Ion pairing in transition metal catalyzed olefin polymerization

Ion pairing is known to play a key role in stoichiometric and catalytic reactions mediated by charged transition metal complexes, especially when they are conducted in low-polar solvents. Molecular olefin polymerization catalysis by group IV metal complexes represents a textbook case in this respect, due to the high electrophilicity of the involved cationic active species and the very low relative permittivity (~ 2) of the commonly used hydrocarbon solvents. As a matter of fact, ion pairing has a huge impact on activity and selectivity of olefin polymerization catalysts. Ion pairing is strictly related to the activation process, since the cationic active species is usually generated by the reaction of a neutral precursor with a suitable cocatalyst (activator). The counterion generated from the latter reaction has to be weakly coordinating in order to disfavor its competition with the olefin for the occupancy of the coordination vacancy of the active metal center. Herein, after having recalled some generalities of olefin polymerization and nature of activators, using ion pairing as a reading key, the main findings concerning with the structure and self-aggregation tendency of ion pairs relevant to olefin polymerization, as deduced by means of advanced NMR techniques, are described. Representative examples of structure/activity relationships are then discussed, suggesting some rationale to interpret the different catalytic behavior of various ion pairs. Finally, cases in which the activation process, besides providing the expected active ion pair, leads to further layers of complexity (in situ ligand modification, dimer formation, etc.) are illustrated.