Reversible supramolecular interactions

Block co-polymer which use reversible supramolecular interactions like hydrogen bonding can form materials with interesting properties. As the interactions are reversible, more control can be exercised and the properties can be minutely controlled. Meijer and coworkers have designed an ureidotriazine (UTr) based systems, which on combination with poly (ethylene/butylenes) give rise to rod-coil systems [14]. 

Reversible supramolecular interactions in polymers can be achieved via several mechanisms, such as hydrogen bonding, exemplified by the ure-idopyrimidinone unit (Bosman et al, 2004 Montarnal et al, 2009), metal coordination chemistry (Fiore et al, 2011 Kumpfer et al, 2010), or k-k stacking, such as that seen with triphenylene units (Buratini et al, 2009). These materials can heal autonomously in a few cases, but in most cases also need stimuli for their healing action such as temperature or pressure. 

Figure 1.2 A schematic representation of the versatility of reversible, supramolecular side-chain modification and selected examples of interactions that can be employed.

The selection of one or more components occurs as function of either internal (the nature and the geometry of the binding subunits, the stoichiometry, etc.) or external factors (nature of the solvent, the presence of specific molecules or ions, etc.). In view of the lability of the reversible molecular and supramolecular interactions (H-bonding, van der Waals, coordinative bonds, etc.) the self-assembly processes may present a number of novel features such as cooperativity, diversity, selection, or adaptation. 

Immobilization of catalysts via supramolecular interactions may open new possibilities for the use of catalysts that were originally developed in homogeneous form, which can then be applied in their immobilized form to facilitate separation of the catalyst and product (III). One possibility is permanent immobilization on an insoluble or soluble support. A second possibility is to use a catalyst in a homogeneous liquid phase and separate it from the solution by supramolecular interactions with a solid support, preferably in a reversible fashion. Both avenues were explored by Reek et al. (II2). 

The interesting polymer-solubility behaviour led us to compare this phenomenon with classical LCST effects. In our case, because of the reversible complex formation between the polymer P20 and CD, the optical effect is based on supramolecular interactions. This means that the discovered pseudo-LCST behaviour is a result of non-covalent interactions between the CD host and polymer guest. Furthermore, in this system competitive inhibition or control of the LCST is possible by addition of other suitable guest molecules of low molecular weight, for example, potassium 1-adamantylcarboxylate. CD complexes these molecules preferably and the polymer precipitates. This special effect cannot be observed in regular LCST systems. 

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... 

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