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 c omplex. The chain initiation starts with coordination of ethylene to t he active catalyst [L2PdCH3](+), followed by ethylene migratory insert ion into the Pd-alkyl band to form a gamma-agostic intermediate, which rearranges to a more stable beta-agostic form. The chain propagation starting with the beta-agostic complex 7 can proceed via pathway A thr ough coordination, migratory insertion, and rearrangement, leading to linear polyethylene, or pathway B, through beta-hydride elimination, o lefin 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 c omplexes are the resting states in the catalytic cycle. Associative di splacement, where the coordinated polymer is associatively replaced fr om the same diimine-Pd-hydrido-olefin complex 7 by ethylene in solutio n, is likely to be a preferable chain transfer pathway.