Supramolecular Polymerizations
Supramolecular Polymerizations
Supramolecular Polymerizations
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Macromol. Rapid Commun. 2002, 23, 511–529 511<br />
Review: Unimers of both natural and synthetic origin<br />
self-assemble into linear, helical, columnar, planar and<br />
three-dimensional structures depending upon the functionality<br />
of supramolecular interactions. Recent reports<br />
describing the mechanism of formation, properties and<br />
possible applications of these systems are critically<br />
reviewed. The assembling of one-dimensional systems<br />
produces equilibrium polymers showing a length distribution<br />
and a degree of polymerization that may far exceed<br />
that of typical condensation polymers. Their growth may<br />
occur by a step-by-step process akin to polycondensation,<br />
and by cooperative processes such as helical growth or<br />
growth coupled to liquid crystallinity. Of particular interest<br />
are functional systems based on the coupling of a<br />
chemical reaction to supramolecular polymerization, and<br />
systems based on a covalent polymer hosted within the<br />
cavity of a supramolecular one. The assembly of two and<br />
three-dimensional systems occurs through a process akin<br />
to crystallization. The supramolecular organization of<br />
amphiphiles such as block copolymers is currently well<br />
described by the mean-field theory of unstable modes in<br />
homogeneous melts. An alternative, less sophisticated<br />
approach considers the growth of specifically designed<br />
building blocks. Possible applications are in areas that<br />
expand the uses of covalent polymers, electrochemical<br />
<strong>Supramolecular</strong> <strong>Polymerizations</strong><br />
Alberto Ciferri<br />
Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy<br />
E-mail: cifjepa@chimica.unige.it<br />
Keywords: assemblies; host-guest systems; hydrogen bonding; supramolecular structures;<br />
Contents<br />
1 Definitions. Classification of <strong>Supramolecular</strong> Polymers<br />
2 The Bond. Site and Shape Recognition<br />
3 Functionality of the Unimer and Dimensionality of<br />
the Assembly<br />
4 Theory of <strong>Supramolecular</strong> Polymerization<br />
4.1 Linear, Helical Assemblies<br />
4.2 Multidimensional Assemblies<br />
5 Polymers from <strong>Supramolecular</strong> Polymerization<br />
5.1 Linear Chains and Applications<br />
5.1.1 H-Bonded Polymers<br />
5.1.2 H-Bonded Networks<br />
5.1.3 Coordination Polymers<br />
5.1.4 Rheology and Polymer-Like Properties<br />
5.2 Helical Chains and Functional Systems<br />
and photonic devices, ion-selective channels, separation<br />
processes, microengines mimicking the performance of<br />
biological systems, storage of sequential information, biocompatible<br />
and patterned surfaces, sensors. A classification<br />
including additional systems that have been described<br />
as supramolecular polymers is presented.<br />
Disk-shaped, three blades molecule: formation of a helical<br />
assembly when the blades assume a propeller-like conformation.<br />
(i Am. Chem. Soc. 2000 and 2001.)<br />
5.3 Columnar and Micellar Assemblies<br />
5.3.1 Evidence for the SLC Mechanism<br />
5.3.2 Helical-Columnar Mechanism<br />
5.3.3 Supermolecules. Tubular Assemblies<br />
5.4 Host/Guest Polymeric Assemblies<br />
5.5 Planar Assemblies<br />
5.6 Composite and 3D Assemblies<br />
6 Topics for Further Investigation<br />
1 Definitions. Classification of <strong>Supramolecular</strong><br />
Polymers<br />
<strong>Supramolecular</strong> polymerization is the process of forming<br />
long sequences of bifunctional unimers linked only by<br />
main-chain noncovalent bonds. Theoretically well<br />
defined growth mechanisms assist the process, and extension<br />
of the underlying concepts to planar and three-<br />
Macromol. Rapid Commun. 2002, 23, No. 9 i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1022-1336/2002/0906–0511$17.50+.50/0
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.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 513<br />
metries are helical, columnar, tubular soluble [25–44] or<br />
fibrous proteins, [45] S-layers, [46] composite systems such<br />
as block copolymers [47–49] and the tobacco mosaic virus<br />
(TMV). [50] Random networks and blends stabilized by<br />
multifunctional supramolecular linkages have also been<br />
reported. [51–55]<br />
Class B. This class includes self-assembled structures<br />
formed by supramolecular binding of monofunctional<br />
unimers. Such unimers cannot undergo open supramolecular<br />
polymerization, but can nevertheless form closed<br />
assemblies involving both low- and high-molecularweight<br />
species. Classical host/guest complexes, [56] base<br />
pairing of simple nucleoside [57] and supermolecules are<br />
low-molecular-weight examples. Polymeric examples<br />
described as SPs include side-chain binding of a monofunctional<br />
unimer to a covalent chain. For instance, Kato<br />
and FrØchet first reported the binding of the monofunctional<br />
mesogen stilbazole to the side chains of a nonmesogenic<br />
polymer functionalized with pendant benzoic<br />
acid groups. [58] Additional examples are double-, and triple-chain<br />
assemblies, and globular structures unable to<br />
grow further when complementary monofunctional sites<br />
are internally saturated.<br />
Class C. A number of SPs displaying novel supramolecular<br />
features were obtained by superimposition of covalent<br />
and supramolecular bonds. These systems are selfassembling<br />
but show irreversible DPs. The supramolecular<br />
organization may either precede, be simultaneous to,<br />
or follow the formation of covalent bonds. Examples of<br />
the first type include the rotaxane and catenane polymers<br />
described by Stoddart and coworkers, [59–61] the growth of<br />
dendrimers though successive generations, [62] and other<br />
attempts to stabilize a supramolecular assembly by the<br />
subsequent formation of covalent bonds. [63, 64] The final<br />
covalent system may retain specific supramolecular features,<br />
or the precursor supramolecular organization may<br />
just be a step of a supramolecularly assisted synthesis of<br />
a complex structure. Examples in which the supramolecular<br />
and the molecular order are simultaneously formed<br />
are the synthesis of dendrons possessing polymerizable<br />
functionality at their focal points, as reported by Percec<br />
and Schlüter. [65, 66] These assemblies display most interesting<br />
composite architectures such as columns of disks<br />
hosting the dendrons, with the main covalent chain running<br />
in the center of each column. [67] Cases in which the<br />
covalent structure occurs before the supramolecular one<br />
include the dendronization of a covalent polymer,<br />
reported for instance by Tomalia and coworkers, [68] and<br />
the self-assembled monolayers (SAMs) regarded as<br />
supramolecular assemblies of short hydrocarbon chains<br />
covalently grafted to a gold surface. [69]<br />
Class D. The class of engineered assemblies includes<br />
systems that do not form spontaneously ordered structures<br />
under normal conditions. Their ordered structurization<br />
is based on controlled methods of deposition or<br />
synthesis. Their classification as SPs can be justified<br />
since elements of supramolecular interaction still assist<br />
the final organization. Examples are the layered assembly<br />
of complementary polyelectrolytes obtained by step-wise<br />
deposition under kinetic control, [70] and polymer brushes<br />
prepared by grafting a polymer chain over a selfassembled<br />
monolayer of an initiator. [71] Both approaches<br />
allow a fine tuning of surface properties, complemented<br />
by patterning possibilities. Tailored performance in applications,<br />
such as biocompatibility, biocatalysis, integrated<br />
optics and electronics, are possible.<br />
[70, 71]<br />
The following sections detail concepts and results relevant<br />
to supramolecular polymerization and SPs in class A<br />
systems. In particular, Section 4 summarizes the theoretical<br />
framework of polymerization mechanisms [1] forming<br />
the basis for the critical analysis of the experimental data<br />
to be presented in Section 5. Some aspects already<br />
described in preceding reviews/analyses by the author [1]<br />
(focusing on mechanisms), by Lehn, [2] Zimmermann and<br />
coworkers, [5] Meijer and coworkers [4] (focusing on chemical<br />
features) are briefly summarized, placing more<br />
emphasis on recent data and concepts.<br />
2 The Bond. Site and Shape Recognition<br />
The cement holding well organized supramolecular structures<br />
requires the description of: (i) interaction between<br />
specific sites, (ii) site distribution and (iii) shape complementarity<br />
of the unimers (cf. ref. [1] for a detailed discussion).<br />
The relevant interactions are schematized in Figure<br />
Figure 2. Forces assisting supramolecular organization.
514 A. Ciferri<br />
Figure 3. (a) Bifunctional units with binding sites along the N<br />
and S (or E and W) directions, yielding linear polymers. (b) Tetrafunctional<br />
units with two sites along N and S, and two sites<br />
along N-E and S-E (same side of the assembly), yielding linear<br />
or helical double chains. (c) Tetrafunctional units with sites at<br />
right angles within the cross-sectional or equatorial area of the<br />
unimer, yielding planar polymers. (d) Hexafunctional units with<br />
sites as in (c), and two additional sites on the flat surfaces or<br />
poles, yielding three-dimensional polymers.<br />
2. Classical supramolecular interactions (Coulombic,<br />
hydrogen and van der Waals bonds) are localized at specific<br />
sites or atoms of the unimers. These sites may be<br />
distributed at discrete locations over the surface of the<br />
unimers: the direction of interaction determines the functionality<br />
of the unimer and, in turn, the dimensionality of<br />
the assembly (cf. next section and Figure 3). These localized<br />
interactions are described by the respective set of<br />
potential functions involving combinations of point<br />
charges, dipolar interaction and separation distances. Several<br />
combinations of the above interactions may occur<br />
over the surface of the unimer, additively contributing to<br />
the overall binding free energy. [56, 72] Cooperative effects<br />
(when the formation of the first pairwise interaction<br />
increases the binding constant at successive sites along<br />
the chain) are also possible. In the case of H-bonds that –<br />
due to their strong directionality – are a primary source<br />
of stabilization of several SPs in class A, the parallel or<br />
antiparallel arrangement of multiple bonds may increase<br />
or decrease the product of the single binding constants on<br />
account of secondary electrostatic interaction.<br />
[5, 6]<br />
In addition to the above site-localized classical interactions,<br />
other stabilizing interactions play an important role<br />
in polymeric assemblies. [1] The solvophobic bond is<br />
responsible for the micellization of amphiphiles in the<br />
presence of a solvent. Even in the absence of a solvent,<br />
the incompatibility of amphiphilic components produces<br />
their ordered microsegregation. These interactions can be<br />
described by thermodynamic parameters that control<br />
micro- and macrophase separations. For instance, the<br />
Flory-Huggins thermodynamic parameter v plays a primary<br />
role in the theoretical description of microsegregation<br />
in block copolymers (cf. ref. [1, 81] and Section 4.2).<br />
The occurrence of liquid crystallinity in solutions of<br />
several polymeric assemblies is an example of hierarchical<br />
structurization (“macroscopic expression of molecular<br />
recognition” [2] ). Again, a thermodynamic effect (e.g.,<br />
volume exclusion resulting from the shape anisotropy of<br />
rigid SPs) is the primary driving force for structurization.<br />
Note that it is convenient to distinguish the role of shape<br />
in the stabilization of individual unimers (shape I effect,<br />
cf. (iii) above) from the role of shape anisotropy in the<br />
development of liquid crystallinity by rigid SPs (shape II<br />
effect, cf. ref. [1] and Section 4.1).<br />
3 Functionality of the Unimer and<br />
Dimensionality of the Assembly<br />
The assessment of unimer functionality is a primary<br />
requirement for determining the dimensionality of the<br />
assembly. Bifunctional rod-like, disk-like or spherical<br />
unimers (see scheme in Figure 3a) having binding sites<br />
pointing toward the North and South directions (or<br />
toward East and West) yield linear polymers. Note that it<br />
is the directionality of the interaction that specifies the<br />
functionality, e.g., the same functionality is assumed if<br />
four H-bonds rather than a single one point in the same<br />
direction.<br />
Increasing the functionality of the unimers produces<br />
more complex structures. The presence of two additional<br />
sites produces extended or helical double chains, [1, 7] provided<br />
the additional sites are located at the same side of<br />
the assembly (e.g., NE and SE, Figure 3b). However, planar<br />
assemblies are expected when four sites point toward<br />
perpendicular directions within a cross-section of cylindrical<br />
and disk-like unimers, or the equatorial plane of a<br />
spherical unimer (Figure 3c). The occurrence of two<br />
additional sites on the flat surfaces of cylinders and disks,<br />
or the poles of a sphere, generates a three-dimensionally<br />
ordered network (Figure 3d).<br />
The symbols +, –, 0, and 9, in Figure 3 indicate the<br />
functionality and refer to any possible localized supramolecular<br />
bond (Coulombic, hydrogen, van der Waals). In<br />
the case of non-localized effects, such as incompatibility<br />
and shape II-induced mesophases, a more uniform distribution<br />
of repulsive interaction ought to be assumed. In a<br />
few cases a polymer is undoubtedly formed, but the specification<br />
of unimer size and shape may not be straightforward.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 515<br />
Figure 4. Theory of linear supramolecular polymerization.<br />
Schematic variation of the length (or DP) of a growing linear or<br />
helical assembly with the unimer concentration C according to<br />
three different growth mechanisms. MSOA: multi-stage open<br />
association, [1, 74] HG: helical growth, [26] SLC: open supramolecular<br />
liquid crystal. [1, 28– 30] C* is the critical concentration for helical<br />
growth, C i is the critical concentration for the formation of<br />
the mesophase. [77]<br />
4 Theory of <strong>Supramolecular</strong> Polymerization<br />
4.1 Linear, Helical Assemblies<br />
There are three different supramolecular polymerization<br />
mechanisms by which a linear or helical polymer could<br />
assemble consistently with the schemes in Figure 3a and<br />
b. [1]<br />
(i) Multistage open association (MSOA) resembles the<br />
step-by-step mechanism of the molecular polycondensation<br />
of bifunctional unimers. [73] The increase in the degree<br />
of polymerization of the growing assembly with the (total)<br />
[1, 74, 75]<br />
unimer concentration C is given by<br />
C L [M0/(4KNa)] N [(DP w) 2 –1] (1)<br />
where M0 is the unimer molar mass, Na the Avogadro’s<br />
number and K is the site binding constant. A plot of<br />
Equation (1) is schematized in Figure 4a. A continuous<br />
increase in DP, or length L of the assembly, with concentration<br />
is expected and the rate of increase is strongly<br />
affected by the binding constant. For K a 10 6 m –1 , corresponding<br />
to one or two H-bonds per site, only oligomers<br />
(DP a 10) occur in dilute (a1%) solution. However, in<br />
the case of the ureidopyrimidone polymers reported by<br />
Meijer and coworkers, [4, 52] characterized by four H-bonds<br />
per site and K A 10 7 , extremely high DPs (e 1000) may<br />
be reached in dilute isotropic solution. The familiar Car-<br />
others equation and the control of DP by monofunctional<br />
unimers [73] apply to MSOA.<br />
(ii) Helical growth (HG) is achieved when step-by-step<br />
growth is reinforced by an intra-assembly cooperative<br />
effect, [26] originating from a peculiar functionality such as<br />
that schematized in Figure 3b. Since more bonds per<br />
unimers occur with respect to the situation in Figure 3a,<br />
Kh A K and a critical unimer concentration C*occurs at<br />
which the MSOA regime encroaches helix formation<br />
with a sudden increase in DP according to [26]<br />
DPn =(Ch/C*) 1/2 N r –1/2 (2)<br />
where r s 1 is the familiar cooperativity parameter<br />
(easily in the order of 10 –8 ) and Ch is the concentration of<br />
helical polymer (increasing when C A C*). Figure 4b<br />
shows the step-jump increase in DP, orL, accompanying<br />
the nucleation of the helix at C*. The theory was intended<br />
to describe the reversible aggregation of globular proteins<br />
attaining extremely high DPs(A4000, cf. Section 5.2).<br />
Van der Schoot and coworkers [76] have recently generalized<br />
Oosawa’s theory to cover general situations with<br />
Kh A K, including high and weakly cooperative helical<br />
aggregation. The theory was applied to the case of columnar<br />
assemblies of chiral discotic molecules (cf. Section<br />
6.3.2).<br />
(iii) Growth-coupled-to-orientation (SLC) is alternatively<br />
described as the open supramolecular liquid crystal.<br />
[77] It is an inter-assembly cooperative process that<br />
encroaches MSOA causing a sudden enhancement of the<br />
step-by-step growth of bifunctional unimers at a critical<br />
concentration C i at which nematic order appears.<br />
[1, 21–23]<br />
The polymer must have considerable chain rigidity, [22] as<br />
expressed by its persistent (q) or deflection length (k),<br />
and the DP attained at C i may be approximated as<br />
DP V q/L0<br />
where L0 is the length of the unimer. The schematization<br />
of the theoretical behavior in Figure 4c shows the sudden<br />
jump of DP, orL, atC i (note C i is generally S C*), followed<br />
by a minor increase in L upon further increasing<br />
the unimer concentration. [23] Values of q for SPs frequently<br />
exceed the lm range, [1] enabling DPs higher than<br />
1000. Note that while attractive interactions stabilize the<br />
assembly along the N-S direction, no lateral (soft) anisotropic<br />
attraction among the formed assemblies is postulated<br />
to occur in the nematic state: only excluded volume<br />
(hard) interaction stabilizes the mesophase. In view of its<br />
fundamental significance, the difference between an open<br />
SLC and a conventional, molecular liquid crystal should<br />
be emphasized. In the latter, no association/dissociation<br />
equilibria accompany the transition from the isotropic<br />
solution: the nematic field has only an orienting effect. In<br />
the open SLC, development of orientation is simultaneous<br />
to an enhancement of supramolecular polymeriza-<br />
(3)
516 A. Ciferri<br />
tion. [77] Growth-coupled-to-orientation could in principle<br />
occur even for systems displaying only soft interactions,<br />
when mesophases are expected in the melt or in very concentrated<br />
solutions. [11] However, if growth due to alternative<br />
mechanisms (MSOA or HG) has produced a wormlike<br />
chain exceeding the persistent length (L A q) at these<br />
high concentrations, further growth by the SLC mechanism<br />
will not be relevant. The original theory, [20, 21] developed<br />
to explain the linear assembly of cylindrical<br />
micelles in nematic solutions, was later extended to discotic<br />
molecules in a wide range of concentrations at<br />
which hexagonal and higher-order phases were pre-<br />
dicted.<br />
[31, 78, 79]<br />
A common feature of the cases considered above is the<br />
occurrence of polydispersity. [80] It is also apparent that<br />
SPs attain average DPs that are concentration-dependent,<br />
but may nevertheless far exceed those obtained in molecular<br />
polycondensation (cf. ref. [73] and Section 5.1.4).<br />
4.2 Multidimensional Assemblies<br />
The above description of supramolecular polymerization<br />
for linear systems has involved the identification of<br />
proper unimers and corresponding growth mechanism.<br />
Unimers with functionality A2 can express strong supramolecular<br />
interactions in two and three dimensions. How<br />
can the approach developed for linear SPs be extended to<br />
multidimensional systems? General thermodynamic considerations<br />
regarding the growth of multidimensional<br />
assemblies were summarized by Israelachvili. [80] The<br />
standard chemical potential per unit (ln 0 ) decreases with<br />
the number n of aggregating units according to<br />
l 0 n = l 0 v + a kT/n p (4)<br />
where l 0 v is the bulk free energy for an infinite aggregate,<br />
a reflects the strength of contact energy, and p is a dimensionality<br />
index (p = 1 for linear systems, 1/2 for discs, 1/3<br />
for spheres). Elaboration of the approach leads to the<br />
expectation that, whenever p a 1, macroscopic aggregates<br />
(n ev) grow abruptly above a critical concentration by<br />
a mechanism corresponding to a crystallization. Thus, at<br />
variance with the linear systems, no concentrationdependent<br />
broad size distributions are expected and there<br />
is no need to invoke a growth-coupled-to-orientation<br />
mechanism. These considerations apply to the growth of<br />
monolayers, single lamella, and also to the growth of<br />
three-dimensional assemblies of unimers having symmetrical<br />
and equivalent functionality. For some 2D systems,<br />
finite-size effects may frustrate growth to infinite assemblies.<br />
[81] The description of the growth of 3D systems is<br />
also more complex whenever a preferential growth direction<br />
occurs, reflecting, for instance, the geometrical anisotropy<br />
of the polymer, or the presence of two components.<br />
In such cases it might be possible to follow the<br />
Figure 5. Schematic assembly of the side and top views of a<br />
rigid polyanion and a cationic surfactant. This assembly may<br />
grow longitudinally by stacking, and laterally by interdigitation<br />
and hexagonal packing taken (taken from ref. [81] ).<br />
modes of growth along the longitudinal and transversal<br />
directions.<br />
The situations that may be encountered are illustrated<br />
by the assembly of a rigid polyanion (e.g., DNA) and a<br />
cationic surfactant in water (Figure 5). [82] The observation<br />
of the conventional nematic phase expected for the DNA/<br />
H2O system is precluded by the strong reduction of solubility<br />
when the surfactant is present. Growth along the<br />
lateral direction (D) occurs by consecutive interdigitation<br />
of DNA helices with bound surfactant and produces<br />
aggregates with hexagonal symmetry. Growth along the<br />
longitudinal direction (L) may also occur due to the<br />
hydrophobic interaction occurring at the exposed North<br />
and South surfaces of the assembly.<br />
Provided L prevails over D, the nematic and hexagonal<br />
phases, crucial for the spatial orientation of the growing<br />
columns, should develop through the linear growthcoupled-to-orientation<br />
mechanism. On the other hand,<br />
when D prevails over L, planar growth leads to aggregates<br />
of infinite size (crystallization). Due to the fact that<br />
the helix and surfactant molecules are not chemically<br />
bound, it ought to be possible, through compositional<br />
control, to monitor growth along the two directions, possibly<br />
evidencing a liquid-crystalline phase, enhancing<br />
growth along the longitudinal direction. In general, one<br />
would expect a difference in the growth rate along the<br />
longitudinal and lateral directions. Indeed, such differences<br />
have been observed in block copolymers, when the<br />
components are chemically bound. [83]<br />
The foregoing considerations justify the empirical<br />
identification of linear growth components even in bulk<br />
composite assemblies, to be discussed in more detail in<br />
Section 5.6. A sounder mean-field approach was developed<br />
to interpret the solid-state morphology of (A)n–(B)m<br />
amorphous diblock copolymers exhibiting a microsegregation<br />
of components into cylindrical, lamellar or spherical<br />
domains. [48] The approach is based on the thermodynamic<br />
incompatibility of copolymer components that cannot<br />
be crystallized, theoretically shown to generate an<br />
unstable mode in an homogeneous, undiluted melt. [84–87]<br />
The formation of interfaces reduces the enthalpic cost of<br />
mixing (measured by the v parameter) but entails an<br />
entropy loss due to chain stretching to fill space uniformly.<br />
The latter depends upon the relative length and
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 517<br />
Figure 6. Mean-field phase diagram for amorphous diblock<br />
copolymers (A)n-(B)m. Main phases are: lamellar (L), hexagonal<br />
cylindrical (H), closed packed spheres (CPS), disordered (DIS).<br />
f is the volume fraction of A segments. (i Am. Chem. Soc.<br />
1996 [87] ).<br />
flexibility of the A and B blocks. On this basis, cubic,<br />
hexagonal, lamellar, and other phases are predicted in<br />
discrete regions of v N N versus m/n phase diagrams as<br />
that illustrated in Figure 6 (N: total number of A and B<br />
units). A review of the most recent elaboration that unifies<br />
the weak [85] and strong [86] segregation regimes is<br />
given in the literature. [87] Noolandi et al. [88] extended the<br />
(self-consistent) mean-field approach to the calculation of<br />
the relative stability of liquid-crystalline phases occurring<br />
in solutions of copolymers composed of three flexible<br />
blocks.<br />
5 Polymers from <strong>Supramolecular</strong><br />
Polymerization<br />
5.1 Linear Chains and Applications<br />
5.1.1 H-Bonded Polymers<br />
This class of systems has attracted considerable interest<br />
due to the intrinsic characteristics of H-bonds, such as<br />
directionality and the possibility of increasing bond<br />
strength by multiple pairwise interactions. Figure 7 illustrates<br />
typical cases in which flexible or rigid segments<br />
are functionalized with groups able to form single or multiple<br />
H-bonds. The binding constant K is the primary<br />
parameter controlling DP in terms of the simple MSOA<br />
mechanism schematized in Figure 4a, predicting, according<br />
to Equation (1), K A 106 –107 m –1 needed for DP above<br />
the oligomeric range. If one takes K = 500 m –1 as an average<br />
value for a single H-bond (reported for the pyridine/<br />
benzoic acid dimerization), [7] it appears that at least 4 Hbonds<br />
are needed to produce DPs of interest. In fact, the<br />
polymer in Figure 7c based on the dimerization of ureidopyrimidone<br />
residues (K =66107 m –1 in CDCl3), [4] was<br />
Figure 7. <strong>Supramolecular</strong> polymers stabilized by main-chain<br />
links based on (a) one, [11] (b) three, [2, 15] and (c) four [4, 52] Hbonds.<br />
reported to attain DP in the order of 1000 in dilute, iso-<br />
[4, 52]<br />
tropic solution.<br />
In the case of polymer (b), based on the 3 H-bond<br />
scheme of rigid anthracene segments terminated by uracil<br />
(A-A) or pyridine residues (B-B), one therefore would<br />
not expect a significant DP to occur in virtue of the<br />
MSOA mechanism. In fact, no evidence for appreciable<br />
DP was reported for isotropic solutions. However, the<br />
development of liquid crystallinity evidenced the formation<br />
of large DPs in moderately concentrated solutions in<br />
organic solvents, [14] likely triggered by the mechanism of<br />
growth-coupled-to orientation.<br />
When the rigid segments in (b) were replaced by flexible,<br />
tartaric acid spacers, the occurrence of liquid crystallinity<br />
was observed only in undiluted (thermotropic) systems.<br />
[13] Polymer (c), based on flexible segments terminated<br />
by diacid (A-A) and dipyridil (B-B) residues forming<br />
single H-bonds, [8–12] displayed analogously only thermotropic<br />
behavior. The occurrence of liquid crystallinity<br />
in the melt does not provide evidence of significant polymerization.<br />
[1] In fact, the role of the growth-coupled-toorientation<br />
mechanism for worm-like chains displaying<br />
soft interaction was shown to be extremely small. [10, 11] On<br />
the other hand, Equation (1) offers compelling evidence<br />
that no large DPs are produced in the case of a single Hbond.<br />
Other linear H-bonded systems of interest include the<br />
polycaps [89] based on the polymerization of capsular host<br />
complexes (calixarenes) functionalized with urea. The<br />
formation of main-chain bonds occurred only when a<br />
guest molecule was hosted in the capsule (CHCl3 acted<br />
both as a capsule host and as a solvent). Whitesides and<br />
coworkers [16] who had discussed the formation of closed<br />
structures by the self-assembly of melanine and cyanuric<br />
acid earlier, reported the formation of linear nanorods by<br />
introducing a mismatch in the spacers of the AA and BB<br />
groups of these monomers. Ladder-type supramolecular<br />
polymers, [10] and ribbon-type polymers have also been<br />
reported. [17]
518 A. Ciferri<br />
Figure 8. (a) Subunits used for supramolecular networks. [10] (b)<br />
PEO/PPO block copolymer networks based on supramolecular<br />
(upper) and covalent crosslinks. [52] (c) Hydrogen-bond-induced<br />
compatibilization in a polymer blend. [53]<br />
5.1.2 H-Bonded Random Networks<br />
In the scheme of Figure 7 all unimers exhibit a complete<br />
match of the donor/acceptor components of either single<br />
or multiple H-bond units. If this match does not occur, or<br />
tetra and bifunctional unimers are mixed, planar or threedimensional<br />
networks are possible. [2] Networks based on<br />
triacids and bipyridine derivatives, or on tetrafunctionalized<br />
pyridine and difunctional benzoic acid compounds<br />
(cf. Figure 8a), have been reported. [10, 51] Single H-bonds<br />
connect chain segments emanating from tetrafunctional<br />
crosslinkages. Meijer et al. [52] have reported functionalized<br />
copolymers of propylene oxide and ethylene oxide<br />
exhibiting a strong four H-bond scheme (Figure 8b).<br />
These networks exhibit peculiar rheological features to<br />
be described below. H-bonding (Figure 8c) between<br />
polymers, such as poly(4-vinylpyridine) and poly(4hydroxystyrene),<br />
[53] was described as a factor promoting<br />
[55, 90]<br />
compatibility in polymer blends.<br />
5.1.3 Coordination Polymers<br />
The scheme of linear coordination polymerization was<br />
discussed by Lehn. [2] The unimers are ditopic ligands<br />
with two binding groups forming main-chain bonds<br />
through metal-ion coordination (Figure 9a). Several metal<br />
binding groups (bidentate, tridentate) and metal ions with<br />
tetra-, penta- and hexa-coordination are available. Among<br />
Figure 9. (a) Schematization of a linear coordination SP showing<br />
bidentate and tridentate metal binding group and metal ions<br />
with tetra-, penta-, and hexa-coordination. [2] (b) Degree of polymerization<br />
of Be(Bu2PO2)2 vs concentration in CHCl3 at room<br />
temperature. The chain backbone is schematized on the right. [18]<br />
(c) Self-assembly of a cobalt porphyrin polymer by coordination<br />
of two covalently attached pyridine ligands. [20]<br />
the earliest reports of soluble, reversible coordination<br />
polymers we find systems based on three-atom-bridging<br />
phosphinate groups connected by tetrahedral metal atoms<br />
reported by Ripamonti and coworkers [18] in 1968. Figure<br />
9b illustrates the variation of DP (by means of vapor<br />
pressure osmometry) with the concentration of beryllium<br />
dibutylphosphinate (Be(Bu2PO2)2) dissolved in CHCl3, a<br />
non-coordinating solvent. Fiber-forming properties, suggesting<br />
larger DPs, were exhibited by anisotropic gels<br />
occurring in more concentrated solution. Substantial evidence<br />
of depolymerization with dilution was observed,<br />
confirming the dynamic reversibility typical of supramolecular<br />
polymers. [4] The chain structure, deduced from Xray<br />
diffraction, is based on the alternate singly and triply<br />
bridged structure shown in Figure 9b.<br />
Among the most recent reports, [20] the functionalized<br />
porphyrin polymer in Figure 9c was shown to attain DP<br />
L 100 in a 7610 –3 m solution in CHCl3 (by means of<br />
size-exclusion chromatography (SEC)). Here, coordination<br />
occurs between the Co atom (hexa-coordination) and<br />
the two pyridine ligands.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 519<br />
Figure 10. Complex viscosity (Pa N s) vs frequency x of polymer<br />
C in Figure 4 between 30 and 558C, and of the equivalent<br />
polymer (bottom, 308C) exhibiting a 2 H-bond (taken from<br />
ref. [52] ).<br />
5.1.4 Rheology and Polymer-Like Properties<br />
It is certainly surprising to observe that SPs based on<br />
bonds that are weaker than covalent ones can nevertheless<br />
attain DP larger than obtained, for instance, from<br />
conventional polycondensation. As indicated above, DP<br />
L 1000 can be expected for polymer (c) in Figure 7 under<br />
thermodynamic equilibrium, whereas DP L 100 requires<br />
the use of irreversible conditions in the case of aliphatic<br />
polyamides. [73] Thus, SPs may exhibit strong growth and<br />
labile bonds. Which applications can be conceived for<br />
such systems? The spontaneous thermodynamic assembly<br />
q disassembly process allows variations of DP in<br />
response to temperature, concentration, and other external<br />
variables. Moreover, the concomitant readjustment of<br />
donor/acceptor partners even under constant values of<br />
these variables renders the SPs truly adaptive, self-healing,<br />
combinatorial materials. [2] It is important to distinguish<br />
cases in which the growth of the SP is controlled by<br />
a non-cooperative mechanism (when changes in the<br />
above variables produce relatively minor DP changes, cf.<br />
Figure 4a) from cases in which cooperative effects are<br />
operative (cf. Figure 4b or c). Materials exhibiting minor<br />
DP alterations under the influence of an external variable,<br />
while still allowing the persistence of appreciable DPs,<br />
offer opportunities in areas of conventional polymers. For<br />
instance, the beneficial value of a relatively large DP on<br />
the mechanical properties will not necessarily be accompanied<br />
by a prohibitively large melt viscosity under processing<br />
conditions, as is the case for covalent polymers<br />
(properties of materials undergoing major changes in DP<br />
due to changes in external variables will be considered in<br />
the following section).<br />
The expectations described above are fully supported<br />
by rheological studies on some of the H-bond systems<br />
described above. Figure 10 illustrates the viscoelastic<br />
Figure 11. Isothermal viscosity and normal forces vs shear rate<br />
for a solution of polycapsules (C L 3% in o-dichlorobenzene)<br />
(taken from ref. [89] ).<br />
Table 1. Applications of linear polymers.<br />
STRONG T-DEPENDENT RHEOLOGY AT LARGE DP: easy<br />
to flow, stronger in use<br />
EXTENDING DP of covalent polymers<br />
RECYCLING: with complete regeneration properties<br />
TUNABLE, SMART MATERIALS: adjusting properties to<br />
environmental variables (switches, etc.)<br />
DIFFERENT CORES: mechanical, conductivity, light emitting,<br />
catalytic properties<br />
SELF-REPAIRING: any structural damage<br />
STRUCTURAL CONTROL: in copolymers, high selectivity,<br />
alternation, chirality<br />
SUPRA e covalent<br />
behavior exhibited by the polymer in Figure 4c under<br />
small oscillatory deformation at various temperatures (M — n<br />
=8610 3 ). [52] The zero-shear viscosity at 308C is comparable<br />
to that shown by an unfunctionalized polydimethylsiloxane<br />
with M — n = 3610 5 , and appears to be<br />
1000 times larger than for the compound based on similar<br />
unimers linked by a 2 H-bond scheme. The strong non-<br />
Newtonian behavior reflects the interplay of polymer viscoelasticity<br />
and chain dissociation at higher temperature.<br />
At low frequency (x) and high temperature (T), the loss<br />
modulus is larger than the storage modulus, while the<br />
reverse was observed at higher x and lower T. Consistent<br />
data was exhibited by the reversible networks displayed<br />
in Figure 8b showing a plateau modulus (5610 5 Pa) six<br />
times larger than that for a corresponding covalent copolymer.<br />
Figure 11 is even a more stringent demonstration<br />
of persistent polymeric behavior in spite of reversible<br />
polymerization. [89] Normal forces attaining values in the<br />
order of 1000 Pa are indisputable evidence of polymeric<br />
behavior, and all data was reversible upon reducing the<br />
shear rate. The rheological behavior of a coordination<br />
polymer (Cu(II) tetraoctanoate in decalin) was inter-
520 A. Ciferri<br />
preted [90] in terms of a theory describing the chain-extending<br />
role of labile crosslinkages. [91]<br />
Applications. Applications of SPs have been suggested<br />
in areas that expand the applicability of covalent polymers<br />
(Table 1). The advantage of an easier processing in<br />
spite of a large DP has been commented above. Particularly<br />
significant is the possibility of increasing the DP of<br />
conventional polymers by main-chain supramolecular<br />
bonds. Under study is the extension of the DP of aromatic<br />
polyamides that are usually produced in the desirable<br />
high-molecular-weight range only by cumbersome syntheses.<br />
[92] Polymers based on long covalent segments<br />
extended by supramolecular bonds should be recyclable<br />
with complete recovery of properties. [92] The subsequent<br />
transformation of supramolecular bonds into covalent<br />
ones should allow the stabilization of large DPs and<br />
structures that would have been extremely difficult to<br />
synthesize directly. [38, 93] The thermodynamic control of<br />
DP and of the topology of networks should allow applications<br />
as tunable, smart materials responding to changes in<br />
variables, such as temperature, stress, and solvent permeation.<br />
[2, 52] By using different covalent segments in A-<br />
A/B-B unimers, the tunability could be expressed in<br />
mechanical, conductivity, light emitting, and catalytic<br />
properties. If changes in the above variables were occurring<br />
accidentally, the SP would have the capability of<br />
self-repairing. The high selectivity of molecular recognition<br />
should allow the selection of proper sequences in<br />
mixtures of more than two complementary unimers.<br />
5.2 Helical Chains and Functional Systems<br />
The original Oosawa theory (cf. Section 4.2) is based on<br />
unimers exhibiting the site distribution shown in Figure<br />
3b. Cooperation arises because each unimer makes two<br />
bonds along the linear sequence and two weaker ones<br />
along the helical pattern. The unimers should be large<br />
enough to avoid possible mismatches in the pattern.<br />
There has been no verification of the model using synthetic<br />
SPs. The model was elaborated to describe the formation<br />
of helices and microtubules (G q F transformation)<br />
by biological SPs. [1, 26] However, the verification of<br />
the helical growth model has been problematic even in<br />
the relatively simple case of actin, due to the simultaneous<br />
dephosphorylation of ATP normally bound to the<br />
protein. A study in which growth could be observed without<br />
the complication of ATP e ADP hydrolysis was performed<br />
by Korn on actin-ADP. [27] Actin polymerization<br />
occurs by increasing the ionic strength or temperature in<br />
isotropic solution at unimer concentrations below<br />
0.04 mg/ml. Filaments exceeding 11 lm, corresponding<br />
to a DP larger than 4000, are attained (higher values<br />
were observed with tubulin). [1, 28] The length distribution<br />
conforms to the most probable distribution and chain<br />
stoppers reduce DP, in line with theory. The diagram in<br />
Figure 12. (a) Concentration of helical (0) and oligomeric<br />
(6) actin vs unimer concentration showing the critical concentration<br />
as predicted by Oosawa’s theory (taken from ref. [29] ). (b)<br />
Schematization of translational movement resulting from the<br />
directional growth of F-actin (taken from ref. [25] ).<br />
Figure 12a exhibits the theoretically predicted occurrence<br />
of a critical unimer concentration at which helical growth<br />
begins, and the concentration of unimers and oligomers<br />
attains a constant value. [29] The recent generalization [76] of<br />
Oosawa’s theory to growth processes enhanced by cooperative<br />
effects has not yet been applied to specific chainlike<br />
sequences. It has however been applied to the growth<br />
of columnar systems to be discussed in the following section.<br />
The overall functioning of actin or tubulin as in vivo<br />
systems invites to consider the way in which the helical<br />
growth process is coupled to the dephosphorylation reaction.<br />
This coupling produces a dynamic function of the<br />
polymer [25, 30] that assists in processes, such as motility,<br />
contraction, and cell division. Figure 12b illustrates the<br />
processes believed to occur during the polymerization of<br />
actin filaments. The critical concentration of G-actin<br />
unimers is defined by the condition of equality between<br />
the sum of the assembly and disassembly rates at the two<br />
ends of the filament (the barbed and the pointed ends).<br />
Under ATP hydrolysis, the depolymerization at one end<br />
may be faster than polymerization at the other end. Thus,<br />
a cycling of actin monomers from one end to the other<br />
occurs during the growth of F-filaments, resulting in a<br />
translation of the polymer (tread-milling effect). A<br />
related dynamic instability effect controls the assembly q<br />
disassembly process of tubulin into microtubules. [25]<br />
Applications. The design principle of functional protein<br />
systems is based on the coupling of a supramolecular<br />
polymerization process to a chemical reaction.<br />
[25, 30]<br />
Understanding and possibly reproducing coupled<br />
mechanisms is the challenging road to microengines and<br />
other functional SPs mimicking mechanical properties of<br />
biological systems and engineered for new practical<br />
applications.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 521<br />
Figure 13. (a) Schematization of the assembly of surfactant<br />
and growth of micellar polymers. (b) Experimental data and theoretical<br />
phase diagram for the discotic amphiphile 2,3,6,7,10,11hexa(1,4,7-trioxoacetyltriphenylene)<br />
in D2O (taken from ref. [31] ).<br />
5.3 Columnar and Micellar Assemblies<br />
5.3.1 Evidence for the SLC Mechanism<br />
In this section we will consider amphiphilic unimers that<br />
unmistakably exhibit nematic phases appearing simultaneously<br />
with the onset of extensive polymerization, thus<br />
revealing a superimposition of the MSOA and SLC<br />
mechanisms. If the shape of the unimer is cylindrical or<br />
disk-like, the resulting SPs will be linear or discotic. In<br />
both cases, theory suggests that strong contact forces and<br />
rigidity along the longitudinal direction are involved (cf.<br />
Section 4.1.3).<br />
Odijk [22, 94] has discussed the experimental verification<br />
of the growth-coupled-to-orientation theory in the case of<br />
conventional spherical micelles that exhibit, upon<br />
increasing the surfactant concentration, the assembling<br />
sequence: dispersed molecules e spherical micelles e<br />
cylindrical (end-capped) micelles e linear polymer, the<br />
growth of linear polymers being associated to the transformation<br />
isotropic e nematic.<br />
The broad features of the experimental phase diagram<br />
were found to be in line with theory, although some discrepancies<br />
remain. [95, 96] Persistence length data confirmed<br />
the rigidity of the micellar polymer, but exhibited consid-<br />
erable scattering with q ranging from L0.02 to 10 lm; a<br />
likely value of L1 lm corresponds to a DP in the order of<br />
20000. There is no data on the formation of the nematic<br />
phase and on persistence length in the case of block copolymer<br />
micelles. Higher-order phases (hexagonal, lamellar,<br />
etc.), however, have been observed. [97, 98] The absence<br />
of a nematic phase was also noticed for several conventional<br />
surfactants. The growth-coupled-to-orientation theory<br />
predicts that the nematic phase can be skipped and<br />
only a direct isotropic e hexagonal columnar phase is<br />
observed for suitable combinations of contact forces and<br />
persistence length. [79] It is also expected that volume<br />
excluded effects [23] and a reduced chain stretching [99]<br />
cause some micellar growth even in isotropic solutions of<br />
block copolymers.<br />
A most detailed evidence for growth-coupled-to-orientation<br />
is provided by discotic molecules: ditopic structures<br />
possessing a disk-shaped core from which a number<br />
of flexible alkyl chains emanate. Molecularly dispersed<br />
disks would be expected to form conventional liquid crystals<br />
in virtue of their large excluded volume. Moreover,<br />
disks are able to aggregate into soluble columns due to a<br />
p-p stacking of the cores and solvophobic interaction of<br />
the side chains. As in other supramolecular polymerizations,<br />
columns may assemble due to the small equilibrium<br />
constant (MSOA), and growth can be reinforced by the<br />
occurrence of cooperative effects. Figure 13b displays<br />
the phase diagram of the discotic amphiphile<br />
2,3,6,7,10,11-hexa(1,4,7-trioxoacetyltriphenylene) in<br />
D20. [31] The diagram represents a match between experimental<br />
data and theoretical lines calculated according to<br />
the theory of growth-coupled-to-orientation, and includes<br />
the prediction of higher-order phases. [68] Growth is seen<br />
to occur simultaneously with the appearance of the<br />
nematic phase at a concentration of L20% at room temperature.<br />
It appears that, for this system, the balance of<br />
contact forces and flexural rigidity favors the occurrence<br />
of the nematic phase. The formation of the hexagonal<br />
phase observed at higher concentration is attributed to an<br />
improved packing efficiency with respect to the nematic<br />
phase. The diagram reveals a hierarchical evolution of<br />
the assembling process through higher-order phases:<br />
disks (I) e columns (N) e hexagonal columnar (H) e<br />
solid.<br />
5.3.2 Helical-Columnar Growth Mechanism<br />
Meijer and coworkers [32, 33] have synthesized a series of<br />
most interesting C3-symmetrical disk-like molecules that<br />
assembles into cylindrical stacks in virtue of both hydrogen<br />
and arene-arene interactions. The molecules shown<br />
in Figure 14a have large aromatic cores, H-bonding<br />
groups, and either achiral or chiral side chains. The latter<br />
varied in their polar character allowing the study of<br />
aggregation in either polar or nonpolar solvents. The for-
522 A. Ciferri<br />
Figure 14. (a) Disk-shaped, three blades molecule prepared by<br />
Meijer and coworkers [4] with side chains having polar, nonpolar,<br />
chiral, achiral character. [32, 33] (b) Formation of a helical assembly<br />
when the blades assume a propeller-like conformation. [100]<br />
(c) Schematic representation of the transition from dispersed<br />
disks to partially ordered and to fully ordered columns. [76]<br />
(i Am. Chem. Soc. 2000 and 2001.)<br />
mation of long columns was shown to occur in very dilute<br />
solution in hexane (10 –6 m), and a large association constant<br />
(10 8 m –1 ) was reported. [32] It was suggested that such<br />
an aggregation reflects the formation of helical columns<br />
through a cooperative process attributed to a conformational<br />
transition from flat to propeller shape for the blades<br />
of each disk, resulting in a maximization of interaction<br />
for the chiral helical assembly (Figure 14b). [100] Experimental<br />
data for the more polar molecules in dilute butanol<br />
(2.4610 –4 m) [33] revealed a sequence of two association<br />
steps upon temperature changes. The postulated process<br />
is schematized in Figure 14c: starting with an isotropic<br />
dispersion of disks, a decrease in the temperature causes<br />
the formation of low-DP achiral aggregates stabilized by<br />
non-cooperative interaction, followed at a lower critical<br />
temperature by the cooperative formation of helical columns<br />
with DP attaining the 1000 range. [76]<br />
The theoretical description of the processes described<br />
above was formulated by van der Schoot and coworkers. [76]<br />
The occurrence of the two regimes was described in terms<br />
of the cooperativity parameter r. The situation r =1is<br />
equivalent to the binding of unimers into disordered aggregates<br />
(i.e. MSOA), while r s 1 describes the subsequent<br />
cooperative formation of ordered aggregates by a HG<br />
mechanism. Essentially, the treatment is a generalization<br />
of the Zimm-Bragg and the Oosawa theories (Kh A K) without<br />
necessarily specifying a detailed molecular model and<br />
a critical nucleus. With respect to Oosawa’s treatment that<br />
focused on the critical concentration C* (cf. Figure 4c),<br />
Figure 15. (a) Deoxyguanosine, its oligomers, and folic acid.<br />
Their assembly in tetrameric disks. [36] (b) Variation of the number<br />
of stacked tetrameric disks with folate concentration in (1)<br />
pure H2O and (2) 1 m NaCl at 308 C. The vertical broken line<br />
indicates the I e H transition (replotted using data taken from<br />
ref. [26] ).<br />
the van der Schoot treatment emphasizes the fractions of<br />
aggregated material and helical bonds as a function of<br />
both temperature and concentration. Rigidity and excluded<br />
volume effects are not introduced and, therefore, liquid<br />
crystallinity does not direct the aggregation of the stacks.<br />
5.3.3 Supermolecules. Tubular Assemblies<br />
The formation of disk-like supermolecules from two or<br />
more complementary components was discussed by several<br />
authors. [2, 34–36, 43, 44] Disk-like supermolecules often<br />
show liquid-crystalline behavior even though the separate<br />
components do not. Moreover, the discotic supermolecules<br />
can form columnar stacks in the melt and in solution<br />
just as the molecular discotics discussed above do.<br />
The relative contributions of MSOA, HG and SLC<br />
mechanisms have not always been characterized adequately.<br />
Gottarelli et al. [35, 36] have investigated the most<br />
interesting assembly of the nucleotide deoxyguanosine,<br />
its oligomers and alkaline folates (Figure 15a). These<br />
compounds form hydrogen-bonded disk-like tetramers in<br />
solution and are able to assemble in columnar stacks of<br />
discrete length and DP. Small-angle neutron scattering<br />
techniques were used to determine the length of the<br />
aggregates in water and in salt solutions. The critical concentrations<br />
for the appearance of the nematic (cholesteric)<br />
and the hexagonal phases were determined by<br />
means of X-ray diffraction. Selected data for the deoxyguanosine<br />
dimer (d(GpG); Figure 15a) and the folate is
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 523<br />
Table 2. Critical concentration and DP in the isotropic phase<br />
for folic acid (selected data taken from ref. [36] ).<br />
Sample Solvent C IN<br />
%<br />
C NH<br />
%<br />
LISO DPISO a) XISO b)<br />
d(GpG) H2O – – 70 15 2.3<br />
d(GpG) H2O +Na + 2.5 18 – – –<br />
d(GpG) H2O +K + 1.5 15 – – –<br />
folate H2O – 35 2.3 1 0.1<br />
folate H2O +Na + 27 35 2.1 9 0.7<br />
a) L/2.35 Š (4.70 for d(GpG)).<br />
b) L/30 Š.<br />
collected in Table 2. The deoxyguanosine derivatives<br />
generally show larger DPs in isotropic solutions and<br />
lower critical concentrations CIN and CNH than the folates.<br />
The presence of NA + and, particularly, K + ions enhances<br />
the stabilization of the aggregates. In the case of folates<br />
in pure H2O, no nematic phase and small DPs were<br />
observed. Due to the considerable diameter of the cylinders<br />
(D L 30 Š) and the thickness of each disk (L = 2.35<br />
Š), the DP in the isotropic phase is extremely small and<br />
the corresponding axial ratio X suggests that thick disks<br />
rather than columns are present. Figure 15b illustrates the<br />
evolution of DP with concentration covering the range<br />
from the isotropic to the hexagonal phase. The smooth<br />
dependence DP vs C does not evidence cooperative<br />
effects in the case of folates. The largest DP (L30), determined<br />
from a 60% (hexagonal) solution, reveals in fact<br />
columns of very small geometric anisotropy (X L 2.3).<br />
The formation of the mesophase may be promoted by the<br />
large excluded volume effect of disks even in the absence<br />
of soft interactions. In fact, simulation studies evidenced<br />
nematic and columnar phases for solutions of extremely<br />
thin disks (0 a L/D a 0.1). [101]<br />
Disk-like supermolecules based on dimers of ureidotriazines<br />
connected by a 4 H-bond scheme similar (but<br />
not identical) to that of the ureidopyrimidone polymers in<br />
Figure 7c were reported by Meijer and coworkers. [34]<br />
These disks stack in columns with loose positional order<br />
and low DP (Figure 16a). Percec and coworkers [44]<br />
reported tubular polymeric assemblies of disks composed<br />
by six tapered molecules of 12-ABG-15C5 complexed<br />
with triflate salt (Figure 16b). The columns assembled<br />
into a hexagonal mesophase revealed as by means of Xray<br />
diffraction from the undiluted system.<br />
Several of the systems described above present a central<br />
cavity into which a covalent polymer can be hosted<br />
or a flow of ions be achieved (cf. also next section). Of<br />
particular interest are nanotubes (Figure 16c) formed by<br />
stacking cyclic peptides connected by H-bonds along the<br />
columnar axis. [37–41] The chemical design of these flat<br />
ring-like peptides was discussed by De Santis and coworkers.<br />
[37] Tubes assembled in solution and the contact<br />
forces for the dimerization (K L 2.5610 3 m –1 ) are too<br />
Figure 16. (a) Monofunctional ureidotriazine disks capable of<br />
assembling into columns. [34] (b) Assembly of tapered 12-ABG-<br />
12C5 into disks, formation of a column of stacked disk, hexagonal<br />
columnar organization. [44] (c) Nanotubules formed by Hbonded<br />
cyclic peptides. [37] (i Am. Chem. Soc. 1994 and 1996.)<br />
small for a large DP unless cooperative effects occur. [38]<br />
Cyclic b-peptides were also considered. [39] The self<br />
assembly of the nanotubules into ion-selective membranes<br />
was discussed as well. [40]<br />
Unimers of most of the systems considered above were<br />
subsequently connected by flexible covalent spacers, producing<br />
main-chain or side-chain SPs that should be<br />
described more appropriately under class C SPs. The<br />
basic ability of the disks to form columnar assemblies<br />
was preserved, but the covalent segments produced<br />
alterations in the stacking details such as the occurrence<br />
of helicity. For instance, a slowly rising helicoidal stack<br />
was produced when the crown ether receptor in Figure<br />
16b was replaced by a flexible endo-receptor (nEO-<br />
PMA) connected as a side-chain to a poly(methyl acrylate)<br />
chain. [42] When the ureidotriazine disks in Figure<br />
16a were main-chain linked through flexible spacers,<br />
helical columns and large DPs were observed. [34]<br />
Applications. The columnar nematic or hexagonal<br />
packing of disk-like molecules and supermolecules could<br />
be exploited as a precursor step for the assembly of large,<br />
oriented structures modeling natural systems. Electronic<br />
mobility due to the p–p interactions along the columnar<br />
axis may be useful for electronic and photonic<br />
devices. [102] Central cavities could be exploited for the<br />
selective hosting of polymer molecules [44] or for ion-<br />
[40, 41]<br />
selective channels.<br />
5.4 Host/Guest Polymeric Assemblies<br />
Covalent polymers can enter a cavity of a columnar<br />
assembly or of single ring-like structures. The result is a<br />
composite host/guest polymeric assembly exhibiting a
524 A. Ciferri<br />
Figure 17. Shish-kebab composites. Schemes for (a) polyrotaxane,<br />
[61] (b) tobacco mosaic virus, [50] and (c) a-cyclodextrin +<br />
poly(ethylene oxide) (taken from ref. [104] ).<br />
shish-kebab-type architecture. The driving force for the<br />
formation of these assemblies is a complex combination<br />
of molecular recognition and supramolecular polymerization.<br />
In fact, the host polymers often promote the supramolecular<br />
polymerization of the guest, or an alteration of<br />
its assembly mode. Three examples are illustrated in Figure<br />
17.<br />
The primary interaction assisting the threading of a<br />
polymer into a single macrocycle cavity, as in the case of<br />
pseudopolyrotaxanes (Figure 17a), is attributed to the<br />
occurrence of appropriately spaced p-rich hydroquinone<br />
rings on the polymer and p-acceptor groups within the tetracationic<br />
cyclophane. [59–61] It has been shown that the<br />
electron donor/acceptor interaction can be monitored by<br />
electrochemically or photochemically induced reduction/<br />
oxidation reactions. Relative motion of the two components<br />
can thus be induced, simulating a molecular microengine.<br />
[103]<br />
The situation of TMV, illustrated in Figure 17b, is<br />
more complex. Here the guest is an RNA molecule and<br />
the host is a hollow columnar assembly composed of<br />
2130 identical tapered protein molecules. The host/guest<br />
systems can be disassembled and reassembled in vitro by<br />
pH changes. However, the host can be reassembled even<br />
without RNA. A very interesting effect is manifested in<br />
the structure of the host when RNA is present. [50] In the<br />
absence of RNA the host is a stack of disks of various<br />
DPs, each disk comprising 17 protein units. However,<br />
formation of a spiral occurs when RNA occupies the cavity.<br />
The proteins of the host then follow a helical pattern<br />
with 16.3 units per turn, and the assembly assumes definite<br />
dimensions (L = 3000, d = 180 Š, X = 16.6) and a<br />
DP of 2310.<br />
The RNA-induced helix formation in an otherwise<br />
stacked systems of disks is reminiscent of the similar<br />
effect described in the preceding section (Figure 16a,b).<br />
It thus appears that supramolecular interactions between<br />
sites on RNA and protein induce spiral formation similar<br />
to that of disks connected to a covalent polymer as side<br />
chains. The complex role of RNA for the whole structure<br />
is evident. RNA acts like a crank-shaft that drives the<br />
proteins bound to it into a helical pattern and simultaneously<br />
provides the information about the proper length<br />
and DP of the host. The assembly mechanism of the overall<br />
TMV structure can thus be described in terms of a<br />
supramolecular polymerization of the columnar assembly,<br />
coupled to the formation of monofunctional sidechain<br />
bonds between RNA and proteins. [77]<br />
Figure 17c illustrates the assembly of a-cyclodextrin<br />
rings over poly(ethylene oxide). This system belongs to<br />
the class of inclusion compounds, or clathrates, that have<br />
aroused considerable interest for separation processes and<br />
for the unique properties of single chains confined in narrow<br />
(d L 6 Š) channels. [104] The stability of the crystalline<br />
adduct is likely to be assisted by pairwise host/guest<br />
interactions, the strength of which is increased within the<br />
small cavity. [56] However, the wide variety of systems<br />
capable of forming inclusion compounds invites to consider<br />
other less specific factors affecting the supramolecular<br />
polymerization of a-cyclodextrin rings threaded<br />
along the polymer chain. These factors might be: (i) relatively<br />
strong contact forces between the surfaces of the<br />
host and (ii) a steric-type effect not so far theoretically<br />
described. In support of (i) one may note that soluble stoichiometric<br />
complexes of the host are known to occur<br />
(e.g., head-to-head dimers of cyclodextrin unable to<br />
assemble into long channels in the crystalline structure).<br />
Concerning (ii) it is plausible that the guest stretches out<br />
(loss of conformation entropy) while simultaneously<br />
assembling individual host molecules. A suppression of<br />
undulation modes of the polymer due to the presence of<br />
rings may lead to an entropically induced effective attraction<br />
between the threaded rings (a similar Casimir-type<br />
effect leads to an attraction between undulating, flexible<br />
membranes). Single host/guest channels could form even<br />
in isotropic solution of more rigid polymers if there is a<br />
favorable balance between the contact energy of cyclodextrin<br />
rings and the chain-conformational entropy. In<br />
concentrated solutions, the resulting rod-like structure<br />
could be favored by the occurrence of a nematic phase.<br />
Applications. The possibility of generating relative<br />
motion of the assembled surfaces by electrochemical or<br />
photochemical stimuli could be exploited in a variety of
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 525<br />
Figure 18. S-layers with hexagonal and square lattice symmetry<br />
derived from TEM. (a) Thermoanaerobacter thermohydrosulfuricus,<br />
and (b) Desulfotomaculum nigrificans (taken from<br />
ref. [46] ).<br />
nano/molecular scale engines. [103] The encapsulation of<br />
polymer molecules within cavities formed by self-assembling<br />
unimers provides systems of interest for separation<br />
processes, for recognizing and storing sequential information,<br />
[50] for orienting and screening single polymer molecules<br />
from similar neighbor interaction. [104]<br />
5.5 Planar Assemblies<br />
Figure 3c illustrates an equatorial distribution of four<br />
binding sites suitable for the formation of planar assemblies.<br />
As discussed in Section 4.2, these assemblies are<br />
expected to grow to large sizes by an intra-assembling<br />
cooperative mechanism akin to crystallization. At variance<br />
with the growth-coupled-to-orientation of linear<br />
systems, the growth of a planar assembly does not require<br />
the simultaneous formation and orientation of other growing<br />
units. Single free-standing, monomolecular layers are<br />
possible. An excellent verification of these expectations<br />
is provided by self-assembling S-layers forming the protective<br />
layer of the external surfaces of bacterial cells,<br />
and enabling the maintenance of a closed lattice during<br />
cell growth and division. [46] The identical constituent proteins<br />
have quasi-spherical form and exhibit an equatorial<br />
distribution of donor/acceptor groups capable of H-bonding<br />
to adjacent unimers. The proteins also posses a southpole<br />
site capable of electrostatic anchoring to the cell surface.<br />
S-layers can be disassembled and reassembled in<br />
vitro, allowing the preparation of purely H-bonded monolayers<br />
standing over an inert surface. The assembly q<br />
disassembly process has been described as a crystalliza-<br />
tion, [46] producing highly organized morphologies such as<br />
those shown in Figure 18. Depending upon the lattice<br />
type, the center-to-center distance of the morphological<br />
units varies from 3 to 30 nm, the thickness of monomolecular<br />
lattices vary from 5 to 25 nm, and the pore size is<br />
between 2 and 8 nm.<br />
Applications. The controllable confinement in definite<br />
areas of nanometric dimensions, coupled to the easiness<br />
of extraction and re-assembly, has allowed applications<br />
in areas of nanotechnology, such as bioanalytical sensors,<br />
templates for superlattices with prescribed symmetry,<br />
electronic and optical devices, matrices for the immobilization<br />
of functional molecules, and biocompatible surfaces.<br />
[46] S-layers recrystallized over solid supports have<br />
also been successfully patterned by using UV radiation<br />
and microlithographic masks. [46]<br />
5.6 Composite and 3D Assemblies<br />
Hexagonal cylindrical and lamellar phases (cf. transmission<br />
electron microscopy (TEM) photographs in Figure<br />
18) are often seen [48] in diblock and multiblock copolymers,<br />
ternary systems, copolymers formed from one unit<br />
that can be crystallized, rod-coil copolymers, [49] and some<br />
biological fibers. [105] For amorphous diblock copolymers<br />
in the cylindrical mode, one block is hexagonally packed<br />
within a matrix of the other block. The lamellar mode is<br />
instead based on alternating layers of A and B. The<br />
lamellar mode is the prevalent feature observed with rodcoil<br />
copolymers. [49, 106] In the case of a helical comb-like<br />
polymer (poly(b-l-aspartate) with paraffinic side chains),<br />
a layered distribution of helices correlated by interdigitation<br />
of the side chains was observed. [107] In the case of<br />
keratin, the fiber cross-section reveals L1 lm long microfibrils<br />
parallel to the fiber axis and hexagonally imbedded<br />
in a disordered S-rich matrix. Each microfibril is composed<br />
of eight protofibrils that are left-hand cables of two<br />
strands, each including two right-hand a-helices. [105]<br />
In the case of amorphous block copolymers, the (selfconsistent)<br />
mean-field theory [87] (cf. Section 4.2 and Figure<br />
6) describes the occurrence of various phases in terms<br />
of parameters pertinent to single copolymer molecules<br />
(compatibility, relative length and flexibility of the two<br />
blocks). This theory has been an eminently successful<br />
one and experimental results for the undiluted melt offer<br />
good support to it. [83, 108–110] Even in the case of block<br />
copolymer solutions, the predicted [88] sequence of phases<br />
upon increasing the concentration (e.g., isotropic e<br />
micellar e cubic e hexagonal e lamellar) revealed simi-<br />
[97, 98]<br />
larities with experimental data.<br />
Within the context of supramolecular polymerization it<br />
is however useful to explore alternative descriptions of<br />
the above structures in terms of a simpler, less sophisticated<br />
approach based on the concept of self-assembling<br />
of specifically designed building blocks. A similar con-
526 A. Ciferri<br />
Figure 19. (a) Micelles of coil-coil diblock copolymers in a<br />
selective solvent undergoing supramolecular polymerization.<br />
Hexagonally packed morphology as determined by means of<br />
TEM. (b) Bilayers of rod-coil diblock copolymers in a solvent<br />
selective for the coil block, growth directions within the plane of<br />
the layer and perpendicular to it are shown. Layered morphology<br />
[48, 112]<br />
as determined by means of TEM.<br />
cept has been used to describe solid arrays displaying<br />
complex and ordered structurizations such as interpenetrating<br />
nets. [111] The present author had suggested [1, 77] that<br />
the three-dimensional solid state morphologies described<br />
above, originating from molecular recognition of similar<br />
blocks, ought to be described in terms of supramolecular<br />
polymerization. The approach requires the identification<br />
of unimers with proper functionality and of their one- or<br />
multidimensional growth mechanism. [112] Relevant to this<br />
end are the considerations set forth in Section 4.2 regarding<br />
the separation of the modes of longitudinal and lateral<br />
growth.<br />
In particular, the formation of the hexagonal phase for<br />
copolymers that cannot be crystallized could be described<br />
as an essentially one-dimensional growth of micellar<br />
unimers according to the mechanism of growth-coupledto-orientation.<br />
By analogy with the processes illustrated<br />
in Figure 13, leading to the hierarchical sequences 5 and<br />
6, it has been suggested that the growth process schematized<br />
in Figure 19a describes the formation of the hexagonal<br />
phase of coil-coil block copolymers. [112] Here it is suggested<br />
that, as for conventional surfactants, a block copolymer<br />
micelle can assume an elongated form playing the<br />
role of a bifunctional unimer. Upon increasing the concentration,<br />
the latter undergoes linear growth simultaneously<br />
with the development of nematic orientation.<br />
This is followed by the hexagonal columnar phase (the<br />
intermediate nematic phase may not appear for suitable<br />
combinations of contact forces and persistence length) at<br />
even higher concentration. The hexagonal phase should<br />
be viewed as an embryo of the final morphology in the<br />
condensed state. The coiled segments may dangle over<br />
the lateral surface in a disordered fashion rather than<br />
interdigitate regularly. Note that while the approach discussed<br />
above does not have the predictive features of the<br />
mean-field theory, it does predict a role of the persistence<br />
length for the primary length scale, which is not predicted<br />
by the latter theory.<br />
The expectation that linear growth controls the formation<br />
of hexagonal columnar mesophases is not limited to<br />
coil-coil copolymers, or to systems with long molecular<br />
axes normal to the growth direction. Bifunctional unimers<br />
unable to grow along the lateral dimensions should in<br />
general be candidates for linear growth. In the case of<br />
keratin fibers, considering that the length of the microfibrils<br />
by far exceeds the length of constituent chains and<br />
falls in the range of the persistent length reported for<br />
similar systems, [1] it is plausible that the assembly of the<br />
fibril is also directed by the growth-coupled-to-orientation<br />
mechanism. The following hierarchical assembly<br />
sequence has therefore been suggested: extrusion of the<br />
low sulfur protein into extracellular fluids e head-to-tail<br />
assembly of shorter unimers into individual microfibrils<br />
to a length related to the persistence length with simultaneous<br />
orientation in the mesophase e stabilization of the<br />
microfibril by internal 1S1S1 bonding and the two non<br />
a-helical terminals e crosslinking of the sulfur-rich component<br />
in the narrow interfibrillar space. [112]<br />
The formation of lamellar structures could also be<br />
described qualitatively in terms of the self-assembly of<br />
specifically designed building blocks. Considering the<br />
case of the single lamella schematized in Figure 19b,<br />
growth akin to crystallization can occur along two perpendicular<br />
in-plane directions: functionality is larger than<br />
two. This mode of growth is at striking variance with the<br />
case of the uni-dimensional growth of cylindrical, bifunctional<br />
unimers considered above. It remains, however, to<br />
be considered how the ordered polymerization along the<br />
direction perpendicular to the lamellar plane is achieved.<br />
It is possible that, as disoriented lamellae grow, a critical<br />
axial ratio is attained at which purely hard interactions<br />
stabilize a nematic phase of the discotic type. This type<br />
of order has been experimentally evidenced during the<br />
graphitization of organic materials when large and growing<br />
planar rings are formed. [113] The coiling segments protruding<br />
from the lamellae may dangle over the surface, as<br />
in the case of cylindrical assemblies.<br />
An alternative assembling mode along the direction<br />
perpendicular to the lamellar plane could be based on<br />
attractive forces, or interdigitation, among the coiled segments.<br />
This interaction does not appear relevant to the<br />
class of copolymers considered above, but was evidenced<br />
for the comb-like poly(b-l-aspartate) with paraffinic side<br />
chains. This system showed nematic order based on clusters<br />
of helices correlated by interdigitation of partly molten<br />
side chains. [106] The polymer formed a complete 3D<br />
structure based on a layered organization of helices with<br />
side chains crystallized in a separate hexagonal lattice.<br />
Applications. Block copolymers are known to represent<br />
an important class of materials allowing a desirable blend
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 527<br />
of properties of different polymers, while preventing the<br />
undesirable (incompatibility-driven) macrophase separation<br />
of unconnected components. [47–49, 106] Pre-assembly<br />
followed by the growth of specifically designed unimers<br />
could represent a novel strategy toward the fabrication of<br />
composite structures with a prescribed distribution of<br />
components.<br />
6 Topics for Further Investigation<br />
Several aspects of fundamental and applied character<br />
appear to need more extensive investigation. Here we<br />
restrict attention to the polymer-like properties of SPs<br />
and the basic polymerization mechanisms.<br />
A main problem is the assessment of DP. A characteristic<br />
feature of these systems is that DP is a function of<br />
concentration. However, the determination of DP at a<br />
given concentration becomes complicated when using<br />
conventional molecular-weight determinations, requiring<br />
extrapolations to infinite dilution. Measurements under<br />
theta conditions should be preferred. The assessment of<br />
DP by means of SEC is also questionable for kinetically<br />
unstable SPs that display weak contact forces. In the case<br />
of actin, a successful determination of the DP distribution<br />
was reported using electron microscopy. [28] A reliable<br />
determination of the complete DP/concentration dependence<br />
may require the evaluation of binding constants and<br />
the application of theoretical expressions valid for a specific<br />
growth mechanism. For instance, Equation (1) can<br />
be used in the low concentration regime when MSOA is<br />
expected to prevail. The analysis of data at higher concentration<br />
could allow the detection of cooperative contributions<br />
arising from the HG or SLC mechanisms.<br />
There are several other parameters that could be determined<br />
by an extension of conventional polymer physical<br />
chemistry. One is the characterization of the rigidity of<br />
the assembly. The usual determination of the persistence<br />
length from solution studies may not be a viable one in<br />
the present context. An alternative approach, suitable for<br />
large values of q, is based on flexural rigidity data<br />
extracted from the thermally driven fluctuation of shape<br />
or end-to-end distance. [114, 115] Calculation approaches are<br />
also possible. [116] Rheological characterization of processing<br />
parameters under shear and elongational flow should<br />
aim to the coupling of polymer viscoelasticity and bond<br />
lability. Analogies should be considered with the behavior<br />
of living polymers, [91] covalent networks exhibiting<br />
labile crosslinkages, including those formed in the<br />
oriented state. The assessment of the mechanical strength<br />
of supramolecular bonds, particularly for the linear Hbond<br />
materials discussed in Section 5.1.1, is another open<br />
topic of great importance, e.g., for the performance of<br />
supramolecularly extended covalent polymers. An evaluation<br />
of the most suitable method [117] for calculating the<br />
elastic constants of SPs is needed. The characterization of<br />
the difference in the properties of reversible and irreversible<br />
polymers and copolymers could be better assessed by<br />
comparative studies on a given SP and on its analog<br />
obtained by transforming the supramolecular main-chain<br />
bonds into covalent ones.<br />
Analysis and more detailed investigations of the<br />
growth mechanisms are also needed. The sudden occurrence<br />
of growth when the nematic phase appears ought to<br />
be documented for other SPs, the rigidity of which needs<br />
to be independently assessed. The theoretical reasons preventing<br />
the observation of a nematic phase for block<br />
copolymers and some other amphiphiles need to be clarified<br />
and experimentally verified. Systems for which the<br />
contact energy, or equilibrium constant, can be systematically<br />
altered (e.g. by altering the number of pairwise<br />
interactions at given functionality) should be considered<br />
for a more stringent test of the relationship between K<br />
and DP. In this context, recent work has shown that<br />
bonds based on DNA base pairing produce well-behaved<br />
SPs. [118] An analysis of model systems in which only the<br />
site distribution is altered could allow an assessment of<br />
the parameters controlling linear versus helical growth.<br />
The detailed analysis of the steps contributing to the formation<br />
of host/guest composites (Section 5.4), and other<br />
hierarchical processes (Section 5.3), should help to elucidate<br />
the scaling up from nanometric to mesoscopic<br />
assemblies.<br />
Acknowledgement: The author expresses his appreciation to<br />
Prof. Paul van der Schoot for clarifying discussions and constructive<br />
criticism.<br />
Received: March 23, 2002<br />
Revised: May 13, 2002<br />
Accepted: May 13, 2002<br />
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