Ethylene polymerization catalyzed by group 4 metal complexes of tridentate heteroscorpionate ligands

Considerable attention has been recently devoted to group 4 metal complexes of N,O polydentate ligands exhibiting electronic properties comparable to those of the cyclopentadienyl ligand: among these the heteroscorpionates are attractive candidates. Herein we review the results reached in olefin polymerization catalysis promoted by group 4 metals complexes of heteroscorpionate ligands, with particular reference to the work carried out in our research group. The ethylene polymerization catalyzed by titanium and zirconium complexes of the (3,5-Bu-t(2)-2-phenoxo)bis(3,5-Me-2-pyrazol-1-yl)methane ligand has been investigated in great detail. A solution structure of the active species was proposed on the basis of variable temperature H-1 NMR spectroscopic evidences. Kinetic mechanisms accounting for the formation of linear and branched polyethylenes obtained by the zirconium complexes have been proposed in the light of the ionic interactions of the active species and steric hindrance at metal centre determined by the heteroscorpionate ligands.