Single to Multiple Site Behavior of Metallocenes through C–H Activation for Olefin Polymerization: A Mechanistic Insight from DFT

Abstract

Density functional theory has been used to study single to multiple site behavior of metallocene catalysts for olefin polymerization using (Cp Pr ) 2 Hf(R) 2 and (Cp Pr ) 2 Zr(R) 2 (here, R = Me and n-butyl group) and boron activators B(C 6 F 5 ) 3 and [CPh 3 ] + [B(C 6 F 5 ) 4 ] − . Detailed pathways were investigated for two steps: (i) catalyst activation and (ii) ethylene and 1-hexene polymerization. For the catalyst activation step, the C−H activation of Cp-substituents (here, the n-propyl group) has also been studied, which leads to another active site named "metal cyclic active site" in comparison to the conventional active site. Among these sites, the formation of a metal cyclic active site is slower with respect to the conventional active site. Additionally, two different routes have been explored for the catalyst activation, depending on the nature of activators, and observed that [CPh 3 ] + [B(C 6 F 5 ) 4 ] − is a better activator with respect to B(C 6 F 5 ) 3 . Moreover, complete insertion (primary and secondary) and termination (chain transfer to monomer and β-H elimination) steps have also been studied for both the active sites. These results suggest that the metal cyclic active site can also produce the ethylene and 1-hexene polymer, which defines the multiple site behavior of metallocene catalysts, which explains the broadening of molecular weight distribution of linear low-density polyethylene-produced metallocene catalysts. Along with hafnocene and zirconocene, the toluene-solvated hafnocene catalyst system has also been investigated for catalyst activation and ethylene polymerization for comparison study. These results indicate that the toluene-solvated hafnocene catalyst system is energetically more facile in comparison to other catalysts. Furthermore, the effect of solvents as well as dispersion interaction on catalyst activation steps has been studied for the (Cp Pr ) 2 M(n-butyl) 2 (M = Hf and Zr) catalyst systems and observed that the dispersion interaction played an important role in the catalyst activation process. In addition to this, the regio-and stereoselective behavior of 1-hexene monomer for the (Cp Pr ) 2 Hf(Me) 2 catalyst has been investigated, and it was found that the metal cyclic active site improves the stereoselective nature of the hafnocene catalyst. Therefore, the current chemical calculations provide insights into the nature of active sites in metallocene catalysts for olefin polymerization.

Preparation of High-Purity Ammonium Tetrakis(pentafluorophenyl)borate for the Activation of Olefin Polymerization Catalysts

“…With the species [ 5 -Ti(CH 2 N(C 18 H 37 ) 2 )] + [B(C 6 F 5 ) 4 ] − , the four NCH 2 protons, the four methyl groups attached to cyclopentadienyl ligand, and the two methyl groups attached to Si are separately observed at 3.29, 2.93, 2.77, 2.51, 2.20, 2.08, 1.88, 1.83, 0.75, and 0.58 ppm with intensity ratios of roughly 1:1:1:1:3:3:3:3:3:3, respectively, along with Ti-CH 2 signals at 0.45 and −0.71 ppm with intensity ratios of roughly 1 and 1, respectively. The same type of complex, [Cp 2 Zr(CH 2 N(C 18 H 37 ) 2 )] + [B(C 6 F 5 ) 4 ] − , was reported for the activation reaction of Cp 2 ZrMe 2 [ 46 , 47 , 48 ], and a similar transformation of [ L -M(Me)] + [B(C 6 F 5 ) 4 ] − -type complexes to other species (via the σ-bond metathesis reaction with formation of other M-C bonds and generation of CH 4 ) has also been reported for other metallocene and post-metallocene complexes [ 31 , 49 , 50 , 51 ]. Action of [Me(C 18 H 37 ) 2 N-H] + [B(C 6 F 5 ) 4 ] − to Hf-based metallocene (e.g., 3 - and 4 -HfMe 2 ), half-metallocene (e.g., 6 -HfMe 2 ), and post-metallocene (e.g., pyridylamido-HfMe 2 ) complexes usually afforded stable [ L -Hf(Me)(NMe(C 18 H 37 ) 2 )] + [B(C 6 F 5 ) 4 ] − type ion pair complexes [ 37 ].…”

Interception of Elusive Cationic Hf–Al and Hf–Zn Heterobimetallic Adducts with Mixed Alkyl Bridges Featuring Multiple Agostic Interactions

“…The two singlets at d H = À0.62 and À0.67 are assigned to the terminalA l-bound methyl moieties because of their scalar coupling with the carbon signals at d C = À5.55 and d C = À6.75, respectively.T he chemical shifts of the remaining methyl group (d H = 0.31, d C = 40.6) support assignment as the bridging moiety( Me18, Figure 1). Startingf rom H9, protona nd carbon resonances of the Cp ligand bearing the nonmetalatedp ropylc hain (10)(11)(12)(13)(14)(15)(16) are individuated by following the scalar connectivity in 1 HCOSY, 1 H, 13 CHSQC, and 1 H, 13 CHMBC NMR spectra. The assignments of resonancesb elongingtothe metalatedC pligand follow the same line of reasoningb ys tarting from the methylene carbon (C1, d C = 57.6).…”

“…In our continuing mechanistic investigations of olefin polymerization catalyzed by hafnocene derivatives, we recently reportedt hat the [(Cp Pr ) 2 HfR][X] ion pair undergoes ar elatively facile CÀHa ctivation of one propyl chain, affording the corresponding cyclometalateds pecies[ Cp Pr Cp CH 2 CH 2 CH 2 Hf] [X] (1). [11] Detailed NMR, [11a] mass spectrometry, [11b] and polymerization studies [11a] indicate that 1 can be formed under relevant polymerization conditions, enabling multiple olefin insertionsi nto its HfÀCH 2 bond. We have hypothesized that these irreversible in situ catalyst alterations may generate several different active species.T he concept of modifying al igand through olefin insertioni sk nown although only as ingle olefin was appended in ap revious example.…”

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

“…Metallocenes are industrially relevant chemical catalysts for polyolefin production. In the past, numerous efforts [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] have been made to understand the behavior of group 4 metallocenes in olefin polymerization. Metallocenes require cocatalysts to make active catalyst species (ion pair formation), [20][21][22][23] and modification in and/or changing cocatalysts lead to significant changes in catalyst performance.…”

“…Metallocenes require cocatalysts to make active catalyst species (ion pair formation), [20][21][22][23] and modification in and/or changing cocatalysts lead to significant changes in catalyst performance. [1,3] Many cocatalyst species have been used in metallocene such as aluminiumbased (MAO, [12,24] Al(R) 3 [25] and Al(C 6 F 5 ) 3 [26,27] ), boron-based (B-(C 6 F 5 ) 3 , [1][2][3]20,21] [CPh 3 ] + [B(C 6 F 5 ) 4 ] À , [1,3] ([B(C 6 F 5 ) 4 ] À [Me 2 NHPh] + ), [28] ). The aluminium-based cocatalysts (MAO and Al(R) 3 ) are extensively used for olefin polymerization.…”