"Probing the Stereoselectivity of the Ruthenium-Catalyzed Ring-Opening Metathesis Polymerization of Norbornene and Norbornadiene Diesters"
Laurence Beaufort, Lionel Delaude, Albert Demonceau, and Alfred F. Noels
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source: e-Polymers
year: 2003
number: P_010
doi: not available
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Abstract:Various NMR techniques have been applied to probe the stereoregularity of polymers prepared by Ring-Opening Metathesis Polymerization (ROMP) of 2,3-dicarboalkoxy-norbornenes and -norbornadienes catalyzed by the [RuCl2(p-cymene)]2 dimer in the presence of TMSD. Direct methods based on the examination of cross-coupling peaks in polymers derived from optically active monomers and indirect methods based on the spectroscopic analysis of polynorbornanes obtained by hydrogenating the parent polyolefins lead to the same conclusion. They both show that high trans, highly isotactic polymers are formed using the ruthenium-arene catalyst precursor. The precise nature of the alkoxy side-chains determines to a large extent the degree of stereocontrol that can be achieved. With monomers bearing small ester groups the ring-opening polymerization proceeds stereospecifically and affords macromolecular products with 100% trans exocyclic double bonds and highly isotactic sequences. An increase in the steric bulkiness of the alkoxy substituents results in the progressive emergence of cis double bonds, together with a loss of tacticity. Such an evolution strongly suggests that ester coordination to the metal center is of prime importance to the catalytic cycle. Chelation of the ruthenium active sites by the alkoxy groups of the monomer most likely occur to make up for the thermal displacement of the p-cymene ligand upon activation. A tentative mechanism involving arene loss, carbene formation, and monomer chelation is proposed to account for the observed stereoselectivities. To date, the exact nature of the active species generated in situ upon activation of the ruthenium-arene dimer [RuCl2(p-cymene)]2 by a diazo compound co-catalyst and a chelating diester monomer remains, however, elusive.
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