Rotaxane-based mechanically linked block copolymers

Article abstract of DOI:10.1002/anie.201103716

We just clicked: A convergent approach consisting of two successive copper(I)-catalyzed azide-alkyne cycloaddition "click" reactions leads to a diblock copolymer in which the two blocks are linked by a rotaxane-type mechanical bond (see scheme). Rotaxane formation is templated by a square-planar Pd^II complex. Copyright

Full text of DOI:10.1002/anie.201103716

DOI: 10.1002/anie.201103716
Polymer Architectures
Rotaxane-Based Mechanically Linked Block Copolymers**
Guillaume De Bo, Julien De Winter, Pascal Gerbaux, and Charles-Andrꢀ Fustin*
Block copolymers constitute an extremely important class of
materials because of their utility in countless applications.[1]
The development of controlled polymerization methods and
the combined use of polymer and organic chemistries have
enabled the production of complex architectures and highly
functional polymers.[2] Recently, block copolymers that bear
an addressable junction have attracted particular interest
because of the added functionality and additional degrees of
control of the properties and the self-assembly behavior of
such copolymers.[3–8] Two main types of junction have been
reported to date: 1) supramolecular junctions based on, for
example, hydrogen bonds, metal–ligand coordination, or
inclusion or host–guest complexes,[3–5] and 2) covalent junc-
tions that are responsive to various kinds of stimuli (light, pH,
or redox).[6–8] Herein we report the synthesis of a diblock
copolymer where the two blocks are linked by a new type of
junction: a mechanical bond of the rotaxane type (1 and
Figure 1).
topological) bond, that is, it allows for large-amplitude
controlled motions of the interlocked components, and for
their relative positioning with respect to each other. These
exceptional properties have made catenanes and rotaxanes
highly promising candidates for the design of molecular
machines that are capable of performing well-defined tasks in
response to various stimuli.[9] Moreover, these molecules also
show great potential for imparting unusual properties to
polymers.[10] Indeed, the unique flexibility and mobility of
catenanes and rotaxanes, and their incorporation into poly-
mers is expected to strongly modify the rheological, mechan-
ical, dynamic, and thermal properties of the polymers. This
promise of polymeric materials exhibiting new properties has
thus generated an intense activity of research that has led to
the creation of polycatenanes and polyrotaxanes of diverse
compositions and architectures.[10] In all these examples, the
topological bonds were incorporated into the repeating units
(in the main chain or as side groups) of the polymers.
Contrary to these examples, here we use a single mechanical
bond of the rotaxane type that is strategically located at the
junction between two polymer blocks (see Figure 1), thus
giving rise to a new kind of diblock copolymers designated as
A-rot-B. In this way, the blocks are linked by a bond that has
the strength of a covalent bond, since the two interlocked
components cannot be separated unless a covalent bond is
broken, but which confers unique degrees of freedom to the
polymer chains since the macrocycle that bears the B block
can potentially move along the whole length of the A block
that acts as the thread. Copolymers of this type would thus
enable motions of much larger amplitude than with conven-
tional “small” rotaxanes, and constitute a first step toward
large, fully synthetic, systems that mimic biological structures,
where large-amplitude motions are commonplaces.[11]
Figure 1. Convergent synthetic strategy toward A-rot-B block copoly-
mers.
The chosen synthetic strategy is depicted in Figure 1.
Among the different possible routes, we selected one that
shows a highly convergent character, and where the key step
(the rotaxanation) is performed on a “small” molecule and
not on a polymer chain end. As a template for the
pseudorotaxane formation, we selected a square-planar PdII
complex built from one tridendate and one monodendate
ligand. This very efficient template has been successfully used
for the preparation of various catenanes[12] and rotaxanes.[13]
Moreover, the template is a neutral complex that can be used
in a variety of organic solvents, and is robust enough to
withstand chemical modifications performed on other parts of
the molecule. Once available, the different building blocks
can be assembled by two successive copper(I)-catalyzed
azide–alkyne cycloaddition (CuAAC) “click” reactions to
produce the A-rot-B diblock copolymer.[14] By following this
strategy, a higher degree of control on the formation of the
topological bond will be achieved, and its convergent nature
Interlocked molecules such as catenanes and rotaxanes
have attracted ever-increasing interest since the discovery in
the late 1980s and early 1990s of efficient synthetic methods to
prepare these molecules. This interest stems largely from the
unique degrees of freedom associated with the mechanical (or
[*] Dr. G. De Bo, Prof. C. A. Fustin
Institute of Condensed Matter and Nanosciences (IMCN)
Bio- and Soft Matter, Universitꢀ catholique de Louvain
Place Pasteur 1, bte L4.01.01, 1348 Louvain-la-Neuve (Belgium)
E-mail: charles-andre.fustin@uclouvain.be
Dr. J. De Winter, Dr. P. Gerbaux
Mass Spectrometry Research Group, University of Mons (UMONS)
Place du Parc 23, 7000 Mons (Belgium)
[**] C.A.F. and P.G. are Research Associates of the FRS-FNRS. J.D.W. is a
Research Fellow of the FRS-FNRS.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103716.
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allows an easy variation of the chemical structure of the
polymer chains.
The first building block is the polymeric thread 4, which is
end-capped by a bulky stopper at one end, and by an azide
moiety at the other. We synthesized the atom transfer radical
polymerization (ATRP) initiator 2 that bears the bulky
stopper (easily prepared from the corresponding phenol, see
the Supporting Information). This procedure allows the facile
synthesis of threads of different length and/or chemical nature
without having to graft the bulky stopper in a subsequent step.
As an example, we chose to polymerize methyl acrylate. The
ATRP polymerization of methyl acrylate from initiator 2 was
performed in anisole with an equimolar CuBr/pentamethyl-
diethylenetriamine (PMDETA) complex as catalyst. A well-
defined poly(methyl acrylate) (PMA) with a polydispersity
index (PDI) of 1.09 and Mn of 5040 gmolÀ1, and bearing a
bulky stopper at one of its extremities (3) was obtained in this
way. Characterization by MALDI mass spectrometry con-
firmed the presence of the expected end groups (see the
Supporting Information). The bromide chain end was then
substituted with NaN3 to quantitatively produce the azide-
terminated polymeric thread 4 (Scheme 1).
Scheme 2. Synthesis of pseudorotaxane 8 by slipping thread 7 through
the cavity of the palladium-complexed macrocycle 6.
the bulky stopper (see the Supporting Information). The
palladium-complexed macrocycle 6 was obtained by adapting
a previously reported procedure that uses chelidamic acid as
starting material.[13a] The pseudorotaxane 8 was formed in
good yield by simply mixing the small thread 7 and the
macrocycle 6 at room temperature for 1 hour (Scheme 2), as
Scheme 1. Synthesis of the polymeric thread 3 by ATRP from an
initiator that bears a bulky stopper.
1
unambiguously shown by H NMR spectroscopy and mass
The next step was to prepare the pseudorotaxane 8, which
bears the second bulky stopper and an alkyne group, in order
to enable attachment of 8 to the polymer thread 4. We
selected a square-planar complex centered on a PdII ion as a
template for the pseudorotaxane. In order to efficiently
template the rotaxane formation, this complex has to be built
from both a tridendate ligand, which is first complexed with
the PdII ion, and from a monodendate ligand.[12,13] In our case,
the tridendate ligand is the 2,6-pyridinedicarboxamide motif
of the macrocycle 5, and the monodendate ligand is the
pyridine station present on the small thread 7 (Scheme 2).
The pseudorotaxane 8 is then assembled by threading
compound 7 through the cavity of macrocycle 6; the pyridine
station displaces the acetonitrile molecule present in 6. The
structure of the small thread 7 is inspired by previous
studies[13a,b,f] that showed that 1) the presence of a bulky
group on one side of the pyridine station, 2) the presence of a
rigid arm (here the resorcinol fragment) on the other side of
the pyridine station, and 3) the 2,6-substitution of the pyridine
ring all favor threading the macrocycle through the cavity and
minimize the chance of an exo complexation, that is, the
spectrometry (see the Supporting Information).
With the building blocks available, the assembly of the
A-rot-B copolymer could be performed. The first step is the
attachment of the pseudorotaxane 8 onto the chain end of the
polymeric thread 4 by the CuAAC reaction to obtain the
macrorotaxane 9 (Scheme 3). This reaction was performed at
room temperature with CuBr/PMDETA as catalyst. As
shown by the mass spectrum of 9 (Figure 2), the clear mass
shift to higher values than those of thread 4 and the very good
agreement between experimental and isotope distributions
confirm the efficiency of the coupling reaction. The Cl group
present on the macrocycle was then substituted by an azide
moiety by reacting the macrorotaxane 9 with NaN3. Aromatic
substitution of the Cl by the azide group occurs smoothly in
this case because of the presence of the two amide groups on
the aromatic cycle and the complexation of the pyridine
nitrogen atom with the palladium center.
The final step toward the A-rot-B copolymer is the
attachment of the B block onto the macrocycle of the
macrorotaxane 10 by the CuAAC reaction. As B block we
selected
a
poly(ethylene oxide) (PEO) chain (Mn =
propargyl
noninterlocked configuration. The small thread
7
was
2000 gmolÀ1, PDI = 1.05) end-capped with
a
obtained in good yield in three steps starting from
2,6-bis(chloromethyl)pyridine and from the phenol form of
group to allow its reaction with the azide group on the
macrocycle. The CuAAC reaction was conducted in chloro-
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Scheme 3. Synthesis of the macrorotaxane 10 by attaching the pseudo-
rotaxane 8 onto the chain end of the polymeric thread 4.
Scheme 4. Last step of the assembly of the PMA-rot-PEO copolymer by
attaching the PEO chain 11 onto the macrocycle of macro-rotaxane 10.
Figure 3. SEC traces of the building blocks 3 (solid gray line), 9
(dashed black line), 11 (dashed gray line) and of the PMA-rot-PEO
copolymer 1 (solid black line). (Eluent: DMF, flow rate: 1 mLminÀ1
,
refractive index detector, normalized intensities are shown.)
shift toward shorter elution times is clearly visible between
each of the three compounds, thus confirming the increase in
molecular weight at each step of the assembly of the
1
copolymer. Moreover, the H NMR spectrum of copolymer
1 (see the Supporting Information) confirms its structure
since the integrated areas under the PMA and PEO signals
match the expected composition of the copolymer.
Figure 2. MALDI mass spectrum of macrorotaxane 9. The expansion
shows the very good agreement between the experimental (lower) and
theoretical isotope distributions (upper).
To obtain further evidence for the obtained structure, the
PMA-rot-PEO copolymer 1 was analyzed by MALDI mass
spectrometry. As shown in Figure 4, the resolution of the
MALDI mass spectrum obtained for copolymer 1 is signifi-
cantly lower that of the macrorotaxane 9. Since compound 1 is
a copolymer of PEO and PMA, the low mass difference
between EO and MA units (2 ꢀ EO = 88 Da and MA =
86 Da) leads to many isobaric ions, thus explaining the
lower resolution. Nevertheless, an interesting global shift with
an m/z difference of 2000 is observed, thus confirming the
grafting of the PEO chain (Mn = 2000 gmolÀ1) onto the
form heated at reflux with the CuBr/PMDETA complex as
catalyst. After purification to remove the residual homo-
polymers, the PMA-rot-PEO copolymer was obtained in a
satisfactory yield, when considering the complexity of the
system (Scheme 4).
Figure 3 shows a comparison of the size-exclusion chro-
matography (SEC) traces of the polymeric thread 3, of the
macrorotaxane 9, and of the PMA-rot-PEO copolymer 1. A
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Figure 4. Comparison of the recorded MALDI mass spectra before and
after attachement of the PEO block to the macrorotaxane. In the
bottom spectrum, signals with low m/z ratios correspond to PMA-
containing fragment ions produced by decomposition of the copoly-
mer (higher laser fluence conditions).
macrorotaxane and therefore the structure of the PMA-rot-
PEO copolymer 1.
In conclusion, we have described the first synthesis of an
original diblock copolymer where the two blocks are linked
by a mechanical bond of the rotaxane type. The proposed
strategy, by virtue of its modular and convergent character,
will allow the preparation of a series of copolymers with
tunable composition. Moreover, it offers several possibilities
of adaptation, for example by replacing the second CuAAC
reaction by a polymerization step that uses a modified
macrorotaxane as macroinitiator. This modification would
further broaden the accessible composition range and could
give access to copolymers with high molecular weights. The
type of copolymers reported here opens promising pathways
to study the sliding motion of a macrocycle along a polymer
chain. Gaining insight in such mechanisms is indeed very
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Received: May 31, 2011
Published online: August 18, 2011
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Keywords: block copolymers · click chemistry · palladium ·
rotaxanes · supramolecular chemistry
.
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