Polymerization of Cyclodextrin Host-Guest Complexes in Water

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. 

While the polymer precipitates, the cyclodextrin stays in solution and can be reused again as host-molecule. 

Variations of this method are possible in several ways. First of all, cyclodextrin which is available on a large scale by enzymatically catalyzed modification of starch can be tailored by chemical reactions. Furthermore, copolymerizations between different host-guest complexes are possible whereby in some cases the reactivity ratios differ from those reported in literature. 

Cyclodextrin can also be used in order to stabilize monomers that would otherwise oxidize upon contact with air. The cyclodextrin host shields the monomer from oxygen in the air. Monomers such as pyrrole can be stored as a cyclodextrin complex for months without any noticeable degradation. The complex is a colorless powder that does not change color over time unlike pure pyrrole, which would oxidize and therefore turn black via yellow. Another advantage of the method is the fact, that the complex is odorless whereas pyrrole itself has an unpleasant smell. 

In general, polypyrrole can be prepared via electrochemical or chemical oxidative polymerization of pyrrole involving different highly reactive intermediates. Also, the pyrrole/cyclodextrin complex can be polymerized in aqueous solution under oxidative conditions by adding potassium peroxodisulfate as an ox- 

Example 3.14a Free Radical Polymerization of Cyciodextrin Host-Guest Complexes of Butyl Acrylate from Homogeneous Aqueous Solution (Precipitation Polymerization) 

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