Ethylene Polymerization Activity of Zr- and Ti-FI Catalysts

The FI ligand structure has a significant influence on the ethylene polymerization activity in particular, modification of the R2 substituent has a dramatic effect on the activity (Table 1) [12, 54, 55], Namely, R2 substituents that are sterically smaller than a f-Bu group [i.e., t -Pr (6), Me (7)] significantly reduce the activity (activity 1 kg mmoF1 Ir1). By contrast, R2 substituents that are sterically larger than the f-Bu group markedly enhance the activity. The activity is thus directly correlated to the steric bulk of the R2 substituent. For example, in the sequence f-Bu (1, 8) adamantyl (9) cumyl (10) 1,1-diphenylethyl (11), the activity increases from 519 (1 R1 = phenyl, R2 = f-Bu) to 2383 kg mmol-1 h-1 (11 R1 = phenyl, R2 = 1,1-diphenylethyl). 

Our rationalization of the significant effect of R2 with regard to the activity enhancement is as follows  

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

Large R2 substituents induce effective ion-separation between the cationic active species and an anionic cocatalyst, which allows more space for ethylene coordination to the metal and for its insertion into the carbon-metal bond. In addition, electronically, the ion separation increases the electrophilicity of the catalytically active species and hence enhances the reactivity toward ethylene. 

Conversely, Ti-FI catalysts with MAO normally show lower (but still high) ethylene polymerization activities and form higher molecular weight PEs relative to the Zr congeners. The ethylene polymerization activities of Ti-FI catalysts were also 

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