Supramolecular Polymerizations

Macromol. Rapid Commun. 2002, 23, 511–529 511 

Review: Unimers of both natural and synthetic origin 

self-assemble into linear, helical, columnar, planar and 

three-dimensional structures depending upon the functionality 

of supramolecular interactions. Recent reports 

describing the mechanism of formation, properties and 

possible applications of these systems are critically 

reviewed. The assembling of one-dimensional systems 

produces equilibrium polymers showing a length distribution 

and a degree of polymerization that may far exceed 

that of typical condensation polymers. Their growth may 

occur by a step-by-step process akin to polycondensation, 

and by cooperative processes such as helical growth or 

growth coupled to liquid crystallinity. Of particular interest 

are functional systems based on the coupling of a 

chemical reaction to supramolecular polymerization, and 

systems based on a covalent polymer hosted within the 

cavity of a supramolecular one. The assembly of two and 

three-dimensional systems occurs through a process akin 

to crystallization. The supramolecular organization of 

amphiphiles such as block copolymers is currently well 

described by the mean-field theory of unstable modes in 

homogeneous melts. An alternative, less sophisticated 

approach considers the growth of specifically designed 

building blocks. Possible applications are in areas that 

expand the uses of covalent polymers, electrochemical 

Supramolecular Polymerizations 

Alberto Ciferri 

Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy 

E-mail: cifjepa@chimica.unige.it 

Keywords: assemblies; host-guest systems; hydrogen bonding; supramolecular structures; 

Contents 

1 Definitions. Classification of Supramolecular Polymers 

2 The Bond. Site and Shape Recognition 

3 Functionality of the Unimer and Dimensionality of 

the Assembly 

4 Theory of Supramolecular Polymerization 

4.1 Linear, Helical Assemblies 

4.2 Multidimensional Assemblies 

5 Polymers from Supramolecular Polymerization 

5.1 Linear Chains and Applications 

5.1.1 H-Bonded Polymers 

5.1.2 H-Bonded Networks 

5.1.3 Coordination Polymers 

5.1.4 Rheology and Polymer-Like Properties 

5.2 Helical Chains and Functional Systems 

and photonic devices, ion-selective channels, separation 

processes, microengines mimicking the performance of 

biological systems, storage of sequential information, biocompatible 

and patterned surfaces, sensors. A classification 

including additional systems that have been described 

as supramolecular polymers is presented. 

Disk-shaped, three blades molecule: formation of a helical 

assembly when the blades assume a propeller-like conformation. 

(i Am. Chem. Soc. 2000 and 2001.) 

5.3 Columnar and Micellar Assemblies 

5.3.1 Evidence for the SLC Mechanism 

5.3.2 Helical-Columnar Mechanism 

5.3.3 Supermolecules. Tubular Assemblies 

5.4 Host/Guest Polymeric Assemblies 

5.5 Planar Assemblies 

5.6 Composite and 3D Assemblies 

6 Topics for Further Investigation 

1 Definitions. Classification of Supramolecular 

Polymers 

Supramolecular polymerization is the process of forming 

long sequences of bifunctional unimers linked only by 

main-chain noncovalent bonds. Theoretically well 

defined growth mechanisms assist the process, and extension 

of the underlying concepts to planar and three- 

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 

dimensional growth is possible. [1] While the assembly 

produced by supramolecular polymerization is a supramolecular 

polymer, [2] the reverse is not true. Several systems 

have been reported and defined as supramolecular 

polymers that do not conform to the growth mechanisms 

and theory of supramolecular polymerization. 

A supramolecular polymer (SP) can be defined broadly 

as a system characterized by non-bonded interactions 

among repeating units. Such a broad definition includes 

all systems that have been described as SPs although, due 

to its general nature, it does not suggest immediately the 

structural features or potential applications of this exciting 

class of new materials. In fact, even a molecular polymer, 

based on covalently bonded repeating units, displays 

high order structural organization controlled by supramolecular 

interactions. The occurrence of supramolecular 

interaction is so widespread that even an organic crystal 

is described as a supramolecular assembly. [3] Attempts to 

restrict the definition of SPs, for instance to systems displaying 

polymer-like properties in dilute solution, have 

been made. [4] The problem is not a simple one since several 

variables control the degree of supramolecular polymerization 

(DPÞ. Moreover, the term supramolecular has 

great appeal since it invites the use of unifying concepts 

that cut across the traditional boundaries between colloid, 

polymer and solid state science. 

Rather than attempting to further clarify the definition 

of SP, it is useful to present a classification of the various 

systems that have been reported. A possible classification, 

based on assembling mechanisms, is schematized in 

Figure 1. It offers a glimpse of the impressive growth that 

has occurred over the past decade and contextually highlights 

the SPs produced by supramolecular polymerization 

that form the core of the present review. The reference 

model of the classical covalent chain resulting from 

molecular polymerization of small bifunctional monomers 

is schematized at the top of Figure 1. The selfassembling 

chain is an open one, meaning that, in principle, 

it can grow to a distribution of large DPs, irreversible 

in solution and under a wide range of external variables. 

Figure 1. Classification of supramolecular polymers. Class A 

(reversible polymers obtained by supramolecular polymerization) 

is the main topic of this article. 

Class A. The major components of this class are equilibrium 

polymers based on processes that can appropriately 

be regarded as supramolecular polymerizations. [1, 2] The 

linear chains are self-assembled, open, growing to a distribution 

of DPs, and in a state of thermodynamic equilibrium 

sensitive to solvent type, concentration and external 

variables. The geometrical shapes in the scheme of 

class A (Figure 1) remind that unimers in supramolecular 

polymerization can be of several forms and sizes. In particular, 

the unimer can be a large supramolecular aggregate, 

a supermolecule, [2] a covalent polymer (e.g., a globular 

protein) in which case the SP will actually be a polymer 

of polymers. In class A we may also include SPs 

based on unimers with functionality A2, when a variety of 

multidimensional assemblies (helical, planar, 3D) 

becomes possible. Examples of linear systems are hydrogen-bonded 

polymers, [5–17] coordination polymers [18–20] 

and also micelles. [21–24] Examples of more complex geo- 

Alberto Ciferri is Chemistry Professor at the University of Genoa, as well as Visiting Professor 

at Duke University, Durham, North Carolina (1975-present). He has authored about 

200 original papers, books and patents mostly in the areas of rubber elasticity, biological 

and synthetic fibers, interactions between salts and macromolecules, liquid crystals, and 

supramolecular assemblies. He received his D.Sc. degree in physical chemistry from the 

University of Rome, and held positions as Scientist at Monsanto Co. and Director of 

Research at the National Research Council. He currently is President of the Jepa-Limmat 

Foundation, supporting advanced education in developing countries.

Supramolecular Polymerizations 513 

metries are helical, columnar, tubular soluble [25–44] or 

fibrous proteins, [45] S-layers, [46] composite systems such 

as block copolymers [47–49] and the tobacco mosaic virus 

(TMV). [50] Random networks and blends stabilized by 

multifunctional supramolecular linkages have also been 

reported. [51–55] 

Class B. This class includes self-assembled structures 

formed by supramolecular binding of monofunctional 

unimers. Such unimers cannot undergo open supramolecular 

polymerization, but can nevertheless form closed 

assemblies involving both low- and high-molecularweight 

species. Classical host/guest complexes, [56] base 

pairing of simple nucleoside [57] and supermolecules are 

low-molecular-weight examples. Polymeric examples 

described as SPs include side-chain binding of a monofunctional 

unimer to a covalent chain. For instance, Kato 

and FrØchet first reported the binding of the monofunctional 

mesogen stilbazole to the side chains of a nonmesogenic 

polymer functionalized with pendant benzoic 

acid groups. [58] Additional examples are double-, and triple-chain 

assemblies, and globular structures unable to 

grow further when complementary monofunctional sites 

are internally saturated. 

Class C. A number of SPs displaying novel supramolecular 

features were obtained by superimposition of covalent 

and supramolecular bonds. These systems are selfassembling 

but show irreversible DPs. The supramolecular 

organization may either precede, be simultaneous to, 

or follow the formation of covalent bonds. Examples of 

the first type include the rotaxane and catenane polymers 

described by Stoddart and coworkers, [59–61] the growth of 

dendrimers though successive generations, [62] and other 

attempts to stabilize a supramolecular assembly by the 

subsequent formation of covalent bonds. [63, 64] The final 

covalent system may retain specific supramolecular features, 

or the precursor supramolecular organization may 

just be a step of a supramolecularly assisted synthesis of 

a complex structure. Examples in which the supramolecular 

and the molecular order are simultaneously formed 

are the synthesis of dendrons possessing polymerizable 

functionality at their focal points, as reported by Percec 

and Schlüter. [65, 66] These assemblies display most interesting 

composite architectures such as columns of disks 

hosting the dendrons, with the main covalent chain running 

in the center of each column. [67] Cases in which the 

covalent structure occurs before the supramolecular one 

include the dendronization of a covalent polymer, 

reported for instance by Tomalia and coworkers, [68] and 

the self-assembled monolayers (SAMs) regarded as 

supramolecular assemblies of short hydrocarbon chains 

covalently grafted to a gold surface. [69] 

Class D. The class of engineered assemblies includes 

systems that do not form spontaneously ordered structures 

under normal conditions. Their ordered structurization 

is based on controlled methods of deposition or 

synthesis. Their classification as SPs can be justified 

since elements of supramolecular interaction still assist 

the final organization. Examples are the layered assembly 

of complementary polyelectrolytes obtained by step-wise 

deposition under kinetic control, [70] and polymer brushes 

prepared by grafting a polymer chain over a selfassembled 

monolayer of an initiator. [71] Both approaches 

allow a fine tuning of surface properties, complemented 

by patterning possibilities. Tailored performance in applications, 

such as biocompatibility, biocatalysis, integrated 

optics and electronics, are possible. 

[70, 71] 

The following sections detail concepts and results relevant 

to supramolecular polymerization and SPs in class A 

systems. In particular, Section 4 summarizes the theoretical 

framework of polymerization mechanisms [1] forming 

the basis for the critical analysis of the experimental data 

to be presented in Section 5. Some aspects already 

described in preceding reviews/analyses by the author [1] 

(focusing on mechanisms), by Lehn, [2] Zimmermann and 

coworkers, [5] Meijer and coworkers [4] (focusing on chemical 

features) are briefly summarized, placing more 

emphasis on recent data and concepts. 

2 The Bond. Site and Shape Recognition 

The cement holding well organized supramolecular structures 

requires the description of: (i) interaction between 

specific sites, (ii) site distribution and (iii) shape complementarity 

of the unimers (cf. ref. [1] for a detailed discussion). 

The relevant interactions are schematized in Figure 

Figure 2. Forces assisting supramolecular organization.


Figure 3. (a) Bifunctional units with binding sites along the N 

and S (or E and W) directions, yielding linear polymers. (b) Tetrafunctional 

units with two sites along N and S, and two sites 

along N-E and S-E (same side of the assembly), yielding linear 

or helical double chains. (c) Tetrafunctional units with sites at 

right angles within the cross-sectional or equatorial area of the 

unimer, yielding planar polymers. (d) Hexafunctional units with 

sites as in (c), and two additional sites on the flat surfaces or 

poles, yielding three-dimensional polymers. 

2. Classical supramolecular interactions (Coulombic, 

hydrogen and van der Waals bonds) are localized at specific 

sites or atoms of the unimers. These sites may be 

distributed at discrete locations over the surface of the 

unimers: the direction of interaction determines the functionality 

of the unimer and, in turn, the dimensionality of 

the assembly (cf. next section and Figure 3). These localized 

interactions are described by the respective set of 

potential functions involving combinations of point 

charges, dipolar interaction and separation distances. Several 

combinations of the above interactions may occur 

over the surface of the unimer, additively contributing to 

the overall binding free energy. [56, 72] Cooperative effects 

(when the formation of the first pairwise interaction 

increases the binding constant at successive sites along 

the chain) are also possible. In the case of H-bonds that – 

due to their strong directionality – are a primary source 

of stabilization of several SPs in class A, the parallel or 

antiparallel arrangement of multiple bonds may increase 

or decrease the product of the single binding constants on 

account of secondary electrostatic interaction. 

[5, 6] 

In addition to the above site-localized classical interactions, 

other stabilizing interactions play an important role 

in polymeric assemblies. [1] The solvophobic bond is 

responsible for the micellization of amphiphiles in the 

presence of a solvent. Even in the absence of a solvent, 

the incompatibility of amphiphilic components produces 

their ordered microsegregation. These interactions can be 

described by thermodynamic parameters that control 

micro- and macrophase separations. For instance, the 

Flory-Huggins thermodynamic parameter v plays a primary 

role in the theoretical description of microsegregation 

in block copolymers (cf. ref. [1, 81] and Section 4.2). 

The occurrence of liquid crystallinity in solutions of 

several polymeric assemblies is an example of hierarchical 

structurization (“macroscopic expression of molecular 

recognition” [2] ). Again, a thermodynamic effect (e.g., 

volume exclusion resulting from the shape anisotropy of 

rigid SPs) is the primary driving force for structurization. 

Note that it is convenient to distinguish the role of shape 

in the stabilization of individual unimers (shape I effect, 

cf. (iii) above) from the role of shape anisotropy in the 

development of liquid crystallinity by rigid SPs (shape II 

effect, cf. ref. [1] and Section 4.1). 

3 Functionality of the Unimer and 

Dimensionality of the Assembly 

The assessment of unimer functionality is a primary 

requirement for determining the dimensionality of the 

assembly. Bifunctional rod-like, disk-like or spherical 

unimers (see scheme in Figure 3a) having binding sites 

pointing toward the North and South directions (or 

toward East and West) yield linear polymers. Note that it 

is the directionality of the interaction that specifies the 

functionality, e.g., the same functionality is assumed if 

four H-bonds rather than a single one point in the same 

direction. 

Increasing the functionality of the unimers produces 

more complex structures. The presence of two additional 

sites produces extended or helical double chains, [1, 7] provided 

the additional sites are located at the same side of 

the assembly (e.g., NE and SE, Figure 3b). However, planar 

assemblies are expected when four sites point toward 

perpendicular directions within a cross-section of cylindrical 

and disk-like unimers, or the equatorial plane of a 

spherical unimer (Figure 3c). The occurrence of two 

additional sites on the flat surfaces of cylinders and disks, 

or the poles of a sphere, generates a three-dimensionally 

ordered network (Figure 3d). 

The symbols +, –, 0, and 9, in Figure 3 indicate the 

functionality and refer to any possible localized supramolecular 

bond (Coulombic, hydrogen, van der Waals). In 

the case of non-localized effects, such as incompatibility 

and shape II-induced mesophases, a more uniform distribution 

of repulsive interaction ought to be assumed. In a 

few cases a polymer is undoubtedly formed, but the specification 

of unimer size and shape may not be straightforward.


Figure 4. Theory of linear supramolecular polymerization. 

Schematic variation of the length (or DP) of a growing linear or 

helical assembly with the unimer concentration C according to 

three different growth mechanisms. MSOA: multi-stage open 

association, [1, 74] HG: helical growth, [26] SLC: open supramolecular 

liquid crystal. [1, 28– 30] C* is the critical concentration for helical 

growth, C i is the critical concentration for the formation of 

the mesophase. [77] 

4 Theory of Supramolecular Polymerization 

4.1 Linear, Helical Assemblies 

There are three different supramolecular polymerization 

mechanisms by which a linear or helical polymer could 

assemble consistently with the schemes in Figure 3a and 

b. [1] 

(i) Multistage open association (MSOA) resembles the 

step-by-step mechanism of the molecular polycondensation 

of bifunctional unimers. [73] The increase in the degree 

of polymerization of the growing assembly with the (total) 

[1, 74, 75] 

unimer concentration C is given by 

C L [M0/(4KNa)] N [(DP w) 2 –1] (1) 

where M0 is the unimer molar mass, Na the Avogadro’s 

number and K is the site binding constant. A plot of 

Equation (1) is schematized in Figure 4a. A continuous 

increase in DP, or length L of the assembly, with concentration 

is expected and the rate of increase is strongly 

affected by the binding constant. For K a 10 6 m –1 , corresponding 

to one or two H-bonds per site, only oligomers 

(DP a 10) occur in dilute (a1%) solution. However, in 

the case of the ureidopyrimidone polymers reported by 

Meijer and coworkers, [4, 52] characterized by four H-bonds 

per site and K A 10 7 , extremely high DPs (e 1000) may 

be reached in dilute isotropic solution. The familiar Car- 

others equation and the control of DP by monofunctional 

unimers [73] apply to MSOA. 

(ii) Helical growth (HG) is achieved when step-by-step 

growth is reinforced by an intra-assembly cooperative 

effect, [26] originating from a peculiar functionality such as 

that schematized in Figure 3b. Since more bonds per 

unimers occur with respect to the situation in Figure 3a, 

Kh A K and a critical unimer concentration C*occurs at 

which the MSOA regime encroaches helix formation 

with a sudden increase in DP according to [26] 

DPn =(Ch/C*) 1/2 N r –1/2 (2) 

where r s 1 is the familiar cooperativity parameter 

(easily in the order of 10 –8 ) and Ch is the concentration of 

helical polymer (increasing when C A C*). Figure 4b 

shows the step-jump increase in DP, orL, accompanying 

the nucleation of the helix at C*. The theory was intended 

to describe the reversible aggregation of globular proteins 

attaining extremely high DPs(A4000, cf. Section 5.2). 

Van der Schoot and coworkers [76] have recently generalized 

Oosawa’s theory to cover general situations with 

Kh A K, including high and weakly cooperative helical 

aggregation. The theory was applied to the case of columnar 

assemblies of chiral discotic molecules (cf. Section 

6.3.2). 

(iii) Growth-coupled-to-orientation (SLC) is alternatively 

described as the open supramolecular liquid crystal. 

[77] It is an inter-assembly cooperative process that 

encroaches MSOA causing a sudden enhancement of the 

step-by-step growth of bifunctional unimers at a critical 

concentration C i at which nematic order appears. 

[1, 21–23] 

The polymer must have considerable chain rigidity, [22] as 

expressed by its persistent (q) or deflection length (k), 

and the DP attained at C i may be approximated as 

DP V q/L0 

where L0 is the length of the unimer. The schematization 

of the theoretical behavior in Figure 4c shows the sudden 

jump of DP, orL, atC i (note C i is generally S C*), followed 

by a minor increase in L upon further increasing 

the unimer concentration. [23] Values of q for SPs frequently 

exceed the lm range, [1] enabling DPs higher than 

1000. Note that while attractive interactions stabilize the 

assembly along the N-S direction, no lateral (soft) anisotropic 

attraction among the formed assemblies is postulated 

to occur in the nematic state: only excluded volume 

(hard) interaction stabilizes the mesophase. In view of its 

fundamental significance, the difference between an open 

SLC and a conventional, molecular liquid crystal should 

be emphasized. In the latter, no association/dissociation 

equilibria accompany the transition from the isotropic 

solution: the nematic field has only an orienting effect. In 

the open SLC, development of orientation is simultaneous 

to an enhancement of supramolecular polymeriza- 

(3)


tion. [77] Growth-coupled-to-orientation could in principle 

occur even for systems displaying only soft interactions, 

when mesophases are expected in the melt or in very concentrated 

solutions. [11] However, if growth due to alternative 

mechanisms (MSOA or HG) has produced a wormlike 

chain exceeding the persistent length (L A q) at these 

high concentrations, further growth by the SLC mechanism 

will not be relevant. The original theory, [20, 21] developed 

to explain the linear assembly of cylindrical 

micelles in nematic solutions, was later extended to discotic 

molecules in a wide range of concentrations at 

which hexagonal and higher-order phases were pre- 

dicted. 

[31, 78, 79] 

A common feature of the cases considered above is the 

occurrence of polydispersity. [80] It is also apparent that 

SPs attain average DPs that are concentration-dependent, 

but may nevertheless far exceed those obtained in molecular 

polycondensation (cf. ref. [73] and Section 5.1.4). 

4.2 Multidimensional Assemblies 

The above description of supramolecular polymerization 

for linear systems has involved the identification of 

proper unimers and corresponding growth mechanism. 

Unimers with functionality A2 can express strong supramolecular 

interactions in two and three dimensions. How 

can the approach developed for linear SPs be extended to 

multidimensional systems? General thermodynamic considerations 

regarding the growth of multidimensional 

assemblies were summarized by Israelachvili. [80] The 

standard chemical potential per unit (ln 0 ) decreases with 

the number n of aggregating units according to 

l 0 n = l 0 v + a kT/n p (4) 

where l 0 v is the bulk free energy for an infinite aggregate, 

a reflects the strength of contact energy, and p is a dimensionality 

index (p = 1 for linear systems, 1/2 for discs, 1/3 

for spheres). Elaboration of the approach leads to the 

expectation that, whenever p a 1, macroscopic aggregates 

(n ev) grow abruptly above a critical concentration by 

a mechanism corresponding to a crystallization. Thus, at 

variance with the linear systems, no concentrationdependent 

broad size distributions are expected and there 

is no need to invoke a growth-coupled-to-orientation 

mechanism. These considerations apply to the growth of 

monolayers, single lamella, and also to the growth of 

three-dimensional assemblies of unimers having symmetrical 

and equivalent functionality. For some 2D systems, 

finite-size effects may frustrate growth to infinite assemblies. 

[81] The description of the growth of 3D systems is 

also more complex whenever a preferential growth direction 

occurs, reflecting, for instance, the geometrical anisotropy 

of the polymer, or the presence of two components. 

In such cases it might be possible to follow the 

Figure 5. Schematic assembly of the side and top views of a 

rigid polyanion and a cationic surfactant. This assembly may 

grow longitudinally by stacking, and laterally by interdigitation 

and hexagonal packing taken (taken from ref. [81] ). 

modes of growth along the longitudinal and transversal 

directions. 

The situations that may be encountered are illustrated 

by the assembly of a rigid polyanion (e.g., DNA) and a 

cationic surfactant in water (Figure 5). [82] The observation 

of the conventional nematic phase expected for the DNA/ 

H2O system is precluded by the strong reduction of solubility 

when the surfactant is present. Growth along the 

lateral direction (D) occurs by consecutive interdigitation 

of DNA helices with bound surfactant and produces 

aggregates with hexagonal symmetry. Growth along the 

longitudinal direction (L) may also occur due to the 

hydrophobic interaction occurring at the exposed North 

and South surfaces of the assembly. 

Provided L prevails over D, the nematic and hexagonal 

phases, crucial for the spatial orientation of the growing 

columns, should develop through the linear growthcoupled-to-orientation 

mechanism. On the other hand, 

when D prevails over L, planar growth leads to aggregates 

of infinite size (crystallization). Due to the fact that 

the helix and surfactant molecules are not chemically 

bound, it ought to be possible, through compositional 

control, to monitor growth along the two directions, possibly 

evidencing a liquid-crystalline phase, enhancing 

growth along the longitudinal direction. In general, one 

would expect a difference in the growth rate along the 

longitudinal and lateral directions. Indeed, such differences 

have been observed in block copolymers, when the 

components are chemically bound. [83] 

The foregoing considerations justify the empirical 

identification of linear growth components even in bulk 

composite assemblies, to be discussed in more detail in 

Section 5.6. A sounder mean-field approach was developed 

to interpret the solid-state morphology of (A)n–(B)m 

amorphous diblock copolymers exhibiting a microsegregation 

of components into cylindrical, lamellar or spherical 

domains. [48] The approach is based on the thermodynamic 

incompatibility of copolymer components that cannot 

be crystallized, theoretically shown to generate an 

unstable mode in an homogeneous, undiluted melt. [84–87] 

The formation of interfaces reduces the enthalpic cost of 

mixing (measured by the v parameter) but entails an 

entropy loss due to chain stretching to fill space uniformly. 

The latter depends upon the relative length and


Figure 6. Mean-field phase diagram for amorphous diblock 

copolymers (A)n-(B)m. Main phases are: lamellar (L), hexagonal 

cylindrical (H), closed packed spheres (CPS), disordered (DIS). 

f is the volume fraction of A segments. (i Am. Chem. Soc. 

1996 [87] ). 

flexibility of the A and B blocks. On this basis, cubic, 

hexagonal, lamellar, and other phases are predicted in 

discrete regions of v N N versus m/n phase diagrams as 

that illustrated in Figure 6 (N: total number of A and B 

units). A review of the most recent elaboration that unifies 

the weak [85] and strong [86] segregation regimes is 

given in the literature. [87] Noolandi et al. [88] extended the 

(self-consistent) mean-field approach to the calculation of 

the relative stability of liquid-crystalline phases occurring 

in solutions of copolymers composed of three flexible 

blocks. 

5 Polymers from Supramolecular 

Polymerization 

5.1 Linear Chains and Applications 

5.1.1 H-Bonded Polymers 

This class of systems has attracted considerable interest 

due to the intrinsic characteristics of H-bonds, such as 

directionality and the possibility of increasing bond 

strength by multiple pairwise interactions. Figure 7 illustrates 

typical cases in which flexible or rigid segments 

are functionalized with groups able to form single or multiple 

H-bonds. The binding constant K is the primary 

parameter controlling DP in terms of the simple MSOA 

mechanism schematized in Figure 4a, predicting, according 

to Equation (1), K A 106 –107 m –1 needed for DP above 

the oligomeric range. If one takes K = 500 m –1 as an average 

value for a single H-bond (reported for the pyridine/ 

benzoic acid dimerization), [7] it appears that at least 4 Hbonds 

are needed to produce DPs of interest. In fact, the 

polymer in Figure 7c based on the dimerization of ureidopyrimidone 

residues (K =66107 m –1 in CDCl3), [4] was 

Figure 7. Supramolecular polymers stabilized by main-chain 

links based on (a) one, [11] (b) three, [2, 15] and (c) four [4, 52] Hbonds. 

reported to attain DP in the order of 1000 in dilute, iso- 

[4, 52] 

tropic solution. 

In the case of polymer (b), based on the 3 H-bond 

scheme of rigid anthracene segments terminated by uracil 

(A-A) or pyridine residues (B-B), one therefore would 

not expect a significant DP to occur in virtue of the 

MSOA mechanism. In fact, no evidence for appreciable 

DP was reported for isotropic solutions. However, the 

development of liquid crystallinity evidenced the formation 

of large DPs in moderately concentrated solutions in 

organic solvents, [14] likely triggered by the mechanism of 

growth-coupled-to orientation. 

When the rigid segments in (b) were replaced by flexible, 

tartaric acid spacers, the occurrence of liquid crystallinity 

was observed only in undiluted (thermotropic) systems. 

[13] Polymer (c), based on flexible segments terminated 

by diacid (A-A) and dipyridil (B-B) residues forming 

single H-bonds, [8–12] displayed analogously only thermotropic 

behavior. The occurrence of liquid crystallinity 

in the melt does not provide evidence of significant polymerization. 

[1] In fact, the role of the growth-coupled-toorientation 

mechanism for worm-like chains displaying 

soft interaction was shown to be extremely small. [10, 11] On 

the other hand, Equation (1) offers compelling evidence 

that no large DPs are produced in the case of a single Hbond. 

Other linear H-bonded systems of interest include the 

polycaps [89] based on the polymerization of capsular host 

complexes (calixarenes) functionalized with urea. The 

formation of main-chain bonds occurred only when a 

guest molecule was hosted in the capsule (CHCl3 acted 

both as a capsule host and as a solvent). Whitesides and 

coworkers [16] who had discussed the formation of closed 

structures by the self-assembly of melanine and cyanuric 

acid earlier, reported the formation of linear nanorods by 

introducing a mismatch in the spacers of the AA and BB 

groups of these monomers. Ladder-type supramolecular 

polymers, [10] and ribbon-type polymers have also been 

reported. [17]


Figure 8. (a) Subunits used for supramolecular networks. [10] (b) 

PEO/PPO block copolymer networks based on supramolecular 

(upper) and covalent crosslinks. [52] (c) Hydrogen-bond-induced 

compatibilization in a polymer blend. [53] 

5.1.2 H-Bonded Random Networks 

In the scheme of Figure 7 all unimers exhibit a complete 

match of the donor/acceptor components of either single 

or multiple H-bond units. If this match does not occur, or 

tetra and bifunctional unimers are mixed, planar or threedimensional 

networks are possible. [2] Networks based on 

triacids and bipyridine derivatives, or on tetrafunctionalized 

pyridine and difunctional benzoic acid compounds 

(cf. Figure 8a), have been reported. [10, 51] Single H-bonds 

connect chain segments emanating from tetrafunctional 

crosslinkages. Meijer et al. [52] have reported functionalized 

copolymers of propylene oxide and ethylene oxide 

exhibiting a strong four H-bond scheme (Figure 8b). 

These networks exhibit peculiar rheological features to 

be described below. H-bonding (Figure 8c) between 

polymers, such as poly(4-vinylpyridine) and poly(4hydroxystyrene), 

[53] was described as a factor promoting 

[55, 90] 

compatibility in polymer blends. 

5.1.3 Coordination Polymers 

The scheme of linear coordination polymerization was 

discussed by Lehn. [2] The unimers are ditopic ligands 

with two binding groups forming main-chain bonds 

through metal-ion coordination (Figure 9a). Several metal 

binding groups (bidentate, tridentate) and metal ions with 

tetra-, penta- and hexa-coordination are available. Among 

Figure 9. (a) Schematization of a linear coordination SP showing 

bidentate and tridentate metal binding group and metal ions 

with tetra-, penta-, and hexa-coordination. [2] (b) Degree of polymerization 

of Be(Bu2PO2)2 vs concentration in CHCl3 at room 

temperature. The chain backbone is schematized on the right. [18] 

(c) Self-assembly of a cobalt porphyrin polymer by coordination 

of two covalently attached pyridine ligands. [20] 

the earliest reports of soluble, reversible coordination 

polymers we find systems based on three-atom-bridging 

phosphinate groups connected by tetrahedral metal atoms 

reported by Ripamonti and coworkers [18] in 1968. Figure 

9b illustrates the variation of DP (by means of vapor 

pressure osmometry) with the concentration of beryllium 

dibutylphosphinate (Be(Bu2PO2)2) dissolved in CHCl3, a 

non-coordinating solvent. Fiber-forming properties, suggesting 

larger DPs, were exhibited by anisotropic gels 

occurring in more concentrated solution. Substantial evidence 

of depolymerization with dilution was observed, 

confirming the dynamic reversibility typical of supramolecular 

polymers. [4] The chain structure, deduced from Xray 

diffraction, is based on the alternate singly and triply 

bridged structure shown in Figure 9b. 

Among the most recent reports, [20] the functionalized 

porphyrin polymer in Figure 9c was shown to attain DP 

L 100 in a 7610 –3 m solution in CHCl3 (by means of 

size-exclusion chromatography (SEC)). Here, coordination 

occurs between the Co atom (hexa-coordination) and 

the two pyridine ligands.


Figure 10. Complex viscosity (Pa N s) vs frequency x of polymer 

C in Figure 4 between 30 and 558C, and of the equivalent 

polymer (bottom, 308C) exhibiting a 2 H-bond (taken from 

ref. [52] ). 

5.1.4 Rheology and Polymer-Like Properties 

It is certainly surprising to observe that SPs based on 

bonds that are weaker than covalent ones can nevertheless 

attain DP larger than obtained, for instance, from 

conventional polycondensation. As indicated above, DP 

L 1000 can be expected for polymer (c) in Figure 7 under 

thermodynamic equilibrium, whereas DP L 100 requires 

the use of irreversible conditions in the case of aliphatic 

polyamides. [73] Thus, SPs may exhibit strong growth and 

labile bonds. Which applications can be conceived for 

such systems? The spontaneous thermodynamic assembly 

q disassembly process allows variations of DP in 

response to temperature, concentration, and other external 

variables. Moreover, the concomitant readjustment of 

donor/acceptor partners even under constant values of 

these variables renders the SPs truly adaptive, self-healing, 

combinatorial materials. [2] It is important to distinguish 

cases in which the growth of the SP is controlled by 

a non-cooperative mechanism (when changes in the 

above variables produce relatively minor DP changes, cf. 

Figure 4a) from cases in which cooperative effects are 

operative (cf. Figure 4b or c). Materials exhibiting minor 

DP alterations under the influence of an external variable, 

while still allowing the persistence of appreciable DPs, 

offer opportunities in areas of conventional polymers. For 

instance, the beneficial value of a relatively large DP on 

the mechanical properties will not necessarily be accompanied 

by a prohibitively large melt viscosity under processing 

conditions, as is the case for covalent polymers 

(properties of materials undergoing major changes in DP 

due to changes in external variables will be considered in 

the following section). 

The expectations described above are fully supported 

by rheological studies on some of the H-bond systems 

described above. Figure 10 illustrates the viscoelastic 

Figure 11. Isothermal viscosity and normal forces vs shear rate 

for a solution of polycapsules (C L 3% in o-dichlorobenzene) 

(taken from ref. [89] ). 

Table 1. Applications of linear polymers. 

STRONG T-DEPENDENT RHEOLOGY AT LARGE DP: easy 

to flow, stronger in use 

EXTENDING DP of covalent polymers 

RECYCLING: with complete regeneration properties 

TUNABLE, SMART MATERIALS: adjusting properties to 

environmental variables (switches, etc.) 

DIFFERENT CORES: mechanical, conductivity, light emitting, 

catalytic properties 

SELF-REPAIRING: any structural damage 

STRUCTURAL CONTROL: in copolymers, high selectivity, 

alternation, chirality 

SUPRA e covalent 

behavior exhibited by the polymer in Figure 4c under 

small oscillatory deformation at various temperatures (M — n 

=8610 3 ). [52] The zero-shear viscosity at 308C is comparable 

to that shown by an unfunctionalized polydimethylsiloxane 

with M — n = 3610 5 , and appears to be 

1000 times larger than for the compound based on similar 

unimers linked by a 2 H-bond scheme. The strong non- 

Newtonian behavior reflects the interplay of polymer viscoelasticity 

and chain dissociation at higher temperature. 

At low frequency (x) and high temperature (T), the loss 

modulus is larger than the storage modulus, while the 

reverse was observed at higher x and lower T. Consistent 

data was exhibited by the reversible networks displayed 

in Figure 8b showing a plateau modulus (5610 5 Pa) six 

times larger than that for a corresponding covalent copolymer. 

Figure 11 is even a more stringent demonstration 

of persistent polymeric behavior in spite of reversible 

polymerization. [89] Normal forces attaining values in the 

order of 1000 Pa are indisputable evidence of polymeric 

behavior, and all data was reversible upon reducing the 

shear rate. The rheological behavior of a coordination 

polymer (Cu(II) tetraoctanoate in decalin) was inter-


preted [90] in terms of a theory describing the chain-extending 

role of labile crosslinkages. [91] 

Applications. Applications of SPs have been suggested 

in areas that expand the applicability of covalent polymers 

(Table 1). The advantage of an easier processing in 

spite of a large DP has been commented above. Particularly 

significant is the possibility of increasing the DP of 

conventional polymers by main-chain supramolecular 

bonds. Under study is the extension of the DP of aromatic 

polyamides that are usually produced in the desirable 

high-molecular-weight range only by cumbersome syntheses. 

[92] Polymers based on long covalent segments 

extended by supramolecular bonds should be recyclable 

with complete recovery of properties. [92] The subsequent 

transformation of supramolecular bonds into covalent 

ones should allow the stabilization of large DPs and 

structures that would have been extremely difficult to 

synthesize directly. [38, 93] The thermodynamic control of 

DP and of the topology of networks should allow applications 

as tunable, smart materials responding to changes in 

variables, such as temperature, stress, and solvent permeation. 

[2, 52] By using different covalent segments in A- 

A/B-B unimers, the tunability could be expressed in 

mechanical, conductivity, light emitting, and catalytic 

properties. If changes in the above variables were occurring 

accidentally, the SP would have the capability of 

self-repairing. The high selectivity of molecular recognition 

should allow the selection of proper sequences in 

mixtures of more than two complementary unimers. 

5.2 Helical Chains and Functional Systems 

The original Oosawa theory (cf. Section 4.2) is based on 

unimers exhibiting the site distribution shown in Figure 

3b. Cooperation arises because each unimer makes two 

bonds along the linear sequence and two weaker ones 

along the helical pattern. The unimers should be large 

enough to avoid possible mismatches in the pattern. 

There has been no verification of the model using synthetic 

SPs. The model was elaborated to describe the formation 

of helices and microtubules (G q F transformation) 

by biological SPs. [1, 26] However, the verification of 

the helical growth model has been problematic even in 

the relatively simple case of actin, due to the simultaneous 

dephosphorylation of ATP normally bound to the 

protein. A study in which growth could be observed without 

the complication of ATP e ADP hydrolysis was performed 

by Korn on actin-ADP. [27] Actin polymerization 

occurs by increasing the ionic strength or temperature in 

isotropic solution at unimer concentrations below 

0.04 mg/ml. Filaments exceeding 11 lm, corresponding 

to a DP larger than 4000, are attained (higher values 

were observed with tubulin). [1, 28] The length distribution 

conforms to the most probable distribution and chain 

stoppers reduce DP, in line with theory. The diagram in 

Figure 12. (a) Concentration of helical (0) and oligomeric 

(6) actin vs unimer concentration showing the critical concentration 

as predicted by Oosawa’s theory (taken from ref. [29] ). (b) 

Schematization of translational movement resulting from the 

directional growth of F-actin (taken from ref. [25] ). 

Figure 12a exhibits the theoretically predicted occurrence 

of a critical unimer concentration at which helical growth 

begins, and the concentration of unimers and oligomers 

attains a constant value. [29] The recent generalization [76] of 

Oosawa’s theory to growth processes enhanced by cooperative 

effects has not yet been applied to specific chainlike 

sequences. It has however been applied to the growth 

of columnar systems to be discussed in the following section. 

The overall functioning of actin or tubulin as in vivo 

systems invites to consider the way in which the helical 

growth process is coupled to the dephosphorylation reaction. 

This coupling produces a dynamic function of the 

polymer [25, 30] that assists in processes, such as motility, 

contraction, and cell division. Figure 12b illustrates the 

processes believed to occur during the polymerization of 

actin filaments. The critical concentration of G-actin 

unimers is defined by the condition of equality between 

the sum of the assembly and disassembly rates at the two 

ends of the filament (the barbed and the pointed ends). 

Under ATP hydrolysis, the depolymerization at one end 

may be faster than polymerization at the other end. Thus, 

a cycling of actin monomers from one end to the other 

occurs during the growth of F-filaments, resulting in a 

translation of the polymer (tread-milling effect). A 

related dynamic instability effect controls the assembly q 

disassembly process of tubulin into microtubules. [25] 

Applications. The design principle of functional protein 

systems is based on the coupling of a supramolecular 

polymerization process to a chemical reaction. 

[25, 30] 

Understanding and possibly reproducing coupled 

mechanisms is the challenging road to microengines and 

other functional SPs mimicking mechanical properties of 

biological systems and engineered for new practical 

applications.


Figure 13. (a) Schematization of the assembly of surfactant 

and growth of micellar polymers. (b) Experimental data and theoretical 

phase diagram for the discotic amphiphile 2,3,6,7,10,11hexa(1,4,7-trioxoacetyltriphenylene) 

in D2O (taken from ref. [31] ). 

5.3 Columnar and Micellar Assemblies 

5.3.1 Evidence for the SLC Mechanism 

In this section we will consider amphiphilic unimers that 

unmistakably exhibit nematic phases appearing simultaneously 

with the onset of extensive polymerization, thus 

revealing a superimposition of the MSOA and SLC 

mechanisms. If the shape of the unimer is cylindrical or 

disk-like, the resulting SPs will be linear or discotic. In 

both cases, theory suggests that strong contact forces and 

rigidity along the longitudinal direction are involved (cf. 

Section 4.1.3). 

Odijk [22, 94] has discussed the experimental verification 

of the growth-coupled-to-orientation theory in the case of 

conventional spherical micelles that exhibit, upon 

increasing the surfactant concentration, the assembling 

sequence: dispersed molecules e spherical micelles e 

cylindrical (end-capped) micelles e linear polymer, the 

growth of linear polymers being associated to the transformation 

isotropic e nematic. 

The broad features of the experimental phase diagram 

were found to be in line with theory, although some discrepancies 

remain. [95, 96] Persistence length data confirmed 

the rigidity of the micellar polymer, but exhibited consid- 

erable scattering with q ranging from L0.02 to 10 lm; a 

likely value of L1 lm corresponds to a DP in the order of 

20000. There is no data on the formation of the nematic 

phase and on persistence length in the case of block copolymer 

micelles. Higher-order phases (hexagonal, lamellar, 

etc.), however, have been observed. [97, 98] The absence 

of a nematic phase was also noticed for several conventional 

surfactants. The growth-coupled-to-orientation theory 

predicts that the nematic phase can be skipped and 

only a direct isotropic e hexagonal columnar phase is 

observed for suitable combinations of contact forces and 

persistence length. [79] It is also expected that volume 

excluded effects [23] and a reduced chain stretching [99] 

cause some micellar growth even in isotropic solutions of 

block copolymers. 

A most detailed evidence for growth-coupled-to-orientation 

is provided by discotic molecules: ditopic structures 

possessing a disk-shaped core from which a number 

of flexible alkyl chains emanate. Molecularly dispersed 

disks would be expected to form conventional liquid crystals 

in virtue of their large excluded volume. Moreover, 

disks are able to aggregate into soluble columns due to a 

p-p stacking of the cores and solvophobic interaction of 

the side chains. As in other supramolecular polymerizations, 

columns may assemble due to the small equilibrium 

constant (MSOA), and growth can be reinforced by the 

occurrence of cooperative effects. Figure 13b displays 

the phase diagram of the discotic amphiphile 

2,3,6,7,10,11-hexa(1,4,7-trioxoacetyltriphenylene) in 

D20. [31] The diagram represents a match between experimental 

data and theoretical lines calculated according to 

the theory of growth-coupled-to-orientation, and includes 

the prediction of higher-order phases. [68] Growth is seen 

to occur simultaneously with the appearance of the 

nematic phase at a concentration of L20% at room temperature. 

It appears that, for this system, the balance of 

contact forces and flexural rigidity favors the occurrence 

of the nematic phase. The formation of the hexagonal 

phase observed at higher concentration is attributed to an 

improved packing efficiency with respect to the nematic 

phase. The diagram reveals a hierarchical evolution of 

the assembling process through higher-order phases: 

disks (I) e columns (N) e hexagonal columnar (H) e 

solid. 

5.3.2 Helical-Columnar Growth Mechanism 

Meijer and coworkers [32, 33] have synthesized a series of 

most interesting C3-symmetrical disk-like molecules that 

assembles into cylindrical stacks in virtue of both hydrogen 

and arene-arene interactions. The molecules shown 

in Figure 14a have large aromatic cores, H-bonding 

groups, and either achiral or chiral side chains. The latter 

varied in their polar character allowing the study of 

aggregation in either polar or nonpolar solvents. The for-


Figure 14. (a) Disk-shaped, three blades molecule prepared by 

Meijer and coworkers [4] with side chains having polar, nonpolar, 

chiral, achiral character. [32, 33] (b) Formation of a helical assembly 

when the blades assume a propeller-like conformation. [100] 

(c) Schematic representation of the transition from dispersed 

disks to partially ordered and to fully ordered columns. [76] 

(i Am. Chem. Soc. 2000 and 2001.) 

mation of long columns was shown to occur in very dilute 

solution in hexane (10 –6 m), and a large association constant 

(10 8 m –1 ) was reported. [32] It was suggested that such 

an aggregation reflects the formation of helical columns 

through a cooperative process attributed to a conformational 

transition from flat to propeller shape for the blades 

of each disk, resulting in a maximization of interaction 

for the chiral helical assembly (Figure 14b). [100] Experimental 

data for the more polar molecules in dilute butanol 

(2.4610 –4 m) [33] revealed a sequence of two association 

steps upon temperature changes. The postulated process 

is schematized in Figure 14c: starting with an isotropic 

dispersion of disks, a decrease in the temperature causes 

the formation of low-DP achiral aggregates stabilized by 

non-cooperative interaction, followed at a lower critical 

temperature by the cooperative formation of helical columns 

with DP attaining the 1000 range. [76] 

The theoretical description of the processes described 

above was formulated by van der Schoot and coworkers. [76] 

The occurrence of the two regimes was described in terms 

of the cooperativity parameter r. The situation r =1is 

equivalent to the binding of unimers into disordered aggregates 

(i.e. MSOA), while r s 1 describes the subsequent 

cooperative formation of ordered aggregates by a HG 

mechanism. Essentially, the treatment is a generalization 

of the Zimm-Bragg and the Oosawa theories (Kh A K) without 

necessarily specifying a detailed molecular model and 

a critical nucleus. With respect to Oosawa’s treatment that 

focused on the critical concentration C* (cf. Figure 4c), 

Figure 15. (a) Deoxyguanosine, its oligomers, and folic acid. 

Their assembly in tetrameric disks. [36] (b) Variation of the number 

of stacked tetrameric disks with folate concentration in (1) 

pure H2O and (2) 1 m NaCl at 308 C. The vertical broken line 

indicates the I e H transition (replotted using data taken from 

ref. [26] ). 

the van der Schoot treatment emphasizes the fractions of 

aggregated material and helical bonds as a function of 

both temperature and concentration. Rigidity and excluded 

volume effects are not introduced and, therefore, liquid 

crystallinity does not direct the aggregation of the stacks. 

5.3.3 Supermolecules. Tubular Assemblies 

The formation of disk-like supermolecules from two or 

more complementary components was discussed by several 

authors. [2, 34–36, 43, 44] Disk-like supermolecules often 

show liquid-crystalline behavior even though the separate 

components do not. Moreover, the discotic supermolecules 

can form columnar stacks in the melt and in solution 

just as the molecular discotics discussed above do. 

The relative contributions of MSOA, HG and SLC 

mechanisms have not always been characterized adequately. 

Gottarelli et al. [35, 36] have investigated the most 

interesting assembly of the nucleotide deoxyguanosine, 

its oligomers and alkaline folates (Figure 15a). These 

compounds form hydrogen-bonded disk-like tetramers in 

solution and are able to assemble in columnar stacks of 

discrete length and DP. Small-angle neutron scattering 

techniques were used to determine the length of the 

aggregates in water and in salt solutions. The critical concentrations 

for the appearance of the nematic (cholesteric) 

and the hexagonal phases were determined by 

means of X-ray diffraction. Selected data for the deoxyguanosine 

dimer (d(GpG); Figure 15a) and the folate is


Table 2. Critical concentration and DP in the isotropic phase 

for folic acid (selected data taken from ref. [36] ). 

Sample Solvent C IN 

% 

C NH 

% 

LISO DPISO a) XISO b) 

d(GpG) H2O – – 70 15 2.3 

d(GpG) H2O +Na + 2.5 18 – – – 

d(GpG) H2O +K + 1.5 15 – – – 

folate H2O – 35 2.3 1 0.1 

folate H2O +Na + 27 35 2.1 9 0.7 

a) L/2.35 Š (4.70 for d(GpG)). 

b) L/30 Š. 

collected in Table 2. The deoxyguanosine derivatives 

generally show larger DPs in isotropic solutions and 

lower critical concentrations CIN and CNH than the folates. 

The presence of NA + and, particularly, K + ions enhances 

the stabilization of the aggregates. In the case of folates 

in pure H2O, no nematic phase and small DPs were 

observed. Due to the considerable diameter of the cylinders 

(D L 30 Š) and the thickness of each disk (L = 2.35 

Š), the DP in the isotropic phase is extremely small and 

the corresponding axial ratio X suggests that thick disks 

rather than columns are present. Figure 15b illustrates the 

evolution of DP with concentration covering the range 

from the isotropic to the hexagonal phase. The smooth 

dependence DP vs C does not evidence cooperative 

effects in the case of folates. The largest DP (L30), determined 

from a 60% (hexagonal) solution, reveals in fact 

columns of very small geometric anisotropy (X L 2.3). 

The formation of the mesophase may be promoted by the 

large excluded volume effect of disks even in the absence 

of soft interactions. In fact, simulation studies evidenced 

nematic and columnar phases for solutions of extremely 

thin disks (0 a L/D a 0.1). [101] 

Disk-like supermolecules based on dimers of ureidotriazines 

connected by a 4 H-bond scheme similar (but 

not identical) to that of the ureidopyrimidone polymers in 

Figure 7c were reported by Meijer and coworkers. [34] 

These disks stack in columns with loose positional order 

and low DP (Figure 16a). Percec and coworkers [44] 

reported tubular polymeric assemblies of disks composed 

by six tapered molecules of 12-ABG-15C5 complexed 

with triflate salt (Figure 16b). The columns assembled 

into a hexagonal mesophase revealed as by means of Xray 

diffraction from the undiluted system. 

Several of the systems described above present a central 

cavity into which a covalent polymer can be hosted 

or a flow of ions be achieved (cf. also next section). Of 

particular interest are nanotubes (Figure 16c) formed by 

stacking cyclic peptides connected by H-bonds along the 

columnar axis. [37–41] The chemical design of these flat 

ring-like peptides was discussed by De Santis and coworkers. 

[37] Tubes assembled in solution and the contact 

forces for the dimerization (K L 2.5610 3 m –1 ) are too 

Figure 16. (a) Monofunctional ureidotriazine disks capable of 

assembling into columns. [34] (b) Assembly of tapered 12-ABG- 

12C5 into disks, formation of a column of stacked disk, hexagonal 

columnar organization. [44] (c) Nanotubules formed by Hbonded 

cyclic peptides. [37] (i Am. Chem. Soc. 1994 and 1996.) 

small for a large DP unless cooperative effects occur. [38] 

Cyclic b-peptides were also considered. [39] The self 

assembly of the nanotubules into ion-selective membranes 

was discussed as well. [40] 

Unimers of most of the systems considered above were 

subsequently connected by flexible covalent spacers, producing 

main-chain or side-chain SPs that should be 

described more appropriately under class C SPs. The 

basic ability of the disks to form columnar assemblies 

was preserved, but the covalent segments produced 

alterations in the stacking details such as the occurrence 

of helicity. For instance, a slowly rising helicoidal stack 

was produced when the crown ether receptor in Figure 

16b was replaced by a flexible endo-receptor (nEO- 

PMA) connected as a side-chain to a poly(methyl acrylate) 

chain. [42] When the ureidotriazine disks in Figure 

16a were main-chain linked through flexible spacers, 

helical columns and large DPs were observed. [34] 

Applications. The columnar nematic or hexagonal 

packing of disk-like molecules and supermolecules could 

be exploited as a precursor step for the assembly of large, 

oriented structures modeling natural systems. Electronic 

mobility due to the p–p interactions along the columnar 

axis may be useful for electronic and photonic 

devices. [102] Central cavities could be exploited for the 

selective hosting of polymer molecules [44] or for ion- 

[40, 41] 

selective channels. 

5.4 Host/Guest Polymeric Assemblies 

Covalent polymers can enter a cavity of a columnar 

assembly or of single ring-like structures. The result is a 

composite host/guest polymeric assembly exhibiting a


Figure 17. Shish-kebab composites. Schemes for (a) polyrotaxane, 

[61] (b) tobacco mosaic virus, [50] and (c) a-cyclodextrin + 

poly(ethylene oxide) (taken from ref. [104] ). 

shish-kebab-type architecture. The driving force for the 

formation of these assemblies is a complex combination 

of molecular recognition and supramolecular polymerization. 

In fact, the host polymers often promote the supramolecular 

polymerization of the guest, or an alteration of 

its assembly mode. Three examples are illustrated in Figure 

17. 

The primary interaction assisting the threading of a 

polymer into a single macrocycle cavity, as in the case of 

pseudopolyrotaxanes (Figure 17a), is attributed to the 

occurrence of appropriately spaced p-rich hydroquinone 

rings on the polymer and p-acceptor groups within the tetracationic 

cyclophane. [59–61] It has been shown that the 

electron donor/acceptor interaction can be monitored by 

electrochemically or photochemically induced reduction/ 

oxidation reactions. Relative motion of the two components 

can thus be induced, simulating a molecular microengine. 

[103] 

The situation of TMV, illustrated in Figure 17b, is 

more complex. Here the guest is an RNA molecule and 

the host is a hollow columnar assembly composed of 

2130 identical tapered protein molecules. The host/guest 

systems can be disassembled and reassembled in vitro by 

pH changes. However, the host can be reassembled even 

without RNA. A very interesting effect is manifested in 

the structure of the host when RNA is present. [50] In the 

absence of RNA the host is a stack of disks of various 

DPs, each disk comprising 17 protein units. However, 

formation of a spiral occurs when RNA occupies the cavity. 

The proteins of the host then follow a helical pattern 

with 16.3 units per turn, and the assembly assumes definite 

dimensions (L = 3000, d = 180 Š, X = 16.6) and a 

DP of 2310. 

The RNA-induced helix formation in an otherwise 

stacked systems of disks is reminiscent of the similar 

effect described in the preceding section (Figure 16a,b). 

It thus appears that supramolecular interactions between 

sites on RNA and protein induce spiral formation similar 

to that of disks connected to a covalent polymer as side 

chains. The complex role of RNA for the whole structure 

is evident. RNA acts like a crank-shaft that drives the 

proteins bound to it into a helical pattern and simultaneously 

provides the information about the proper length 

and DP of the host. The assembly mechanism of the overall 

TMV structure can thus be described in terms of a 

supramolecular polymerization of the columnar assembly, 

coupled to the formation of monofunctional sidechain 

bonds between RNA and proteins. [77] 

Figure 17c illustrates the assembly of a-cyclodextrin 

rings over poly(ethylene oxide). This system belongs to 

the class of inclusion compounds, or clathrates, that have 

aroused considerable interest for separation processes and 

for the unique properties of single chains confined in narrow 

(d L 6 Š) channels. [104] The stability of the crystalline 

adduct is likely to be assisted by pairwise host/guest 

interactions, the strength of which is increased within the 

small cavity. [56] However, the wide variety of systems 

capable of forming inclusion compounds invites to consider 

other less specific factors affecting the supramolecular 

polymerization of a-cyclodextrin rings threaded 

along the polymer chain. These factors might be: (i) relatively 

strong contact forces between the surfaces of the 

host and (ii) a steric-type effect not so far theoretically 

described. In support of (i) one may note that soluble stoichiometric 

complexes of the host are known to occur 

(e.g., head-to-head dimers of cyclodextrin unable to 

assemble into long channels in the crystalline structure). 

Concerning (ii) it is plausible that the guest stretches out 

(loss of conformation entropy) while simultaneously 

assembling individual host molecules. A suppression of 

undulation modes of the polymer due to the presence of 

rings may lead to an entropically induced effective attraction 

between the threaded rings (a similar Casimir-type 

effect leads to an attraction between undulating, flexible 

membranes). Single host/guest channels could form even 

in isotropic solution of more rigid polymers if there is a 

favorable balance between the contact energy of cyclodextrin 

rings and the chain-conformational entropy. In 

concentrated solutions, the resulting rod-like structure 

could be favored by the occurrence of a nematic phase. 

Applications. The possibility of generating relative 

motion of the assembled surfaces by electrochemical or 

photochemical stimuli could be exploited in a variety of


Figure 18. S-layers with hexagonal and square lattice symmetry 

derived from TEM. (a) Thermoanaerobacter thermohydrosulfuricus, 

and (b) Desulfotomaculum nigrificans (taken from 

ref. [46] ). 

nano/molecular scale engines. [103] The encapsulation of 

polymer molecules within cavities formed by self-assembling 

unimers provides systems of interest for separation 

processes, for recognizing and storing sequential information, 

[50] for orienting and screening single polymer molecules 

from similar neighbor interaction. [104] 

5.5 Planar Assemblies 

Figure 3c illustrates an equatorial distribution of four 

binding sites suitable for the formation of planar assemblies. 

As discussed in Section 4.2, these assemblies are 

expected to grow to large sizes by an intra-assembling 

cooperative mechanism akin to crystallization. At variance 

with the growth-coupled-to-orientation of linear 

systems, the growth of a planar assembly does not require 

the simultaneous formation and orientation of other growing 

units. Single free-standing, monomolecular layers are 

possible. An excellent verification of these expectations 

is provided by self-assembling S-layers forming the protective 

layer of the external surfaces of bacterial cells, 

and enabling the maintenance of a closed lattice during 

cell growth and division. [46] The identical constituent proteins 

have quasi-spherical form and exhibit an equatorial 

distribution of donor/acceptor groups capable of H-bonding 

to adjacent unimers. The proteins also posses a southpole 

site capable of electrostatic anchoring to the cell surface. 

S-layers can be disassembled and reassembled in 

vitro, allowing the preparation of purely H-bonded monolayers 

standing over an inert surface. The assembly q 

disassembly process has been described as a crystalliza- 

tion, [46] producing highly organized morphologies such as 

those shown in Figure 18. Depending upon the lattice 

type, the center-to-center distance of the morphological 

units varies from 3 to 30 nm, the thickness of monomolecular 

lattices vary from 5 to 25 nm, and the pore size is 

between 2 and 8 nm. 

Applications. The controllable confinement in definite 

areas of nanometric dimensions, coupled to the easiness 

of extraction and re-assembly, has allowed applications 

in areas of nanotechnology, such as bioanalytical sensors, 

templates for superlattices with prescribed symmetry, 

electronic and optical devices, matrices for the immobilization 

of functional molecules, and biocompatible surfaces. 

[46] S-layers recrystallized over solid supports have 

also been successfully patterned by using UV radiation 

and microlithographic masks. [46] 

5.6 Composite and 3D Assemblies 

Hexagonal cylindrical and lamellar phases (cf. transmission 

electron microscopy (TEM) photographs in Figure 

18) are often seen [48] in diblock and multiblock copolymers, 

ternary systems, copolymers formed from one unit 

that can be crystallized, rod-coil copolymers, [49] and some 

biological fibers. [105] For amorphous diblock copolymers 

in the cylindrical mode, one block is hexagonally packed 

within a matrix of the other block. The lamellar mode is 

instead based on alternating layers of A and B. The 

lamellar mode is the prevalent feature observed with rodcoil 

copolymers. [49, 106] In the case of a helical comb-like 

polymer (poly(b-l-aspartate) with paraffinic side chains), 

a layered distribution of helices correlated by interdigitation 

of the side chains was observed. [107] In the case of 

keratin, the fiber cross-section reveals L1 lm long microfibrils 

parallel to the fiber axis and hexagonally imbedded 

in a disordered S-rich matrix. Each microfibril is composed 

of eight protofibrils that are left-hand cables of two 

strands, each including two right-hand a-helices. [105] 

In the case of amorphous block copolymers, the (selfconsistent) 

mean-field theory [87] (cf. Section 4.2 and Figure 

6) describes the occurrence of various phases in terms 

of parameters pertinent to single copolymer molecules 

(compatibility, relative length and flexibility of the two 

blocks). This theory has been an eminently successful 

one and experimental results for the undiluted melt offer 

good support to it. [83, 108–110] Even in the case of block 

copolymer solutions, the predicted [88] sequence of phases 

upon increasing the concentration (e.g., isotropic e 

micellar e cubic e hexagonal e lamellar) revealed simi- 

[97, 98] 

larities with experimental data. 

Within the context of supramolecular polymerization it 

is however useful to explore alternative descriptions of 

the above structures in terms of a simpler, less sophisticated 

approach based on the concept of self-assembling 

of specifically designed building blocks. A similar con-


Figure 19. (a) Micelles of coil-coil diblock copolymers in a 

selective solvent undergoing supramolecular polymerization. 

Hexagonally packed morphology as determined by means of 

TEM. (b) Bilayers of rod-coil diblock copolymers in a solvent 

selective for the coil block, growth directions within the plane of 

the layer and perpendicular to it are shown. Layered morphology 

[48, 112] 

as determined by means of TEM. 

cept has been used to describe solid arrays displaying 

complex and ordered structurizations such as interpenetrating 

nets. [111] The present author had suggested [1, 77] that 

the three-dimensional solid state morphologies described 

above, originating from molecular recognition of similar 

blocks, ought to be described in terms of supramolecular 

polymerization. The approach requires the identification 

of unimers with proper functionality and of their one- or 

multidimensional growth mechanism. [112] Relevant to this 

end are the considerations set forth in Section 4.2 regarding 

the separation of the modes of longitudinal and lateral 

growth. 

In particular, the formation of the hexagonal phase for 

copolymers that cannot be crystallized could be described 

as an essentially one-dimensional growth of micellar 

unimers according to the mechanism of growth-coupledto-orientation. 

By analogy with the processes illustrated 

in Figure 13, leading to the hierarchical sequences 5 and 

6, it has been suggested that the growth process schematized 

in Figure 19a describes the formation of the hexagonal 

phase of coil-coil block copolymers. [112] Here it is suggested 

that, as for conventional surfactants, a block copolymer 

micelle can assume an elongated form playing the 

role of a bifunctional unimer. Upon increasing the concentration, 

the latter undergoes linear growth simultaneously 

with the development of nematic orientation. 

This is followed by the hexagonal columnar phase (the 

intermediate nematic phase may not appear for suitable 

combinations of contact forces and persistence length) at 

even higher concentration. The hexagonal phase should 

be viewed as an embryo of the final morphology in the 

condensed state. The coiled segments may dangle over 

the lateral surface in a disordered fashion rather than 

interdigitate regularly. Note that while the approach discussed 

above does not have the predictive features of the 

mean-field theory, it does predict a role of the persistence 

length for the primary length scale, which is not predicted 

by the latter theory. 

The expectation that linear growth controls the formation 

of hexagonal columnar mesophases is not limited to 

coil-coil copolymers, or to systems with long molecular 

axes normal to the growth direction. Bifunctional unimers 

unable to grow along the lateral dimensions should in 

general be candidates for linear growth. In the case of 

keratin fibers, considering that the length of the microfibrils 

by far exceeds the length of constituent chains and 

falls in the range of the persistent length reported for 

similar systems, [1] it is plausible that the assembly of the 

fibril is also directed by the growth-coupled-to-orientation 

mechanism. The following hierarchical assembly 

sequence has therefore been suggested: extrusion of the 

low sulfur protein into extracellular fluids e head-to-tail 

assembly of shorter unimers into individual microfibrils 

to a length related to the persistence length with simultaneous 

orientation in the mesophase e stabilization of the 

microfibril by internal 1S1S1 bonding and the two non 

a-helical terminals e crosslinking of the sulfur-rich component 

in the narrow interfibrillar space. [112] 

The formation of lamellar structures could also be 

described qualitatively in terms of the self-assembly of 

specifically designed building blocks. Considering the 

case of the single lamella schematized in Figure 19b, 

growth akin to crystallization can occur along two perpendicular 

in-plane directions: functionality is larger than 

two. This mode of growth is at striking variance with the 

case of the uni-dimensional growth of cylindrical, bifunctional 

unimers considered above. It remains, however, to 

be considered how the ordered polymerization along the 

direction perpendicular to the lamellar plane is achieved. 

It is possible that, as disoriented lamellae grow, a critical 

axial ratio is attained at which purely hard interactions 

stabilize a nematic phase of the discotic type. This type 

of order has been experimentally evidenced during the 

graphitization of organic materials when large and growing 

planar rings are formed. [113] The coiling segments protruding 

from the lamellae may dangle over the surface, as 

in the case of cylindrical assemblies. 

An alternative assembling mode along the direction 

perpendicular to the lamellar plane could be based on 

attractive forces, or interdigitation, among the coiled segments. 

This interaction does not appear relevant to the 

class of copolymers considered above, but was evidenced 

for the comb-like poly(b-l-aspartate) with paraffinic side 

chains. This system showed nematic order based on clusters 

of helices correlated by interdigitation of partly molten 

side chains. [106] The polymer formed a complete 3D 

structure based on a layered organization of helices with 

side chains crystallized in a separate hexagonal lattice. 

Applications. Block copolymers are known to represent 

an important class of materials allowing a desirable blend


of properties of different polymers, while preventing the 

undesirable (incompatibility-driven) macrophase separation 

of unconnected components. [47–49, 106] Pre-assembly 

followed by the growth of specifically designed unimers 

could represent a novel strategy toward the fabrication of 

composite structures with a prescribed distribution of 

components. 

6 Topics for Further Investigation 

Several aspects of fundamental and applied character 

appear to need more extensive investigation. Here we 

restrict attention to the polymer-like properties of SPs 

and the basic polymerization mechanisms. 

A main problem is the assessment of DP. A characteristic 

feature of these systems is that DP is a function of 

concentration. However, the determination of DP at a 

given concentration becomes complicated when using 

conventional molecular-weight determinations, requiring 

extrapolations to infinite dilution. Measurements under 

theta conditions should be preferred. The assessment of 

DP by means of SEC is also questionable for kinetically 

unstable SPs that display weak contact forces. In the case 

of actin, a successful determination of the DP distribution 

was reported using electron microscopy. [28] A reliable 

determination of the complete DP/concentration dependence 

may require the evaluation of binding constants and 

the application of theoretical expressions valid for a specific 

growth mechanism. For instance, Equation (1) can 

be used in the low concentration regime when MSOA is 

expected to prevail. The analysis of data at higher concentration 

could allow the detection of cooperative contributions 

arising from the HG or SLC mechanisms. 

There are several other parameters that could be determined 

by an extension of conventional polymer physical 

chemistry. One is the characterization of the rigidity of 

the assembly. The usual determination of the persistence 

length from solution studies may not be a viable one in 

the present context. An alternative approach, suitable for 

large values of q, is based on flexural rigidity data 

extracted from the thermally driven fluctuation of shape 

or end-to-end distance. [114, 115] Calculation approaches are 

also possible. [116] Rheological characterization of processing 

parameters under shear and elongational flow should 

aim to the coupling of polymer viscoelasticity and bond 

lability. Analogies should be considered with the behavior 

of living polymers, [91] covalent networks exhibiting 

labile crosslinkages, including those formed in the 

oriented state. The assessment of the mechanical strength 

of supramolecular bonds, particularly for the linear Hbond 

materials discussed in Section 5.1.1, is another open 

topic of great importance, e.g., for the performance of 

supramolecularly extended covalent polymers. An evaluation 

of the most suitable method [117] for calculating the 

elastic constants of SPs is needed. The characterization of 

the difference in the properties of reversible and irreversible 

polymers and copolymers could be better assessed by 

comparative studies on a given SP and on its analog 

obtained by transforming the supramolecular main-chain 

bonds into covalent ones. 

Analysis and more detailed investigations of the 

growth mechanisms are also needed. The sudden occurrence 

of growth when the nematic phase appears ought to 

be documented for other SPs, the rigidity of which needs 

to be independently assessed. The theoretical reasons preventing 

the observation of a nematic phase for block 

copolymers and some other amphiphiles need to be clarified 

and experimentally verified. Systems for which the 

contact energy, or equilibrium constant, can be systematically 

altered (e.g. by altering the number of pairwise 

interactions at given functionality) should be considered 

for a more stringent test of the relationship between K 

and DP. In this context, recent work has shown that 

bonds based on DNA base pairing produce well-behaved 

SPs. [118] An analysis of model systems in which only the 

site distribution is altered could allow an assessment of 

the parameters controlling linear versus helical growth. 

The detailed analysis of the steps contributing to the formation 

of host/guest composites (Section 5.4), and other 

hierarchical processes (Section 5.3), should help to elucidate 

the scaling up from nanometric to mesoscopic 

assemblies. 

Acknowledgement: The author expresses his appreciation to 

Prof. Paul van der Schoot for clarifying discussions and constructive 

criticism. 

Received: March 23, 2002 

Revised: May 13, 2002 

Accepted: May 13, 2002 

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