Formation of polypseudorotaxane networks by cross-linking the quadruple hydrogen bonded linear supramolecular polymers via bisparaquat molecules

Article abstract of DOI:10.1039/c1cc14559b

The creation of novel crown ether-paraquat polypseudorotaxane networks, constructed by bisparaquat monomers threading into the cavity of the crown ether units of linear supramolecular polymers that are formed based on the quadruple hydrogen bonded unit ur

Full text of DOI:10.1039/c1cc14559b

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COMMUNICATION
Formation of polypseudorotaxane networks by cross-linking the
quadruple hydrogen bonded linear supramolecular polymers via
bisparaquat moleculesw
Shao-Lu Li,a Tangxin Xiao,a Bingjie Hu,b Yajie Zhang,a Feng Zhao,a Ya Ji,a Yihua Yu,b
Chen Lina and Leyong Wang*a
Received 26th July 2011, Accepted 12th August 2011
DOI: 10.1039/c1cc14559b
The creation of novel crown ether–paraquat polypseudorotaxane
networks, constructed by bisparaquat monomers threading into the
cavity of the crown ether units of linear supramolecular polymers
that are formed based on the quadruple hydrogen bonded unit
ureidopyrimidinone (Upy) in the concentrated solution, is described.
the creation of complicated supramolecular architectures.7
Very recently, we have reported the formation of novel linear
supramolecular polymers via the combination of two types of
classical binding interactions: quadruple hydrogen bonding
ureidopyrimidinone (UPy) and crown ether–paraquat recog-
nition motifs in the main chain.8 Therefore, we envisioned that
bifunctional ureidopyrimidinone monomers can self-assemble
into linear supramolecular polymers at high concentrations.9
If bifunctional ureidopyrimidinone monomers are bridged
with crown ether units, the quadruple hydrogen bonded linear
supramolecular polymers which are constructed with crown
ether units are formed, and if such formed linear supramolecular
polymers are utilized as polymeric backbones, plus if bisparaquat
is utilized as the axes, polypseudorotaxane networks might be
obtained in the concentrated solution. In that respect, we
synthesized two types of monomers: bifunctional Upys P1
bridged with a crown ether unit and bisparaquat D1. Finally we
investigated the polypseudorotaxanes formation in solution
(Fig. 1). The reason to select the ester-containing crown ether
is that the ester-substituted BPP34C10 derivatives show an
increased binding ability toward the electron-deficient bipyri-
dinium unit in the form of pseudorotaxane.10
Polypseudorotaxanes/polyrotaxanes1 are mechanically inter-
locked compounds constructed by incorporating multiple
pseudorotaxane/rotaxane moieties into the polymeric frame-
work. They have attracted great attention in the past years
owing to their potential applicability in the material chemistry,
electrochemistry, life science, and biotechnology.2 For most of
the reported polypseudorotaxanes and polyrotaxanes, at least
one covalent polymer was used as a component. Whereas, the
threading efficiency of macrocycles or axes to the covalent
polymers is relatively low and is significantly influenced by the
affinity of the subunits, temperature, concentration, and polymer
chain length.3 While up to now, there is only one report using
the non-covalent polymers as the polymeric backbones. In
2009, Huang and co-workers reported the formation of linear
main-chain polypseudorotaxanes with supramolecular polymer
backbones via two self-sorting host–guest motifs.4 Generally,
the supramolecular polymer backbones assembled from
monomeric units are reversible and stimuli responsive, which
can exhibit polymeric properties in solutions as well as in the
bulk.5 Therefore, this work provides an effective method to
construct new polypseudorotaxanes which might be possessing
some unique, adaptive properties.
1
Firstly, H NMR spectroscopy was used to investigate the
formation of the linear supramolecular polymer and the cross-
linked polypseudorotaxanes in acetonitrile-d3–chloroform-
d (1 : 1, v/v). For the individual P1, the Upy N–H signals
showed a large downfield shift together with a little lower
intensity (between 10 and 13.5 ppm), which gave direct evidence
for the UPy unit dimerization in this mixed solvent.7f,11 That
indicated the linear polymer formation at relatively high
concentrations (Fig. S2, ESIw). Upon mixing 0.5 equiv. D1
to the P1 solution, the UPy N–H signals shifted little compared
with the individual P1. The proton signals of crown ether and
paraquat subunits had only one set of signals and were
concentration dependent, since the complexation between
crown ether and paraquat shows a fast-exchange process. As
the concentration of P1 increased, the ethoxy protons, the
aromatic protons of the crown ether, and the paraquat all
shifted upfield progressively, which revealed the concentration
dependent percentage of the complexed species of the crown
ether–paraquat moieties (Fig. 2). The association ratios
The polypseudorotaxane/polyrotaxane networks6 are the most
fascinating class of cross-linked polymers due to their unique
structure and specific properties. The combination of different
non-covalent interactions is a convenient and efficient way for
a Key Laboratory of Mesoscopic Chemistry of MOE, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, China. E-mail: lywang@nju.edu.cn; Fax: +86 25-83317761
b Shanghai Key Laboratory of Magnetic Resonance, Department of
Physics, East China Normal University, Shanghai 200062, China
w Electronic supplementary information (ESI) available: Experimental
details, 1H NMR, DOSY, NOESY, and UV-Vis spectra of individual
compounds or mixtures of P1 and 0.50 equiv. D1, the association
constant Ka of crown ether–paraquat. See DOI: 10.1039/c1cc14559b
c
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constant size, the slope should tend to 1,12 this might be caused
by the quite long, bulky linkage of bis-Upys P1. The cyclic
structures formed in the diluted solutions, in the range of
4.0–40.0 mM, are predominantly multimeric macrocycles,
which is deduced from the DOSY measurements (Fig. S5
and S6). The multimeric macrocycles are large, bulky and
can associate with each other to some extent through the
entanglement of the macrocycles. When the concentration
increased to above 40 mM, a sharp rise of the slope (2.56) is
observed, indicating a transition from the cyclic structures to
the linear supramolecular polymers of increasing size.
Furthermore, in order to investigate the cross-linking of the
hydrogen bonded linear supramolecular polymers, the viscosity
data of P1 and D1 varied from 0.1 to 0.5 equiv. mixtures at
different concentrations were obtained. Firstly, we found that
the reduced viscosity (VR) of P1 with addition of 0.5 equiv. D1
varied nearly exponentially upon increasing the monomers
concentration, suggesting formation of the polypseudorotaxane
networks. While with the reducing amount of D1 in the same
P1 solution, the reduced viscosity decreased gradually. By
contrast, especially from 40 mM onward, individual P1 samples
showed a relatively much lower value at the same concentrations
(Fig. 3b). This could be mainly due to the increased cross-
linking ratio of the linear supramolecular polymer backbones
with bisparaquat monomers by threading into the cavity of the
crown ether units of linear supramolecular polymers above and
the increased percentage of the complexed crown ether and
paraquat units at higher concentrations. While below 40 mM,
the reduced viscosity has similar values for the individual P1
and the mixtures, indicating that the cyclic oligomers are the
predominant species below 40 mM, which is in accordance
with the double logarithmic plot of individual P1.
Fig. 1 Graphical representation of the construction of polypseudo-
rotaxane networks from monomers P1 and D1.
between the crown ether moieties and the paraquat groups of
the P1 and 0.50 equiv. D1 were calculated under different
concentrations (Table S1), which proves that the [3]pseudo-
rotaxane structure had been incorporated leading to cross-
linking (at least 70% based on the association ratio of 87.4%
at 220 mM P1).6e The NOESY NMR spectrum of P1 and
0.50 equiv. D1 mixtures confirmed that the paraquat moieties
threaded into the cavity of the crown ether (Fig. S4w). All
evidence above supports the cross-linking of the hydrogen
bonded linear supramolecular polymers via the bisparaquat to
form the polypseudorotaxane networks at relatively high
concentrations.
1
The diffusion ordered H NMR spectroscopy (DOSY) is a
useful technique to investigate the size of the aggregates in
solution.6c,13 Thus as shown in Fig. 4, the diffusion coefficient
D of individual P1 and mixtures of P1 and 0.5 equiv. D1 at
different concentrations were measured. As the P1 monomers
concentration increased, the diffusion constant decreased to
smaller values gradually. For the individual P1, the diffusion
constant of the assemblies decreased B13 fold from 4.0 mM
to 120.0 mM, indicating the formation of extended, high-
molecular-weight polymeric aggregates at relatively high con-
centrations. With the addition of bisparaquats into the solution
of P1, the paraquat groups can thread into the cavity of the crown
The formation of the large supramolecular polypseudo-
rotaxane networks was also examined by the viscosity studies.
Fig. 3a shows a change in slope at B40 mM for the double
logarithmic plot of individual P1. At low concentrations, the
curve slope is 1.48, while for the non-interacting assemblies of
Fig. 3 (a) Specific viscosity Zsp of chloroform/acetonitrile (1 : 1, v/v)
solutions of individual P1 versus the monomer concentration at 27 1C;
(b) reduced viscosity VR of chloroform/acetonitrile (1 : 1, v/v) solutions
of individual P1 (m) and mixtures of P1 with 0.10–0.50 equiv. of D1,
versus the P1 concentration at 27 1C, the inset shows the reduced
viscosity changes with the addition of 0–0.50 equiv. D1 to the P1
(137.7 mM) solution.
Fig. 2 Partial 1H NMR spectra (300 MHz, 1 : 1 CDCl3–CD3CN,
297 K) of (a) P1; mixtures of P1 and 0.50 equiv. D1 at different
P1 concentrations: (b) 4.0, (c) 16.0, (d) 40.0, (e) 72.2, (f) 106.4,
(g) 158.6, (h) 220 mM; and (i) D1. Signals affiliated with solvents are
denoted by star symbols.
c
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individual P1 at the same concentrations from 4.0 mM to
120.0 mM, suggesting the increase in the average size of the
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structures to form the polypseudorotaxane networks.
In summary, we have constructed polypseudorotaxane
networks with the cross-linking of linear quadruple hydrogen
bonded supramolecular polymer backbones by the bisparaquat
molecules in solution. This was confirmed by the combination
of various techniques, such as variable-concentration 1H NMR,
NOESY, DOSY, UV-Vis spectra, and viscosity measurement.
The reversible and tunable supramolecular polymer backbones
would potentially bring unique, applicable properties to these
traditional polypseudorotaxane structures. The construction
of supramolecular [2]cantenane and new polypseudorotaxanes
based on the ureidopyrimidione unit is now underway in our lab.
We gratefully thank the financial support of National Natural
Science Foundation of China (No. 20602017, 20932004,
21072093), National Basic Research Program of China
(2007CB925103, 2011CB808600), the Doctoral Fund of Ministry
of Education of China (20090091110017).
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Chem. Commun., 2011, 47, 10755–10757 10757