Control of cis-selectivity and tacticity in ring opening metathesis polymerization using ruthenium metathesis catalysts

A series of ruthenium metathesis catalysts contg. a cyclometalated N- heterocyclic carbene (NHC) ligand were examd. in the ring opening metathesis polymn. (ROMP) of norbornene- and norbornadiene- derived monomers. In general, the resulting polymers were found to be highly cis with syndiotactic- biased cis and trans regions, while blockiness calcns. showed that trans double bonds occurred randomly throughout the polymers. These structural trends suggest that the controlling factor in cis selectivity and tacticity is the combined influence of the stereogenic ruthenium center and the steric environment surrounding the alkylidene on monomer approach. These conclusions are further supported by preliminary computational studies, which are ongoing and will also be discussed. It is expected that these results will provide invaluable insight into the mechanism and mode- of- action of cyclometalated ruthenium metathesis catalysts and will be instrumental in the design of future catalysts for cis- selective metathesis transformations