Density Functional Study of the Mechanism of the Palladium(II)-Catalyzed Ethylene Polymerization Reaction

The hybrid density functional B3LYP method has been used to study the mechanism of ethylene polymerization catalyzed by the Pd(II) diimine complex. The chain initiation starts with coordination of ethylene to the active catalyst [L2PdCH3]+, followed by ethylene migratory insertion into the Pd−alkyl bond to form a γ-agostic intermediate, which rearranges to a more stable β-agostic form. The chain propagation starting with the β-agostic complex 7 can proceed via pathway A through coordination, migratory insertion, and rearrangement, leading to linear polyethylene, or pathway B, through β-hydride elimination, olefin rotation, and reinsertion to give a branched alkyl intermediate, which can then coordinate ethylene and produce branched polyethylene. The rate-determining step is olefin insertion, and the olefin alkyl complexes are the resting states in the catalytic cycle. Associative displacement, where the coordinated polymer is associatively replaced from the same diimine−Pd−hydrido−olefin complex 7 by ethy...