Synthesis of polymeric ladders by topochemical polymerization

Article abstract of DOI:10.1039/c3cc47379a

Two polymeric ladders were synthesized by topochemical polymerization. The critical assemblies with multiple reactive centers were characterized by single crystal X-ray diffraction. Approximately 64% and 70% of the mass of the two polymeric ladders can be

Full text of DOI:10.1039/c3cc47379a

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Synthesis of polymeric ladders by topochemical
polymerization†
Cite this: Chem. Commun., 2014,
50, 1218
Xiaodong Hou, Zhihan Wang, Joseph Lee, Erin Wysocki, Casey Oian,
Jennifer Schlak and Qianli R. Chu*
Received 26th September 2013,
Accepted 21st November 2013
DOI: 10.1039/c3cc47379a
www.rsc.org/chemcomm
Two polymeric ladders were synthesized by topochemical polymer-
ization. The critical assemblies with multiple reactive centers were
characterized by single crystal X-ray diffraction. Approximately 64%
and 70% of the mass of the two polymeric ladders can be derived from
biomass, respectively.
Polymeric ladders are high value synthetic targets and are predicted
to have high thermal/chemical stability because the chains do not
automatically fall apart if one covalent bond is broken.1,2 Polymeric
ladders are constructed with monomers containing more than two
Fig. 1 DSC curves of monomer I (left); UV-Vis spectra of I (right).
reactive centers. The challenge lies in synthesizing a polymeric curves of first cooling and second heating were also consistent with
ladder by the classical one-pot approach because of the potential the completion of the polymerization. The polymerization could be
for cross linking between the monomers. Herein, we report two reproduced under conventional conditions in near quantitative yield
polymeric ladders synthesized by topochemical polymerization.3–8
by heating monomer I at 150 1C for a week. UV-Vis spectra showed
A monomer used in this study is compound I with two that monomer I absorbed irradiation between 260 nm and 300 nm
reactive diene groups connected by an unreactive 1,4-phenylene in a chloroform solution and between 240 nm and 340 nm in the
diester linker (Scheme 1). The facile synthesis of monomer I solid state (Fig. 1). The powder of monomer I polymerized slowly at
was accomplished in 90% yield by activating sorbic acid with room temperature under UV irradiation using a high-pressure Hg
thionyl chloride and then coupling with hydroquinone.
lamp. The reaction can be accelerated and was finished in 99% yield
A variety of physical properties were studied to determine if within 16 h by UV irradiation at 120 1C, well below the melting point
compound I is a viable candidate for polymerization. It was found to of monomer I. Raising the temperature increased thermal vibration
be a reactive white powder with a melting point of 178–180 1C. The of the monomer in the solid state; therefore, heating sped up the
differential scanning calorimetry curve (DSC, Fig. 1) showed an topochemical polymerization.3–8 FT-IR spectra of monomer I com-
endothermic peak at around 179 1C corresponding to its melting pared with its polymers showed a shift of CQO stretching vibration
point. The exothermic peak to the right of the endothermic peak was from 1717 cmÀ1 to 1740 cmÀ1 (see ESI†). The anti-symmetric
consistent with an exothermic polymerization event. The featureless stretching of C–C at 1008 cmÀ1 shifted to a higher wavenumber
1014 cmÀ1, and the relative intensity of the peak for the isolated
trans CQC at 970 cmÀ1 increased after polymerization. These
changes showed that the polymerization broke the conjugation
between the carbonyl and diene in monomer I.
To further investigate the solid-state polymerization, high quality
Scheme 1 Synthesis of monomer I from sorbic acid.
crystals of monomer I were obtained by slowly cooling a saturated
DMSO or acetone solution. The structure was determined by single
crystal X-ray diffraction. Monomer I self-assembled into columnar
complexes based on p–p stacking as shown in Fig. 2. Within the
assembly, the closest reactive atoms are the 1st diene carbon and the
4th diene carbon in the nearest neighboring monomer as marked in
Scheme 1 and Fig. 2a. These two adjacent reactive sp2 hybridized
Department of Chemistry, University of North Dakota, Grand Forks, ND 58202,
USA. E-mail: chu@chem.und.edu; Fax: +1 701-777-2331; Tel: +1 701-777-3941
† Electronic supplementary information (ESI) available: Experimental procedure,
full characterization and spectra, and crystallographic data. CCDC 957993 and
957994. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc47379a
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Fig. 3 The crystal structure of the monomer II: (a, b) side view of the p–p
stacking column; (c) top view of 8 p–p stacking columns.
Fig. 2 The crystal structure of the reactive monomer I: (a) side view of the
p–p stacking column showing the reactive centers (only five molecules are
shown for simplicity); (b) side view of the p–p stacking column in space filling
style; (c) top view of 13 p–p stacking columns in capped stick style. (Colors are
introduced to illustrate the packing of these supramolecular columns.)
further supports the completion of the topochemical reaction
and the formation of the molecular ladder.
To explore the scope of the reaction, monomer II was synthesized
(Scheme 2). Due to the presence of the two three-carbon alkyl chains,
the above monomer II has improved solubility in many organic
solvents such as acetone, acetonitrile and ethyl acetate when com-
pared with monomer I. X-ray quality crystals of monomer II were
obtained by slow evaporation of an acetone solution. The structure
determined by single crystal X-ray diffraction is shown in Fig. 3. The
critical p–p stacking of the monomer remained in this crystal
structure. The distances between the centroids of the aromatic rings
shown in the p–p stacking columns in Fig. 2b and 3b are 5.60 and
5.49 Å, respectively. The 1st diene carbon and the 4th one in the
adjacent monomers are only 3.62 Å from each other. The closest
reactive sp2 carbons between self-assemblies are more than 5 Å away,
so the alkenes between self-assemblies are unlikely to react (Fig. 3c).
Although the packing of these p–p assemblies of II is different from
that of monomer I (Fig. 2c and 3c), the solid-state reactivity of
monomer II is comparable to that of monomer I. The topochemical
UV polymerization of monomer II was finished within 64 h at 70 1C,
below its melting point 107–108 1C.
carbons are only separated from each other by 3.66 Å and their p
orbitals are well overlapping each other. The distance and orientation
are ideal to form a new carbon–carbon bond in the solid state upon UV
irradiation or heating.9–11 The powder XRD pattern of the synthesized
monomer I without further processing was nearly identical to that of
the calculated one from the single crystals (see ESI†), making this
crystal structure suitable to analyze and interpret the solid-state poly-
merization for both crystals and powder. After the photopolymerization,
the polymer maintained its crystallinity. The powder XRD pattern of the
polymer was similar to that of monomer I; therefore, it is consistent
with the theory that the solid-state polymerization was a topochemical
reaction involving minimum movement of the atoms.
Due to the preorganization of the reactive centers in the self-
assembly,monomer I is photoreactive allowing each monomer in
close enough proximity to react with its two nearest neighbors. UV
irradiation and heating of the self-assemblies furnished polymeric
products. As outlined in the graphical abstract, the 1,4-photopoly-
merization is proposed.2a,4a Essentially, this process occurs via an
intra-assembly reaction. The reactive alkene sp2 carbons between
self-assemblies are too distant for polymerization and their p orbitals
are not overlapping (Fig. 3c). The formation of the polymeric
ladder was confirmed by solid-state 13C NMR, which showed the
disappearance of two sp2 hybridized carbons of the diene con-
current with the appearance of two sp3 hybridized carbons at
around 43.6 and 56.4 ppm. Hydrolysis of the polymeric product
of I yielded hydroquinone and no unreacted monomer was
observed by NMR (see ESI†). The result of product hydrolysis
Topochemical reactions offer a unique opportunity to synthesize
molecules and macromolecules with regio- and stereo-specificity.9–13
Scheme 2 Synthesis of monomer II from leaf aldehyde.
Chem. Commun., 2014, 50, 1218--1220 | 1219
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could be a useful reaction to make interesting targets with
potential applications.2a Furthermore, the polymeric ladders
exemplified herein will provide access to other covalently
bonded ladder materials, and allow researchers to further
explore their unique properties such as mechanical strength.
This work was supported by the Doctoral New Investigator
grants of the American Chemical Society Petroleum Research
Fund (PRF 52705-DNI7) and the National Science Foundation
ND EPSCoR program (EPS-0814442). The authors thank
Dr Jeffrey Simpson of MIT for solid-state NMR and Dr Angel
Ugrinov of NDSU for powder XRD.
Notes and references
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Fig. 4 Stereochemical structure of the polymeric ladders: (a) a structural
formula showing the chiral centers; (b) a model of the polymeric ladder
built from the crystal structure of monomer I to illustrate the stereo-
chemistry (only four repeating units are shown for simplicity).
3 (a) Q. Chu, D. C. Swenson and L. R. MacGillivray, Angew. Chem., Int.
Ed., 2005, 44, 3569–3572; (b) L. R. MacGillivray, G. S. Papaefstathiou,
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movement of atoms, stereoregular polymeric ladders are proposed
as products (Fig. 4). The four carbon–carbon bonds that each
monomer forms with its two adjacent neighbors all involve chiral
centers that are generated stereo-specifically. Besides their stereo-
regularity, the two polymeric ladders have shown excellent chemical
stability, which is also important for future applications. They are
insoluble in all the common organic solvents. The ladders can
tolerate strong acids such as concentrated HCl and TFA, and organic
bases such as Et3N (even when refluxed overnight). However, these
ladder polymers are degradable in aqueous potassium hydroxide
and can be oxidized by concentrated sulfuric acid.
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´
Another significant characteristic of the polymeric ladders is
their partial environment friendly origin.14 The polymeric ladder I
is approximately 64% by mass from the renewable starting material
sorbic acid. Sorbic acid is commercially available and can be
produced from triacetic acid lactone, which is a versatile bio-
renewable molecule with potential to be a platform chemical for
the production of commercially valuable chemical intermediates
and end products.15 The polymeric ladder II is approximately
70% by mass renewable but synthesized from leaf aldehyde and
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In conclusion, two stereoregular polymeric ladders were synthe-
sized from plant-derived materials. Other starting materials and
reagents or their preparations were also readily available and
inexpensive. Monomer powder I directly obtained from the synthesis
was not an amorphous solid, but was indeed microcrystals, which
were suitable for topochemical polymerization. The above character-
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Chem., 2012, 14, 1850–1853.
16 http://www.eere.energy.gov/bioenergy/, Top Value Added Chemicals
align well with a known supramolecular ladder, which was synthe-
sized from a two-dimensional p-xylylenediammonium disorbate,
from Biomass Volume I - Results of Screening for Potential Candi-
showing that the topochemical 1,4-polymerization of diene
dates from Sugars and Synthesis Gas, (accessed in Oct. 2013).
1220 | Chem. Commun., 2014, 50, 1218--1220
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