Methyl-Branched\nPolyethylene Produced by Hafnocene\nComplexes Exhibiting Unconventional Dialkoxy Bridges

Research on the homogeneous\npolymerization of propylene\nemploying\ngroup IV metallocene complexes primarily focuses on structures like <i>rac</i>-dimethylsilanediylbis(4-phenyl-7-methoxy-2-methylindenyl)hafnium\ndichloride (<i>rac</i>-<b>I</b>), featuring the long-established <i>Spaleck</i>-type −SiMe<sub>2</sub>– bridge between\ntwo indenyl fragments, while such complexes are rarely used in ethylene\npolymerization. This work describes the syntheses and polymerization\nperformances of two hafnocene complexes, <i>rac</i>-ethane-1,2-diyl((2-methyl-7-phenyl-1<i>H</i>-inden-4-yl)oxy)hafnium dichloride (<i>rac</i>-<b>II</b>) and <i>rac</i>-propane-1,3-diyl((2-methyl-7-phenyl-1<i>H</i>-inden-4-yl)oxy)hafnium dichloride (<i>rac</i>-<b>III</b>), bearing unconventional dialkoxy bridges of varying\nlengths. Single-crystal X-ray diffraction experiments enabled the\ncomparison of characteristic geometric parameters between the two\nsynthesized hafnocenes and <i>Spaleck</i>-type complex <i>rac</i>-<b>I</b>. Upon activation with different cocatalysts, <i>rac</i>-<b>II</b> and <i>rac</i>-<b>III</b> yielded polyethylene with exclusively methyl branches, contrary\nto long-chain branches, which are usually formed when group IV metallocene\ncomplexes are applied for ethylene polymerization. Thus, a branching\nmechanism related to the prominent chain-walking mechanism, which\nis frequently observed for late transition-metal complexes, is proposed.\nFurthermore, the degree of branching could be regulated by adjusting\nthe employed cocatalysts, polymerization temperatures, and bridge\nlengths, allowing the control of the polymers’ melting transition\ntemperatures.