Double ethene insertion

For a series of model acyl of the type [Pd(COMe)(C2H4)(P - P)]+ it has been found that the insertion of ethene into the Pd-acyl bond with formation of a /3-chelate (Eq. 20) is irreversible and that the energy barrier is ca. 12 kcal/mol [52,55,56]. From thermodynamic and kinetic data, Schultz et al. calculated that the insertion of ethene into a Pd-alkyl bond (double ethene insertion) could occur every ca. 105 CO insertions into the same bond [52], which accounts for the strict alternating chain growing. 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

C-NMR spectroscopy showed that cyclopentene is incorporated in the copolymer chain through a 1,2-insertion, without ring-opening metathesis. This is in contrast to the homopolymerization of cyclopentene where 1,3-insertion was observed. The 1,2-enchainment by only double bond opening is a result of an easy coordination to the zirconium center when the last insertion was an ethene unit. The p-hydride elimination of a cyclopentene unit at the end of the growing chain, which is needed to form a 1,3-enchained cyclopentene unit, is relatively slow compared to a next ethene insertion. Therefore, it is... 

To investigate this idea, which would confirm and summarize a large class of widely accepted mechanisms, the insertion reactions of (Z)- and (E)-butene with catalytic systems based on the C2-symmetric Me2Si(l-Ind)2 ligand and on the ((-symmetric Me2Si(Cp)(9-Flu) ligand have been recently studied. A double approach based on combined quantum mechanics/molecular mechanics (QM/MM) techniques and on selected ethene/2-butene copolymerization runs has recently been utilized.94... 

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