A DENSITY-FUNCTIONAL AND MOLECULAR MECHANICS STUDY OF BETA-HYDROGEN TRANSFER IN HOMOGENEOUS ZIEGLER-NATTA CATALYSIS

Models of catalytic complexes, of the type Cp(2)ZrC(2)H(5)(+) and Cp(2 )ZrC(4)H(9)(+) (Cp = eta(5)-C5H5), for the P-hydrogen transfer process es in homogeneous Ziegler-Natta polymerizations have been studied usin g density functional methods. We investigated the geometries and the e nergetics of the processes corresponding to the P-hydrogen transfer ei ther to the metal or to the monomer. A preference for the transfer to the monomer is shown by the noticeably smaller activation barrier. The analysis has been extended through molecular mechanics calculations t o models of the catalytic complexes based on the ligands BenzInd and M eBenzInd (BenzInd = (CH3)(2)Si(benz[e]indenyl)(2) and MeBenzInd = (CH3 )(2)Si(2-methylbenz[e]indenyl)(2)). This analysis, in agreement with e xperimental results, indicates that the beta-hydrogen transfer to the monomer is more difficult in the presence of the MeBenzInd ligand due to the nonbonded interactions of the 2-methyl substituents with the C- alpha of both the monomer and the growing chain.