Catalyst Activation and Chain Transfer Agents in Metallocene Catalysts for Ethylene Polymerization: A Computational Investigation

Abstract

A density functional theory (DFT) study has been carried out to investigate the effect of various cocatalysts/chain transfer agents (such as Al(R) 3 , Mg(R) 2 , (R)Mg(Cl), Zn(R) 2 , (R)Zn(Cl), B(C 6 F 5 ) 3 , and Al(C 6 F 5 ) 3 , here, R = alkyl group) on catalyst activation and ethylene polymerization including chain transfer steps using (Cp R, R' ) 2 M(Cl) 2 (M = Zr, Hf and R, R' = alkyl group) metallocenes. Initially, the catalyst alkylation step (where one of the chlorine atoms of the catalyst is replaced by an alkyl group of the cocatalyst) has been studied using the Al(Me) 3 , Mg(Me) 2 , (Me)Mg(Cl), Zn(Me) 2 , and (Me)Zn(Cl) moieties, and it was observed that the AlMe 3 species is more effective for the alkylation step. Furthermore, a modification in the metallocene, such as (Cp R, R' ) 2 Zr(Cl) 2 , where R and R' = H, Me, Et, n-propyl, isopropyl, iso-butyl, tert-butyl, and different combinations of those, has been performed to evaluate the effect of different alkyl substituents on the alkylation process, whereby it was found that the mono-substituted metallocene favors the alkylation in comparison to bi-substituted metallocene. Moreover, a detailed fundamental understanding of the ion pair formation step was gained by incorporating various steric and electronic factors of different alkyl groups, including fluorinated alkyl group bound on the Cp ligand. These results suggest that only B(C 6 F 5 ) 3 and Al(C 6 F 5 ) 3 activators could form the ion pairs by abstracting a methyl group from the metallocene. Additionally, the ethylene polymerization has also been investigated using B(C 6 F 5 ) 3 or Al(C 6 F 5 ) 3 cocatalyst, where B(C 6 F 5 ) 3 was found to be more effective towards polymerization. Furthermore, the Al(Me) 3 , Mg(R) 2 , (Me)Mg(Cl), Zn(R) 2 , and (Me)Zn(Cl) species have been employed as chain transfer agent (CTA). It has been observed that the magnesium-and zinc-based species are more likely to behave as CTAs, and this process can be improved by changing the alkyl substituents on them. These chemical calculations shed light on catalyst alkylation and ethylene polymerization, including the chain transfer process, in metallocenes.

Transition Metal–(μ-Cl)–Aluminum Bonding in α-Olefin and Diene Chemistry

“…The kinetically favorable route is dependent on the ligand structure of the catalyst: the front pathway is preferred for Cp 2 ZrCl 2 ( Figure 11 ) but is sterically hindered for Me 2 C(Cp)(Flu)Zrl 2 , thereby preferring the side pathway [ 123 ]. Similar results have also been obtained in a more recent study by Kumawat and Gupta [ 124 ]. Zr01 was also detected by IR spectroscopy during studies of the activation of Cp 2 ZrCl 2 by MAO [ 125 ].…”

“…One can assume that the dissolution of Zr26 occurs through the formation of the Zr26 – i Bu 3 Al complex with weak Cl–Al coordination. Notably, a similar complexation of L 2 ZrCl 2 with metal alkyls was recently discussed by Kumawat and Gupta in their study on the DFT modeling of zirconocene activation and chain transfer [ 124 ].…”

DFT studies on the effect of additives on stereoselectivity in the polymerization of styrene catalyzed by rare-earth metal complexes

“…The overall mechanisms of catalyst activation have been reported in detail. 41 As shown in Scheme 1, the complex a Sc showed syndiotactic selectivity for styrene. However, the polymerization product displayed atacticity upon addition of an organic additive (AOB) ( b Sc ), and chain transfer occurred in toluene solution.…”

Solution Structure and Dynamics of Hf–Al and Hf–Zn Heterobimetallic Adducts Mimicking Relevant Intermediates in Chain Transfer Reactions

“…Heterobimetallic adducts featuring bridging alkyl/hydride/chloride moieties are usually proposed as catalyst resting states or intermediates in these transformations. − Although several heterobimetallic adducts having hydride, methyl, or chloride bridges have been intercepted and characterized, ,,− very few studies examine systems with alkyl bridges longer than methyl or systems with inequivalent alkyl bridges − critical to olefin polymerization, CCTP, and CSP processes.…”