Catalyst Nuclearity Effects on Stereo- and Regioinduction in Pyridylamidohafnium-Catalyzed Propylene and 1-Octene Polymerizations

In comparison to monometallic controls, bimetallic olefin polymerization catalysts often exhibit superior performance in terms of higher polyolefin M_w, higher comonomer incorporation, and higher polar comonomer tolerance. However, using cooperating catalyst centers to modulate stereoselectivity in α-olefin polymerizations is relatively unexplored. In this contribution, the monometallic Hf(IV) complex, L¹-HfMe₂ (catalyst A, L¹ = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline), and homo-bimetallic Hf(IV) complexes, L²-Hf₂Me₅ (catalyst B) and L²-Hf₂Me₄ (catalyst C) (L² = N,N′-{[naphthalene-1,4-diylbis(pyridine-6,2-diyl)]bis[(2-isopropylphenyl)methylene)]bis(2,6-diisopropylaniline}), are activated with Ph₃C⁺B(C₆F₅)₄^- and investigated in propylene and 1-octene homopolymerizations. In propylene polymerizations, the conformationally flexible catalyst B-derived bimetallic dicationic catalyst produces higher M_w polypropylene (up to 7.8×), higher total stereo- and regiodefect densities (up to 3.5×), and lower T_m (by as much as ∼14 °C) versus the monometallic catalyst A-derived control. In 1-octene polymerizations, the conformationally flexible catalyst B-derived bimetallic dicationic catalyst induces greatly reduced isotacticity (23% reduction in [mmmm]) versus the catalyst A-derived monometallic control ([mmmm] > 99%). Interestingly, conformationally flexible catalyst B-derived cationic bimetallic Hf catalysts are also known to undergo rapid intramolecular/intermetal methyl exchange and to exhibit strong Hf···Hf cooperative enchainment effects in ethylene homo- and copolymerizations. Steric effects and intramolecular intermetal chain transfer likely both contribute to the increased isotactic polyolefin stereo- and regiodefect content.