Microstructure of ring-opened polymers and copolymers of norbornadiene

Abstract

A variety of homopolymers of norbornadiene and copolymers with norbornene have been synthesized using a range of olefin metathesis catalysts. By employing hex-1-ene as chain-transfer agent, the molecular weight of these materials has been regulated to levels that permit high solubilities in suitable solvents. As a result, high-quality 13C and 1H NMR spectra of these poly(norbornadienes) have been obtained of a quality hitherto observed only for analogous poly(norbornenes). These spectra have been fully assigned and the detailed microstructure of the homopolymers and copolymers established. One of the most significant observations is that OsCl 3 as catalyst affords essentially all- cis polymers of norbornadiene, in contrast to norbornene, where the cis content is <50%. Furthermore, the copolymer of the monoene and diene is also high- cis and compositionally random, a result which indicates that the metallacarbene propagating complex holds one norbornadiene molecule as a diendo chelating ligand, which remains intact as a spectator species throughout the reaction. The steric bulk of this extra ligand then constrains the incoming monoene or diene monomer to form metallacyclobutanes in the cis mode. In some cases, for example RuCl 3, this chelating effect is so dramatic that it effectively renders the catalyst almost inert.