Suzuki polycondensation with a hairpin monomer

Article abstract of DOI:10.1021/ol9016047

Two straight monomers were subjected to an AA/BB-type Suzuki polycondensation with a hairpin-shaped 1,8-anthrylene monomer as the counterpart leading to a novel polyarylene which should have the preferred conformation of a folded chain. The molar masses w

Full text of DOI:10.1021/ol9016047

ORGANIC
LETTERS
2009
Vol. 11, No. 18
4112-4115
Suzuki Polycondensation with a Hairpin
Monomer
Yogesh Sangvikar,† Karl Fischer,‡ Manfred Schmidt,‡ A. Dieter Schlu¨ter,† and
Junji Sakamoto*,†
Laboratory of Polymer Chemistry, Department of Materials, ETH Zurich,
Wolfgang-Pauli-Strasse 10, HCI G523, CH-8093 Zurich, Switzerland, and Institute of
Physical Chemistry, Johannes Gutenberg UniVersity of Mainz, Jakob-Welder-Weg 11,
D-55099 Mainz, Germany
sakamoto@mat.ethz.ch
Received July 14, 2009
ABSTRACT
Two straight monomers were subjected to an AA/BB-type Suzuki polycondensation with a hairpin-shaped 1,8-anthrylene monomer as the
counterpart leading to a novel polyarylene which should have the preferred conformation of a folded chain. The molar masses were determined
by gel permeation chromatography and dynamic light scattering and found to be Mw ) 14,000 and Mn ) 7,000. MALDI-TOF MS analysis of a
fraction provides a fingerprint of the step-growth nature of this polymerization.
The polymerization of bifunctional p-phenylene monomers
based on successive C-C couplings leads to poly(p-
phenylene)s. Their backbones are relatively stiff1 and thus
unlikely to fold back on themselves tightly. The rigid rodlike
chains rather tend to align in a nematic or a smectic fashion.2
Figure 1a (top) represents the copolymerization of AA- and
BB-type p-phenylene-derived monomers, e.g., by Suzuki
polycondensation (SPC).3-5 Because of the functional groups
† ETH Zurich.
‡ Johannes Gutenberg University of Mainz.
Figure 1. Impact of kinked monomers on polymer conformation
(1) Vanhee, S.; Rulkens, R.; Lehmann, U.; Rosenauer, C.; Schulze, M.;
Ko¨hler, W.; Wegner, G. Macromolecules 1996, 29, 5136.
(a) and possible structures formed by chain-folding with hairpin
turns (b) which include ribbons (top) cylinders (medium) and tubes
(bottom) depending on concrete monomer structures and intramo-
lecular interactions such as hydrogen bonding.
(2) See, for example: (a) Witteler, H.; Lieser, G.; Wegner, G.; Schulze,
M. Makromol. Chem. Rapid Commun. 1993, 14, 471. (b) Harre, K.; Wegner,
G. Polymer 2006, 47, 7312.
(3) For a recent comprehensive review on Suzuki-Miyaura cross-
coupling, see: Miyaura, N. Metal-Catalyzed Cross-Coupling Reactions of
Organoboron Compounds. In Metal-Catalyzed Cross-Coupling Reactions,
2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2005;
controlling the regiochemical course of the polymerization,
straight polyarylenes are the necessary product. On the other
hand, if any one of the monomers is replaced by a
m-phenylene6 (Figure 1a, medium), the resultant polyphe-
Vol. 1, p 41
.
(4) For initial reports on rational syntheses of solubile poly(p-phenylene)
derivatives by SPC, see: (a) Rehahn, M.; Schlu¨ter, A. D.; Wegner, G.; Feast,
W. J. Polymer 1989, 30, 1060. (b) Rehahn, M.; Schlu¨ter, A. D.; Wegner,
G. Makromol. Chem. 1990, 191, 1991
.
(5) For a recent comprehensive review on SPC, see: Sakamoto, J.;
Rehahn, M.; Wegner, G.; Schlu¨ter, A. D. Macromol. Rapid Commun. 2009,
(6) Kandre, R.; Feldman, K.; Meijer, H. E. H.; Smith, P.; Schlu¨ter, A. D.
Angew. Chem., Int. Ed. 2007, 46, 4956.
30, 653
.
10.1021/ol9016047 CCC: $40.75
Published on Web 08/13/2009
 2009 American Chemical Society

nylene is kinked so that it can attain conformations ranging
from the most extended zigzag one to the more compact
helical and globular ones.7 The many conformations ener-
getically within reach for such a polymer are considered a
key factor for why a reported poly(p,m-phenylene) is
amorphous in bulk and exhibits an outstanding toughness
which is even comparable to that of polycarbonate.6 Trig-
gered by this success, we set out to further explore kinked
monomers and selected a hairpin-shaped one. This forces
the bond vector to periodically flip its orientation during the
polymerization resulting in an enforced chain-folding (Figure
1a, bottom).8,9 Note that this chain-folding coincides with
polymerization, which is in contrast to “foldamers”, whose
chain-folding is a subsequent step driven by noncovalent
interactions or by changes of physical environment such as
solvent polarity.10 Depending on the particular monomer
structures, the resultant polymers can not only be pseudo
2D coils11 but also be shaped into ribbons,12 cylinders,13
and tubes14 with assistance from intramolecular interactions
such as hydrogen bonding (Figure 1b). Attractive monomers
include “para”-bifunctionalized polyaromatic hydrocarbons,
porphyrins and shape-persistent macrocycles.15 Evidently,
the main advantage of the present strategy over the supramo-
lecular assembly approaches13,14 is that the products can be
isolated as discrete entities because of their covalent back-
bone. The most critical hurdle in this project is seen in the
polymerizability of 1,8-disubstituted anthracenes, which have
a hydrogen peri to the coupling sites and so far were never
successfully used in SPC. We felt encouraged to seriously
test this and the entire concept because of our recent
multigram scale synthesis of highly pure 1,8-anthracene (2).
which is obviously a “hairpin” monomer.16
It should be noted that on average they have obtained only
low oligomeric products,18 whereas one key interest of our
work is to arrive at as high molar mass products as possible.
Our results accidentally allowed an interesting and rather
deep insight into the mechanism of SPC of this concrete case
which will also be described.
The monomer syntheses were carried out according to
literature procedures,16,19,20 and the analytically pure com-
pounds were obtained on a gram scale (see the Supporting
Information). First, the straight 1a and the hairpin 2 were
subjected to SPC. Despite the two hexyloxy chains of 1a,
the product, oligomer/polymer 3a, precipitated from the
polymerization mixture as the reaction time progressed. The
solubility of 3a turned out to be so low that its molar mass
could not be properly determined in common solvents. The
chloroform-soluble part of the products could nevertheless
be analyzed by MALDI-TOF mass spectrometry. Compared
to the numerous unpublished such spectra of SPC products
that were recorded over the years in the authors’ group, the
present spectrum looks like a textbook example. Most
remarkably, it shows all possible monomer combinations for
the oligomers up to n ) 6. There is no preferential formation
of a dimer of 1a and 2 to give an AB-type monomer, the
prerequisite for considerations toward SPC operating ac-
cording to a chain growth instead of a step-growth mecha-
nism as was put forward by Yokozawa and others for some
transition-metal-mediated cross-coupling polymerizations
including certain SPC cases.21 Furthermore, all oligomers
(7) Zhang, A.; Sakamoto, J.; Schlu¨ter, A. D. Chimia 2008, 62, 776.
(8) Sangvikar, Y. Sakamoto, J. Schlu¨ter, A. D. Chimia 2008, 62, 678.
See also ref 5 in which Figure 2 is used.
(9) For spontaneous chain folding, e.g., in polyethylene, see: Keller,
A.; O’Connor, A. Nature 1957, 180, 1289.
p-Phenylene diboronates 1a or 1b were selected as simple
counter monomers for these orienting studies (Scheme 1a).
(10) For a review, see: Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes,
T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893.
(11) See, for example: (a) Sastri, V. R.; Schulman, R.; Roberts, D. C.
Macromolecules 1982, 15, 939. (b) Vogel, T.; Blatter, K.; Schlu¨ter, A. D.
Makromol. Chem. Rapid Commun. 1989, 10, 427. (c) Schu¨rmann, B. L.;
Enkelmann, V.; Lo¨ffler, M.; Schlu¨ter, A. D. Angew. Chem., Int. Ed. Engl.
1993, 32, 123. (d) Thomas, S. W.; Long, T. M.; Pate, B. D.; Kline, S. R.;
Thomas, E. L.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 17976.
(12) See, for example: Yu, T.-B.; Bai, J. Z.; Guan, Z. Angew. Chem.,
Int. Ed. 2009, 48, 1097.
Scheme 1
.
Alternating Copolymerization of Straight and Hairpin
Monomers by SPC
(13) See, for example: Dou, X.; Pisula, W.; Wu, J.; Bodwell, G. J.;
Mu¨llen, K. Chem.sEur. J. 2008, 14, 240.
(14) See, for example: Ghadira, M. R. AdV. Mater. 1995, 7, 675.
(15) For a review, see: Grave, C.; Schlu¨ter, A. D. Eur. J. Org. Chem.
2002, 3075.
(16) Kissel, P.; Weibel, F.; Federer, L.; Sakamoto, J.; Schlu¨ter, A. D.
Synlett 2008, 1793.
(17) Morisaki, Y.; Imoto, H.; Miyake, J.; Chujo, Y. Macromol. Rapid
Commun. 2009, 30, 1094.
(18) The highest molar masses given in ref 17 (Mn ) 4,000; Pn ) 5.4)
refer to a fraction which represents 44% of the entire product mass. It is
reasonable to assume that the remaining material is of lower molar mass
and that the entire product has a representative Pn lower than 5.4. The authors
additionally provide NMR-based molar mass data. For this purpose, the
products were end-capped with appropriate groups. However, this method
can overestimate the molar mass because it is not evident that all chains
still have intact end groups at the time they were subjected to the end-
capping reaction.
(19) Vahlenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994, 195,
1933
.
(20) Traser, S.; Wittmeyer, P.; Rehahn, M. e-Polym. 2002. no. 032
.
(21) (a) Yokoyama, A.; Suzuki, H.; Kubota, Y.; Ohuchi, K.; Higash-
imura, H.; Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236. (b)
Beryozkina, T.; Boyko, K.; Khanduyeva, N.; Senkovskyy, V.; Horecha,
M.; Oertel, U.; Simon, F.; Stamm, M.; Kiriy, A. Angew. Chem., Int. Ed.
2009, 48, 2695.
A related recently published approach by Morisaki and Chujo
et al.17 using a xanthane derivative rather than an anthracene
led us disclose these findings in the present communication.
Org. Lett., Vol. 11, No. 18, 2009
4113

Figure 2. MALDI-TOF mass spectrum of the chloroform-soluble part of the SPC product 3a (matrix: trans-2-[3-(4-tert-butylphenyl)-2-
methyl-2-propenylidene]malononitrile (DCTB) with AgOTf) as fingerprint of step-growth mechanism (X ) 2,5-dihexyloxy-1,4-phenylene,
Y ) 1,8-anthrylene, B(pin) ) boron pinacolate, OTf ) triflate).
have the two end groups, the commonly accepted mechanism
of SPC requires (Figure 2). Thus, this spectrum supports the
view that the growth mechanism of SPC is step-growth in
this case. Also, the recently reported preferential 2-fold
oxidative addition22 does not seem to dominantly apply here.
In contrast to the spectrum in Figure 2, related MALDI mass
spectra often exhibit rather complex end group situations
which do not normally allow any such mechanistic conclu-
sions to be drawn. For a beautiful example by Janssen
et al., see ref 23.
fractions A-C were obtained by preparative GPC in isolated
yields of 91% (508 mg) for A, 4% (21 mg) for B, and 5%
(28 mg) for C.261H NMR and MS analyses proved fraction
C to consist of a ca. 1:1 atropisomeric mixture of the cyclic
1
dimers 4 and 5 (Scheme 1b). The H NMR signals of the
tri(ethyleneoxy) chains of these products are shifted upfield
in comparison to those of monomer 1b, which is attributed
to shielding effects caused by the adjacent anthrylene
moieties. The side chains are obviously positioned over these
units.
In order to let the products grow further without precipita-
tion, monomer 1b carrying the more flexible and longer
tri(ethyleneoxy) chains was designed and used instead of 1a.
The polymerization was performed under previously reported
SPC conditions (Pd[P(p-tolyl)3]3, NaHCO3, THF/water, 80
°C) with or without additional use of a Buchwald ligand.24,25
No precipitation was observed this time. The presence or
absence of the additional ligand did not have a sizable effect.
After workup, the crude product mixture was analyzed by
gel permeation chromatography (GPC) using chloroform as
eluent to give the trimodal main trace in Figure 3. Three
The cyclic dimers were used to assign the NMR spectrum
of the higher molar mass fraction A (Figure 4). Though
Figure 4.
1H NMR spectra of fractions C (a) and A (b). Starred
signals are from residual chloroform and impurities.
substantially broader, the signals of the polymer fraction appear
at very similar chemical shifts, supporting the polymer structure
3b as a whole but specifically also confirm its alternating
character. In addition, the UV absorption spectrum of fraction
A is nearly identical with the reference (Figure 5). The emission
Figure 3. GPC elution curve of the crude product mixture 3b from
monomers 1b and 2 using chloroform as eluent and an indication
how the fractionation was performed. Inset: GPC chromatogram
of fraction A using THF as eluent.
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Org. Lett., Vol. 11, No. 18, 2009

the slow mode is due to aggregate formation in HFIP.
Assuming a rodlike conformation, a hydrodynamically
effective diameter of 3b of d ) 2.0 nm, and a Schulz-Flory
molar mass distribution function, a weight average contour
length Lw ) 15-20 nm is calculated.27 Since the length per
repeat unit is l ) 0.5 nm, the weight average degree of
polymerization Pw is in the range of Pw ≈ 30 to 40 or Pn ≈
15 to 20, respectively. These numbers compare well to the
GPC results. Obviously, the larger mass per unit length of
3b as compared to the polystyrene standard (leading to a
shorter contour length) is compensated by the larger hydro-
dynamic volume due to the increased chain stiffness of 3b.
Finally, fraction B was not investigated.
Figure 5. UV absorption (solid lines) and emission spectra (broken
lines) of fractions A (black) and C (gray).
In conclusion, compound 2 with the peri-hydrogen atom
between the two functional groups mediating the growth is
in fact a viable monomer for SPC. For the counter monomer
1a mostly insoluble material 3a was obtained. The chloroform-
soluble part, however, allowed an unexpectedly deep insight
into the operative growth mechanism, proving it to be step-
growth in nature. For monomer 1b, soluble material 3b was
obtained which consisted of the new atropisomeric cyclic
dimers 4 and 5 and polymeric material with a weight average
molar mass of approximately 14000 g/mol referring to 90%
of the entire product mass. The structure of the resulting
polymer 3b was confirmed by NMR, UV absorption, and
emission spectroscopies. Given the complexity of the present
system and the fact that no 1,8-disubstituted anthracenes or
related compounds have ever been successfully subjected to
SPC, polymer 3b is considered a major step forward and
suggests that truly high molar mass material may be
achievable in the future after careful optimization. Whether
the structural features realized in the present polymer are
sufficient to enforce the aimed at secondary structure, which
would be the one with all phenylene units forming a
columnar stack, needs to be proven in the steps to come.
wavelength of fraction A is in a similar range as fraction C,
but its overall intensity is lower than the reference’s intensity
at a similar concentration (Figure 5). This can be attributed
to the lower rigidity of 3b due to its open linear structure
facilitating nonradiative deactivation. It should be mentioned
that the high molar mass onset of fraction A’s main peak
was not separated off because it was expected to be caused
by aggregates of individual chains which were not broken
apart by chloroform. This was proven correct by rerecording
this fraction now dissolved in THF. There were no detectable
mass losses by filtering the THF solution through Millipore
LCR filters (0.45 µm pore size) prior to injection into the
GPC apparatus. As the inset in Figure 3 shows, a nearly
monomodal elution curve was obtained. Based on polysty-
rene calibration, the average molar mass was estimated to
Mw ) 14000 (Pw ) 24) and Mn ) 7000 (Pn ) 12). Note
that this refers to 90% of the entire product mass and can
therefore be considered almost representative. The same
fraction was also analyzed by dynamic light-scattering (DLS)
in hexafluoro-2-propanol (HFIP) and in THF. Whereas in
HFIP a bimodal correlation function was observed with
particle radii of Rh ) 3.3 nm and Rh ) 65 nm, in THF a
monomodal decay led to a hydrodynamic radius of Rh )
3.9 nm. Obviously, the fast mode in HFIP solution corre-
sponds to the radius measured in THF and represents the
dimension of nonaggregated chains of polymer 3b, whereas
Acknowledgment. We thank Prof. P. Walde and his co-
workers (ETH Zurich) for help with the UV/vis absorption
and fluorescence measurements, D. Paripovic and P. Kissel
(ETH Zurich) for skillful help in some of the syntheses, M.
Colussi (ETH Zurich) for GPC analysis, and Dr. X. Zhang
and L. Bertschi (ETH Zurich) for MALDI-TOF MS mea-
surements.
(22) (a) Sinclair, D. J.; Sherburn, M. S. J. Org. Chem. 2005, 70, 3730.
(b) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127, 10006. (c) Weber,
S. K.; Galbrecht, F.; Scherf, U. Org. Lett. 2006, 8, 4039.
(23) Jayakannan, M.; van Dongen, J. L. J.; Janssen, R. A. J. Macro-
molecules 2001, 34, 5386.
Supporting Information Available: All experimental
procedures including NMR and HR-MS for new compounds
as well as the DLS correlation functions.This material is
available free of charge via the Internet at http://pubs.acs.org.
(24) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L.
J. Am. Chem. Soc. 2005, 127, 4685.
(25) Kandre, R.; Schlu¨ter, A. D. Macromol. Rapid Commun. 2008, 29,
1661.
OL9016047
(26) These values are calculated by ignoring possible presence of end
functional groups of the products.
(27) Schmidt, M. Macromolecules 1984, 17, 553.
Org. Lett., Vol. 11, No. 18, 2009
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