Ion Pair Dynamics in Metallocene Catalysts

The anion dissociates, and the coordinatively unsaturated metal center then picks up a monomer molecule for subsequent enchainment. This dissociative model has been favored in the past [16, 21-23, 27-28] since it allows a convenient explanation of the observed polymer stereochemistry by considering only the roles of the ligand and the alkyl chain in the cationic metallocene complex. However, anion dissociation opposes the electrostatic attraction between cation and anion and is therefore energetically expensive. So does it operate at all  

Even if anion dissociation can be shown to be prevalent, there will be a strong tendency for anion binding. If anion re-association is slow, many monomer units may be consumed and polymerised before anion binding interrupts the process. On the other hand, if enchainment is slow, anion recombination can be expected to occur after each insertion step, to form a possibly long-lived resting state. Which mechanism applies  

These suggestions have not been without criticism, the objection being that these studies were carried out at significantly higher concentrations than are typically employed under catalytic conditions and are therefore probably not relevant to 

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