Surface-modified Zirconium Oxide Clusters and their Use as Components for Inorganic-Organic Hybrid Materials
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Surface-modified Zirconium Oxide Clusters and their Use as Components for InorganicOrganic Hybrid Materials Yu Gao, Denise Silvia Dragan, Myhedin Jupa, Franz René Kogler, Michael Puchberger and Ulrich Schubert Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria ABSTRACT Pre-formed transition metal oxide clusters with methacrylate ligands were polymerized in the presence of organic co-monomers to form inorganic-organic hybrid polymers. The number of polymerizable ligands and thus the crosslinking density of the hybrid polymers can be varied for a given cluster type by partially exchanging the methacrylate ligands for non-reactive ones. The properties of the hybrid polymers can also be varied by adjusting the conditions of the polymerization reaction, especially by a pre-polymerization procedure. It is shown that the thermal stability of polystyrene crosslinked by Zr6O4(OH)4(OMc)12 is thus greatly improved.
INTRODUCTION The concept of inorganic-organic hybrid materials exploded with the birth of soft inorganic chemistry processes, where mild synthetic conditions allow versatile access to chemically designed hybrid materials by the combination of organic and inorganic groupings on a molecular or nanometer scale. Our approach to a new type of inorganic-organic hybrid materials is a two-step process. In the first step, structurally well defined transition metal oxide clusters are prepared by carefully controlled hydrolysis and condensation of methacrylate- or acrylate-substituted metal alkoxides, where the water is generated in situ by an esterification reaction. Depending on the employed methacrylic acid/metal alkoxide ratio, clusters of different size and shape crystallize in almost quantitative yields from the reaction mixtures [1]. The pre-formed methacrylate-substituted clusters, as structurally well-defined nanosized building blocks, are then reacted with organic co-monomers in free radical polymerizations. The resulting polymers are class II hybrid materials [2], i.e. the organic and inorganic entities are connected by covalent bonds. Several methacrylate-substitute transitional metal oxide clusters of the general formula MaOb(OH/OR)c(OMc)d (OMc= methacrylate) were prepared by this route. The polymerization of Zr6(OH)4O4(OMc)12 (Zr6), Zr4O2(OMc)12 (Zr4), Ti6O4(OR)8(OMc)8 and Ti4O2(OR)6(OMc)6 with e.g. methyl methacrylate (MMA), styrene or methacrylic acid, and some of the properties of the resulting hybrid polymers were previously reported [3,4]. The properties not only depend on the cluster/organic monomer ratio, as expected, but also on the kind of the employed cluster [4]. In this article, we describe two other possibilities to tailor the properties of the hybrid polymers:
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(i) the variation of the number of polymerizable groups for a given cluster type, and (ii) the optimization of the conditions of the polymerization reaction. This is mainly discussed for the zirconium oxide cluster Zr6(OH)4O4(OMc)12 (Zr6) as an example.
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