Separating Electronic from Steric Effects in Ethene/α-Olefin Copolymerization: A Case Study on Octahedral [ONNO] Zr-Catalysts

Abstract

Four Cl/Me substituted [ONNO] Zr-catalysts have been tested in ethene/α-olefin polymerization. Replacing electron-donating methyl with isosteric but electron-withdrawing chlorine substituents results in a significant increase of comonomer incorporation. Exploration of steric and electronic properties of the ancillary ligand by DFT confirm that relative reactivity ratios are mainly determined by the electrophilicity of the metal center. Furthermore, quantitative DFT modeling of propagation barriers that determine polymerization kinetics reveals that electronic effects observed in these catalysts affect relative barriers for insertion and a capture-like transition state (TS).

Heteroleptic Calcium Complexes Supported by a Phenoxy-Imine NON Ligand: Polymerization of Cyclic Esters by a Ligand-Assisted, Activated-Monomer Mechanism

“…Attempts to evaluate the stereoelectronic influence of the para -substituent of tridentate N , N , O -phenoxyimine zinc complexes toward rates of LA polymerization were made by Lin et al; however, the rapid activity of all the initiators meant that the individual rates were indistinguishable . Similar observations have been widely observed, however, upon variation of the ortho-position steric bulk; this is most likely due to this position lying closer to the active metal center. , More broadly, remote substitution has been demonstrated to affect many classes of polymerization reaction, e.g., ROP and olefin polymerization. , …”

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

“…The kinetic selectivity of a catalyst for comonomer enchainment vs ethylene in ethylene/α-olefin copolymerizations is critical in catalyst optimization and producing polyethylenes with controlled compositions and rheological/mechanical properties . Despite this importance, understanding catalytic comonomer enchainment selectivity is still primarily empirical, based on correlations of ligand and counteranion steric effects, cation electrophilicity, ion pairing, etc. − In the present ethylene/1-octene copolymerizations, under identical reaction conditions, homogeneous 1-octene selection scales as L Naph -HfMe 2 > L Me -HfMe 3 ≈ L H -HfMe 3 , with the highest selectivity achieved at 50% (Table and Figure B). As noted above, the homogeneous copolymerization rates are greater than the homopolymerization rates with only minor dispersion in numbers, L Naph -HfMe 2 ≥ L H -HfMe 3 ≥ L Me -HfMe 3 .…”

Hafnium vs. Zirconium, the Perpetual Battle for Supremacy in Catalytic Olefin Polymerization: A Simple Matter of Electrophilicity?

“…Following the protocol proposed in [ 34 ], dichloride metallocenes were fully optimized using the Gaussian 16 software package (Gaussian 16, Revision A.1, Gaussian, Inc., Wallingford, CT, USA) [ 35 ], in combination with the OPTIMIZE routine of Baker [ 36 , 37 ] and the BOpt software package [ 38 ], at the TPSSTPSS [ 39 ]/cc-pVDZ(-PP) [ 40 , 41 , 42 ] level of theory, using a small core pseudo-potential on Zr [ 43 , 44 ]. The protocol has been successfully used, in combination with M06-2X [ 45 ] single-point energy (SP) corrections, to address several polymerization-related problems: absolute barrier heights for propagation [ 46 ], comonomer reactivity ratios [ 47 , 48 , 49 ], metal–carbon bond strengths under polymerization conditions [ 50 , 51 , 52 , 53 ], electronic and steric tuning of molar mass capability [ 54 ], and quantitative structure–activity relationship (QSAR) modeling [ 23 , 25 , 31 , 32 , 33 ]. The density-fitting approximation (Resolution of Identity, RI) [ 55 , 56 , 57 , 58 ] and standard Gaussian16 quality settings were used at the optimization stage and SP calculations.…”