The Hafnium-Pyridyl Amido-Catalyzed Copolymerization of Ethene and 1-Octene: How Small Amounts of Ethene Impact Catalysis

Copolymerization of ethene and 1-octene is catalyzed by the Hf-pyridyl amido complex used in chain-shuttling polymerization. Active site counts and the molar mass distribution of catalystbound polymeryls are determined by chromophore quench-labeling. Catalytic data for copolymerization (1-octene consumption, active site counts, molar mass distributions of polymers, and I-2-labeling of Znpolymeryls) are compared with similar data for the homopolymerization of 1-octene. Such comparisons reveal that small amounts of ethene have a significant impact on catalysis. The rate of 1-octene consumption increases in a copolymerization similar to 3-fold compared to a homopolymerization; this likely results from in situ ligand modification. The first insertion of alkene occurs into the Hf-naphthyl bond of the Hf-pyridyl amido catalyst; in copolymerization, competition between ethene and 1-octene for this insertion creates multiple active species. We propose that ethene insertion into the Hf-naphthyl bond leads to a faster polymerization catalyst than insertion of 1-octene. In the presence of diethyl zinc, the homopolymerization of 1-octene produces broader molar mass distributions than those seen for the copolymerization of octene and ethene. Narrowing of the distribution by ethene presumably reflects increased reversibility of chain transfer between Hf and Zn. I-2-labeling of Zn-polymeryls and subsequent NMR chain end analysis reveals that more than one polymer chain per Zn is produced under copolymerization conditions. The steric requirements for reversible chain transfer are assessed by I-2-labeling of sequential copolymerization reactions.