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

Two novel silyl-bridged C2-symmetric 2-methyl-4-aryl-7-methoxy-substituted 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 [Ph3C]­[B­(C6F5)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 [Ph3C]­[B­(C6F5)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 1H 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.