Dual-Chain Polymerization at an Early Transition-Metal Single-Site Catalyst

• Published in: ACS catalysis 2022-09, Vol.12 (17), p.10680-10689
• Main Authors: McDaniel, Tanner, Smith, Nicholas E., Cueny, Eric, Landis, Clark R.
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.2c01240

The hafnium complex (NoctNdipp)­Hf­(Me)3 (1, NoctNdipp = N-octyl-N′-(2,6-diisopropylphenyl)-1,4-diaza-2,3,3-trimethyl-1-butene) contains a bidentate imino-anilido ligand. This complex undergoes protonolysis by the ammonium Brønsted acid [HN­(Me)­(C18H37)2]­[B­(C6F5)4] to produce a competent alkene polymerization precatalyst, [(NoctNdipp)­Hf­(Me)2(N­(Me)­(C18H37)2)]­[B­(C6F5)4] (2). Unlike most alkene polymerization precatalysts, 2 contains two Hf–Me groups, which have the potential to initiate and propagate two polymer chains simultaneously. Herein, we demonstrate that 2 does indeed grow two polymer chains at each hafnium center. Additional unusual characteristics for an early transition-metal catalyst include: (1) the number-average polymer molar mass (M n) is independent of the concentration of monomer; (2) linear plots of monomer concentration as a function of time during polymerization indicate apparent zeroth-order dependence of rate on the monomer concentration; and (3) apparent first-order dependence of steady-state rate of polymerization on the initial concentration of monomer. While saturation behavior with respect to monomer concentration is common with late transition-metal-derived catalysts, propagation rate laws for early transition-metal complexes overwhelmingly are first-order in monomer concentration. Based on operando NMR kinetics, end group analysis, active site counting with 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 2.