Ultrarigid Indenyl-based Hafnocene Complexes for the Highly Isoselective Polymerization of Propene: Tunable Polymerization Performance Adopting Various Sterically Demanding 4-Aryl Substituents

Two novel silyl-bridged C₂-symmetric (2-methyl-4-aryl-7-methoxy) substituted bisindenyl based ansa-hafnocene complexes of varied steric demand (I, 4-phenyl; II, 4-[(3′,5′-methyl)-phenyl]) were synthesized and examined in the coordinative polymerization of propene. Both complexes enable a comparative study with the state of the art homogeneous metallocene catalyst (III, 4-[(3′,5′-tert-butyl)-phenyl]) for high melting ultrahigh molecular weight isotactic polypropylene. All three activated complexes exhibit extremely concise stereoregularity along with high molecular weights and high melting transitions at low to moderate polymerization temperatures. Increased sterical encumbrance of the 4-aryl substituent prevents the process of chain release reactions more effectively, especially due to enhanced reduction of β-methyl elimination. Accordingly, end group analysis disclosed the highest selectivity toward allylic chain ends as a result of β-methyl elimination with the less sterically encumbered complex I. Examination of the catalytic activity of I-III disclosed considerable impact of the varied 4-aryl substituents on the maximum productivity with respect to the applied polymerization conditions considering the combined influence of activation, monomer diffusion rate, catalyst deactivation, and rate of chain growth.