Post Tagged with: "metallocenes"

Sandwiches, Gluttons and Picky Eaters

This post is contributed by John Spevacek, an industrial polymer chemist and the author of the blog “It’s the Rheo Thing

Quintus guest-blogged recently on that iconic sandwich molecule, ferrocene, an iron atom sandwiched between two cyclopentadiene rings. Ferrocene is the first discovered and best known of a broader class of molecules called metallocenes, molecules in which a metal atom is sandwiched between two aromatic ligands (not necessarily cyclopentadienes). The applications of ferrocene at present are rather limited, but that is not the case with metallocenes. I thought I would expand on this subject by showing the particular usefulness of these molecules – the metallocenes – to polymer chemistry. Most people, including chemists, have little idea how important these molecules are to their everyday life. The molecules themselves are not polymerized, but instead are catalysts for the polymerization of olefins such as ethylene and propylene.

 Before we can get into the reaction details, I first need to explain for the stereochemistry of polymers and why it is import. In a isotactic polymer, all the monomers have been added to the chain in the same orientation:

while in an atactic polymer, the orientation is random:

This stereochemistry is critical to the mechanical properties of a polymer. Atactic propylene is easy to make, but is a pile of goo that you can use as a pretty bad adhesive and not much else. The isotactic version however, can crystallize and that then builds the strength of the material. Crystalline polypropylene is a good strong material that we use every day in food packaging, dishwasher safe food containers, carpeting, nonwoven fabrics, ropes and hundreds of other uses.

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