Supramolecular Polymerizations
Supramolecular Polymerizations
Supramolecular Polymerizations
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512 A. Ciferri<br />
dimensional growth is possible. [1] While the assembly<br />
produced by supramolecular polymerization is a supramolecular<br />
polymer, [2] the reverse is not true. Several systems<br />
have been reported and defined as supramolecular<br />
polymers that do not conform to the growth mechanisms<br />
and theory of supramolecular polymerization.<br />
A supramolecular polymer (SP) can be defined broadly<br />
as a system characterized by non-bonded interactions<br />
among repeating units. Such a broad definition includes<br />
all systems that have been described as SPs although, due<br />
to its general nature, it does not suggest immediately the<br />
structural features or potential applications of this exciting<br />
class of new materials. In fact, even a molecular polymer,<br />
based on covalently bonded repeating units, displays<br />
high order structural organization controlled by supramolecular<br />
interactions. The occurrence of supramolecular<br />
interaction is so widespread that even an organic crystal<br />
is described as a supramolecular assembly. [3] Attempts to<br />
restrict the definition of SPs, for instance to systems displaying<br />
polymer-like properties in dilute solution, have<br />
been made. [4] The problem is not a simple one since several<br />
variables control the degree of supramolecular polymerization<br />
(DPÞ. Moreover, the term supramolecular has<br />
great appeal since it invites the use of unifying concepts<br />
that cut across the traditional boundaries between colloid,<br />
polymer and solid state science.<br />
Rather than attempting to further clarify the definition<br />
of SP, it is useful to present a classification of the various<br />
systems that have been reported. A possible classification,<br />
based on assembling mechanisms, is schematized in<br />
Figure 1. It offers a glimpse of the impressive growth that<br />
has occurred over the past decade and contextually highlights<br />
the SPs produced by supramolecular polymerization<br />
that form the core of the present review. The reference<br />
model of the classical covalent chain resulting from<br />
molecular polymerization of small bifunctional monomers<br />
is schematized at the top of Figure 1. The selfassembling<br />
chain is an open one, meaning that, in principle,<br />
it can grow to a distribution of large DPs, irreversible<br />
in solution and under a wide range of external variables.<br />
Figure 1. Classification of supramolecular polymers. Class A<br />
(reversible polymers obtained by supramolecular polymerization)<br />
is the main topic of this article.<br />
Class A. The major components of this class are equilibrium<br />
polymers based on processes that can appropriately<br />
be regarded as supramolecular polymerizations. [1, 2] The<br />
linear chains are self-assembled, open, growing to a distribution<br />
of DPs, and in a state of thermodynamic equilibrium<br />
sensitive to solvent type, concentration and external<br />
variables. The geometrical shapes in the scheme of<br />
class A (Figure 1) remind that unimers in supramolecular<br />
polymerization can be of several forms and sizes. In particular,<br />
the unimer can be a large supramolecular aggregate,<br />
a supermolecule, [2] a covalent polymer (e.g., a globular<br />
protein) in which case the SP will actually be a polymer<br />
of polymers. In class A we may also include SPs<br />
based on unimers with functionality A2, when a variety of<br />
multidimensional assemblies (helical, planar, 3D)<br />
becomes possible. Examples of linear systems are hydrogen-bonded<br />
polymers, [5–17] coordination polymers [18–20]<br />
and also micelles. [21–24] Examples of more complex geo-<br />
Alberto Ciferri is Chemistry Professor at the University of Genoa, as well as Visiting Professor<br />
at Duke University, Durham, North Carolina (1975-present). He has authored about<br />
200 original papers, books and patents mostly in the areas of rubber elasticity, biological<br />
and synthetic fibers, interactions between salts and macromolecules, liquid crystals, and<br />
supramolecular assemblies. He received his D.Sc. degree in physical chemistry from the<br />
University of Rome, and held positions as Scientist at Monsanto Co. and Director of<br />
Research at the National Research Council. He currently is President of the Jepa-Limmat<br />
Foundation, supporting advanced education in developing countries.