Effects of the Linking of Cyclopentadienyl and Ketimide Ligands in Titanium Half‐Sandwich Olefin Polymerization Catalysts

Abstract

The role of the ketimide ligand geometry in Ti half‐sandwich complexes and the consequent effects in olefin polymerization catalysis (ethylene, styrene, 1‐hexene polymerization, and ethylene/1‐hexene copolymerization) were investigated under various conditions. [CpTiCl2(N=CtBu2)] (1; Cp=η5‐cyclopentadienyl) was used as a reference compound for comparison with the recently described complex [{η5‐C5H4CMe2CMe2C(tBu)=N‐κN}TiCl2] (2 a) and a new derivative that has a longer linker between Cp and the ketimide, [{η5‐C5H4CH2CH2CMe2C(tBu)=N‐κN}TiCl2] (9). The presence of a distorted intramolecularly tethered ketimide moiety reduces the polymerization activity significantly in systems that contain Al‐based cocatalysts (methylaluminoxane, triisobutylaluminum). However, in Al‐free systems both types of compounds provided active polymerization catalysts. Notably, the recently reported activation system Et3SiH/B(C6F5)3 was for the first time demonstrated to activate Ti complexes for ethylene and 1‐hexene (co)polymerization catalysis by hydride transfer.

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

“…Under comparable conditions, compounds 8 and 9 activated by SiHB produced polyethylene with melting temperature 134 °C and 135 °C, respectively, with a yet lower activity of 3 kg PE (mol Ti · h · bar) –1 . However, the use of a sixfold excess of B(C 6 F 5 ) 3 resulted in an increase in activity of precatalyst 9 nearly by a factor of 7 (Table , run 3), similarly as reported earlier for the Cp 2 TiCl 2 /SiHB system …”

“…In contrast, compounds 8 – 11 withstand heating in toluene to 100 °C for 24 h, neither decomposing to the dichlorides CpCp′TiCl 2 , nor dissociating to homometallic dimers. Therefore compounds 8 , 9 , and 11 were selected for examination of their potential in forming catalysts for polymerization of ethylene using MMAO, trimethylaluminum or the B(C 6 F 5 ) 3 /Et 3 SiH system (SiBH), for the precatalyst activation. On the other hand, it is expected that binuclear complexes will dissociate under the influence of activation agents; hence, Cp 2 TiCl 2 , [CpCrCl 2 ] 2 and their metal equimolar mixture (Ti + Cr) were also examined as precatalysts.…”

“…None of these sites should be, however, based on chromium, since the (CpCrCl 2 ) 2 /SiHB system showed extremely low activity (Table , run 8) and a trimodal PE distribution (Figure S5). Moreover, the Cp 2 TiCl 2 /SiHB system also produced bimodal polyethylenes in toluene . Activation of titanocene derivatives with the SiHB co‐catalyst is currently under investigation and will be the subject of a forthcoming paper.…”

“…The thermally stable complexes 8 , 9 , and 11 of suitable solubility in toluene were activated with MMAO to compare their behavior with the generally used titanocene and chromium systems . The activation of the above complexes was also examined by an aluminum‐free borane/hydrosilane system B(C 6 F 5 ) 3 /Et 3 SiH (SiHB), which has been recently used to activate titanium half‐sandwich ketimide complexes and zirconocene dihalides …”

“…Experiments with activating 8 , (CpCrCl 2 ) 2 , or Cp 2 TiCl 2 with AlMe 3 in toluene revealed almost negligible polymerization activity (Table S13), and absence of any ethylene oligomers in the products. For the case of the B(C 6 F 5 ) 3 /Et 3 SiH (SiHB) system, it was found that it activates the half‐sandwich titanium ketimide complex [Cp( t Bu 2 C=N)TiCl 2 ] to polymerize ethylene with good activity [210 kg PE (mol Ti · h · bar) –1 ], however, the catalytic system Cp 2 TiCl 2 /SiHB under the same conditions exerted activity of only 5 kg PE (mol Ti · h · bar) –1 . Under comparable conditions, compounds 8 and 9 activated by SiHB produced polyethylene with melting temperature 134 °C and 135 °C, respectively, with a yet lower activity of 3 kg PE (mol Ti · h · bar) –1 .…”