Rhodium-Catalyzed Stitching Polymerization of 1,5-Hexadiynes and Related Oligoalkynes

Article abstract of DOI:10.1002/anie.201901148

A new mode of polymerization, rhodium-catalyzed stitching polymerization, has been developed for the synthesis of π-conjugated polymers with bridged repeating units from nonconjugated 1,5-hexadiynes containing both terminal and internal alkyne moieties as monomers. The polymerization proceeded smoothly with a high degree of stitching efficiency under mild conditions, and 1,5,9-decatriyne and 1,5,9,13-tetradecatetrayne monomers could also be employed. The present polymerization strategy would be particularly beneficial for the synthesis of polymers consisting of a repeating unit that is difficult to prepare as a stable monomer because it does not require the use of a preformed bridged π-conjugated monomer.

Full text of DOI:10.1002/anie.201901148

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Accepted Article
Title: Rhodium-Catalyzed Stitching Polymerization of 1,5-Hexadiynes
and Related Oligoalkynes
Authors: Sho Ikeda and Ryo Shintani
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901148
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10.1002/anie.201901148
Angewandte Chemie International Edition
COMMUNICATION
Rhodium-Catalyzed Stitching Polymerization of 1,5-Hexadiynes
and Related Oligoalkynes
Sho Ikeda[a] and Ryo Shintani*[a]
Ar
Abstract: A new mode of polymerization, rhodium-catalyzed stitching
polymerization, has been developed for the synthesis of π-conjugated
polymers with bridged repeating units from non-conjugated 1,5-
Ar
Ar
R
Rh
hexadiynes containing both terminal and internal alkyne moieties as
monomers. The polymerization proceeded smoothly with high degree
of stitching efficiency under mild conditions, and 1,5,9-decatriyne and
1,5,9,13-tetradecatetrayne monomers could also be employed. The
present polymerization strategy would be particularly beneficial for the
synthesis of polymers consisting of a repeating unit that is difficult to
prepare as a stable monomer because it does not require the use of
a preformed bridged π-conjugated monomer.
R
Rh
A
intermolecular insertion
(terminal alkyne)
intermolecular insertion
(terminal alkyne)
Ar
intramolecular
insertion
intramolecular
insertion
Ar
Ar
Rh
Rh
Rh
R
R
R
(internal alkyne)
(internal alkyne)
Ar
B
C
D
Ar
Ar
Bridged π-conjugated compounds are intensively investigated as
useful structural motifs for functional organic materials due to their
optoelectronic properties derived from the extended π-systems
with rigid planar structures.[1] They also often appear in (a part of)
repeating units of π-conjugated (co)polymers,[2,3] which are
typically synthesized by condensation (co)polymerization of
preformed bridged π-conjugated monomers. Although this
synthetic process is a powerful and reliable approach, applicable
repeating units are inherently limited to stable and isolable
structures as monomers.
Recently, we developed a rhodium/diene-catalyzed stitching
reaction as a new strategy for the intermolecular synthesis of
polycyclic bridged π-conjugated compounds through multiple
carbon–carbon bond formations via successive insertion of
alkynes.[4] This method allows for the construction of a rigid planar
structure and extension of π-conjugation at the same time in a
single catalytic process. Based on this attractive feature of the
stitching reaction, we imagined that polymers having a bridged π-
conjugated repeating unit could be readily synthesized from easily
accessible non-conjugated oligoalkynes without the necessity of
preparation of corresponding π-conjugated monomers
beforehand, and this would be particularly advantageous when
the repeating unit is difficult to prepare as a stable monomer. In
this context, herein we disclose our new polymerization strategy,
rhodium-catalyzed stitching polymerization, for the synthesis of π-
conjugated polymers with bridged repeating units by employing
1,5-hexadiynes and their extended triynes and a tetrayne as
monomers which possess both terminal and internal alkyne
moieties.[5–8]
Ar
Ar
Ar
Ar
R
Ar
Ar
Ar
E
Scheme 1. Schematic representation of rhodium-catalyzed stitching
polymerization of 1-aryl-1,5-hexadiyne.
terminal alkyne of the monomer to give alkenylrhodium(I)
intermediate A.[10] Successive insertion of a terminal alkyne of
another monomer would lead to dienylrhodium(I) species B. This
then undergoes intramolecular insertion of an internal alkyne of
the initially incorporated monomer in a 6-exo-dig fashion to form
a
6-membered ring with concomitant generation of
trienylrhodium(I) species C. Subsequent 6-membered ring-
forming intramolecular insertion of an internal alkyne of the
secondly incorporated monomer gives tetraenylrhodium(I)
species D possessing a conjugated bicyclic structure. Repeating
this 4-step sequence would eventually lead to π-conjugated
polymer E.
As a starting point, we employed 1-(4-methylphenyl)-1,5-
hexadiyne as the monomer and found that the polymerization
proceeded smoothly in the presence of [{Rh(OH)(cod)}2] (3 mol%
Rh) in THF at 60 °C to give 81% yield of poly-1a after
reprecipitation (Table 1, entry 1). The number-averaged
molecular weight (Mn) was calculated to be 5400 g mol−1 with PDI
of 1.8 using size-exclusion chromatography against a polystyrene
standard. In comparison, the presence of a phosphine ligand such
as PPh3 or binap significantly lowered the polymerization
efficiency (entries 2 and 3). The structural information of poly-1a
was mainly obtained by 13C NMR analysis. Figure 1 shows 13C
NMR spectra of (a) monomer 1a and (b) poly-1a prepared in
Table 1, entry 1. According to these spectra, poly-1a showed
essentially no residual peaks in the sp-carbon region, indicating
that both terminal and internal alkynes of monomer 1a were
successfully incorporated into the main chain as alkenyl sp2-
carbons. The chemical shifts of methylene carbons were also
significantly downfield-shifted, which corresponds to the change
of propargylic carbons to allylic carbons.
A schematic representation of the proposed rhodium-catalyzed
stitching polymerization pathway of 1-aryl-1,5-hexadiyne as a
model monomer is illustrated in Scheme 1.[9] Thus, a catalytic
amount of organorhodium(I) complex undergoes insertion of a
[a]
S. Ikeda, Prof. Dr. R. Shintani
Division of Chemistry, Department of Materials Engineering Science
Graduate School of Engineering Science
Osaka University, Toyonaka, Osaka 560-8531 (Japan)
E-mail: shintani@chem.es.osaka-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201901148
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COMMUNICATION
Table 1. Rhodium-catalyzed polymerization of 1a.
[{Rh(OH)(cod)}2]
(3 mol% Rh)
Ar
13C
(1)
13C
Ar
13C
[{Rh(OH)(cod)}2]
(3 mol% Rh)
ligand (3.3 mol%)
n
THF, 60 °C, 22 h
Ar
Ar
1a-13C (Ar = 4-MeC6H4)
poly-(1a-13C)
(0.25 M)
Ar
83% yield
Mn 6400 g mol–1, Mw/Mn 1.9
n
THF, 60 °C, 22 h
Ar
1a (Ar = 4-MeC6H4)
poly-1a
(0.25 M)
CDCl3
[a]
[a]
Entry
Ligand
None
PPh3
Yield [%]
Mn (g mol–1
)
Mw/Mn
1.8
1
2
3
81
42
23
5400
6000
5900
2.2
Binap
2.4
[a] Determined using size-exclusion chromatography against a polystyrene
standard.
(a)
i
c
h
a
i
e
f
k
d
Figure 2. Inverse gated 1H decoupling 13C NMR spectrum of poly-(1a-13C)
a
b
g
d
j
obtained in Eq. (1) (176 MHz in CDCl3 at 25 °C).
k
c
j
h
g
f
e
21.9
b
30.5
CDCl3
33.9
132.2
(b)
Ar
Ar
136.5
sp2 carbons
methyl carbons
Ar
Ar
34.6 34.5
21.9
tri-1a (Ar = 4-MeC6H4)
methylene carbons
Figure 3. Selected 13C NMR chemicals shifts (ppm) of tri-1a by DFT calculation
at the B3LYP/6-31G(d) level of theory (as transoid–transoid conformer).
Figure 1. 13C NMR spectra of (a) monomer 1a and (b) poly-1a obtained in Table
1, entry 1 (101 MHz in CDCl3 at 30 °C).
bolded carbon–carbon single bonds in Figure 3.[12]
The present stitching polymerization could also be applied to
other 1-aryl-1,5-hexadiynes. For example, monomers having an
electron-donating group (1b) or an electron-withdrawing group
(1c and 1d) at the 4-position of the aryl group gave the
corresponding stitched polymers poly-1b, poly-1c, and poly-1d
in 71–80% yield with Mn of 3000–9700 g mol–1 [Eq. (2)].
Polymerization of monomers 1e and 1f having 3,5-dialkylphenyl
group also proceeded smoothly to give corresponding poly-1e
and poly-1f in 71–90% yield with somewhat higher molecular
weight (Mn 15600–21000 g mol–1). The 13C NMR spectra of these
polymers suggest that the main chain structures are the same as
that of poly-1a with essentially no residual peaks in the sp-carbon
region.[12]
To gain more insights into the structure of poly-1a, we prepared
monomer 1a-13C possessing a 13C-labeled carbon at one of the
internal alkyne carbons, and conducted the present
polymerization reaction to give poly-(1a-13C) in 83% yield with Mn
of 6400 g mol–1 and PDI of 1.9 [Eq. (1)]. As indicated by the
quantitative 13C NMR spectrum in Figure 2, although some
labeled internal alkyne peak remained at 90 ppm,[11] more than
95% of the labeled carbon was shifted to sp2-carbon region as two
sets of peaks (140 ppm and 134 ppm) with almost equal intensity,
which supports the proposed stitched structure of poly-1a. In
addition, we estimated the 13C NMR chemical shifts of model
trimer tri-1a by DFT calculations and selected values of the
middle section are shown in Figure 3.[12] Based on these data,
observed signals for sp3-carbon region of poly-1a are consistent
with the proposed structure, and two sets of peaks in poly-(1a-
13C) at 140 ppm and 134 ppm with almost equal intensity can also
be reasonably explained. It is worth noting that the peak at 140
ppm actually consists of two peaks (140.5-139.6 ppm and 139.6-
138.0 ppm) in the area ratio of ca. 1:3, and this can be understood
by considering the existence of transoid/cisoid conformers at the
[{Rh(OH)(cod)}2]
(3 mol% Rh)
Ar
(2)
Ar
n
THF, 60 °C, 22 h
Ar
1b: Ar = 4-MeOC6H4
1c: Ar = 4-MeO2CC6H4
1d: Ar = 4-CF3C6H4
1e: Ar = 3,5-Me2C6H3
1f: Ar = 3,5-tBu2C6H3
(0.25 M)
poly-1b: 80% yield, Mn 5200 g mol–1, Mw/Mn 2.2
poly-1c: 72% yield, Mn 3000 g mol–1, Mw/Mn 1.9
poly-1d: 71% yield, Mn 9700 g mol–1, Mw/Mn 1.4
poly-1e: 71% yield, Mn 21000 g mol–1, Mw/Mn 1.8
poly-1f: 90% yield, Mn 15600 g mol–1, Mw/Mn 1.6
a
a
a 6 mol% of rhodium catalyst was used in the presence of 30 mol% of cod.
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10.1002/anie.201901148
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COMMUNICATION
CDCl3
poly-1a
ꢀꢁꢅ
ꢀꢁꢄ
ꢀꢁꢃ
ꢀꢁꢂ
ꢀ
poly-2a
poly-3
poly(4-methylphenylacetylene)
propargylic
carbons
allylic carbons
sp carbons
250
350
450
550
650
Figure 4. Inverse gated 1H decoupling 13C NMR spectrum of poly-2a obtained
wavelength (nm)
in Eq. (3) (176 MHz in CDCl3 at 25 °C).
Figure 5. UV-vis spectra of poly-1a (black line; at 1.6 x 10–2 g/L), poly-2a (gray
line; at 2.1 x 10–2 g/L), poly-3 (black broken line; at 1.3 x 10–2 g/L), and poly(4-
methylphenylacetylene) (gray broken line; at 1.2 x 10–2 g/L) in THF at 25 °C.
In addition, we also examined the polymerization of triynes 2 for
the synthesis of polymers possessing a longer bridged π-
conjugated repeating unit. As shown in Eq. (3), polymerization of
2a took place efficiently to give poly-2a in 70% yield with Mn of
7100 g mol–1 and PDI of 2.2. According to the quantitative 13C
NMR spectrum of this polymer (Figure 4), some residual peaks
were observed in the sp-carbon region and peaks observed at 20
and 19 ppm could be assigned as propargylic carbons. Based on
the area ratio of sp2, sp, and sp3 carbon peaks, however,
incorporation efficiency of internal alkynes into the main chain was
estimated to be as high as 93%. This indicates that the desired
stitching polymerization of triyne 2a did proceed successfully for
the most part with some minor defects. Similarly, poly-2b was
obtained in 68% yield with Mn of 11200 g mol–1 and PDI of 1.8,
and its incorporation efficiency of internal alkynes into the main
chain was estimated to be 86%.[12] Furthermore, we found that
even tetrayne 3 could be employed for the present stitching
polymerization, although the incorporation efficiency of internal
alkynes became somewhat lower. Thus, poly-3 was obtained in
69% yield with Mn of 5000 g mol–1 and PDI of 2.0 [Eq. (4)], and ca.
82% of internal alkynes were stitched into the main chain
according to the same analysis as that for poly-2.[12]
at 434 nm and its absorption edge reached up to ca. 615 nm. We
also found that absorption edge showed bathochromic shift by
extending the bridged π-conjugation unit as demonstrated with
poly-2a (ledge = ca. 630 nm) and poly-3 (ledge = ca. 660 nm). In
comparison, cis-poly(4-methylphenylacetylene) [Mn = 30000 g
mol–1, PDI = 1.3], a related π-conjugated polymer possessing no
bridged π-conjugation units, showed absorption maximum at 329
nm with absorption edge of ca. 520 nm. These results strongly
suggest that polymers obtained in the present stitching
polymerization show effective extension of π-conjugation by
introduction of rigid bridged structures during the polymerization
reaction.
We have also begun to explore post-derivatization of these
polymers. For example, poly-1f was found to be readily oxidized
by treatment with DDQ to give poly(2,6-naphthylene vinylene)
poly-1f-ox in 94% yield [Eq. (5)], which showed hypsochromic
shift in the UV-vis absorption spectrum (lmax = 355 nm whereas
lmax = 496 nm for poly-1f) and was found emissive (lmax = 483
nm whereas non-emissive for poly-1f) (Figure 6).[14,15] The same
oxidation protocol was also applied to poly-2b derived from triyne
2b to give poorly soluble polymer poly-2b-ox, which is
presumably the corresponding poly(2,8-chrysenylene vinylene)
based on its UV-vis absorption and emission spectra [Eq. (6) and
Figure 7]. These results demonstrate the potential utility of poly-
1–3 as promising precursors for poly(arylene vinylene)s
possessing an extended polyaromatic hydrocarbon repeating
unit.[16]
[{Rh(OH)(cod)}2]
(3 mol% Rh)
Ar
(3)
n
THF
Ar
60 °C, 22 h
Ar
2a: Ar = 4-MeC6H4
2b: Ar = 3,5-tBu2C6H3
(0.25 M)
poly-2a: 70% yield, Mn 7100 g mol–1, Mw/Mn 2.2
(93% incorporation of internal alkynes)
poly-2b: 68% yield, Mn 11200 g mol–1, Mw/Mn 1.8
(86% incorporation of internal alkynes)
[{Rh(OH)(cod)}2]
(6 mol% Rh)
Ar
Ar
THF, 60 °C, 22 h
Ar
DDQ
(5)
n
n
toluene
3: Ar = 4-MeC6H4
110 °C, 22 h
Ar
poly-1f (Ar = 3,5-tBu2C6H3)
Ar
Ar
(0.25 M)
poly-1f-ox: 94% yield
(4)
n
Ar
Ar
Ar
DDQ
poly-3: 69% yield
Mn 5000 g mol–1, Mw/Mn 2.0
(82% incorporation of internal alkynes)
(6)
n
n
toluene
110 °C
22 h
Ar
Ar
poly-2b (Ar = 3,5-tBu2C6H3)
poly-2b-ox: 98% yield
These polymers can also be regarded as cis-poly(acetylene)s
with rigid repeating units, and we briefly examined their optical
properties using UV-vis spectrophotometry.[13] As shown in Figure
5, poly-1a derived from diyne 1a possessed absorption maximum
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10.1002/anie.201901148
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COMMUNICATION
Support has been provided in part by JSPS KAKENHI Grant
Number JP18H04653 (Hybrid Catalysis), the Asahi Glass
Foundation, and the Mitsubishi Foundation.
poly-1f
(absorption)
poly-1f-ox (absorption)
poly-1f-ox (emission)
ꢃ
ꢀꢁꢂ
ꢀ
Keywords: diyne • oligoalkyne • π-conjugated polymer •
polymerization • rhodium catalyst
[1]
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[2]
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wavelength (nm)
Figure 6. UV-vis spectra of poly-1f (black line; at 3.0 x 10–2 g/L) and poly-1f-
ox (black broken line; at 2.7 x 10–2 g/L), and fluorescence spectrum of poly-1f-
ox (gray line; at 5.1 x 10–3 g/L; excited at 355 nm) in THF at 25 °C.
[3]
poly-2b
(absorption)
poly-2b-ox(absorption)
poly-2b-ox(emission)
ꢃ
ꢀꢁꢂ
ꢀ
[4]
[5]
[6]
a) R. Shintani, R. Iino, K. Nozaki, J. Am. Chem. Soc. 2016, 138, 3635–
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Kishikawa, K. Nakamura, T. Tsuda, K. Nozaki, Chem. Commun. 2019,
55, 1072–1075.
250
350
450
wavelength (nm)
550
650
For reviews on rhodium-catalyzed polymerization of terminal alkynes: a)
M. Shiotsuki, F. Sanda, T. Masuda, Poly. Chem. 2011, 2, 1044–1058; b)
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Figure 7. UV-vis spectra of poly-2b (black line; at 3.1 x 10–2 g/L) and poly-2b-
ox (black broken line; soluble part), and fluorescence spectrum of poly-2b-ox
(gray line; soluble part; excited at 338 nm) in THF at 25 °C.
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1995, 28, 6662–6666.
In summary, we have developed a new mode of polymerization,
rhodium-catalyzed stitching polymerization, for the synthesis of π-
conjugated polymers with bridged repeating units by employing
1,5-hexadiynes containing both terminal and internal alkyne
moieties as monomers. The polymerization proceeded smoothly
with high degree of stitching efficiency under mild conditions, and
1,5,9-decatriyne and 1,5,9,13-tetradecatetrayne monomers could
also be employed. Because the use of preformed bridged π-
conjugated monomers is not required, this method could be
particularly beneficial for the synthesis of polymers consisting of
a repeating unit that is difficult to prepare as a stable monomer.
Future studies will include investigation of the properties of
obtained new polymers, further improvement of this new type of
polymerization reaction, and application to the preparation of
functional polymers that are inaccessible using other existing
methods.
[7]
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H. Yang, Y. Jin, Y. Du, W. Zhang, J. Mater. Chem. A 2014, 2, 5986–5993.
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Czekelius, Indian J. Chem. 2009, 48B, 1704–1708; For examples: b) F.
Cataldo, Polym. Int. 1993, 30, 375–379; c) W. Zhang, M. Shiotsuki, T.
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Acknowledgements
[8]
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COMMUNICATION
Hayashi, Organometallics 2011, 30, 2342–2348; See also: e) S. Okada,
K. Hayamizu, H. Matsuda, A. Masaki, H. Nakanishi, Bull. Chem. Soc. Jpn.
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[13] See the Supporting Information for substituent effects on the UV-vis
spectra of poly-1.
[14] a) P. J. Lynch, L. O’Neill, D. Bradley, H. J. Byrne, M. McNamara,
Macromolecules 2007, 40, 7895–7901; b) L. O’Neill, P. J. Lynch, N.
McGoldrick, H. J. Byrne, M. McNamara, J. Luminescence 2012, 132,
2217–2223; For substituent effects of poly(arylene vinylene)s on the UV-
vis absorption, see: c) C. A. Young, S. Saowsupa, A. Hammack, A. A.
Tangonan, P. Anuragudom, H. Jia, A. C. Jamison, S. Panichphant, B. E.
Gnade, T. R. Lee, J. Appl. Polym. Sci. 2014, 131, 41162/1–41162/12.
[15] Although it is difficult to quantify the degree of oxidation precisely, based
on the changes of 1H and 13C NMR spectra (see the Supporting
Information; particularly disappearance of allylic proton and carbon
signals) as well as UV-vis absorption spectra in Figure 6 upon DDQ
oxidation, it can be concluded that the oxidation proceeded successfully.
[16] For reviews: a) F. R. Denton, III, P. M. Lahti, Plastics Engineering 1998,
49, 61–102; b) T. Haque, K. Nomura, Catalysts 2015, 5, 500–517.
[9]
For mechanistic studies on the rhodium-catalyzed polymerization of
phenylacetylenes: a) Y. Kishimoto, P. Eckerle, T. Miyatake, M. Kainosho,
A. Ono, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1999, 121, 12035–
12044; b) Z. Ke, S. Abe, T. Ueno, K. Morokuma, J. Am. Chem. Soc. 2011,
133, 7926–7941; c) M. Angoy, M. V. Jiménez, F. J. Modrego, L. A. Oro,
V. Passarelli, J. J. Pérez-Torrente, Organometallics 2018, 37, 2778–
2794.
[10] The initiation group R in Scheme 1 could be either 6-aryl-1,5-hexadiyn-
1-yl or H. See Ref. [9] for proposed initiation mechanisms.
[11] At this stage, we cannot conclude whether the residual internal alkyne
peak is partly derived from the initiation terminal.
[12] See the Supporting Information for details.
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10.1002/anie.201901148
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
Sho Ikeda, Ryo Shintani*
Ar
Rh-catalyst
Ar
Page No. – Page No.
n
Stitching
Polymerization
Ar
Rhodium-Catalyzed Stitching
Polymerization of 1,5-Hexadiynes and
Related Oligoalkynes
n
Ar
n
n = 0, 1, 2
A new mode of polymerization, rhodium-catalyzed stitching polymerization, has
been developed for the synthesis of π-conjugated polymers with bridged repeating
units from non-conjugated oligoalkynes containing both terminal and internal alkyne
moieties as monomers.
This article is protected by copyright. All rights reserved.