Microwave-accelerated synthesis of lengthy and defect-free poly(m-phenyleneethynylene)s via AB′ and A2 + BB′ polycondensation routes

Article abstract of DOI:10.1039/b312762a

A novel microwave-assisted polycondensation protocol involving an in-situ activation/coupling scheme has been developed and applied to the preparation of lengthy poly(m-phenyleneethynylene)s without diyne defects via either AB′ or A₂ + BB′ approaches.

Full text of DOI:10.1039/b312762a

Microwave-accelerated synthesis of lengthy and defect-free
poly(m-phenyleneethynylene)s via ABA and A2 + BBA polycondensation
routes†
Anzar Khan and Stefan Hecht*
Institut für Chemie/Organische Chemie, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany.
E-mail: shecht@chemie.fu-berlin.de; Fax: 49 30 8385 2551; Tel: 49 30 8385 2451
Received (in Cambridge, UK) 13th October 2003, Accepted 5th December 2003
First published as an Advance Article on the web 5th January 2004
A novel microwave-assisted polycondensation protocol involv-
ing an in-situ activation/coupling scheme has been developed
and applied to the preparation of lengthy poly(m-phenyl-
eneethynylene)s without diyne defects via either ABA or A2 + BBA
approaches.
Most importantly, the reaction conditions12 are mild and tolerant of
most functional groups allowing for the preparation of a variety of
functional PmPE derivatives.13 Particularly noteworthy are the low
temperatures required for the reaction in combination with
microwave acceleration.14,15
In order to develop suitable polycondensation conditions, two
pairs of ABA monomers (1a,b) and A2 monomers (2a,b) carrying
non-polar, racemic 2-ethylhexyl as well as polar triethylene glycol
side chains were synthesised (Scheme 2).†
In the initial thermal polycondensation reactions, several param-
eters were varied to test for suitability of the method and to obtain
a reasonable starting point for further optimisation. It was found
that the use of a freshly prepared Pd(PPh3)4 catalyst gave superior
results as compared to other sources for the catalytically active
Pd(0)-species such as Pd(PPh3)2Cl2 or Pd2(dba)3. A catalyst loading
of 6 mol% was found to be optimal. Furthermore, the amount of
added water, necessary to deprotect the silylated acetylene, was
increased to 1.0 equiv. as compared to the substoichiometric
amount of 0.4 equiv. used in the original procedure.12
Oligo(phenyleneethynylene)s (OPE) and poly(phenyleneethyny-
lene)s (PPE) represent attractive building blocks for the generation
of functional nanostructures. While the para-linked OPE1 and PPE2
combine a linear architecture with interesting optoelectronic
properties, meta-connected OPE have been explored in a variety of
kinked and branched topologies including foldamers,3 macro-
cycles,4 and dendrimers.5 Despite the exciting progress made in the
development of palladium-catalysed alkynylation6 as well as
acyclic diyne metathesis (ADIMET) polymerisation,2,7 defect-free
PPE having high degrees of polymerisation (DP) are still difficult to
prepare, in particular in the meta- and ortho-connected forms.8 It
should be noted that the absence of diyne defects is crucial to both
control over secondary structure3 as well as stability of the
material.
In a subsequent focused screening of reaction conditions, ABA
monomers 1a,b were polymerised (Table 1, entries 1–12) to allow
for runs on a small scale while maintaining the necessary
stoichiometric balance. Polycondensation of 1a at room tem-
perature in benzene yielded 3a with an expected2 DP = 20 (entry
1). However, microwave irradiation did not accelerate the reaction
(entry 2) presumably due to the low dipole moment of the solvent
used. Since the initial one-pot procedure12 gave excellent results
not only in benzene but also in acetonitrile as a polar solvent, we
subsequently focused on monomer 1b in order to explore
microwave-accelerated polycondensation chemistry.15 During the
polycondensation of 1b in acetonitrile at room temperature it was
found that further increasing the water content gave rise to
significantly increased chain lengths (entries 3–5). Turning to
microwave-accelerated conditions reduced the reaction times
considerably. In addition, temperature, time, and concentration
were varied showing the expected effects on the outcome of the
polycondensation (entries 6–12). The resulting, optimised poly-
condensation conditions involve the use of 10 equiv. of water,
microwave irradiation at 40 °C for 4 h, and the use of a highly
concentrated reaction mixture to afford polymers 3b with respect-
able chain-lengths (DP ~ 80) and polydispersities (PDI) typical for
polycondensation reactions.
Here, we describe a new approach to access lengthy poly(m-
phenyleneethynylene)s (PmPE) having no diyne defects.9 Our
concept (Scheme 1) involves a rate-limiting in-situ deprotection
step to minimize the concentration of free terminal acetylene in the
polymerisation mixture and therefore limit competing side reac-
tions such as bimolecular diyne defect formation. This one-pot
activation/coupling procedure can be carried out starting from
either ABA monomers, which inherently are stoichiometrically
balanced, or A2 monomers in combination with commercially
available trimethylsilylacetylene (TMSA)10 as the BBA monomer.11
Extending the concept to A2 monomers, 2a and 2b were
subjected to the polycondensation conditions in the presence of an
equimolar amount of TMSA (entries 13 and 14). Again, microwave
heating yielded superior results giving polymer 3b with a
respectable DP and PDI. Due to the commercial availability of
TMSA and the rapid synthetic access to aryl diiodides, this A2 +
Scheme 1 Defect-free PPE by in-situ activation/coupling sequence starting
from either ABA or A2 + BBA monomers.
† Electronic supplementary information (ESI) available: monomer synthe-
ses, polycondensation procedures, and polymer characterization. See http:
//www.rsc.org/suppdata/cc/b3/b312762a/
Scheme 2 PmPE via ABA and A2 + BBA polycondensation approaches.
300
C h e m . C o m m u n . , 2 0 0 4 , 3 0 0 – 3 0 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Table 1 Selected polycondensation experiments according to ABA and A2 + BBA approaches illustrated in Scheme 2a
b
b
Entry
Monomer
H2O/equiv. T/°C
t/h
Mw
Mn
PDIb
DP (Pn)b
Yieldc (%)
1
2
3
4
5
6e
7e
8e
9e
10e
11e
12e,f
13
14
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
2a
2b
1
1
1
4
10
1
2
2
4
4
25
72
1
72
72
72
8
9300
5500
5200
3800
9500
1.78
1.44
1.71
2.85
2.23
1.64
1.45
3.74
3.63
2.23
1.93
2.71
1.78
1.69
20
15
33
35
69
30
48
55
61
80
79
78
20
64
94
83
88
55
60
59
70
51
81
39
88
85
94
80
40 (mw)d
25
16200
29100
44800
14300
20300
61000
63900
52000
44400
62000
9300
25
25
10000
20100
8700
14000
16300
17600
23300
23000
22800
5200
40 (mw)
40 (mw)
40 (mw)
50 (mw)
60 (mw)
30 (mw)
40 (mw)
25
4
8
15
15
15
4
72
4
10
10
1
1
40 (mw)
31400
18500
a Conditions: 1 equiv. AB monomer (1a,b), 6 mol% Pd(PPh3)4, 6 mol% CuI, 6 equiv. DBU in 4 mL of solvent (C6H6 for 1a, 2a and CH3CN for 1b, 2b);
in the case of A2 monomers (2a,b), 1 equiv. TMSA was added as well. b According to GPC in THF at 40 °C. c Isolated yield after precipitation into MeOH
(1a,2a) or Et2O (1b,2b) and short silica column in CH2Cl2. d Microwave heating. e GPC in NMP containing 0.5 wt% LiBr at 70 °C. f In 3.3 mL of
CH3CN.
BBA approach has great potential for the preparation of various PPE
derivatives on a large scale.
(MPI Golm) for some GPC measurements, and to the Reissig group
(FUB) for use of the microwave apparatus. Generous support by the
Sofja Kovalevskaja Award of the Alexander von Humboldt
Foundation, endowed by the Federal Ministry of Education and
Research (BMBF) within the Program for Investment in the Future
(ZIP) of the German Government, is gratefully acknowledged.
In most cases, monomer conversion is almost quantitative,
however, a considerable amount (up to 26% by GPC) of low
molecular weight compounds, presumably oligomeric macro-
cycles, is being formed as well. In the case of amphiphilic polymer
3b, some material is inevitably lost during precipitation into diethyl
ether. Hence, the isolated yields range from 83–94% for 3a to
39–88% for 3b.
The resulting polymers were thoroughly analysed by 13C-NMR
spectroscopy in order to detect potential diyne defects. In polymer
3b, such a defect would give rise to two characteristic signals at:
80.6 4 d 4 79.7 and 75.4 4 d 4 74.7 ppm as indicated by 13C-
NMR spectral data of diacetylene-linked oligomers having the
exact same substitution pattern.16 Gratifyingly, even after extensive
scanning of concentrated samples no signals that could be attributed
to diyne defects were found while the acetylene carbon is clearly
visible at d ~ 89 ppm (Fig. 1).
Notes and references
1 J. M. Tour, Acc. Chem. Res., 2000, 33, 791.
2 U. H. F. Bunz, Chem. Rev., 2000, 100, 1605.
3 (a) J. C. Nelson, J. G. Saven, J. S. Moore and P. G. Wolynes, Science,
1997, 277, 1793. For a comprehensive review, consult (b) D. J. Hill, M.
J. Mio, R. B. Prince, T. S. Hughes and J. S. Moore, Chem. Rev., 2001,
101, 3893.
4 D. Zhao and J. S. Moore, Chem. Commun., 2003, 807.
5 J. S. Moore, Acc. Chem. Res., 1997, 30, 402.
6 For a review, consult: (a) E.-I. Negishi and L. Anastasia, Chem. Rev.,
2003, 103, 1979. See also (b) R. R. Tykwinski, Angew. Chem., Int. Ed.,
2003, 42, 1566.
Both the absence of diyne defects as well as the observed strong
dependence of polymer chain length on the water content indicate
the validity of our initial working hypothesis (Scheme 1) where
deprotection represents the rate-limiting step. Further insight into
7 U. H. F. Bunz, Acc. Chem. Res., 2001, 34, 998.
8 For PmPE, DP 5 25 (see ref. 2). For the only notable exception, see: L.
Arnt and G. N. Tew, J. Am. Chem. Soc., 2002, 124, 7664.
9 For a related A2 + BA2 approach to para-linked PPE, see: (a) H. Haeger
and W. Heitz, Macromol. Chem. Phys., 1998, 199, 1821. For a related
(A2 + 2BBA) + A2 approach to para-linked PPE, see (b) V. A. Solomin
and W. Heitz, Macromol. Chem. Phys., 1994, 195, 303. For A2 + BBA
approaches to para-linked PPE using Stille couplings, see (c) G.
Giardina, P. Rosi, A. Ricci and C. Lo Sterzo, J. Polym. Sci., Part A:
Polym. Chem., 2000, 38, 2603; (d) R. Pizzoferrato, M. Berliocchi, A. Di
Carlo, P. Lugli, M. Venanzi, A. Micozzi, A. Ricci and C. Lo Sterzo,
Macromolecules, 2003, 36, 2215.
1
polycondensation kinetics was obtained using an in-situ H-NMR
experiment that points to fast deprotection and coupling steps
leading to the rapid build-up of short oligomers typical for
polycondensation processes.†
Ongoing work in our laboratories is focused on expanding the
scope of the synthetic route outlined herein as well as evaluating
both folding and aggregation behavior of amphiphilic PmPE.
We are grateful to Maria Francesca Lagana (University of
Bologna) for synthesising starting materials, to Markus Antonietti
10 In an A2 + B2 approach, acetylene has been used to prepare PmPE with
carboxyl side chains: C.-J. Li, W. T. I. V. Slaven, Y.-P. Chen, V. T. John
and S. H. Rachakonda, Chem. Commun., 1998, 1351.
11 The reactive sites have been labeled using polycondensation terminol-
ogy. In this case, A refers to aryl iodide, B to free terminal acetylene, and
BA to TMS-protected terminal acetylene moieties.
12 The reaction conditions are inspired by: M. J. Mio, L. C. Kopel, J. B.
Braun, T. L. Gadzikwa, K. L. Hull, R. G. Brisbois, C. J. Markworth and
P. A. Grieco, Org. Lett., 2002, 4, 3199.
13 Amphiphilic PmPE based on cinnamate moieties have been synthesised
utilizing this A2 + BBA approach and used to prepare organic nanotubes:
S. Hecht and A. Khan, Angew. Chem., Int. Ed., 2003, 42, 6021.
14 For an example of microwave-assisted Sonogashira–Hagihara-type
coupling reactions, see: M. Erdelyi and A. Gogoll, J. Org. Chem., 2001,
66, 4165.
15 One rare example of a microwave-enhanced transition metal catalysed
polycondensation is given in: K. R. Carter, Macromolecules, 2002, 35,
6757.
16 Y. Tobe, N. Utsumi, K. Kawabata, A. Nagano, K. Adachi, S. Araki, M.
Sonoda, K. Hirose and K. Naemura, J. Am. Chem. Soc., 2002, 124,
5350.
Fig. 1 13C-NMR spectrum of PmPE 3b (126 MHz, 116459 scans, CDCl3, 25
°C). The inset shows the region of potential diacetylene defects.16
C h e m . C o m m u n . , 2 0 0 4 , 3 0 0 – 3 0 1
301