Polymers from functional clusters

An emerging sub-class of inorganic-organic hybrid materials is polymers which are crosslinked by inorganic clusters. To prepare such materials, clusters with functional organic ligands are polymerized with organic co- monomers. In previous work we were using carboxylate-substituted titanium and zirconium oxo clusters for the crosslinking of polymers, i.e. clusters having no inherent physical properties. The clusters influence the properties of the resulting hybrid materials by a combination of nanofiller and crosslinking effects. In the proposed project, we want to add a third dimension to the properties by extending this work towards clusters which additionally provide special magnetic or optical properties to the hybrid polymers due to their inherent physical properties. The resulting materials will thus combine the properties of organic polymers, such as processibility, elasticity etc. with that of the inorganic nanobuilding blocks. The first part of the project therefore consists of preparing functional clusters with functional (polymerizable) ligands. We will concentrate on three classes of clusters, viz. iron clusters representing magnetic clusters, zinc and cadmium sulfide clusters which may supplement interesting optical properties to the hybrid materials, and the largely unexplored lanthanoid clusters, which have special optical and/or magnetic properties. We mainly want to modify known cluster types in a way that functional ligands are introduced. This will be done by several methods and for several types of ligands. The use of new ligand types allows introducing new functional organic groups to the cluster surface and thus polymerization methods which were not applicable with the carboxylate-substituted clusters. The hybrid polymers will be prepared by polymerizing mixtures of the appropriately substituted functional clusters and organic co-monomers. We will mainly employ radical polymerizations, but also step-polymerizations, i.e. to obtain polyester or polyamide-based hybrid polymers. Major issues for the characterization of the polymers are the distribution of the clusters, properties due to the filler and crosslinker effect, such as swelling behavior, thermal stability or mechanical properties, as well the properties introduced by the clusters, i.e. magnetic or optical properties. The use of functional clusters for the preparation of hybrid polymers thus has several interwoven aspects that provide a special challenge compared with our previous work: to employ new cluster types as nanosized building blocks, the use of other types of ligands to introduce polymerizable surface groups, the influence of the functional/functionalized clusters on the polymerization reactions and new polymerization options.

Molecular clusters are compounds which contain more than two metal atoms which, in most cases, are connected by non-metallic bridges, such as oxygen or sulfur atoms (as in the current project). The surface atoms of the clusters must be covered by organic groups (ligands) to stabilize the clusters and prevent their growth to larger entities. We are using these ligands to introduce organic functionalities which allow polymerizing the clusters in the presence of organic co-monomers. This results in inorganic-organic hybrid materials with clusters as the inorganic component, where the properties of the organic polymer and that of the clusters are combined. Special focus in the current project was on clusters with special physical properties, such as optical or magnetic properties. We selected iron oxide, cadmium sulfide and yttrium oxide clusters to study some fundamental aspects connected with the use of such clusters in polymerization reactions. The most important issue is the cluster stability during the polymerization reactions. The cluster core must be retained, and the ligands should not undergo uncontrolled exchange reactions. Otherwise materials with unpredictable and non-reproducible structures and properties are obtained. We found that nitrogen-based ligands, such as pyridine derivatives, result in iron oxide clusters that are stable in solution and can be polymerized under certain conditions. Radical polymerization in pyridine solution with small cluster proportions resulted in hard polymers in which the employed cluster was retained. In the case of cadmium sulfide clusters we did not succeed in finding possibilities to prevent cluster rearrangement. When dissolved in organic solvents (which is necessary for the polymerization reactions), the clusters grow and eventually form cadmium sulfide nanoparticles. We achieved, however, to elucidate the mechanism of the cluster growth and to isolate a few large clusters formed as intermediates. Some new yttrium oxo clusters were isolated and structurally characterized, which could be suitable for future modification by polymerizable ligands.