Guest-host polymers

We mentioned in the previous section that the bulk NLO properties of a poled-polymer host-guest system might be expected to be proportional to the chromophore fi/3 multiplied by the number density of chromophores. However, for high loadings of molecules with large pi, the d33 and r33 coefficients do not continue to increase linearly with chromophore loading, but exhibit maxima these are attributed to competition between the interaction of the chromophores and the poling... 

Much attention has recently been focused on organoboronic acids and their esters because of their practical usefulness for synthetic organic reactions including asymmetric synthesis, combinatorial synthesis, and polymer synthesis [1, 3, 7-9], molecular recognition such as host-guest compounds [10], and neutron capture therapy in treatment of malignant melanoma and brain tumor ]11]. New synthetic procedures reviewed in this article wiU serve to find further appHcations of organoboron compounds. 

G. Vamvounis and S. Holdcroft, Enhancing solid-state emission from conjugated polymers via self-forming host-guest systems, Adv. Mater., 16 716-719, 2004. 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

Very recently a new method was developed that opens the possibility to polymerize even hydrophobic monomers in aqueous solution. This method is based on the finding that hydrophobic monomers can be made water-soluble by incorporation in the cavities of cyclodextrins. It has to be mentioned that no covalent bonds are formed by the interaction of the cyclodextrin host and the water-insoluble guest molecule. Obviously only hydrogen bonds or hydrophobic interactions are responsible for the spontaneous formation and the stability of these host-guest complexes. X-ray diffraction pattern support this hypothesis. Radical polymerization then occurs via these host-guest complexes using water-soluble initiators. Only after a few percent conversion the homogeneous solution becomes turbid and the polymer precipitates. 

Arshady R, Mosbach K. S3mthesis of substrate-selective polymers by host-guest polymerization. Maktomol Chem 1981 182 687-692. 

If one polymer has much higher molecular weight than the other, a model host-guest is commonly applied (Figure 9.8). Smaller guest molecules are absorbed on the host molecule. Because hydrophobic interactions take place between created blocks, the molecule of the complex becomes more compact. Similar intermolecular interactions can lead to precipitation. It seems probable that similar process takes place at the very be-... 

In view of the potential technological importance of noncentrosymmetric organic crystals, several approaches have been evolved to artificially achieve noncentrosym-metry, which include electric field poling of polymers, self-assembly of molecular layers, Langmuir-Blodgett assembly of films and host-guest interaction in noncentrosymmetric hosts (Marder et al, 1994). Prediction and/or control of the three-dimensional structure of crystals, given only the information of molecular properties, however, remains difficult at present. 

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