New sterically hindered unbridged zirconocene complexes with 1,2-diphenylcyclopentadienyl ligands: synthesis, structure and properties as olefin polymerization catalysts

New unbridged zirconocene complexes, bis(1,2-diphenylcyclopentadienyl)zirconium dichloride (3) and bis(4-methy-1,2-diphenylcyclopentadienyl)zirconium dichloride (4) have been synthesized in high yield by reaction of ZrCl4 with the lithium salt of 1,2-diphenylcyclopentadiene (1) or 4-methyl-1,2-diphenylcyclopentadiene (2), respectively. X-ray crystallographic analysis reveals both complexes are in racemic-like conformation with near C2 symmetry in solid state. When activated with methylaluminoxane (MAO), 3 and 4 exhibit moderate catalytic activities for ethylene polymerization at relatively low Al:Zr ratios, producing high molecular weight (>106) polyethylenes with high melting transition temperatures. Atactic propylene oligomers with average molecular weight of ∼103 g mol−1 were produced by both 3 and 4 at relatively high catalytic activities upon activation with MAO.