repetitive units. However, the challenge lies on the suitable polymerization method
due to the high structural complexity of daisy chain monomers. Stoddart and
co-workers [35a] employed the highly selective click reaction to covalently connect
daisy chain monomers (Fig. 3c), realizing the polymerization of rotaxane muscles
for the first time. However, the degree of polymerization was not large enough
(n = 11 for polymer 4 [35a]; n = 22 for polymer 5 [35b]), which may be due to the
formation of cyclic oligomers and low solubility of the formed polymers [11g].
Supramolecular polymerization seems to be a more promising strategy for the
amplification of rotaxane muscles. Giuseppone and co-workers were the first who
realized supramolecular polymers consisting of thousands of rotaxane muscles
(n = 3000 for polymer 6) [35c]. In their system, noncovalent metal ion coordination
simultaneously provided good solubility and high affinity for supramolecular polymerization; meanwhile, the improved rigid structure of the daisy chain monomer
inhibited the formation of cyclic oligomers. Another example, reported by the same
group, is the hierarchical self-assembly of daisy chain monomers via multiple
hydrogen-bonding interactions (polymer 7) [35e]. In this case, the hydrogenbonding connected daisy chain linear polymers could be further assembled into
fiber-like structures in a larger scale via the self-assembly of alkyl side chains. The
obtained muscle-like fibers could be actuated by external acid/base stimuli. Importantly, the contraction and stretching motion of bistable poly[n]daisy chain could be
observed under an electron microscope. This work is a significant step toward the
construction of artificial muscle fibers by bottom-up approaches, advancing further
the amplification of single-molecular motion into the macroscopic world.
Recently, our group has reported a self-assembled poly[n]daisy chain system
based on the well-known 2-ureido-4-pyrimidinone (Upy) quadruple hydrogenbonding unit (Polymer 8) [35f]. In this work, the photo-initiated formation of poly
[n]daisy chain was demonstrated for the first time, involving photo-labile coumarin
moieties as protecting groups of the Upy terminals. When protected by the coumarins, Upy-terminated daisy chain monomers could not form polymers even at high
concentrations. After removal of the coumarin moieties using UV light irradiation,
the exposed Upy units were efficiently assembled the daisy chain monomers into
poly[n]daisy chains via their strong hydrogen-bonding interactions. The importance
of this work is in the introduction of photo-initiated polymerization, which allows for
the remote-controlled assembly of poly[n]daisy chains that could have potential use
in biological applications.
11.2.4 Hetero[n]rotaxanes
As a platform for the construction of functional molecules, rotaxanes need increasingly complex structure in order to perform multiple and diverse tasks within a single
molecule [3]. Construction of hetero[n]rotaxane, a rotaxane with two or more kinds
of macrocycles, is a significant route toward advanced rotaxanes with high structural
complexity [12, 26c]. The simultaneous introduction of two different types of
macrocycles would bring much possibility for multiple steady states in a
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C.-X. Zhao et al.
due to the high structural complexity of daisy chain monomers. Stoddart and
co-workers [35a] employed the highly selective click reaction to covalently connect
daisy chain monomers (Fig. 3c), realizing the polymerization of rotaxane muscles
for the first time. However, the degree of polymerization was not large enough
(n = 11 for polymer 4 [35a]; n = 22 for polymer 5 [35b]), which may be due to the
formation of cyclic oligomers and low solubility of the formed polymers [11g].
Supramolecular polymerization seems to be a more promising strategy for the
amplification of rotaxane muscles. Giuseppone and co-workers were the first who
realized supramolecular polymers consisting of thousands of rotaxane muscles
(n = 3000 for polymer 6) [35c]. In their system, noncovalent metal ion coordination
simultaneously provided good solubility and high affinity for supramolecular polymerization; meanwhile, the improved rigid structure of the daisy chain monomer
inhibited the formation of cyclic oligomers. Another example, reported by the same
group, is the hierarchical self-assembly of daisy chain monomers via multiple
hydrogen-bonding interactions (polymer 7) [35e]. In this case, the hydrogenbonding connected daisy chain linear polymers could be further assembled into
fiber-like structures in a larger scale via the self-assembly of alkyl side chains. The
obtained muscle-like fibers could be actuated by external acid/base stimuli. Importantly, the contraction and stretching motion of bistable poly[n]daisy chain could be
observed under an electron microscope. This work is a significant step toward the
construction of artificial muscle fibers by bottom-up approaches, advancing further
the amplification of single-molecular motion into the macroscopic world.
Recently, our group has reported a self-assembled poly[n]daisy chain system
based on the well-known 2-ureido-4-pyrimidinone (Upy) quadruple hydrogenbonding unit (Polymer 8) [35f]. In this work, the photo-initiated formation of poly
[n]daisy chain was demonstrated for the first time, involving photo-labile coumarin
moieties as protecting groups of the Upy terminals. When protected by the coumarins, Upy-terminated daisy chain monomers could not form polymers even at high
concentrations. After removal of the coumarin moieties using UV light irradiation,
the exposed Upy units were efficiently assembled the daisy chain monomers into
poly[n]daisy chains via their strong hydrogen-bonding interactions. The importance
of this work is in the introduction of photo-initiated polymerization, which allows for
the remote-controlled assembly of poly[n]daisy chains that could have potential use
in biological applications.
11.2.4 Hetero[n]rotaxanes
As a platform for the construction of functional molecules, rotaxanes need increasingly complex structure in order to perform multiple and diverse tasks within a single
molecule [3]. Construction of hetero[n]rotaxane, a rotaxane with two or more kinds
of macrocycles, is a significant route toward advanced rotaxanes with high structural
complexity [12, 26c]. The simultaneous introduction of two different types of
macrocycles would bring much possibility for multiple steady states in a
284
C.-X. Zhao et al.