C–H Activation and Olefin Insertion as Sources of Multiple Sites in Olefin Polymerization Catalyzed by Cp^AlkylHf(IV) Complexes

Abstract

Intramolecular activation of hydrocarbyls to form metallacyclic complexes is a relatively fast process in cationic hafnocene catalysts bearing propyl-substituted Cp ligands. The resulting metallacycles are effective 1-hexene polymerization catalysts with activities comparable to that of the nonmetalated precursor. Ad hoc polymerizations of 1-hexene, using (Cp Pr )2HfMe2 as catalyst precursor, allow the isolation and characterization, via nuclear magnetic resonance (NMR) and matrix-assisted laser desorption ionization (MALDI) techniques, of polymers containing (Cp CH 2–CH 2–CR 3 )2HfCl2 (R = H or polymeryl) units. The polymeryl substitutions arise from irreversible incorporation of polymer chains onto the cyclopentadienyl ligand substituent(s) via metallacycle intermediates. As a consequence of such “self-modification”, multiple active sites are generated by a nominally single-site catalyst; this may explain the broadening of the molecular weight distribution (MWD) and chemical composition distribution (CCD) observed in olefin polymerization.

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

“…In our continuing mechanistic investigations of olefin polymerization catalyzed by hafnocene derivatives, we recently reported that the [(Cp Pr ) 2 HfR][X] ion pair undergoes a relatively facile C−H activation of one propyl chain, affording the corresponding cyclometalated species [Cp Pr Cp Hf] [X] ( 1 ) . Detailed NMR, mass spectrometry, and polymerization studies indicate that 1 can be formed under relevant polymerization conditions, enabling multiple olefin insertions into its Hf−CH 2 bond. We have hypothesized that these irreversible in situ catalyst alterations may generate several different active species.…”

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.…”

Toluene and α-Olefins as Radical Scavengers: Direct NMR Evidence for Homolytic Chain Transfer Mechanism Leading to Benzyl and “Dormant” Titanium Allyl Complexes

“…The existence of Ti benzyl- or allyl-type complexes can be inferred from the polymer microstructure, but to the best of our knowledge, no direct experimental evidence has ever been reported for their formation under polymerization conditions. NMR spectroscopy has emerged as a suitable technique to trap and detect elusive species, ,, and 1-X 2 (X = Me, Bn) was regarded as the ideal candidate for such studies . Herein, we report NMR evidence for the successful capture of these Ti species under catalytic conditions.…”