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.

Getting back to the metal catalysts, let’s start by considering just a metal atom such as titanium that is able to catalyze the polymerization of propylene. The problem with titanium and pretty much any other metal is that they have too many orbitals sticky out in too many directions. They react with anything that comes near regardless of the angle of attack. They are gluttonous monsters, the kind that we are all well-advised to keep away from their mouths. For polymerization, this gluttony can lead to problems. While polymerizing propylene for instance, a non-specific catalyst will randomly orient the propylene molecules as the chain builds, resulting in atactic material.

So how can we take advantage of the catalytic properties of a metal, but make sure that the metal isn’t a gluttonous monster? By starting to tie up the various orbitals with ligands. This was first done by using chlorine atoms to make TiCl4. That was a great start, so great as to allow a couple of chemists, Karl Ziegler and Guilio Natta to have dinner with the King of Sweden on a winter’s night in December, 1963. Without these catalysts, polypropylene would still be just a laboratory curiosity.

But improvements could be made to these catalysts as the chlorine atoms just floated around the central titanium atom too much. In the 1980’s, metallocenes entered the party. Compare the structure of TiCl4 and that of a representative metallocene catalyst:

 

Note that the aromatic ligands are not based on cyclopentadiene, but indene instead. And what’s more, there is an ethylene bridge (called an ansa group, ansa being the Greek word for handle) between the two aromatic ligands to stabilize their position relative to each other. The end result is a catalyst that is extremely limited in a geometric sense as to how it can be used to polymerize olefins – truly a picky eater and not a glutton. Yet surprisingly, despite the geometric limitations, the rate at which polymerizations occur is much faster than with the older Ziegler-Natta catalysts.

But more the speed, the outcome of all of this is greater control over the polymerization process. The polymers have a much narrower distribution of molecular weights and the crystallinity can be higher as well. Also, the ability to micromanage the introduction of comonomers is better, which means that the mechanical properties of the polymer can be tailored in ways that are otherwise impossible to achieve.

This is a very brief introduction to this subject as there are cocatalysts that were never discussed, and the details of the reaction mechanism, and the various generations of the catalyst over time and… My point here was to show the usefulness of these visually striking sandwich molecules. Being catalysts and not endproducts, they are little known and seldom appreciated by the chemistry community.

For more details, I recommend the following:

The Polymer Science Learning Center at the University of Southern Mississippi has a great page on metallocenes that is approachable to anyone who has studied organic chemistry. (The entire Macrogalleria website in fact is terrific.)

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