Main directional supramolecular interaction

This chapter reviews main-chain- and side-chain-functionalized supramolecular polymers that were synthesized by ROMP and acyclic diene metathesis (ADMET) polymerization. Emphasis is placed on polymers assembled using directional supramolecular interactions, such as H-bonding, M-L coordination, and inclusion complexation, or a combination thereof. 

Dramatic examples have also been reported for main-chain supramolecular polymers (SPs Lehn 1993 Ciferri 2005 Fig. 3.1), in which specific and directional molecular recognition events between end groups define the main chain of a linear polymeric assembly. Although main-chain SPs had been created and characterized previously (Broze et al. 1983 Fouquey et al. 1990 Alexander et al. 1993 Bladon and Griffin 1993 St Pourcain and Griffin 1995), it was a groundbreaking paper in 1997 that demonstrated the mechanical potential of supramolecular interactions and catalyzed much of the current interest in the field (Sijbesma et al. 1997). 

With the introduction of supramolecular polymers, which are polymers based on monomeric units held together with directional and reversible secondary interactions, the playground for polymer scientists has broadened and is not restricted to macromolecular species, in which the repetition of monomeric units is mainly governed by covalent bonding. The importance of supramolecular interactions within polymer science is beyond discussion and dates back to the first synthesis of synthetic polymers the... 

In Agarwal s model, the supramolecular interactions just tie the component chains together and do not bear any load as they are lying perpendicular to the flow direction. The main reasons for the improved shear stability are the redistribution of the hydrodynamic force (because of topology) to the associated chains and the hydrodynamic shielding effect. This may help us to understand the mech-anochemistry of biomacromolecules such as double strand DNAs. 

Figure 2.7 Scheme of the orientation of a polymer backbone, and of a transition dipole (/I) with respect to the polymer chain, within a nanofiber whose longitudinal axis is along the Z direction. Here, X, Y and Z indicate the axes taken as reference in the macroscopic sample, tp defines the angle between the direction of the polymer chain and the Z direction, and depends on the assembly or nanofabrication process used to produce the nanofiber, and on the characteristics of the molecular system used (steric hindrance, supramolecular interactions, etc.) and flow (De number). The dipole transition moment is oriented at an angle, tp, with respect to the main chain axis (for instance, for polyfiuorenes tp is of about 20°). Other azimuthal angles, not shown here, complete the description of the polymer chain and dipole transition moment directions. 

Hydrogen bonding is the most important directional Interaction responsible for supramolecular construction [17], Appendix 1 illustrates several typical hydrogen bond patterns which are present in two-component molecular crystals. Appendix 2 exemplifies a variety of reported cocrystals selected mainly fiom the recent papers [18], Not only strong hydrogen bonds (i.e, 0—H "0, 0—H- N,... 

Supramolecular polymers are a relatively new class of polymers in which monomeric repeating units are held together with directional and reversible (noncova-lent) secondary interactions (Lehn, 2000), unlike conventional macromolecular species in which repetition of monomeric units is mainly governed by covalent bonding. A schematic comparison of a covalent polymer and a supramolecular polymer is shown in Fig. 1.3. 

DCC is thus rooted in supramolecular chemistry [7], being based on two of its main themes, self-assembly in the generation of the library constituents and molecular recognition in their interaction with the target entity. Self-assembly in a multicomponent system is a combinatorial process with a search-procedure directed by the kinetic and thermodynamic parameters imposed by the nature of the components and their connections. 

Hydrogen bonds (H-bonds) are ideal noncovalent interactions to construct supramolecular nanoporous architectures since they are highly selective and directional [16]. H-bonds are formed when a donor with an available acidic hydrogen atom interacts with an acceptor that carries available nonbonding electron lone pairs. The strength of the H-bond depends mainly on the solvent and number and sequence of the H-bond donors and acceptors. Various supramolecular polymer materials have been developed which use H-bonds as structural element to position molecules. After removal of these molecular templates, a porous material is obtained to fabricate molecule specific systems. 

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