2020/04/01 by Quan H. Tran, Maurice Brookhart, Olafs Daugulis · 1 citation
Chemistry · #Organometallic Complex Synthesis and Catalysis #Asymmetric Hydrogenation and Catalysis #Catalytic Cross-Coupling Reactions
paper · doi:10.1021/jacs.0c02045
openalex publication_date 2020/04/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30
New neutral nickel and palladium ethylene polymerization catalysts have been prepared that incorporate an anionic ( N,O ) chelating ligand. Extensive axial shielding is provided by two 3,5-dichloroaryl moieties in a “sandwich” orientation. Such shielding results in an exceptionally slow rate of chain transfer relative to migratory insertion in the nickel catalyst, and thus highly controlled polymerization of ethylene is observed, leading to lightly branched ultra-high molecular weight polyethylene with M n values up to 4.1 × 10 6 g/mol. The analogous palladium catalysts provide the means for a detailed mechanistic study of chain propagation in an electronically asymmetric neutral palladium catalyst. Both isomers of the methyl ethylene complex can be generated and observed at low temperatures allowing experimental elucidation of mechanistic details of chain propagation probed in other electronically asymmetric systems only through DFT studies or by examination of model studies. The barrier to migratory insertion in these complexes is ca. 19.2 kcal/mol. Investigation of the equilibration of the methyl ethylene isomers in the presence of excess ethylene showed the isomerization rate is dependent on ethylene concentration. This is the first direct proof that isomerization in these alkyl ethylene intermediates is catalyzed by ethylene. Furthermore, isomer equilibration is much faster than migratory insertion so that the barriers for insertion of individual isomers cannot be determined.