Excellent Stereoregular Isotactic Polymerizations of Propylene with C2-Symmetric Silylene-Bridged Metallocene Catalysts

New C2-symmetric silylene-bridged metallocene compounds, [Me2Si(η5-RnC5H4−n)(η5-R′nC5H4−n)]MCl2 (M = Zr, Hf; Rn, R′n = Me, t-Bu, 2,4-Me, 2,3,5-Me3), were synthesized together with C1-symmetric ones (M = Zr, Hf; Rn = 2,3,5-Me3, 2,4-Me2, 3,4-Me2, t-Bu, Me; R′n = Me, H) for comparison and employed as the catalysts for isotactic polymerization of propylene in combination with methylaluminoxane. The C2-symmetric metallocenes bearing methyl groups at 2- or 5-positions gave polymers of the highest melting point (160–163 °C) and stereospecificity ([mmmm]: 97–99%).