Catalyst Keeps Polymers Short and Sweet

The plastics industry depends on catalysts to link tens of thousands of identical chemical groups into the polymer chains that are the basis of plastics. Now chemists have created a new catalyst with a talent for keeping those chains short to make industrially vital molecules known as alpha-olefins, which serve as a feedstock for making everything from soaps and detergents to garbage bags. The new catalyst, described in today's issue of the Journal of the American Chemical Society, does its job with less waste and under simpler conditions than currently used catalysts.

The new molecule belongs to a class of catalysts known as metallocenes, in which a central metal atom is surrounded by a pair of five-membered carbon rings that can themselves be linked to other groups. Together, the rings and dangling groups force polymer building blocks to bind to the metal in a particular orientation that creates a well-ordered chain. In most metallocene catalysts, this chain growth continues on and on until the metal atom manages to yank an atom, usually a hydrogen, completely off the polymer, allowing the polymer to break free of the metal and stopping its growth. However, the chain is normally snipped only after the polymer has grown to great length.

A team from the University of Rochester in New York--including chemists Guillermo Bazan, Jonathan Rogers, and Caroline Sperry--sped up the chain-snipping process by boosting the central metal's hydrogen-swiping ability. To do so, the researchers inserted a boron atom into each of the catalyst's carbon rings. To each boron they then attached oxygen atoms linked to other small hydrocarbon groups. The borons, as it turns out, end up snatching electrons from the metal atom in the center--in this case zirconium--leaving it starved of electrons. As a result, the zirconium atom tries even harder to swipe a hydrogen from the growing polymer chain. It usually succeeds quickly, causing the polymer chain to break off almost as soon as it starts growing, thereby creating the short-chain alpha-olefin.

"It's some very nice work," says Richard Kemp, a chemist at Union Carbide in Houston. He adds that besides producing alpha-olefins more efficiently, the new metallocene works at ambient pressure, whereas current catalysts require high pressures and expensive, high-strength reactors. The new catalysts are not ready for the factory floor just yet, however. Unlike the current variety, the new metallocene catalysts are difficult and expensive to produce, at least for now.