New Neutral Nickel and Palladium Sandwich Catalysts: Synthesis of Ultra-High Molecular Weight Polyethylene (UHMWPE) via Highly Controlled Polymerization and Mechanistic Studies of Chain Propagation

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 Mn values up to 4.1 × 106 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.