Precipitated silica polymer applications

The use of particulate fillers in polymers has a long history, and they continue to play a very important role today. Despite the apparent commodity status, the area is still very dynamic and considerable changes have occurred in applications over the last few years. The most significant of these have been the dramatic growth in the use of precipitated silica in energy efficient tyre applications and the emergence of nano-filler technologies. 

Extremely small particles are not that difficult to prepare, and have been known for a very long time, usually prepared in the forms of sols or smokes (fumes). The challenge is to isolate them in a form that is easily handled and readily dispersible. Some type of agglomeration/aggregation is generally used to isolate the particles, but this is usually partly irreversible, resulting in products that cannot be redispersed into a polymer in their nanoparticle form. Carbon blacks and most fumed and precipitated silicas are examples of such products. One way around this is to form the particles directly in the polymer, or in a liquid monomeg but this has restricted applicability, partly because there are usually coproducts from the particle formation process to deal with. It is only recently that sufficient commercial incentive has been identified to focus real attention on this problem. 

Soluble dendrimers bearing catalytic centers located at the periphery can be covalently attached onto the surface of conventional solid supports (such as polymer beads or silica gels), leading to another type of solid-supported dendrimer catalyst. It is expected that this type of immobihzed catalysts would combine the advantages of both the traditional supported catalysts and the dendrimer catalysts. First, the catalytically active species at the dendrimer surface are more easily solvated, which makes the catalytic sites more available in the reaction solutions (relative to cross-hnked polymers). Second, the insoluble supported dendrimers are easily removed from the reaction mixtures as precipitates or via filtration (relative to soluble dendrimers). These solid-supported peripheraUy functionalized chiral dendrimer catalysts have attracted much attention over the past few years [12, 113], but their number of applications in asymmetric catalysis is very limited. 

---