Titanocenes in Olefin Polymerization: Sustainable Catalyst System or an Extinct Species?

Abstract

Two novel silyl-bridged C 2 -symmetric 2-methyl-4-aryl-7-methoxysubstituted bis-indenyl-based titanocene complexes with varied steric demand (a, 4-(3′,5′-dimethyl)phenyl; b, 4-(3′,5′-di-tert-butyl)phenyl) were synthesized, characterized, and examined in the coordination polymerization of propene. Both adapted ligand structures have proven their capability as precise catalysts in the formation of stereodefect-and regiodefect-free isotactic polypropylene. Several activation pathways to the catalytically active, cationic complexes were analyzed in terms of catalytic activity and stability, taking into account the influence of polymerization temperature, monomer concentration, polymerization time, and type of applied scavenger. The overall lowest activities were observed using the methylaluminoxane (MAO) activated catalyst. The two-step activation mechanism of in situ alkylation with an excess of triisobutylaluminum (TIBA) and subsequent addition of [Ph 3 C][B(C 6 F 5 ) 4 ] resulted in moderate productivities of the respective catalyst systems. However, the highest catalytic activities were observed when eliminating the in situ alkylation step by application of bis-methylated titanocenes in combination with [Ph 3 C][B(C 6 F 5 ) 4 ]. The latter activation mechanism in combination with the sterically more encumbered ligand framework b results in the most productive titanium-based metallocene catalyst for the polymerization of propene to date, at least with respect to reasonable reaction times. The determined molecular weights of the produced polymers were significantly affected by the ligand structures a and b but were only negligibly influenced by the applied activation method. End-group analysis via 1 H NMR spectroscopy disclosed a chain release mechanism dominated by β-hydride elimination. In accordance with the observed accurate stereo-and regiocontrol mechanism, extraordinarily high melting transitions of up to 170 °C (ex reactor) underline the remarkable potential of these titanium-based catalyst systems in the polymerization of propene.

On the Nature of the Lewis Acidic Sites in “TMA‐Free” Phenol‐Modified Methylaluminoxane

“…[5,[32][33][34][35][36] The [AlMe 2 ] + fragments would be (primarily) released from sites carrying a "structural" TMA molecule (Scheme 2). [5,16,29,31] The presence of residual TMA is sometimes undesirable in MAO, since it promotes chain termination via irreversible transalkylation with the active species, leading to reduced polymer molecular weights, [37][38][39] and it is often involved in catalyst deactivation/decomposition [40][41][42] and formation of dormant species. [38,[43][44][45][46][47] Hence, several modifications of MAO have been proposed to minimize or completely remove the TMA component.…”

“…The presence of residual TMA is sometimes undesirable in MAO, since it promotes chain termination via irreversible transalkylation with the active species, leading to reduced polymer molecular weights, and it is often involved in catalyst deactivation/decomposition and formation of dormant species , . Hence, several modifications of MAO have been proposed to minimize or completely remove the TMA component.…”

Chromocene–Cyclopentadienyltitanium Trichloride Ion Pairs and Their Rearrangement to Titanocene Chloride–Cyclopentadienylchromium Dichlorides – Ethylene Polymerization Tests

“…While the MAO system operates via methyl transfer/abstraction, the SiHB functions by hydride transfer/abstraction, affording an unstable titanocene hydride species that easily undergoes reduction to paramagnetic Ti III . Such Ti III species have been proposed to be inactive in olefin polymerization , . In order to oxidize the inactive Ti III species into active Ti IV ones chloroform which has been shown to chlorinate 8 to Cp 2 TiCl 2 and (CpCrCl 2 ) 2 (vide supra) was used as a co‐activation agent.…”

Metalation Studies on Titanocene Dithiolates

“…However, it was also found that the titanium catalysts were rather quickly deactivated and the corresponding zirconium compounds showed much higher stability and catalytic activity. Thus, the mainstream research on metallocene catalysts stopped for titanium-based metallocenes nearly completely until recently when the concepts of "Green Chemistry" and "Sustainable Catalysis" came into play [9][10][11][12]. Another area of "applied research" opened up after the discovery that Cp 2 TiCl 2 showed antitumor activity [13][14][15].…”