Synthesis of the stable ordered conjugated polymer poly(dibromodiacetylene) from an explosive monomer

Article abstract of DOI:10.1002/anie.201504713

Dibromobutadiyne is an extremely unstable compound that explodes at room temperature, even under inert atmosphere. This instability has limited the studies of dibromobutadiyne almost entirely to spectroscopic characterization. Here we report an approach t

Full text of DOI:10.1002/anie.201504713

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International Edition: DOI: 10.1002/anie.201504713
German Edition: DOI: 10.1002/ange.201504713
Topochemical Polymerization
Synthesis of the Stable Ordered Conjugated Polymer
Poly(dibromodiacetylene) from an Explosive Monomer
Hongjian Jin, Christopher N. Young, Gary P. Halada, Brian L. Phillips, and Nancy S. Goroff*
Abstract: Dibromobutadiyne is an extremely unstable com-
pound that explodes at room temperature, even under inert
atmosphere. This instability has limited the studies of dibro-
mobutadiyne almost entirely to spectroscopic characterization.
Here we report an approach to control the reactivity of
dibromobutadiyne, via topochemical reaction in cocrystals,
leading to the ordered polymer poly(dibromodiacetylene),
PBDA. At low temperatures (À15 to À188C), dibromobuta-
diyne can form cocrystals with oxalamide host molecules
containing either pyridyl or nitrile side groups, in which
halogen bonds align the dibromobutadiyne monomers for
topochemical polymerization. The cocrystals with the bis-
(nitrile) oxalamide host undergo complete ordered polymeri-
zation to PBDA, demonstrated by solid-state MAS-NMR,
Raman, and optical absorption spectroscopy. Once formed, the
polymer can be separated from the host; unlike the monomer,
PBDA is stable at room temperature.
a conjugated polymer containing only carbon and bromine,
the first PDA with bromine substituents.
The solid-state polymerization of diynes is a well-estab-
lished reaction that provides a path to the highly ordered
PDAs, in which the conjugated backbone contains alternating
double and triple carbon–carbon bonds. Polymerization of
crystalline monomers provides PDAs with excellent optical
and electronic properties, leading to many different applica-
tions, including colorimetric,[5] chemical[6] and biological
sensors,[7] as well as nanoelectronics.[8]
Achieving a high degree of polymerization in a solid-state
reaction requires an appropriate spatial arrangement of the
monomers. As discovered by Wegner[9] and further described
by Baughman,[10] if diynes are aligned properly in the solid
state at a distance commensurate with the repeat distance in
the target polymer, they can polymerize topochemically
(according to their arrangement in space). However, few
diynes will align appropriately by themselves for this reaction.
For diynes that do not self-organize in this way, Fowler and
Lauher have developed a host–guest approach to make
topochemical polymerization possible.[11] The oxalamide
functional group forms self-complementary hydrogen bond-
ing networks with a repeat distance (4.9–5.0 Š) that corre-
sponds to the spatial repeat required for 1,4-topochemical
polymerization. Using non-covalent interactions between the
diyne as guest and an appropriate oxalamide host, Fowler and
Lauher have shown that the hydrogen bonding of the
oxalamides can determine the repeat distance of the diyne
monomers, aligning them for polymerization.
In 2006, we reported the first and only synthesis of a PDA
with single-atom side groups—polydiiododiacetylene
(PIDA),[12] prepared by the host–guest approach using
halogen bonding, the non-covalent interaction between
a halogen atom (Lewis acid) and a Lewis base. Diiodobuta-
diyne forms cocrystals with host 4 or 6 (Figure 1) that are well
aligned, allowing for spontaneous polymerization at room
temperature.[12,13] The abundance of transition metal-cata-
lyzed coupling reactions led us to explore PIDA as a precursor
to other PDAs via post-polymerization modification. How-
ever, the iodine atoms of PIDA are extremely labile, and the
polymer undergoes deiodination (carbonization) under mild
conditions, including room-temperature reaction with amines
or other bases.[14] PIDA is therefore not an appropriate
substrate for post-polymerization coupling to make new
PDAs.
D
ibromobutadiyne was first reported in 1930 as an
extremely unstable and explosive compound.[1] At room
temperature, it decomposes explosively in seconds, although
it is stable in solution below À308C when kept in the dark.[2]
Research on dibromobutadiyne has thus been quite lim-
ited.[2,3] More generally, bromoalkynes are highly reactive and
unstable. Two years ago, Frauenrath and co-workers reported
the first clean solid-state reactions of a bromodiyne and
a related bromotriyne.[3b] They discovered that in single
crystals, these glycosylated bromopolyynes each undergo an
unprecedented multistep dimerization/rearrangement. How-
ever, the more common solid-state reaction of diynes, namely
the polymerization to form a polydiacetylene (PDA), has not
been reported previously for any bromodiyne. Likewise, there
are very few examples in the literature of bromine-containing
polymers, and the ones that have been reported have
saturated backbones with bromine substituents on the side
arms, rather than directly bonded to the backbone.[4] We
describe here the controlled, ordered polymerization of the
explosive monomer dibromobutadiyne (C4Br2), providing
[*] H. Jin, Prof. N. S. Goroff
Department of Chemistry, Stony Brook University
Stony Brook, NY 11794-3400 (USA)
E-mail: Nancy.Goroff@stonybrook.edu
C. N. Young, G. P. Halada
Department of Materials Science and Engineering
Stony Brook University
Fortunately, polydibromodiacetylene (PBDA) should be
much more stable than PIDA, offering the possibility of post-
polymerization reaction. In model studies in our lab, trans-
dibromoalkenes have shown to be much more resistant to
dehalogenation than trans-diiodoalkenes.[15] Dibromoalkenes
Stony Brook, NY 11794-2275 (USA)
B. L. Phillips
Department of Geosciences, Stony Brook University
Stony Brook, NY 11794-2100 (USA)
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only limited reaction; they do not change color, and X-ray
diffraction indicates that compound 2 continues to be present
primarily as monomer. Upon warming to room temperature,
the cocrystals change from purple to black, indicating
increased polymerization. The Raman spectrum (Figure 2D),
a sensitive tool for detecting the conjugated enyne chain of
a PDA, demonstrates the presence of polymer. However, X-
ray diffraction on warmed samples has been unsuccessful,
suggesting increased mosaicity in the material.
The partial success in preparing cocrystals of compounds 2
and
3 encouraged us to pursue cocrystals with bis-
(nitrile)oxalamide hosts 4, 5 and 6. To try to form cocrystals,
host 4, 5 and 6 were each mixed with diyne 2, in a range of
solvents including THF, dichloromethane, chloroform, meth-
anol, acetonitrile, acetone, toluene and ethyl acetate. Regard-
less of conditions, hosts 4 and 6 were found in each case to
precipitate separately from monomer 2, accompanied by
rapid decomposition of the diyne. In most solvents, host 5 also
precipitated on its own. However, in dichloromethane at
À188C, host 5 and diyne 2 together form deep blue cocrystals.
Even in the freezer, these crystals slowly turn from deep blue-
purple to a dark brown. With slow warming to 208C over 24 h,
the crystals instead change color from blue-purple to a cop-
pery, metallic appearance (Figure 2C), which according to
our previous experience suggests a high degree of polymer-
ization.[12,13]
Figure 1. PIDA, PBDA and monomer 1 and 2, with Lewis-basic host
molecules 3–6.
A comparison of the Raman spectra of 2·3 and 2·5
cocrystals (Figure 2D,E) shows excitation occurring at the
same energies in each sample (1030, 1441, and 2104 cmÀ1),
consistent with the polymer PBDA. However, the spectrum
obtained from the 2·5 cocrystal demonstrates lower back-
ground fluorescence, greater peak intensities, and an
enhanced signal to noise ratio, further support for an
increased degree of polymerization.
Solid-state NMR studies confirm that the copper-colored
cocrystals with host 5 contain fully polymerized PBDA. In
solid-state cross-polarization magic-angle-spinning (CP-
MAS) 13C NMR experiments (Figure 3B), the slowly
warmed 2·5 cocrystals show only two peaks besides those
for host 5: a sharp peak at 103 ppm and a broader, partially
overlapping signal at 108 ppm (Figure 3B), corresponding to
the alkyne and bromoalkene carbons of the polymer PBDA,
respectively. These chemical shifts are consistent with what is
observed in PIDA (alkyne peak at 110 ppm) and in previously
reported bromoalkenes (100–110 ppm).[18] In the solid state,
coupling to the quadrupolar bromine nucleus broadens the
peak corresponding to the alkene carbon. Monomer 2, which
in solution has NMR peaks at 66.7 and 39.0 ppm,[2b] is not
visible in the spectrum, indicating that the sample is > 90%
polymerized.
have been used previously as substrates for transition metal-
catalyzed coupling reactions.[16] However, the preparation and
ordered polymerization of dibromobutadiyne represents
a significant challenge. As described above, the monomer is
highly unstable and requires special handling. In addition,
bromine is both less electrophilic and less polarizable than
iodine, leading to much weaker halogen-bonding interactions.
Dibromobutadiyne is therefore a particularly challenging
substrate for assembly of ordered cocrystals.
Our approach to the preparation of dibromobutadiyne
cocrystals uses bis(pyridyl)oxalamide and bis(nitrile)-
oxalamide hosts 3, 4, 5 and 6. Due to the stronger halogen
bonding to pyridine, bis(pyridyl)oxalamide 3 was tried first.
Cocrystals were prepared in a cold bath at À158C to slow the
decomposition of dibromobutadiyne (2). A variety of sol-
vents, including THF, dichloromethane, chloroform, metha-
nol, acetonitrile, acetone, toluene and ethyl acetate, were
tested, but in each case, host 3 and guest 2 precipitated out as
separate solids. However, in a mixture of dichloromethane
and methanol at À158C, blue crystals were observed (Fig-
ure 2A). Dichloromethane and methanol form an 8:1 azeo-
trope, with boiling point of 388C, compared to 408C for
dichloromethane and 658C for methanol alone.[17] In the
azeotrope, the host is much more soluble than in dichloro-
methane alone, while monomer 2 is more stable than in
methanol alone. In addition, the lower boiling point of the
azeotrope accelerates crystal formation, reducing decompo-
sition of monomer.
CP-MAS NMR can also shed light on the partially
polymerized cocrystals of monomer 2, both those that contain
host 3 that have been allowed to react as fully as possible and
those that contain host 5 but have been kept at low temper-
ature to prevent complete polymerization. Interestingly, these
samples give very different NMR signatures.
Upon warming to room temperature, the cocrystals with
pyridyl host 3 turn black, and their Raman spectrum matches
PBDA, although at lower intensity than the warmed 2·5
Monomer 2 and host 3 form a cocrystal in a 1:1 ratio
(Figure 2B), with a repeat distance (5.05 Š) that is slightly
longer than ideal for topochemical polymerization. Kept in
the freezer for as long as a month, these cocrystals undergo
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Figure 2. A) Morphology of cocrystal 2·3. B) Structure of cocrystal 2·3 determined by X-ray diffraction. C) Morphology of cocrystal PBDA·3.
D) Raman spectrum of cocrystal 2·3 after warming to 208C. E) Raman spectrum of cocrystal PBDA·5.
cocrystals. Consistent with these observations, these crystals
give a CP-MAS NMR spectrum which clearly shows the
presence of both polymer (peaks at 103 and 108 ppm) and
monomer (67 ppm). Possible oligomeric intermediates are
not readily discernable.
does not show the presence of either monomer or PBDA, but
instead contains a broad set of signals covering the range of
60–90 ppm. This broad envelope is consistent with partially
reacted oligomers.
The difference in behavior between cocrystals with
pyridyl host 3 and nitrile host 5 can be attributed to the
differing amount of strain associated with polymerization in
these crystals. The presence of monomer in the warmed 2·3
cocrystals indicates a barrier to initiating the reaction. Once
started, reaction continues through enough steps to produce
material with the same NMR signature as the polymer. In
contrast, in the cocrystals with nitrile host 5, even at low
In contrast, low-temperature solid-state 13C CP-MAS
NMR experiments on the 2·5 cocrystals suggest a different
polymerization pattern (Figure 3C). These cocrystals were
examined at À208C, after storage for several days at À158C.
Over those few days, they had already undergone partial color
change, with some of the sample turning dark brown and
some still deep purple. The NMR spectrum of this sample
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In conclusion, we have prepared the stable conjugated
bromine-substituted polymer poly(dibromodiacetylene) by
taming and directing the reactivity of the explosive monomer
dibromobutadiyne. Future effort will focus on exploring the
properties of this new, extremely simple polymer and
examining it as a precursor to other polydiacetylenes via
post-polymerization modification.
Experimental Section
Safety note: Dibromobutadiyne (2) is explosive at room temperature.
To reduce risks from explosion, compound 2 was prepared only in
quantities of less than 200 mg. Dibromobutadiyne should be used
immediately, or kept in solution at ꢀ 08C or as a solid at ꢀ À408C.
When the diyne was prepared, it was immediately mixed with host
solutions, pipetted into an evaporating dish cooled to À788C and
transferred to the chiller.
General procedure for growing crystals: The crystallization
experiments were performed in a Fisher Scientific Isotemp Refriger-
ated/Heated Bath Circulator. The circulator was filled with a cooling
bath of water and ethylene glycol (50:50 v/v), which has a freezing
point of À308C. After the bath was cooled to À158C, an evaporating
dish containing a solution of diyne 2 and host was placed in the
circulator, so that the bottom of the evaporating dish was just below
the surface of the bath. To block ambient light, the evaporating dish
was covered with aluminum foil, poked with holes to promote
evaporation.
After cocrystals form in the cold bath, the crystals must be rinsed
with pentane to remove excess monomer from the surface. Otherwise,
excess diyne can quickly decompose, and potentially explode, when
warmed.
Dibromobutadiyne (2): To a 250 mL round-bottom flask were
added 1,4-bis(trimethylsilyl)-1,3-butadiyne (0.10 g, 0.5 mmol), N-
bromosuccinimide (0.19 g, 1.1 mmol), AgNO3 (0.17 g, 1.0 mmol),
and acetone (100 mL). The round-bottom flask was wrapped with
aluminum foil, and the mixture was stirred at room temperature for
4 h. The solvent was removed in vacuo at 08C, and the mixture was
poured into cooled pentane (200 mL). A short silicon plug (pentane)
was used to remove excess catalyst and byproduct. Solvent was
removed in vacuo while holding the sample at 08C, resulting in
a white flaky solid. Caution: the flaky solid melts and decomposes in
seconds at room temperature; therefore, keep it cold and use it
immediately after it is prepared.
Figure 3. CP-MAS 13C NMR of cocrystals. A) Cocrystal 2·3 after warm-
ing to 208C in atmosphere. B) Cocrystal 2·5 after warming to 208C in
atmosphere. C) Cocrystal 2·5 held at <À108C in freezer; spectrum
acquired at À208C. Red stars represent polymer peaks, blue star
represents monomer peak, and purple star represents partially reacted
material.
Oxalamide hosts 3–6 were prepared according to literature
procedures.[20]
Cocrystal 2·3: X-ray quality cocrystals were grown by dissolving
guest 2 ( ꢁ 30 mg, 0.14 mmol) and host 3 (15 mg, 0.056 mmol) in 3 mL
of CH2Cl2/MeOH (10:1 to 19:1), which was then allowed to evaporate
slowly at À158C for 3 days. Once the cocrystals formed, they were
warmed slowly to room temperature over 1 day.
Cocrystal 2·5: Cocrystals were grown by dissolving guest 2
( ꢁ 30 mg, 0.14 mmol) and host 5 (15 mg, 0.054 mmol) in 3 mL of
CH2Cl2, which was then allowed to evaporate slowly at À188C for
2 days. Once the cocrystals formed, half were transferred to the
freezer and held at À158C until they could be examined by MAS-
NMR at À208C. The other half were warmed slowly to room
temperature over 1 day and stored for 7 days, then examined by
MAS-NMR at room temperature.
temperature, the monomers all react to form oligomeric
species, well before individual chains are long enough to
appear as PBDA in the NMR spectrum.
Once prepared, PBDA can be isolated from the cocrystal
by dissolving away host 5 in dichloromethane to leave
insoluble shiny green fibers (Figure 4A). Raman and NMR
data (Figure 4B,C) confirm that this material is indeed the
polymer PBDA. Mass balance experiments indicate isolation
of 87% of the expected polymer, providing a lower bound on
the degree of polymerization in the sample.[19] As hoped,
poly(dibromodiacetylene) is significantly more stable than
poly(diiododiacetylene). PBDA is stable to shock pressure
and mild heating, and chemically tolerant to the addition of
many bases, including pyridine and triethylamine.
X-Ray diffraction: Crystals were selected and mounted on glass
fibers using epoxy cement. Each crystal was centered, and the X-ray
intensity data were measured on an Oxford Gemini A diffractometer
with graphite-monochromated Mo irradiation. The structures were
solved by direct methods and refined by using full-matrix least-
squares methods (SHELX97).
MAS-NMR: Solid-state 13C NMR spectra were acquired with
a Varian Infinity-plus 500 spectrometer, operating at 125.68 and
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499.79 MHz for 13C and 1H respectively, and a Varian double-
resonance T3-type MAS probe assembly configured for 4 mm
(outside diameter) rotors. Spinning rates of 13–15 kHz, a 50 kHz
transverse 1H field, and a linear ramp of the 13C field of Æ 7 kHz about
the first sideband match condition were used for the 13C{1H} CP-MAS
Keywords: conjugated polymers · halogens ·
low-temperature reaction · polydiacetylene · polymerization
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14690–14695
Angew. Chem. 2015, 127, 14903–14908
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experiments. The 1H! C contact time was 10 ms and the relaxation
delay was 2 s. The 13C chemical shifts are reported with respect to the
methylene carbon of adamantane, taken to be + 38.5 ppm. Fully
relaxed 13C single-pulse MAS-NMR spectra were taken at a 14 kHz
spinning rate with direct excitation by a 4-ms pulse (908 pulse was 5 ms)
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Published online: October 8, 2015
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