Catenane gel: Synthesis of high molecular weight poly[2]catenanes by Sonogashira coupling polymerization

Article abstract of DOI:10.1016/j.tetlet.2012.03.101

A first catenane polymer gel was obtained by Sonogashira coupling reaction of diiodo[2]catenane and 4,4′-diethynylbiphenyl. Because of the linearity and rigidity of the ethynylene group formed in the coupling reaction, the formation of undesired cyclic dimer was inhibited, and a very high molecular weight (M_n > 2.3 × 10⁵) poly[2]catenane was synthesized. The polymer was gelated without any chemical cross-linkage and swelled in DMF.

Full text of DOI:10.1016/j.tetlet.2012.03.101

Tetrahedron Letters 53 (2012) 2805–2808
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catenane gel: synthesis of high molecular weight poly[2]catenanes by
Sonogashira coupling polymerization
⇑
Toshiki Hagiwara , Yasuhiro Murano, Yuki Watanabe, Toru Hoshi, Takashi Sawaguchi
Department of Materials and Applied Chemistry, College of Science and Technology, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A first catenane polymer gel was obtained by Sonogashira coupling reaction of diiodo[2]catenane and
4,40-diethynylbiphenyl. Because of the linearity and rigidity of the ethynylene group formed in the cou-
pling reaction, the formation of undesired cyclic dimer was inhibited, and a very high molecular weight
(Mn > 2.3 ꢀ 105) poly[2]catenane was synthesized. The polymer was gelated without any chemical cross-
linkage and swelled in DMF.
Received 3 February 2012
Revised 17 March 2012
Accepted 26 March 2012
Available online 29 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
[2]Catenane
Catenane polymer
Sonogashira coupling
Polymerization
Gels
Interest in novel polymers with interlocked structures such as
catenane or rotaxane in their main chain has increased consider-
ably over the last decade.1 Polyrotaxanes are studied widely for
their synthesis, properties, and application.2 In particular, the
slide-ring gel is known to exhibit many interesting properties
due to its interlocked structure.2e For catenanes, owing to their
higher degree of mobility, the development of novel properties
such as a very large loss modulus, low activation energy for viscous
flow, and rapid stress relaxation3 is expected with their introduc-
tion into polymers. However, examples of catenane polymers are
limited because of the many steps and low yield associated with
catenane preparation. In particular, polycatenanes composed only
of the catenane unit have not yet been synthesized. Poly[2]caten-
anes can be synthesized from [2]catenane (two-ring catenane),1a
but there are no reports on polymers with some properties charac-
teristic of the catenane structure.
To synthesize poly[2]catenane, functional [2]catenanes bearing
polymerizable functional groups on each ring are required. Because
the functional groups can be introduced to amide-type catenanes
by using commercially available 5-substituted isophthalic acids as
a starting material, we previously synthesized various functional
amide-type [2]catenanes and poly[2]catenanes.4,5 However, cyclic
dimerization took place in competition with polymerization, and a
high molecular weight polymer was not obtained. For example,
37% cyclic dimer was formed in the case of polyesterification of
dicarboxy[2]catenane and dihydroxy[2]catenane. Therefore, we
devised the strategy of using a rigid linker to connect the monomeric
[2]catenanes. The rigid linker was expected to inhibit the formation
of cyclic oligomers and yield high molecular weight poly[2]caten-
anes. Indeed, formation of the cyclic dimer was decreased to 18%
by azide-alkyne 1,3-dipolar cycloaddition polymerization between
azido[2]catenane and diethynyl linker to form a rigid 1,2,3-triazole
ring.5 We selected an ethynylene group as the next rigid linkage.
The ethynylene group is not only rigid but also linear and is formed
by the Sonogashira coupling reaction between aryl halides and termi-
nal alkynes catalyzed by palladium and copper.6
In this communication, we report the first synthesis of poly[2]caten-
anes by Sonogashira coupling to obtain a very high molecular weight
polymer.
Sonogashira coupling gives an ethynylene group from aryl halides
and terminal alkynes. Electron-withdrawing groups on aryl halides
are known to cause the reactivity to increase, but have the reverse ef-
fect in the case of alkynes.7 Therefore, diiodo[2]catenane, which is an
electron-withdrawing groups-containing aryl halide, was selected as
the monomer. 5-Iodoisophthalic acid, prepared from 5-aminoisoph-
thalic acid via diazonium salt,8 was converted into diamidodiamine 1
in quantitative yield through amidation with 4-aminobenzylamine
using the dicyclohexylcarbodiimide (DCC)/1-hydroxybenzotriazole
(HOBt) method.9 The desired diiodo[2]catenane 2 was obtained in
21% yield from 1 and sebacoyl dichloride by amidation under high
dilution conditions (Scheme 1). In the 1H NMR spectra of catenane
2 and uncatenized macrocycle 3, a high field shift of benzylic and aro-
matic protons caused by the catenane structure was clearly observed
⇑
Corresponding author.
E-mail address: hagiwara.toshiki@nihon-u.ac.jp (T. Hagiwara).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.101

2806
T. Hagiwara et al. / Tetrahedron Letters 53 (2012) 2805–2808
O
NH
2
NH
COOH
COOH
NH
NH
2
H N
2
I
I
DCC, HOBt
THF
99%
O
NH
2
1
O
O
O
O
O
NH
O
O
NH
NH
COCl
COCl
NH
NH
NH
NH
O
O
NH
O
HN
HN
I
I
+
I
Et N
NH
NH
O
3
THF/CHCl
3
O
2 (21%)
3
Scheme 1. Preparation of diiodo[2]catenane 2.
O
O
NH
Pd(PPh ) Cl ,
3 2
2
NH
NH
O
O
NH
O
CuI, PPh
3
HN
HN
Ar
Ar
2
+
Et N, DMF
3
NH
NH
O
4a-c
O
,
n
O
5a-c
,
Ar :
a
b
c
Scheme 2. Preparation of poly[2]catenane 5.
Table 1
Synthesis of poly[2]catenanes 5 from diiodo[2]catenane 2 and diethynylaryls 4 by
Diiodo[2]catenane 2
Sonogashira coupling polymerizationa
b
Entry
Aryls 4
Polymer 5
Yield (%)
Mn
Mwb/Mn
1
2
3
4
4a
4b
4c
4c
5a
5b
5c
68
71
69
88
1.1 ꢀ 104
5.3 ꢀ 104
7.8 ꢀ 104
1.5 ꢀ 104
2.1
2.9
3.2
2.0
Catenane dimer in 5d
5dc
Poly[2]catenane 5a
a
Polymerization was carried out at 60 °C for 2 d using 50
lmol of 2 and 4 with
10 mol % of Pd/Cu catalyst. Only entry 3, 100 lmol of 2, and 4c were used with
10 mol % of catalyst.
5b
b
Number-average molecular weight (Mn) and weight-average molecular weight
(Mw) were determined by GPC based on a standard polystyrene using DMF as an
eluent.
5c
c
Polymer 5d was obtained from diazido[2]catenane and 4c by 1,3-dipolar
cycloaddition.5
5d
in the spectrum of catenane 2.10 In addition, the molecular ion peak of
catenane 2 appeared at m/z = 1355.2 ([M+Na]+) in the MALDI-TOF MS
spectrum.
As [2]catenane linkers, diethynylaryls were used to form rigid
linkages. Sonogashira coupling polymerizations of diiodocatenane
2 and diethynylaryls 4 were carried out in N,N-dimethylformamide
(DMF) with 10 mol % of Pd/Cu catalyst [dichlorobis(triphenylphos-
3
4
5
6
7
Log M
Figure 1. GPC curves of diiodo[2]catenane 2 (dotted line) and poly[2]catenanes 5
prepared from 2 and diethynylaryls 4 as a linker (5a from 4a: solid line, 5b from 4b:
dash-dotted line, 5c from 4c: broken line, and 5d from diazido[2]catenane and 4c
by 1,3-dipolar cycloaddition5: narrow dotted line).
phine)palladium:copper(I)
iodide:triphenylphosphine = 1:1:2]
(Scheme 2). The results of these reactions are summarized in Table
1 and GPC chromatograms of the products are shown in Figure 1.
As expected, the formation of the dimeric product was inhibited
considerably as compared to the 1,3-dipolar cycloaddition poly-
merization reported previously,5 and the rigid, linear linkers were
shown to be effective for obtaining the desired high molecular
weight polymer. When 4,40-diethynylbiphenyl (4c)11 was used as
the linker, the highest number-average molecular weight of
Mn = 7.1 ꢀ 104 was obtained, and the amount of dimer calculated
from the GPC peak area was only 2% in the total product. Mn of
the polymer from p-diethynylbenzene (4b) was the second highest,
but that of m-diethynylbenzene (4a) with a slightly bent orienta-
tion of the two ethynyl groups was low.

T. Hagiwara et al. / Tetrahedron Letters 53 (2012) 2805–2808
2807
molecular weight polymer (Mn = 2.3 ꢀ 105) in the DMF-soluble
fraction (Fig. 2). This is an extremely high molecular weight among
the reported poly[2]catenanes, and the obtained gel of the polymer
should have an even higher molecular weight fraction that is not
soluble in any conventional solvent including DMF. To our knowl-
edge, the highest reported molecular weight of poly[2]catenane for
Table 2
Synthesis of poly[2]catenane 5c from diiodo[2]catenane 2 and 4,40-diethynylbiphenyl
4c by Sonogashira coupling polymerizationa
b
Entry
Catalyst (mol %)
Yield (%)
Mn
Mwb/Mn
1
2
3
10
5
1
69
64
7.8 ꢀ 104
9.0 ꢀ 104
(2.3 ꢀ 105)c
3.2
5.4
Quant. (gelated)
(3.4)c
a
phenanthroline-type
[2]catenane-containing
polymer
(Mn = 8.1 ꢀ 105) was by Shimada.12 Our result is the highest for
amide-type [2]catenane polymers, and is the first report of a caten-
ane-containing gel.
a
Polymerization was carried out at 60 °C for 2 d by using 100
with the prescribed amount of Pd/Cu catalyst.
lmol of 2 and 4c
b
Mn and Mw were determined by GPC based on a standard polystyrene using
DMF as an eluent.
Photographs of the catenane gel are shown in Figure 3. The gel
contains catalysts and salts from the preparative reaction. After
soaking in DMF for 24 h, the gel swelled in the solvent (swelling
ratio: 342% in weight), and most of the impurities were removed
(Fig. 3, right). The catenane gel did not exhibit sol-gel transition
under the boiling point of DMF (153 °C). The high stability of the
gel may be suggesting that the gel was formed by some chemical
cross-links or topological cross-links,13 not just by simple hydro-
gen bondings. After the solvent was removed from the swelled
gel by supercritical CO2 extraction, an insoluble polymer was
recovered quantitatively compared with theoretical amount of
the polymer.
Sonogashira coupling is known to involve the oxidative addition
of aryl halide to Pd(0) and the transmetalation of alkynyl group
from cuprate to palladium followed by reductive elimination to
form an alkynyl aryl. The Pd(0) species is generated from Pd(II)
by oxidative coupling of two alkynes.6b,c From this mechanism,
the relationship between the catalyst amount and the Mn of the
polymer is explained as follows. When using a large amount of
Pd(II) catalyst, corresponding amounts of alkyne are consumed to
generate Pd(0), and the equimolarity of the two monomers is lost.
Conversely, a small amount of the catalyst consumes only a small
fraction of alkyne, and a high-Mn polymer is obtained with high
monomer equimolarity.
c
Trace amount of DMF-soluble part was extracted from the gelation product.
Diiodo[2]catenane 2
5c (catalyst 10%)
Mn = 7.8 × 104
5c (catalyst 1%)
Mn = 2.3 × 105
5c (catalyst 5%)
(DMF-soluble part)
Mn = 9.0 × 104
3
4
5
6
7
Log M
In conclusion, we synthesized a very high molecular weight
(Mn > 2.3 ꢀ 105) poly[2]catenane by Sonogashira coupling of
diiodo[2]catenane 2 and 4,40-diethynylbiphenyl 4c. This is the
highest Mn polymer obtained from amide-type [2]catenane and
the first catenane-containing gel. Further investigations of the
mechanical and chemical properties of the synthesized polymer
are in progress.
Figure 2. GPC curves of diiodo[2]catenane 2 (dotted line) and poly[2]catenanes 5c
prepared over various amounts of the catalyst (10%: broken line, 5%: dash-dotted
line, and 1%: solid line).
Next, catenane 2 was polymerized with 4c as the fixed linker
using various amounts of the catalyst. The Mn of the polymer in-
creased with decreasing catalyst amount, and a reaction mixture
was obtained that finally gelated when 1 mol % of the catalyst
was used (Table 2). It was difficult to dissolve the poly[2]catenane
gel completely in any solvent such as DMF, dimethyl sulfoxide, 1-
methyl-2-pyrrolidinone, and tetrahydrofuran. From the gelation
product, a trace amount of soluble fraction was extracted by
DMF, and GPC measurements confirmed the presence of a high
Acknowledgment
We thank Professor Hiizu Iwamura at the Graduate School of
Nihon University for valuable advice given during the preparation
of this manuscript.
Figure 3. Photographs of the catenane gel (left) and swelled in DMF (right).

2808
T. Hagiwara et al. / Tetrahedron Letters 53 (2012) 2805–2808
Supplementary data
Estimated structure of the cyclic dimer:
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.03.101.
O
N
N
N
N
O
N
N
H
References and notes
N
H
N
O
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N
N
O
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O
O
O
NH
NH
NH
O
NH
O
O
HN
HN
N
N N
N
N N
NH
NH
O
n
O