MAUREEN ROUHI
Facile access to cyclic polyethylenes has been realized with a cyclic ruthenium polymerization catalyst. The work could lead to new materials and may help solve some unanswered questions in polymer science.
Robert H. Grubbs, a professor of chemistry at California Institute of Technology, and graduate students Christopher W. Bielawski and Diego Benitez have prepared high-molecular-weight cyclic polyethylenes through ring-opening metathesis polymerization of cis-cyclooctene, catalyzed by a cyclic ruthenium complex [Science, 297, 2041 (2002)]. The reaction forms a cyclic polyoctenamer, which readily precipitates after addition of acetone or methanol and is isolated simply by filtration. Hydrogenation of the product yields cyclic polyethylenes.
Using this method, the Caltech chemists have prepared cyclic polyethylenes with molecular weights of up to 1,200 kilodaltons. And using various characterization techniques, they have shown that the physical properties of the cyclic polymers are distinct from those of linear analogs.
Ring polymers are not new, but they have been difficult to prepare. Typically, they are made by intramolecular cyclization of linear precursors at very low concentrations or by shifting certain polymerization reactions to favor cyclic products. With either route, ring closure is usually incomplete, side reactions are common, and linear impurities must be removed by elaborate procedures. Neither route easily applies to high-molecular-weight cyclic polyethylenes.
How to make cyclic polyethylenes easily posed “an interesting question,” Grubbs says. The key, he adds, is using not only a cyclic monomer, which has no ends, but also a cyclic—endless—catalyst.
With cyclic polyethylenes at hand, Grubbs says, “we can now answer the fundamental question: Does a cyclic polymer behave like the linear version even when you get to really high molecular weights? And the answer is, no. Whether that translates into practical application is for somebody else to decide. We’ve got the material; now people can test it.”
