Nature of the Active Catalyst in the Hafnium-Pyridyl Amido-Catalyzed Alkene Polymerization

Abstract

We report the true nature of active catalysts in the polymerization of 1-octene catalyzed by the racemic hafnium-pyridyl amido cation (I). Chromophore quench-labeling (CQL) studies reveal that only ca. 50% of the catalyst actively participates in the polymer chain growth. The mass spectral analysis of the pyridine ligands obtained postpolymerization demonstrates that about one-half of the ligand is modified by the insertion of 1-octene into the Hf–naphthyl bond of I. The fraction of octyl-modified ligands correlates with active-site counts, as determined by the correlation of CQL studies and the mass spectral analysis of ligands during in situ activation experiments. The analysis of reaction kinetics and active-site counts were obtained using two different activation methods: (1) activation with the ammonium salt [HNMe(C18H37)2][B(C6F5)4] followed by the polymerization of 1-octene or (2) activation with [Ph3C][B(C6F5)4] followed by 1-octene polymerization in the presence of differing amounts of NMe(C18H37)2. These studies indicate that different diastereomers of activated rac-1 exhibit vastly different polymerization properties; one diastereomer (Ia) initiates rapidly, while the other diastereomer (Ib) initiates much more slowly. Accordingly, the cause of 50% active-site counts in the 1-catalyzed polymerization of 1-octene is stereochemical in origin. Related achiral hafnium-pyridyl amido complexes exhibit much higher active-site counts, thus further supporting the stereochemical origin of reduced active-site counts.