Zirconocenes as Initiators for Carbocationic Isobutene Homo- and Copolymerizations

The zirconocene complexes Cp 2 ZrMe 2 and Cp* 2 ZrMe 2 activated with B(C 6 F 5 ) 3 initiate the carbocationic polymerization of isobutene and isobutene−isoprene copolymerizations to IIR rubbers at temperatures as high as −30 °C. Unlike conventional metal halide initiators, these metallocene-based initiator systems produce both homo- and copolymers of broadly similar molecular weights. Copolymers prepared in the presence of ca. 2 mol % isoprene show a diene incorporation rate of 1.4−1.7%, with the typical 1,4-trans structure. Comparison of the effectiveness of zirconocene dialkyls with that of the metallocene hydrolysis products Cp* 2 Zr(OH) 2 and (Cp 2 ZrMe) 2 (μ-O) in the presence of B(C 6 F 5 ) 3 suggests that initiation by traces of protons is less efficient than initiation by cationic metallocene alkyl species, while oxo-bridged complexes (Cp‘ 2 ZrMe) 2 (μ-O)/B(C 6 F 5 ) 3 (Cp‘ = C 5 H 5 or C 5 H 4 SiMe 3 ) are inactive.