Surface vs Homogeneous Organo-Hafnium Catalyst Ion-Pairing and Ligand Effects on Ethylene Homo- and Copolymerizations

Heterogeneous
catalysts have long dominated polyethylene and polypropylene
production, but understanding their catalysis is challenged by uncertainties
in active site structures and percentages. Surface-bound organometallic
catalysts are an emerging strategy to combine successful homogeneous
catalysts having well-understood structures, relatively high percentages
of active sites, and exceptional control of selectivity, with the
attractions of surface catalysts, to transition promising homogeneous
systems to large-scale heterogeneous ones. Nevertheless, surface-bound
olefin polymerization catalysts typically produce ultrahigh Mw’s but with low activity and comonomer
selectivity. Here, we report the systematic synthesis and characterization
of a series of pyridylamido–Hf complexes and their corresponding
surface catalysts chemisorbed on sulfated alumina (AlS) and zirconia
(ZrS). Comparative ethylene homo- and 1-octene copolymerizations reveal
similar activity and 1-octene selectivity trends in the homogeneous
and heterogeneous systems. For the surface pyridylamido–Hf
catalyst series, large variations in activity (up to 10×) and
1-octene incorporation (up to 28×) are achieved by ligand and
support manipulation. Interestingly, while the homogeneous catalysts
exhibit positive comonomer effects in ethylene/1-octene copolymerization,
the surface catalysts behave oppositely. Extended X-ray absorption
fine structure (EXAFS) reveals significantly elongated Hf···O
bond distances vs typical Hf–O covalent bonds (2.06 vs 1.97
Å). Density functional theory (DFT) analysis of the heterolytic
ion pair separation enthalpies, olefin insertion energetics, and NBO/Bader
charges also suggest electrostatic Hf cation–anionic support
binding and catalytic patterns, which are modulated by the ion-pairing
energetics and ligand architecture.