Methyl-Branched Polyethylene Produced by Hafnocene Complexes Exhibiting Unconventional Dialkoxy Bridges

Abstract

Research on the homogeneous polymerization of propylene employing group IV metallocene complexes primarily focuses on structures like rac-dimethylsilanediylbis(4-phenyl-7-methoxy-2-methylindenyl)hafnium dichloride (rac-I), featuring the long-established Spaleck-type −SiMe₂– bridge between two indenyl fragments, while such complexes are rarely used in ethylene polymerization. This work describes the syntheses and polymerization performances of two hafnocene complexes, rac-ethane-1,2-diyl((2-methyl-7-phenyl-1H-inden-4-yl)oxy)hafnium dichloride (rac-II) and rac-propane-1,3-diyl((2-methyl-7-phenyl-1H-inden-4-yl)oxy)hafnium dichloride (rac-III), bearing unconventional dialkoxy bridges of varying lengths. Single-crystal X-ray diffraction experiments enabled the comparison of characteristic geometric parameters between the two synthesized hafnocenes and Spaleck-type complex rac-I. Upon activation with different cocatalysts, rac-II and rac-III yielded polyethylene with exclusively methyl branches, contrary to long-chain branches, which are usually formed when group IV metallocene complexes are applied for ethylene polymerization. Thus, a branching mechanism related to the prominent chain-walking mechanism, which is frequently observed for late transition-metal complexes, is proposed. Furthermore, the degree of branching could be regulated by adjusting the employed cocatalysts, polymerization temperatures, and bridge lengths, allowing the control of the polymers’ melting transition temperatures.