A Theoretical Outlook on the Stereoselectivity Origins of Isoselective Zirconocene Propylene Polymerization Catalysts

Abstract

The first three insertion steps of propylene for isoselective metallocenes from the one-carbon-bridged cyclopentadienyl-fluorenyl {Cp/Flu} and silicon-bridged ansa-bis(indenyl) {SBI} families were computed by using a theoretical method implementing the B3PW91 functional in combination with solvent corrections incorporated with the Solvation Model based on Density (SMD) continuum model. For C -symmetric {Cp/Flu}-type metallocenes, two mechanisms of stereocontrol were validated theoretically: more facile and more stereoselective chain "stationary" insertion (or site epimerization backskip) and less stereoselective alternating mechanisms. For the C -symmetric {SBI}-type system, the computation results were in complete agreement with the sole operating chain migratory insertion mechanism. The thermochemical data obtained through the study were used to predict microstructures of polypropylenes by using three-parameter and one-parameter statistical models for the two metallocene systems, respectively. The calculated meso/rac pentad distributions were found to be in good agreement with those determined experimentally for isotactic polypropylene samples obtained at different polymerization temperatures.

Propylene Polymerization and Deactivation Processes with Isoselective {Cp/Flu} Zirconocene Catalysts

“…Since the development of isoselective polymerization of propylene with C 1 -symmetric cyclopentadienyl-fluorenyl {Cp/Flu}-metallocene catalysts [1-4], many academic groups have set out to find "the" ideal structure that would produce polypropylene with the highest degree of isotacticity and regiocontrol, well-controlled molecular weight characteristics, and high productivity. This performance race has resulted in the synthesis of a vast number of structures with various substitution patterns [5][6][7][8][9], for the purpose of studying the influence of ligand architecture on the above parameters [10][11][12].…”

“…-The stereoregularities of the PPs obtained from {Cp/Flu}-based catalysts can vary over a broad range, depending on the substituents both on the Cp and Flu moieties[10,11,[18][19][20]. The new metallocenes with bulky 3-R substituents on the Cp ligand (3a,b,e) afforded polypropylenes featuring pentad [m] 4 values (86.2-88.6%) higher than those observed with {Cp/Flu}-2 (82.1%).…”

“…On attempting to find out the connection between the lower productivities of some {Cp/Flu}-based systems and their propensity towards affording regioirregular insertions, a closer inspection of the 13 C{ 1 H} NMR spectra of the PPs produced with 3a-c,e-h and the reference system {Cp/Flu}-2 was carried out (Table 2). For a more complete set of data, a complementary analysis was conducted with the polymers obtained with some previously reported analogous {Cp/Flu}-based systems (Scheme 6; Table 3, entries 1-7) [11,[25][26][27]. The respective resonances derived from the 2,1-and 1,3-insertions and those from the terminal nBu chain-ends were identified, quantified, and used to plot the combined bar diagram (Figure 8).…”

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

“…Computational chemistry has contributed significantly to the identification of some rationale in the relationships between catalyst structure and properties [10][11][12]. The demystification of the origins of stereoselectivity is generally considered as a successful example of rational understanding [13][14][15][16], although some aspects are still debated [17][18][19][20][21][22]. Conversely, other catalyst properties, including comonomer affinity, are far less understood.…”

Extent of structural change during the reaction and its relationship to isoselectivity in polypropylene polymerization with ansa‐zirconocene/borate catalyst: A computational study

“…performed a DFT study on the isoselectivity of ansa‐zirconocene and proposed that this is affected by not only the zirconocene structure but also by agostic interactions and cocatalysts . Solvent‐corrected DFT calculations were applied to a series of energy data concerning propylene polymerization using zirconocene, and the results were used to simulate the meso/rac rate . DFT‐based statistical analysis was performed to separate various contributions related to stereo‐/regioselectivity in olefin polymerization …”