Tuning the Relative Energies of Propagation and Chain Termination Barriers in Polyolefin Catalysis through Electronic and Steric Effects

Abstract

A computational exploration of the predicted molecular weights for Ti‐ and Zr‐catalyzed olefin polymerizations shows that there are considerable opportunities for electronic tuning. Ligand variation mainly affects the propagation rate, whereas chain transfer to the monomer is hardly affected by electronic factors. The results are analyzed in terms of the effects of ligand variation on the relative energies of “connected couples” of reactant local minima and the corresponding transition states on the basis of the Hammond postulate and the Curtin–Hammett principle. For the constrained‐geometry catalysts (CGCs) and bis(amido) systems studied, better donating ligands increase the preference for a “planar” metal environment and produce higher molecular weights.

An Integrated High Throughput Experimentation/Predictive QSAR Modeling Approach to ansa-Zirconocene Catalysts for Isotactic Polypropylene

“…Moving from these studies, we implemented a pool of seven descriptors, all related intuitively to simple electronic or steric properties of neutral LZrX 2 precatalysts that are easy to quantify with DFT methods ( Figure 4 and SI ) using previously established protocols (see experimental section for details) [ 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 ]. The six steric descriptors screen different regions of space around the catalyst, that were selected based on well-established olefin insertion and chain transfer transition state structures ( SI, Table S4 ).…”

“…Following the protocol proposed in Reference [ 65 ], all pre-catalysts were optimized at the TPSSTPSS/cc-pVDZ(-PP) [ 66 , 67 , 68 , 69 ] level of theory, using a small core pseudo potential on Zr [ 70 , 71 ]. The protocol has been successfully used, in combination with M06-2X [ 72 ] single-point energies (SP), to address several polymerization related problems: i.e., absolute barrier heights for propagation [ 73 ], comonomer reactivity ratios [ 74 , 75 ], metal-carbon bond strengths under polymerization conditions [ 76 , 77 , 78 ], electronic and steric tuning of M W capability [ 79 ], and QSAR modeling [ 13 ]. The density fitting approximation (Resolution of Identity, RI) [ 80 , 81 , 82 , 83 ] and standard Gaussian16 quality settings [Scf = Tight and Int(Grid = Fine)] were used throughout.…”

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.…”

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

“…ClCl binds ethene slightly more strongly than MeMe, likely due to the higher electrophilicity of its metal center (vide supra). The insertion barrier for ClCl is lower by 0.7 kcal/mol compared to MeMe, suggesting some electronic influence also in this respect [82]. Furthermore, BBRA activation ClCl binds ethene slightly more strongly than MeMe, likely due to the higher electrophilicity of its metal center (vide supra).…”

“…To unravel the connection between the catalyst structure and comonomer affinity, DFT studies have been carried out on complexes R 1 R 2 . The computational protocol (see Materials and Methods for details) has been previously benchmarked for group IV precatalysts and TSs relevant to olefin polymerization [54,74,[80][81][82], including specifically those for predicting reactivity ratios in copolymerization [33,39]. A facfac geometry for all complexes and TSs has been considered, which is generally accepted to be the most stable for the neutral complexes and the cationic active species; interconversion between active facfac and inactive mermer configurations of the naked cationic complex is generally assumed to represent a rapid pre-equilibrium with respect to chain propagation [43][44][45]83].…”