry [year]
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The existence of p-biquinodimethane species was confirmed by
1
in-situ H NMR experiment. It suggests that the mechanism of
5
Gilch polymerization is also valid for an extremely twisted
biphenyl monomer.
4
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]