Preparation and structure of a tubular addition polymer: A true synthetic nanotube

Article abstract of DOI:10.1021/ja209792f

The structure of a synthetic nanotube prepared by the solid-state polymerization of a stacked column of diacetylene-based macrocycles has been determined. A polyether macrocycle monomer with two parallel diacetylene functionalities was prepared. Its cryst

Full text of DOI:10.1021/ja209792f

Communication
pubs.acs.org/JACS
Preparation and Structure of a Tubular Addition Polymer: A True
Synthetic Nanotube
Te-Jung Hsu, Frank W. Fowler, and Joseph W. Lauher*
Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400, United States
S
* Supporting Information
ABSTRACT: The structure of a synthetic nanotube
prepared by the solid-state polymerization of a stacked
column of diacetylene-based macrocycles has been
determined. A polyether macrocycle monomer with two
parallel diacetylene functionalities was prepared. Its crystal
structure revealed that the compound crystallizes with
structural parameters suitable for topochemical polymer-
ization. Slow annealing of a single crystal for 35 days
brought about a single-crystal-to-single-crystal polymer-
Figure 1. One possible route to a tubular polymer is via a
ization resulting in the first experimentally determined
structure of a tubular addition polymer.
topochemical polymerization of a diacetylene-containing macrocycle.
For the reaction to proceed, the macrocycles would need to be spaced
near the 4.9 Å expected for polymers with neighboring diacetylene
C1−C4′ carbon atoms close to the van der Waals contact distance of
3.4 Å. The application of heat (or radiation) should bring about the
polymerization.
arbon nanotubes are widely regarded as the prototypical
C
and star players in today’s nanotechnology revolution.1,2
These marvels of chemistry are readily available in kilogram
quantities, and potential applications abound. Single-walled
carbon nanotubes have a diameter of essentially one nanometer
but can have lengths in the millimeter range. From a structural
point of view, it is difficult to imagine a better molecular tube.
Chemists have considered this a challenge as they have made
many attempts to design and synthesize their own imaginative
versions of tubular molecules. In the nanoworld, the structures
have often been based upon the ideas of molecular self-
assembly and supramolecular chemistry. Numerous tubular
structures have been prepared by the self-assembly of a wide
variety of substrates. Macrocyclic peptides, carbohydrates, and
many purely synthetic monomers form tubular structures via
hydrogen bonds.3−5 Various molecules and polymers have been
found to coil naturally into helical structures with a tubular
core.6,7 Aromatic macrocycles have been designed to self-
assemble via π−π stacking to give tubes.8,9 Tubular structures
held together by covalent bonds have been formed from
cyclodextrins threaded on to a polyethyleneglycol chain.10
Ghadiri used olefin metathesis to dimerize peptide macrocycles,
yielding short tubular structures.11
What kind of monomer would be needed to give a tubular
addition polymer? The obvious candidate would be a
macrocycle, one that would self-assemble into a polymerizable
stack. Since our own work has been focused on diacetylenes, we
considered a macrocycle with a pair of diacetylene function-
alities (Figure 1). The difficulty is controlling the spacing. For
polymerization to take place, the monomers need to be spaced
at a distance near 4.9 Å.
This approach is not new. All sorts of diacetylene
macrocycles are known in the literature12−15 The first reported
attempt at macrocycle polymerization was by Vollhardt and
Youngs,13 but their system had a long repeat distance of over 6
Å. More recently, Shimizu12 reported the polymerization of a
diacetylene macrocycle that self-assembled via amide hydrogen
bonds. Their spacing was 4.98 Å, very close to the ideal, but a
loss of crystallinity prevented the determination of the polymer
structure. Nagasawa16 just reported the polymerization of
various gels also formed from amide diacetylene macrocycles.
We designed a system based upon π−π stacking of an
aromatic system. A simple π−π stack directly aligned in a
direction perpendicular to an aromatic ring would give a van
der Waals spacing comparable to that in graphite (3.4 Å).
However, a slipped π−π stack is much more common, and
distances in excess of 4 Å are found. A number of aromatic
derivatives of hexa-2,4-diyne-1,6-diol are known to stack at
distances commensurate with the desired 4.9 Å spacing.17 We
reasoned that a macrocycle built from aromatic rings and hexa-
2,4-diyne-1,6-diol might give such a stacking. This led us to
One promising approach is the preparation of a diacetylene-
based tubular addition polymer (Figure 1).12 An advantage of
tubular addition polymers is that they would be robust because
of the continuous network of covalent bonds throughout the
tubular network. This is a property that they would share with
carbon nanotubes. However, a potential advantage of tubular
addition polymers that would not shared with conventional
carbon nanotubes is that they would be prepared by organic
synthesis, which would allow for continuous structural variation
for different applications.
Received: October 18, 2011
Published: December 13, 2011
© 2011 American Chemical Society
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Communication
Figure 2. Synthesis of macrocycle 1 from readily available reagents. In the crystalline state, the compound undergoes a topochemical polymerization
to give the tubular polymer 2. Drawing 2 A shows the original 34-atom macrocycle as it appears embedded within the polymer. After polymerization,
a new and smaller 30- atom macrocycle 2 B can also be identified. The final drawing 2 C should be compared to the actual structure determined by
single-crystal X-ray diffraction (Figure 3D).
design compound 1, a 34-atom macrocyclic ether with two
potentially polymerizable diacetylene functionalities.
accompanied by a color change to a deep-red, nearly black
color. Crystals were examined on the X-ray diffractometer at
various stages as the crystallinity was lost, but no structural
change could be detected before the crystals became
amorphous.
Differential scanning calorimetry (DSC) studies of 1 showed
a single exothermic event at 106 °C consistent with a solid-state
polymerization reaction. The cooling and reheating curves of
the polymerized material were featureless, indicating a
nonreversible solid- state change.
Compound 1 was synthesized in three steps from relatively
common starting materials (Figure 2). The difficult step was
the last step, where the terminal diacetylene 3 was subjected to
a cyclic dimerization using a copper-catalyzed oxidative Hay
coupling to give the desired macrocycle 1 in 46% yield. A
smaller 10% yield of the cyclomonomer was isolated, a small
amount of the cyclic trimer was characterized, and the rest of
the yield was likely polymeric material. The overall yield of
compound 1 starting from 1,3-diiodobenzene was 34%.
The next step in the project required the preparation of
single crystals of sufficient size and quality for a single-crystal X-
ray diffraction study. A number of solvent systems were
investigated, the best turning out to be 1:1 methylene chloride/
hexane. Slow evaporation of a solution of 1 in this mixed
solvent at room temperature gave pale-orange needles. When
the evaporation was carried out at an elevated temperature of
40°, some of the crystals had an initial orange-red color (Figure
3A) but in a short time developed a darker color with a blue
cast. Subsequent experiments showed that these two different
crystalline forms were two different polymorphs of macrocycle
1.
Single-crystal X-ray diffraction experiments revealed that the
crystals formed at room temperature were monoclinic (Figure
3B), with the molecules of macrocyclic monomers 1 stacked in
accordance with the design shown in Figure 1. The repeat
distance of 5.09 Å was longer than the ideal repeat distance of
4.9 Å but within the range reported for other successful
polymerizations.18 Indeed, the crystals were very sensitive to
heat. Gentle heating (50 °C) rapidly converted the crystals to
an amorphous (and nondiffracting) state. When the crystals
were kept at room temperature, they became completely
amorphous in about 30 days. These transformations were
The crystals of 1 grown at 40 °C, the second polymorph,
belong to the triclinic space group P1 The molecules of the
̅
triclinic form had a different molecular conformation than the
molecules of the original monoclinic form, but they had a
similar stacked structure (Figure 3C). The molecular repeat
distance in the triclinic form was 4.84 Å, quite close to the ideal
value of 4.9 Å. Even though the repeat distance within the
triclinic polymorph was a bit closer to the ideal value, the
triclinic crystals were somewhat more robust than the
monoclinic crystals. Initial experiments showed that the triclinic
crystals were stable at room temperature but rapidly became
amorphous upon heating to 50 °C.
In previous work with noncyclic diacetylenes, we found that
slow annealing of a diacetylene monomer crystal at a relatively
low temperature can bring about a single-crystal-to-single-
crystal polymerization that is otherwise unobservable.19
Following this idea, one particularly well formed crystal of
the triclinic polymorph of 1 was chosen, and the monomer
structure was verified using single-crystal X-ray diffraction. The
crystal was then subjected to slow annealing at 40° in a
temperature-controlled oven. At various intervals the crystal
was removed from the oven and returned to the diffractometer,
and the structure was redetermined. Signs of polymerization
could be seen after 3 days of heating, as additional peaks
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Communication
Figure 4. (A) The macrocycle monomers have a rectangular cross
section of 7.5 Å × 9.5 Å. (B) The monomers stack at an angle such
that the cross section of the tubular polymer 2 is reduced with a 3.22 Å
contact between opposite methylene groups.
Figure 3. (A) Freshly formed crystals of the triclinic polymorph of
macrocycle 1 grown at 40°. Upon sitting, these crystals developed a
darker blue cast. The longest crystals in the picture were about 2 mm
long with a thickness of only a few hundredths of a millimeter. (B)
Observed molecular stacking in the monoclinic form of monomer 1.
The repeat distance of 5.09 Å was longer than the ideal polymerization
value of 4.9 Å. (C) Observed molecular stacking in the triclinic form of
monomer 1. The observed repeat distance of 4.84 Å was much closer
to the ideal value. (D) Structure of polymer 2 obtained by slow
annealing of the triclinic crystals at 40°.
This is in keeping with the well-known principle “Natura
abhorret vacuum”:20 one simply does not expect to find empty
space within condensed matter. The molecular tilting is
expected from a packing perspective, because direct stacking
of flat molecules would bring atoms into direct “bump to
bump” contact. Tilting removes these unfavorable contacts and
also brings the neighboring C1 and C4′ atoms into the close
contact needed for the polymerization step (Figure 1).
It is interesting that two polymorphs of monomer 1 were
isolated. The conformations of the molecules in the two
polymorphs are different. Each has inversion symmetry with
four independent ether C−O bonds. In the monoclinic
polymorph, three of these bonds are gauche and one is anti.
In the triclinic polymorph, all four independent C−O bonds are
gauche. Despite this major conformation difference, both
polymorphs have the molecules stacked in accordance with the
parameters needed for the polymerization. The monoclinic
form has a longer repeat distance of 5.09 Å versus a distance of
4.84 Å in the triclinic form. The important neighboring C1−
C4′ contact between potential reacting atoms (3.7 Å) is shorter
in the monoclinic structure than in the triclinic structure (3.9
Å). The “ideal” distances are 4.9 Å for the repeat and 3.4 Å for
the contact. Both the monoclinic and triclinic forms appear to
undergo the polymerization reaction, but only for the triclinic
form is this a crystal-to-crystal process. The repeat distance
appears to be the most important parameter. An incommensu-
rate monomer−polymer repeat distance would naturally lead to
more crystal motion and presumably be more likely to yield an
amorphous solid.18
appeared in the diffraction-determined electron density maps.
As the heating continued day by day, the percentage of polymer
could be seen to increase, accompanied by a decrease in
monomer content as well as a decrease in crystal quality. After
35 days of annealing at 40 °C, the crystal was returned to the
diffractometer for the 20th and last successful crystal structure
determination. This final structural analysis of the annealed
crystal showed only the desired polymer structure (Figure 3D).
Because of a lack of data, the structure of polymer 2 was not of
high quality, but the polymerization appeared to be
substantially complete. Further heating of the crystal gave
only a scattering of diffraction data so the annealing experiment
was brought to an end.
The molecular structure of polymer 2 (Figure 3D) is in
complete accordance with the design shown in Figure 1. The
macrocyclic monomers underwent a double-addition polymer-
ization reaction to give a tubular polymer. The inner cross
section of an idealized monomer is a roughly 11 Å × 12 Å oval
(or roughly a 1 nm circle), which is large enough for the
monomer to hold a guest molecule within its interior. However,
the molecules crystallize to form a tube with an empty interior,
with the macrocycle adopting a conformation with a smaller 7.5
Å × 11.5 Å cross section (Figure 4A). Furthermore, the rings
tilt when they stack, closing down the tube interior and
bringing opposing methylene groups into contact (Figure 4B).
Freshly made samples of 1 rapidly acquired color indicative
of the polymerization even though the amount of polymer was
at a level not detected by the single-crystal X-ray studies.
However, evidence of this initial polymerization could be seen
in a Raman spectrum of a fresh sample, which showed the two
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Journal of the American Chemical Society
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(10) Harada, A.; Hashidzume, A.; Yamaguchi, H.; Takashima, Y.
Chem. Rev. 2009, 109, 5974.
strong resonance-enhanced bands characteristic of polydiace-
tylenes.12 The Raman spectrum of the fully polymerized
crystals of 2 showed these two bands, at 1484 and 2095 cm−1,
at a much greater intensity.
(11) Hartgerink, J. D.; Clark, T. D.; Ghadiri, M. R. Chem.Eur. J.
1998, 4, 1367.
(12) Xu, Y. W.; Smith, M. D.; Geer, M. F.; Pellechia, P. J.; Brown, J.
C.; Wibowo, A. C.; Shimizu, L. S. J. Am. Chem. Soc. 2010, 132, 5334.
(13) Baldwin, K. P.; Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.;
Vollhardt, K. P. C.; Youngs, W. J. Synlett 1995, 1215.
(14) Haley, M. M.; Brand, S. C.; Pak, J. J. Angew. Chem., Int. Ed. Engl.
1997, 36, 836.
Once the polymerization occurred, we found the crystals to
be totally insoluble in a wide range of standard solvents. One
would expect an isolated tube to be a flexible molecule able to
incorporate solvent or other small molecules within the interior.
Because of the complete lack of solubility, we have not yet been
able to verify this hypothesis. The synthesis of 2 is
straightforward from readily available starting materials, and
more soluble analogues should be possible using similar
synthetic methodology. Soluble analogues should have the
open void space that would increase the potential applicability
of molecular tubes.21
Should one call polymer 2 a synthetic nanotube? We believe
so. The interior is 1 nm across in the long direction, and the
outer dimensions are about 1 nm × 2 nm; the length is
indeterminately long. In an idealized conformation, the tubes
would have a roughly circular cross section of about 1 nm, very
similar to that of a single-walled carbon nanotube.
(15) Suzuki, M.; Comito, A.; Khan, S. I.; Rubin, Y. Org. Lett. 2010,
12, 2346.
(16) Nagasawa, J.; Yoshida, M.; Tamaoki, N. Eur. J. Org. Chem. 2011,
2247.
(17) Wegner, G. Z. Naturforsch., B: J. Chem. Sci. 1969, B 24, 824.
(18) Lauher, J. W.; Fowler, F. W.; Goroff, N. S. Acc. Chem. Res. 2008,
41, 1215.
(19) Li, Z.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 2009, 131,
634.
(20) Rabelais, F. Gargantua and Pantagruel; 1564; Vol. 5.
(21) Dalgarno, S. J.; Thallapally, P. K.; Barbour, L. J.; Atwood, J. L.
Chem. Soc. Rev. 2007, 36, 236.
The significance of this new polymer is that it is the first
example of a structurally characterized tubular addition polymer
and that a general strategy for synthesizing any tubular polymer
has been confirmed. One can imagine that the power of organic
synthesis may lead to similar polymers that can be tailor-
designed for a wide range of applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization details, Raman spectra of 1 and
2, DSC study of 1, color photographs of crystals of 1 and 2,
representations of the molecular conformations of 1, and CIF
files for the two polymorphs of monomer 1 and the structure of
polymer 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Joseph.Lauher@stonybrook.edu
■
ACKNOWLEDGMENTS
■
We acknowledge the National Science Foundation for their
support of this work (CHE 0911338) and for their support of
the purchase of our X-ray diffractometer (CHE-0840483). We
thank Xiao Wei Li for the DSC measurement.
REFERENCES
■
(1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002,
297, 787.
(2) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev.
2006, 106, 1105.
(3) Gattuso, G.; Menzer, S.; Nepogodiev, S. A.; Stoddart, J. F.;
Williams, D. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 1451.
(4) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew.
Chem., Int. Ed. 2001, 40, 988.
(5) Pasini, D.; Ricci, M. Curr. Org. Synth. 2007, 4, 59.
(6) Hecht, S.; Khan, A. Angew. Chem., Int. Ed. 2003, 42, 6021.
(7) Yashima, E.; Maeda, K.; Furusho, Y. Acc. Chem. Res. 2008, 41,
1166.
(8) Hill, J. P.; Jin, W. S.; Kosaka, A.; Fukushima, T.; Ichihara, H.;
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004,
304, 1481.
(9) Moore, J. S. Acc. Chem. Res. 1997, 30, 402.
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