Researches also have been conducted on the polymer-based supramolecular gels,
in which the cross-linked network was constructed by the host-guest interaction
between the pillar[n]arene and corresponding guests. Wang and coworkers
constructed a redox-responsive gel based on copolymers containing pillar[6]arene
moieties and ferrocene derivatives (Fig. 18) [27]. Herein, the copolymers containing
pillar[6]arene moieties and ferrocene derivatives were prepared. No supramolecular
gels were formed by mixing these two polymers in an organic solvent because
no complexes were formed between the ferrocenes and pillar[6]arenes moieties.
However, the addition of an oxidant to the mixture triggered the formation of
a supramolecular gel because the ferrocenes were converted to ferrocenium cations,
which could be included by pillar[6]arene, resulting in the formation of cross-linked
network. Moreover, this gel to sol transition could be achieved by the addition of
competitive host (native pillar[6]arene) and competitive guest (cobaltocenium hexafluorophosphate), which could destroy the formed stable inclusion complex, leading
to the destruction of supramolecular gels.
Liao and coworkers constructed another type of polymer-based supramolecular
gels (Fig. 19) [35]. Firstly, they synthesized a copolymer with pendant pillar[5]arene
via free radical polymerization. Herein, a ditopic guest molecule with the pyridinium
cation was synthesized for the cross-linker, which was different with the previous
research by using the copolymer containing the guest molecules. The supramolecular
polymeric gel based on the pyridinium-pillar[5]arene motif was fabricated readily by
mixing the bifunctionalized guest with the copolymer. On the basis of the competition
of host-guest interactions, such a gel could be transformed to sol by addition of
competitive host (native pillar[5]arene) or guest molecules (butanedinitrile). Besides
host-guest interactions, ordered stacking of pillararenes played an important role in
construction of this supramolecular gel, which could realize the gel-sol transition by
increasing temperature and the reverse process by decreasing temperature.
13.3.3 Application for the Nano-carrier
With the development of supramolecular chemistry, supramolecular delivery
systems are providing incredible opportunities for biomedicine, especially for the
diagnosis and treatment of diseases, due to the dynamic nature of noncovalent
interaction leading to the diversification of delivery systems [36]. The combination
of macrocyclic molecules and the covalent polymers could provide more external
stimuli responsiveness than the traditional polymeric drug delivery systems.
Inspired by biodegradable and biocompatible polyglutamic acid, Wang and
coworkers synthesized a butyl-ammonium group functionalized polymer (polymer
3) with the biotin moiety as the targeted unit (Fig. 20) [37]. Due to the host-guest
interaction between butyl-ammonium group and the anionic pillar[5]arene (WP5),
the side-chain supramolecular polymer could be obtained, which could further selfassemble into vesicles thanks to the electrostatic interaction and hydrophobic effect.
Furthermore, this hollow assembly could encapsulate the hydrophilic anticancer
drug mitoxantrone (MTZ). And the release of the encapsulated MTZ could be
13 Pillararene-Based Supramolecular Polymer
365
in which the cross-linked network was constructed by the host-guest interaction
between the pillar[n]arene and corresponding guests. Wang and coworkers
constructed a redox-responsive gel based on copolymers containing pillar[6]arene
moieties and ferrocene derivatives (Fig. 18) [27]. Herein, the copolymers containing
pillar[6]arene moieties and ferrocene derivatives were prepared. No supramolecular
gels were formed by mixing these two polymers in an organic solvent because
no complexes were formed between the ferrocenes and pillar[6]arenes moieties.
However, the addition of an oxidant to the mixture triggered the formation of
a supramolecular gel because the ferrocenes were converted to ferrocenium cations,
which could be included by pillar[6]arene, resulting in the formation of cross-linked
network. Moreover, this gel to sol transition could be achieved by the addition of
competitive host (native pillar[6]arene) and competitive guest (cobaltocenium hexafluorophosphate), which could destroy the formed stable inclusion complex, leading
to the destruction of supramolecular gels.
Liao and coworkers constructed another type of polymer-based supramolecular
gels (Fig. 19) [35]. Firstly, they synthesized a copolymer with pendant pillar[5]arene
via free radical polymerization. Herein, a ditopic guest molecule with the pyridinium
cation was synthesized for the cross-linker, which was different with the previous
research by using the copolymer containing the guest molecules. The supramolecular
polymeric gel based on the pyridinium-pillar[5]arene motif was fabricated readily by
mixing the bifunctionalized guest with the copolymer. On the basis of the competition
of host-guest interactions, such a gel could be transformed to sol by addition of
competitive host (native pillar[5]arene) or guest molecules (butanedinitrile). Besides
host-guest interactions, ordered stacking of pillararenes played an important role in
construction of this supramolecular gel, which could realize the gel-sol transition by
increasing temperature and the reverse process by decreasing temperature.
13.3.3 Application for the Nano-carrier
With the development of supramolecular chemistry, supramolecular delivery
systems are providing incredible opportunities for biomedicine, especially for the
diagnosis and treatment of diseases, due to the dynamic nature of noncovalent
interaction leading to the diversification of delivery systems [36]. The combination
of macrocyclic molecules and the covalent polymers could provide more external
stimuli responsiveness than the traditional polymeric drug delivery systems.
Inspired by biodegradable and biocompatible polyglutamic acid, Wang and
coworkers synthesized a butyl-ammonium group functionalized polymer (polymer
3) with the biotin moiety as the targeted unit (Fig. 20) [37]. Due to the host-guest
interaction between butyl-ammonium group and the anionic pillar[5]arene (WP5),
the side-chain supramolecular polymer could be obtained, which could further selfassemble into vesicles thanks to the electrostatic interaction and hydrophobic effect.
Furthermore, this hollow assembly could encapsulate the hydrophilic anticancer
drug mitoxantrone (MTZ). And the release of the encapsulated MTZ could be
13 Pillararene-Based Supramolecular Polymer
365