Polymers reinforced by covalently bonded inorganic clusters

Organically modified transition-metal oxide clusters (OMTOCs) are structurally well-defined nanosized building blocks for the preparation of a new type of inorganic-organic hybrid materials. There are two principal methods to synthesize OMTOCs with polymerizable organic groups attached to their surface: the groups can either be grafted to a preformed cluster (surface modification method) or introduced during the cluster synthesis (in situ method). The advantages and disadvantages of both methods are discussed. OMTOCs with polymerizable surface groups can be polymerized with or without an organic copolymer to yield polymers which are-in most cases-highly cross-linked by the inorganic constituents. The structural integrity and the properties of the clusters are retained upon incorporation into the polymers. Because there are hitherto only a few examples of cluster-reinforced polymers, restricted to tungsten, tin, titanium, and zirconium oxide clusters, the article reviews the early stages of an interesting new approach to inorganic-organic hybrid polymers and highlights possibilities for future materials syntheses.