Chemistry

ROMP with Restraint

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Science  12 Aug 2005:
Vol. 309, Issue 5737, pp. 991
DOI: 10.1126/science.309.5737.991b

Ring-opening metathesis polymerization (ROMP) yields a versatile range of linear polymers from cyclic olefin starting materials. The reaction is driven by relief of the bonding strain inherent to the geometry, as a metal catalyst pries open the monomer rings and stitches them together one by one. Whereas molybdenum and tungsten catalysts are more active, ruthenium compounds can react selectively with a C=C bond in the presence of many other groups, such as ketones and esters. The tradeoff for such a tolerant catalyst is reduced reactivity toward low-strain rings, such as cyclopentene and cycloheptene, which are appealing substrates because they can be functionalized symmetrically to yield regioregular polymers.

Hejl et al.show that by careful tuning of catalyst and monomer concentrations, some of these rings can be coaxed open by Ru-based systems. In particular, they achieve >80% yields for polymerization of the unsubstituted 5- and 7-membered cyclic alkenes, and >60% yields for several ketone- and ester-substituted variants. The authors used density functional theory to calculate the strain in each monomer variant and found that the threshold for Ru-catalyzed ROMP is a minimal strain of 3.4 to 4.4 kcal/mol. — JSY

Macromolecules 10.1021/ma0501287 (2005).

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