A Systematic Study of the Temperature-Induced Performance Decline of ansa-Metallocenes for iPP

Abstract

Highly accurate high-throughput experimentation (HTE) data for a set of 21 silicon-bridged C 2-symmetric ansa-zirconocenes in propene homopolymerization were collected and were used to develop quantitative structure – activity relationship (QSAR) models for several performance indicators at high polymerization temperature (T p = 100 °C) by using chemically meaningful descriptors. Most notably, stereoselectivity is well described by a two-descriptor model linking the quadrant model for stereoselectivity (sterics) with the chain epimerization model (electronics). The catalysts show widely varying temperature responses, most notably on stereoselectivity and molar mass capability, while the regioselectivity response is uniformly weak. Soft conformational locks lose their performance rapidly while hard conformational locks offer enhanced performance even at high temperatures. The quest for high-temperature stable, well-performing ansa-zirconocenes will unquestionably lead to systems with enhanced rigidity.

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

“…The catalysts Hf-1 to Hf-5 were screened in propene homopolymerization on a PPR48 platform at 60 and 100 °C, and the polymer samples were characterized by 13 C NMR spectroscopy for stereo- and regioerror quantification and by GPC for molecular weight determination ( Table 1 ). Data for the Zr derivatives Zr-1 to Zr-5 , generated under identical polymerization conditions, are provided for comparison, and taken from references [ 23 , 24 , 25 , 32 ].…”

“…It should be noted that the stereoselectivity (ΔΔ G ‡ exp, stereo ) of the hafnocenes at 100 °C follows solely electrophilicity trends, as measured by q MCl2, NPA ( R 2 = 0.87; Figure 2 ), much better than zirconocenes ( R 2 = 0.69; Figure 2 ). As a matter of fact, the QSAR model for predicting the stereoselectivity of zirconocenes at 100 °C is based on the mathematical combination of two descriptors: (1) a 3D-steric descriptor, measuring the difference in buried volume of the occupied and empty quadrants of the catalyst active pocket (Δ% V Bur, see Table 2 ) and (2) an electronic descriptor being the natural population analysis (NPA) charge on the metal dichloride fragment ( q ZrCl2, NPA , see Table 1 ), introduced for accounting for the occurrence of chain epimerization [ 32 ]. The nice correlation for hafnocenes between ΔΔ G ‡ exp, stereo and q MCl2, NPA in Figure 2 indicates that at 100 °C chain epimerization is the dominant stereoerror source for Hf-based metallocenes.…”

“…Therefore, it appears that the different stereoselectivity of hafnocenes and zirconocenes simply reflects the slightly different electrophilicity of the two metals, with the more electrophilic Hf showing an earlier onset of chain epimerization via a series of BHE and reinsertion events. It should be noted here that variation of the substituents on the metallocene backbone influences the electrophilicity of the active species [ 32 ], but often in non-trivial ways, as direct electronic effects can be coupled with indirect electronic effects (e.g., from distortion).…”

“…The cells (5.0 mL working volume) feature 800-rpm magnetically coupled stirring and individual online reading/control of temperature, pressure, monomer uptake, and uptake rate. The setup and the operating protocol are described in full detail in [ 26 , 27 ] and the Supporting Information , and they have been used successfully in various homogenous and heterogeneous polymerization studies [ 16 , 23 , 25 , 28 , 29 , 30 , 31 , 32 , 33 ]. Polymerization conditions are detailed in the Supplementary Materials Table S1 .…”

“…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.…”