and pillar[6]arene, they exhibited different binding affinities with different guest
molecules. Traditionally, the pillar[5]arene had a better binding ability with the
secondary ammonium salt, while the bulky guest molecules, such as paraquat and
positively charged DABCO derivatives, preferred to binding with pillar[6]arene
(Fig. 4) [10]. Therefore, the same strategy with the previous research, the secondary
ammonium salt was modified to the pillar[5]arene to form the A-B-type monomer,
whose binding constant was determined to be (2.40 Æ 0.20) Â 10
3 M
À1 in the
chloroform. Driven by the strong host-guest interaction between these two motifs,
the linear supramolecular polymer was formed. In addition, the obtained supramolecular polymer had anion responsiveness due to the ion-pair effect.
As well known, the decrease in the ion-pair interaction would increase the binding
ability, and the inverse proposition was also correct [5c, 11]. Therefore, the addition of
Cl
À into the polymer solution, the formed supramolecular polymer would disassemble
for the formation of tight ion pairs between the secondary ammonium cation and
halide anion. If the imidazole unit was incorporated into the pillar[5]arene, a pHresponsive supramolecular polymer could be successfully constructed, in which the
difference in the binding constants between pillar[5]arene/imidazolium motif and
pillar[5]arene/imidazole motif played the vital role, whose values were
(1.0 Æ 0.3) Â 10
4 M
À1 and (2.3 Æ 0.2) Â 10
2 M
À1
, respectively, in chloroform
[12]. These results indicated the pH would affect the degree of polymerization of these
molecules, which meant in the acidic solution the higher-molecular-weight supramolecular polymer would be formed than in the neutral solution, concluded by the
theoretical relationship between the association constant and degree of polymerization.
Besides the single constituent to form the A-B-type supramolecular polymer, the
hybrid polymers could be formed by adding another host molecule. Along with this
line, Wang and coworkers synthesize a novel host molecules, in which the cryptand
was fused into the pillar[5]arene, thus affording a host molecule with two cavities with
different binding abilities (Fig. 5) [13]. From the fundamental research in the hostguest property, it was delighted to find that the cryptand cavity could selectively bind
pyridinium cation and the pillar[5]arene cavity selectively bound the neutral guest with
the cyano group at the end in an orthogonal manner. Therefore, the orthogonal
supramolecular polymer could be successfully constructed by addition of two homoditopic guests with pyridinium cation and cyano group into the host solution.
13.2.2 Supramolecular Polymer Formed by Hydrogen Bonding and
Halogen Bond
Hydrogen bond, as an extensively exited noncovalent interaction, has been one
of the research hotspots and widely applied into molecular recognition, construction
of foldamers and helix structures, crystal engineering, as well as the supramolecular
polymer. Among all types of hydrogen bond structures, ureidopyrimidinone (UPy)
has attracted tremendous attention due to the high binding constant (10
7 M
À1 in
chloroform), self-complementary structures, and easy access [7c]. Wang and
coworkers firstly introduced this motif into the pillar[5]arene-based supramolecular
13 Pillararene-Based Supramolecular Polymer
345
molecules. Traditionally, the pillar[5]arene had a better binding ability with the
secondary ammonium salt, while the bulky guest molecules, such as paraquat and
positively charged DABCO derivatives, preferred to binding with pillar[6]arene
(Fig. 4) [10]. Therefore, the same strategy with the previous research, the secondary
ammonium salt was modified to the pillar[5]arene to form the A-B-type monomer,
whose binding constant was determined to be (2.40 Æ 0.20) Â 10
3 M
À1 in the
chloroform. Driven by the strong host-guest interaction between these two motifs,
the linear supramolecular polymer was formed. In addition, the obtained supramolecular polymer had anion responsiveness due to the ion-pair effect.
As well known, the decrease in the ion-pair interaction would increase the binding
ability, and the inverse proposition was also correct [5c, 11]. Therefore, the addition of
Cl
À into the polymer solution, the formed supramolecular polymer would disassemble
for the formation of tight ion pairs between the secondary ammonium cation and
halide anion. If the imidazole unit was incorporated into the pillar[5]arene, a pHresponsive supramolecular polymer could be successfully constructed, in which the
difference in the binding constants between pillar[5]arene/imidazolium motif and
pillar[5]arene/imidazole motif played the vital role, whose values were
(1.0 Æ 0.3) Â 10
4 M
À1 and (2.3 Æ 0.2) Â 10
2 M
À1
, respectively, in chloroform
[12]. These results indicated the pH would affect the degree of polymerization of these
molecules, which meant in the acidic solution the higher-molecular-weight supramolecular polymer would be formed than in the neutral solution, concluded by the
theoretical relationship between the association constant and degree of polymerization.
Besides the single constituent to form the A-B-type supramolecular polymer, the
hybrid polymers could be formed by adding another host molecule. Along with this
line, Wang and coworkers synthesize a novel host molecules, in which the cryptand
was fused into the pillar[5]arene, thus affording a host molecule with two cavities with
different binding abilities (Fig. 5) [13]. From the fundamental research in the hostguest property, it was delighted to find that the cryptand cavity could selectively bind
pyridinium cation and the pillar[5]arene cavity selectively bound the neutral guest with
the cyano group at the end in an orthogonal manner. Therefore, the orthogonal
supramolecular polymer could be successfully constructed by addition of two homoditopic guests with pyridinium cation and cyano group into the host solution.
13.2.2 Supramolecular Polymer Formed by Hydrogen Bonding and
Halogen Bond
Hydrogen bond, as an extensively exited noncovalent interaction, has been one
of the research hotspots and widely applied into molecular recognition, construction
of foldamers and helix structures, crystal engineering, as well as the supramolecular
polymer. Among all types of hydrogen bond structures, ureidopyrimidinone (UPy)
has attracted tremendous attention due to the high binding constant (10
7 M
À1 in
chloroform), self-complementary structures, and easy access [7c]. Wang and
coworkers firstly introduced this motif into the pillar[5]arene-based supramolecular
13 Pillararene-Based Supramolecular Polymer
345