The trimethyl hafnium complex (N^octN^dipp)Hf(Me)₃(N^octN^dipp =N-octyl)-N’-(2,6-diisopropylphenyl)-1,4-diaza-2,3,3-trimethyl-1-butene) features a bidentate imino-anilido ligand and undergoes protonolysis by the ammonium Brønsted acid [HN(CH₃)(C₁₈H₂₇)][B(C₆F₅)₄]. This protonolysis produces the cationic hafnium dimethyl complex [(N^octN^dipp)Hf(Me)₂][B(C₆F₅)₄], a competent alkene polymerization catalyst that demonstrates unusual kinetics. Polymerization of 1-octene as catalyzed by this activated complex demonstrates several interesting characteristics: 1) linear plots of 1-octene as a function of time, suggesting an apparent zeroth-order dependence of rate on monomer concentration during the reaction; 2) a first-order dependence of steady-state rates on the initial concentration of 1-octene; 3) number-average polymer molar mass (M_n) that is independent of the initial concentration of 1-octene. Whereas polymerization with late transition metal-derived catalysts commonly exhibits rate laws with saturation behavior with respect to alkene concentration, propagation rate laws for early transition metal complexes overwhelmingly are first order in alkene. Late transition metal complexes generally exhibit such behavior due to their ability to bind alkenes favorably and reversibly prior to irreversible insertion, a phenomenon that is less favorable with a d⁰, Hf(IV) complex. An additional distinctive feature of catalysis by [(N^octN^dipp)Hf(Me)₂][B(C₆F₅)] is the possibility that two polymer chains may simultaneously grow from a single metal center. Elucidating the number of active polymerization sites per metal center is key to developing a comprehensive mechanistic picture of catalysis. Utilizing operando NMR kinetics, end-group analysis, active site counting using chromophore quench labels, iodine quenching studies, and gel permeation chromatography, we propose a unified kinetic and mechanistic model for the polymerization of 1-octene with these unusual catalysts.

Clark Landis

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