Computed Propagation and Termination Steps in [(Cycloocta-2,6-dien-1-yl)RhIII(polymeryl)]+ Catalyzed Carbene Polymerization Reactions

This paper discloses the DFT-computed pathways for chain propagation, chain transfer, and chain termination during carbene polymerization catalyzed by cationic [(cycloocta-2,6-dien-1-yl)­RhIII(alkyl)]+ species. In contrast to carbene polymerization calculated for neutral [(cod)­RhI(alkyl)]+ catalysts, chain propagation at the cationic [(cycloocta-2,6-dien-1-yl)­RhIII(alkyl)]+ species is clearly competitive with β-hydride elimination, thus explaining the formation of high molecular weight polymers. Computed chain-end-controlled chain propagation reveals a clear preference for syndiotactic polymerization. Chain transfer involving alcohol-mediated protonolysis is computed to be a more favorable pathway than β-hydride elimination. These results are all in agreement with experimental observations. Chain propagation from species with a stereoerror at the α-carbon atom of the growing chain is substantially slower compared to propagation from syndiotactic species without stereoerrors, providing a possible explanation for the experimentally observed low initiation efficiencies of the Rh catalysts in carbene polymerization reactions. These new computational insights, combined with experimental results disclosed in earlier reports, largely clarify the mechanism of Rh-mediated carbene polymerization reactions.