Efficient synthesis of rigid ladder polymers via palladium catalyzed annulation

Article abstract of DOI:10.1021/ja5110415

We report a new method to synthesize rigid ladder polymers using efficient palladium catalyzed annulation reactions with low catalyst loading (1 mol %). Rigid ladder polymers with benzocyclobutene backbone linkages can be synthesized from copolymerization of readily accessible aryl dibromides and norbornadiene or polymerization of AB type monomers bearing norbornene and aryl bromide or triflate moieties. High molecular weight (10-40 kDa) rigid ladder polymers can be obtained with complete monomer conversions. Diverse monomers also gave different, fixed ladder polymer conformations. The ladder polymers exhibited excellent thermal stability, high carbonization yield, and large intrinsic porosity.

Full text of DOI:10.1021/ja5110415

Communication
pubs.acs.org/JACS
Efficient Synthesis of Rigid Ladder Polymers via Palladium Catalyzed
Annulation
Sheng Liu, Zexin Jin, Yew Chin Teo, and Yan Xia*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
rigid ladder polymers in nearly quantitative conversions using
efficient Pd catalyzed annulation reaction.
ABSTRACT: We report a new method to synthesize rigid
ladder polymers using efficient palladium catalyzed
annulation reactions with low catalyst loading (1 mol
%). Rigid ladder polymers with benzocyclobutene back-
bone linkages can be synthesized from copolymerization of
readily accessible aryl dibromides and norbornadiene or
polymerization of AB type monomers bearing norbornene
and aryl bromide or triflate moieties. High molecular
weight (10−40 kDa) rigid ladder polymers can be
obtained with complete monomer conversions. Diverse
monomers also gave different, fixed ladder polymer
conformations. The ladder polymers exhibited excellent
thermal stability, high carbonization yield, and large
intrinsic porosity.
Catellani and co-workers explored Pd catalyzed reactions
between aryl bromides and norbornene (NB).19 A stable
palladacycle 1 from C−H activation on the arene can be formed
in the catalytic cycle, which can undergo reductive elimination to
form an interesting benzocyclobutene (BCB) product 2,20−23 as
well as undergo pathways B and C, leading to other products
(Scheme 1).19 Although rarely explored in small molecule
Scheme 1. Pd Catalyzed Annulation: Possible Products and
Selectivity
central challenge in polymer chemistry is to precisely and
Aefficiently control polymer architectures, which strongly
influence the physical properties and applications of polymeric
materials. In contrast to conventional polymers connected by
single bonds, ladder polymers represent a unique architecture,
where each repeat unit is connected by two bonds,1,2 resulting in
much more restricted chain conformations. Rigid ladder
polymers have recently been shown to be very promising
materials for efficient gas separation, and the high rigidity was
critical for selectivity in gas separation.3,4 We also envision a wide
range of applications for rigid ladder polymers resulting from the
high anisotropy and intrinsic porosity, including dielectric
materials and nanostructured materials. However, very few
methods exist to synthesize ladder polymers by simultaneous
formation of two-bond linkages. Schluter and Swager groups
̈
pioneered the synthesis of ladder polymers via Diels−Alder
reactions,5−7 but the polymerizations required very high pressure
and/or temperature. McKeown and Budd groups recently
synthesis20−23 and never in polymer synthesis, we were intrigued
by the BCB product and envisioned it as a novel motif to build
rigid ladder polymers. Furthermore, we hypothesized that
blocking the 2,5-positions of the aryl halide may inhibit the
reaction pathways B and C due to steric congestion and give only
the desired BCB product. Exclusive selectivity and high efficiency
for the annulation pathway is a prerequisite to utilize this reaction
for polymer synthesis.
developed ladder polymer synthesis based on Troger’s base
̈
formation4,8 and double aromatic nucleophilic substitution.3,9
Multistep monomer syntheses were often required and no
stereocontrol could be achieved in the few reported methods.
We became interested in exploring powerful and versatile
transition metal catalysis to synthesize novel polymers from
readily accessible building blocks. Transition metal catalysis has
continuously stimulated the emergence of new polymer
chemistry to gain access to novel polymers and new
materials.10−14 New catalytic methods also enabled diverse
monomers to be used for the synthesis of new types of
polymers.15−18 Herein, we report the first catalytic synthesis of
Our initial attempt on a model reaction between 2-
bromotoluene and norbornadiene (NBD) using 10 mol %
Pd(PPh3)4 and K2CO3 resulted in complete consumption of
Received: October 27, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja5110415 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
starting materials but led to complex product mixtures. To test
our hypothesis of blocking reaction pathways B and C, we started
with commercially available 2,5-dimethyl-bromobenzene and
NBD. Gratifyingly, after initial optimization, the desired BCB
product 3 was formed in >98% conversion using Pd(OAc)2,
PPh3, and Cs2CO3 at 130 °C, and the reaction was highly
stereoselective with exclusively the exo-isomer formed, as
indicated by the nuclear Overhauser effect (NOE) experiment
(see Supporting Information (SI) Figure S4). The reaction was
sensitive to the sterics of the phosphine ligand as bulkier ligands
gave lower conversions (Table 1, entry 3−4). N-Heterocyclic
Table 2. Synthesis of Various Benzocyclobutene Ladder
Polymers
Table 1. Optimization of a Model Reaction for Ladder
a
Polymerization
b
c
entry
ligand
PPh3
temp (°C)
cat (mol %)
conv (%)
1
2
3
4
5
6
7
8
130
130
130
130
130
115
100
115
115
115
5
>98
94
PCy3
5
PtBu3
5
68
P(o-Tol)3
5
<5
d
NHC
5
<5
PPh3
PPh3
PPh3
PPh3
PPh3
5
>98
80
5
1
>98
91
e
9
0.1
0.1
f
10
>98
a
a
b
b
c
Reactions were performed using Pd(OAc)2, 2 equiv of ligand to Pd,
entry
monomer(s)
4a + NBD
Mn (kDa)
PDI
DP
and 1 equiv Cs2CO3, at [NBD] = 0.1 M in toluene for 5 h in a sealed
1
2
3
4
29
13
16
36
13
12
14
1.38
1.34
1.58
1.36
1.66
1.28
1.28
150
31
b
tube, unless otherwise noted. Based on the mole ratio of Pd to halide.
4b + NBD
4a/4b (9:1) + NBD
4c + NBD
4d + NBD
4e
c
d
1
Determined by H NMR of crude reaction mixture. PEPPSI-IPr Pd
e
f
52
catalyst used. Reaction run for 24 h. Reaction run in THF for 24 h.
160
44
d
5
e
carbene (NHC) based PEPPSI catalyst, although highly efficient
in many cross-coupling reactions,24 gave very low conversion
(Table 1, entry 5). Under optimized conditions, we could lower
the catalyst loading to only 0.1 mol % and reaction temperature
to 115 °C and still obtain 91% and complete conversions to 3 in
toluene and THF, respectively, after 24 h. The turnover number
(TON) was measured to be 1176 and turnover frequency (TOF)
to be 49 h−1. The low catalyst loading and the absence of
detectable side reactions open the door for the application of this
annulation reaction to polymer chemistry and scalable materials
synthesis.
6
62
d
7
4f
91
a
Reaction was conducted using 1 mol % Pd(OAc)2, 2 mol % PPh3,
and 1 equiv of Cs2CO3, at [monomer] = 0.5 M in toluene at 115 °C
for 24 h in a sealed tube, unless otherwise noted. Determined by
GPC MALLS analysis in THF. Degree of polymerization = Mn/MW
of repeat unit. [monomer] = 1.0 M, reaction run for 12 h. Reaction
run in THF.
b
c
d
e
We next proceeded with polymerization of NBD and various
aryl dibromides (Table 2). Polymers were obtained with
complete monomer conversions for attempted aryl dibromides
4a−d. The resulting ladder polymers can be dissolved in THF,
and their absolute MW was measured by gel permeation
chromatography (GPC) coupled with a multiangle laser light
scattering (MALLS) detector. The efficient polymerization
resulted in ladder polymers with high MWs in the range of
10−40 kDa and relatively narrow, monomodal MW distribution
(Table 2 and Figure 1). The high MW and relatively low PDI are
particularly gratifying since the polymerization is presumably
1
step-growth polymerization. H and 13C NMR spectroscopy
Figure 1. GPC traces of synthesized ladder polymers in Table 2.
indicated exclusive BCB ladder backbone linkage and the
expected end group signals of the olefin from NB and of the
arene protons from aryl bromide and debrominated arene
B
dx.doi.org/10.1021/ja5110415 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(Figure 2 and see SI). The resulting ladder polymers are
generally soluble in nonpolar solvents, such as THF, DCM, and
Figure 3. Minimum energy segment (6 repeat units) conformations of
ladder polymers, poly(4a), poly(4d), and poly(4f), using molecular
dynamics simulation.
ladder backbones as indicated by 1H and 13C NMR spectra (see
SI). Their polymerizations needed to be stopped after 12 h to
ensure the polymer’s solubility in THF for GPC characterization.
Prolonged polymerizations of 4e and 4f resulted in polymers that
were only soluble in chloroform, which presumably had higher
MW. Interestingly, monomer 4f induces a fixed bending angle of
∼120° at each repeating unit of poly(4f), resulting in a kinked
ribbon backbone conformation (Figure 3). The kinked ribbon
backbone results in a more expanded conformation. This was
evidenced from comparing the GPC traces of poly(4e) and
poly(4f), which have similar absolute MW and PDI (Mn = 12 and
14 kDa, respectively), but poly(4f) clearly shifted to an earlier
elution time (Figure 1), indicating its larger hydrodynamic
volume than that of poly(4e).
In addition, we were also able to synthesize ladder polymers
with only NB end groups, when NB was intentionally used in 10
mol % excess to aryl bromide (see SI Figure S25). NB is a
convenient reactive group that can be used for cross-linking or
synthesizing block copolymers.
The rigid, nonconjugated ladder backbone renders these
polymers’ interesting properties. We took polymer poly(4a) as
an example for detailed property investigation. Differential
scanning calorimetry (DSC) analysis of poly(4a) showed no
phase transition up to 200 °C, indicating extremely glassy
backbone. Thermal gravimetric analysis (TGA) indicated that
the polymer was stable up to 315 °C, and lost only 14% weight at
496 °C, which we attributed to the loss of the two methyl
substituents. Upon further heating under argon, this polymer can
be carbonized with nearly 81% mass retention at 800 °C (see SI
Figure S1). In comparison, a common carbon precursor polymer,
polyacrylonitrile, has a carbonization yield around 50%.25 The
very high carbonization yield of poly(4a) is a result of the high
degree of unsaturation of the polymer. Furthermore, frustrated
packing of rigid ladder chains resulted in large intrinsic porosity
with the surface area of 620 m2/g for poly(4a) measured by
Brunauer−Emmett−Teller (BET) analysis (see SI Figure S2).
We also performed density functional theory (DFT) calculations
(B3LYP/6-31G* basis set) on the bond angle between BCB and
NB motifs. The rapidly increasing energy barrier to the bond
bending suggested high rigidity for the BCB linkage (see SI
Figure S3). The combined solubility, thermal stability, high
rigidity, and large porosity may make robust BCB−NB ladder
polymers promising materials for a range of engineering
applications.
Figure 2. 1H NMR spectra of model compound 3 (a) and poly(4a) (b);
13C NMR spectra of model compound 3 (c) and poly(4a) (d) with their
assignments in CDCl3.
chloroform. Uniquely, the straight BCB linkage keeps the
backbone of poly(4a−c) in a bowing-ribbon conformation
(Figure 3). The overall polymer chain may resemble a disc shape,
which we plan to study in future scattering experiments. The
biphenyl monomer 4d represents one type of easily accessible
monomers that can give the resulting ladder polymers a certain
degree of bending freedom in two dimensions resulting from the
restricted biphenyl bond (Figure 3). Furthermore, these
polymers can be synthesized on gram scale thanks to the facile
access to a large number of aryl bromides, paving the way for
their materials applications.
To eliminate the need for precise loading of 1:1 ratio of AA and
BB types of monomers for step-growth polymerization, we also
synthesized AB-type monomers 4e and 4f that contain NB and
aryl triflate or bromide groups. Both 4e and 4f polymerized with
no detectable monomer remaining and produced the expected
In summary, we report efficient synthesis of rigid ladder
polymers via Pd catalyzed annulation between various readily
available aryl bromides and NBD. We found blocking
accessibility of other reaction pathways produced ladder
C
dx.doi.org/10.1021/ja5110415 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(20) Catellani, M.; Chiusoli, G. P.; Ricotti, S. J. Organomet. Chem. 1985,
296, C11−C15.
(21) Catellani, M.; Ferioli, L. Synthesis 1996, 6, 769−772.
(22) Hulcoop, D. G.; Lautens, M. Org. Lett. 2007, 9, 1761−1764.
(23) Chai, D. I.; Thansandote, P.; Lautens, M. Chem.Eur. J. 2011, 17,
8175−8188.
(24) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int.
Ed. 2007, 46, 2768−2813.
(25) Burchell, T. D. Carbon Materials for Advanced Technologies;
Elsevier Science: Oxford, U.K., 1999.
polymers with rigid BCB linkage in quantitative conversions at
low catalyst loading. We are currently expanding monomer
scopes to control polymer conformation and porosity and
characterizing the long-range chain conformations. Exploration
of these ladder polymers’ applications is also underway.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, 1H and 13C NMR spectra, TGA and BET
characterizations, and calculated bending barrier for the ladder
linkage. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*yanx@stanford.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Army Research Laboratory
and the U.S. Army Research Office under grant number
W911NF-14-1-0062 and Stanford University. We thank Prof.
Y. Cui for using Micromeritics ASAP 2020 and Prof. T. D. P.
Stack for using computer cluster. Y.C.T. thanks A*STAR
Singapore for a graduate fellowship.
REFERENCES
■
(1) Overberger, C. G.; Moore, J. A. Advanves in Polymer Science;
Springer: Berlin, Germany, 1970.
(2) Zhou, Q.; Yan, S.; Han, C. C.; Xie, P.; Zhang, R. Adv. Mater. 2008,
20, 2970−2976.
(3) Bezzu, C. G.; Carta, M.; Tonkins, A.; Jansen, J. C.; Bernardo, P.;
Bazzarelli, F.; McKeown, N. B. Adv. Mater. 2012, 24, 5930−5933.
(4) Carta, M.; Malpass-Evans, R.; Croad, M.; Rogan, Y.; Jansen, J. C.;
Bernardo, P.; Bazzarelli, F.; McKeown, N. B. Science 2013, 339, 303−
307.
(5) Schluter, A.-D. Adv. Mater. 1991, 3, 282−291.
̈
(6) Thomas, S. W.; Long, T. M.; Pate, B. D.; Kline, S. R.; Thomas, E. L.;
Swager, T. M. J. Am. Chem. Soc. 2005, 127, 17976−17977.
(7) Chen, Z.; Amara, J. P.; Thomas, S. W.; Swager, T. M.
Macromolecules 2006, 39, 3202−3209.
(8) Carta, M.; Malpass-Evans, R.; Croad, M.; Rogan, Y.; Lee, M.; Rose,
I.; McKeown, N. B. Polym. Chem. 2014, 5, 5267−5272.
(9) Budd, P. M.; Elabas, E. S.; Ghanem, B. S.; Makhseed, S.; McKeown,
N. B.; Msayib, K. J.; Tattershall, C. E.; Wang, D. Adv. Mater. 2004, 16,
456−459.
(10) Bielawski, C. W.; Grubbs, R. H. Prog. Polym. Sci. 2007, 32, 1−29.
(11) Schrock, R. R. Acc. Chem. Res. 2014, 47, 2457−2466.
(12) Matyjaszewski, K.; Tsarevsky, N. V. J. Am. Chem. Soc. 2014, 136,
6513−6533.
(13) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009, 109,
4963−5050.
(14) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.;
Coates, G. W. Chem. Rev. 2014, 114, 8129−8152.
(15) Shintani, R.; Iino, R.; Nozaki, K. J. Am. Chem. Soc. 2014, 136,
7849−7852.
(16) Ito, S.; Takahashi, K.; Nozaki, K. J. Am. Chem. Soc. 2014, 136,
7547−7550.
(17) Sugiyama, Y.-K.; Kato, R.; Sakurada, T.; Okamoto, S. J. Am. Chem.
Soc. 2011, 133, 9712−9715.
(18) Lee, I.-H.; Kim, H.; Choi, T.-L. J. Am. Chem. Soc. 2013, 135,
3760−3763.
(19) Catellani, M.; Motti, E.; Della Ca’, N. Acc. Chem. Res. 2008, 41,
1512−1522.
D
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