of an external stimulus such as light, heat, or change in pH. These stimuliresponsive metallopolymers could be affected in many different ways. As a
consequence, the stimuli-responsive metallopolymers are not only interesting
candidates for self-healing polymers but also for several other application fields
such as sensors [69].
In addition to stimulation by redox processes, the properties of a metallopolymer
can also be influenced by other stimuli. An interesting change in metallopolymers
was shown by Peng et al. [70]. The authors used the external stimulus light in
order to influence the properties of an iron metallopolymer; a redox process
from iron(III) to iron(II) could be induced by illumination and this process
changed the consistency of the polymer. In this case, a metallopolymer gel,
which contains iron(III) ions, was converted into a liquid polymer by simply
reducing the iron ions. The subsequent re-oxidation by air led to the original
“solid state” system.
Furthermore, the supply of thermal energy could also be used to influence the
properties of a metallopolymer and to yield the desired effects. For this purpose,
Zhou and coworkers utilized phase-separated ruthenium-containing polymers
with two glass transition temperatures [71]. The transitions induced the a kind of
mobility that is required for the self-healing process [72].
Beside the mentioned external stimuli, there are also several other possibilities
that can influence the polymer properties and structures, leading to changes on
the molecular scale and thus, to other changes in the properties of the
metallopolymer. For example, Beck and Rowan showed that mechanical energy
is adequate (e.g., simple shaking) for generating a change in the properties of the
polymer [73]. The authors used an oligo(ethylene gylcol), which was functionalized
with two 2,6-bis(1’-methylbenzimidazolyl)pyridine units at the termini. In a second
step, a metallopolymer was formed by the addition of a lanthanide (lanthanum or
europium) and a transition metal ion (cobalt or zinc). In further studies it could be
shown that these metallopolymers could also be influenced by other external stimuli
[74, 75], e.g., by changes in pH, light, and temperature.
This all-embracing example shows that many different stimuli can affect a
metallopolymer to result in the desired properties. In particular, the reversibility
of the metal–ligand interaction and the mobility of a metallopolymer are key factors
for the implementation of self-healing properties [76, 77]. Additionally, the
addressability by other stimuli allows the possibility of triggering healing processes
in metallopolymers.
4 Self-Healing Metallopolymers
The above-mentioned properties of metallopolymers are the basic requirements for
the generation of self-healing behavior. As a consequence, it is possible to generate
a reversible system and to introduce self-healing mechanisms, which is the principle of intrinsic self-healing systems [78–81].
Metallopolymers as an Emerging Class of Self-Healing Materials
245
consequence, the stimuli-responsive metallopolymers are not only interesting
candidates for self-healing polymers but also for several other application fields
such as sensors [69].
In addition to stimulation by redox processes, the properties of a metallopolymer
can also be influenced by other stimuli. An interesting change in metallopolymers
was shown by Peng et al. [70]. The authors used the external stimulus light in
order to influence the properties of an iron metallopolymer; a redox process
from iron(III) to iron(II) could be induced by illumination and this process
changed the consistency of the polymer. In this case, a metallopolymer gel,
which contains iron(III) ions, was converted into a liquid polymer by simply
reducing the iron ions. The subsequent re-oxidation by air led to the original
“solid state” system.
Furthermore, the supply of thermal energy could also be used to influence the
properties of a metallopolymer and to yield the desired effects. For this purpose,
Zhou and coworkers utilized phase-separated ruthenium-containing polymers
with two glass transition temperatures [71]. The transitions induced the a kind of
mobility that is required for the self-healing process [72].
Beside the mentioned external stimuli, there are also several other possibilities
that can influence the polymer properties and structures, leading to changes on
the molecular scale and thus, to other changes in the properties of the
metallopolymer. For example, Beck and Rowan showed that mechanical energy
is adequate (e.g., simple shaking) for generating a change in the properties of the
polymer [73]. The authors used an oligo(ethylene gylcol), which was functionalized
with two 2,6-bis(1’-methylbenzimidazolyl)pyridine units at the termini. In a second
step, a metallopolymer was formed by the addition of a lanthanide (lanthanum or
europium) and a transition metal ion (cobalt or zinc). In further studies it could be
shown that these metallopolymers could also be influenced by other external stimuli
[74, 75], e.g., by changes in pH, light, and temperature.
This all-embracing example shows that many different stimuli can affect a
metallopolymer to result in the desired properties. In particular, the reversibility
of the metal–ligand interaction and the mobility of a metallopolymer are key factors
for the implementation of self-healing properties [76, 77]. Additionally, the
addressability by other stimuli allows the possibility of triggering healing processes
in metallopolymers.
4 Self-Healing Metallopolymers
The above-mentioned properties of metallopolymers are the basic requirements for
the generation of self-healing behavior. As a consequence, it is possible to generate
a reversible system and to introduce self-healing mechanisms, which is the principle of intrinsic self-healing systems [78–81].
Metallopolymers as an Emerging Class of Self-Healing Materials
245