Probing the Stereoselectivity of the Ruthenium-Catalyzed Ring-Opening Metathesis Polymerization of Norbornene and Norbornadiene Diesters

The ring-opening metathesis polymerization (ROMP) of two optically active 2,3-dicarboalkoxynorbornadienes derived from ( S )-(−)-2-methyl-1-butanol and ( R )-(−)-2-butanol were carried out in the presence of the [RuCl 2 ( p- cymene)] 2 catalyst precursor activated by trimethylsilyldiazomethane (TMSD). 1 H and 13 C NMR analyses showed that a high degree of stereoregularity was achieved, and homonuclear proton−proton COSY spectroscopy indicated that the major trans fractions of the polymers were most likely isotactic, while the minor cis fractions were syndiotactic. Ring-opened metathesis polymers were also made from 2,3-dicarbomethoxynorbornadiene and exo,exo -2,3-dicarbomethoxy-5-norbornene. They were hydrogenated into the corresponding polynorbornanes using diimide. The NMR spectra of the reduced materials confirmed that the unsaturated parent polymers had an all-trans highly isotactic microstructure. A tentative mechanism involving arene loss, carbene formation, and monomer chelation is proposed to account for the observed stereoselectivities.