Facing Unexpected Reactivity Paths with ZrIV-Pyridylamido Polymerization Catalysts

This work provides original insights to the better understanding of the complex structure-activity relationship of ZrIV-pyridylamido-based olefin polymerization catalysts and highlights the importance of the metal-precursor choice (Zr(NMe2)4 vs. Zr(Bn)4) to prepare precatalysts of unambiguous identity. A temperature-controlled and reversible ?-bond metathesis/protonolysis reaction is found to take place on the ZrIV-amido complexes in the 298-383 K temperature range, changing the metal coordination sphere dramatically (from a five-coordinated tris-amido species stabilized by bidentate monoanionic {N,N-} ligands to a six-coordinated bis-amido-mono-amino complexes featured by tridentate dianionic {N-,N,C-} ligands). Well-defined neutral ZrIV-pyridylamido complexes have been prepared from Zr(Bn)4 as metal source. Their cationic derivatives [ZrIV{N-,N,C-}Bn]+[B(C6F5)4]- have been successfully applied to the room-temperature polymerization of 1-hexene with productivities up to one order of magnitude higher than those reported for the related HfIV state-of-the-art systems. Most importantly, a linear increase of the poly(1-hexene) Mn values (30-250 kg mol-1) has been observed upon catalyst aging. According to that, the major active species (responsible for the increased Mn polymer values) in the aged catalyst solution, has been identified.