Chemistry Reference
In-Depth Information
One of the less explored families of fullerene polymers is the main-chain one.
Here, the C
60
spheres are directly allocated in the polymer backbone forming a
necklace-type structure. Unfortunately, the double addition on the C
60
moieties
results in the formation of complex regioisomeric mixtures (up to eight isomers)
and also the formation of cross linking products by multiple additions can occur.
The preparation of these in-chain polymers can be carried out by direct reaction
between the C
60
unit and a suitable symmetrically difunctionalized monomer or by
means of polycondensation between a fullerene bisadduct (or a mixture) and a
difunctionalized monomer. In contrast, side-chain polymers, sometime called
on-
chain
or “
charm-bracelet
,” represent the most studied family of polyfullerenes and
show the C
60
pending from the main polymer chain. A century of studies on
polymers has been exploited in the binding of C
60
to all the “classic” families
of polymers such as polystyrenes [
79
,
80
], polyacrylates [
81
,
82
], polyethers [
83
],
polycarbonates [
84
], polysiloxanes [
85
], and polysaccharides [
86
] in the search for
improved processability and enhanced properties, with a wide range of potential
applications. In this family can also be included the “double-cable” polymers
[
87
,
88
], in which the
-conjugated semiconducting polymer (p-type cable) with
electron-donating characteristics contains electron-accepting fullerene units cova-
lently connected (n-type cable), with remarkable advantages for construction of
photovoltaic devices. The synthesis of the members belonging to this family can be
achieved by direct introduction of fullerene itself (or a C
60
-derivative) into a
preformed polymer, or by homo-/co-polymerization of a suitable C
60
-derivative.
Moreover, for the double-cable polymers, electropolymerization is also possible.
Finally, the most recent family of macromolecular fullerene is that composed by
supramolecular polymers created through any type of self-assembly and via revers-
ible interactions of one or more types of components. Interestingly, these reversible
interactions can often allow supramolecular polymers to equilibrate thermally with
their monomers or oligomers. There are several ways to obtain such supramolecular
assemblies; among them several systems may be obtained by interactions between
functionalized polymers and C
60
derivatives or fullerene itself or through the
assembly of self-complementary C
60
derivatives. More recently, assemblies
between ditopic concave guests and [60]fullerene by means of concave-convex
complementary interactions have also been reported [
89
,
90
].
π
3.2 Properties and Applications
Despite hundreds of examples of polyfullerenes having been reported over the last
two decades, to date this class of smart material has not had a real application.
However, the progress achieved year by year reveals new potential applications and
improved properties with respect to early examples, or even with respect to the state
of the art, as in the case of photovoltaic applications. In the present section, some of
the most promising and recent applications for C
60
-polymers will be shown. Since
1991 fullerene polymers have been studied as active materials in membranes both
for gas separation [
91
,
92
] and for proton exchange fuel cells [
93
], and also as active
