Zirconocenes as Initiators for Carbocationic Isobutene Homo- and Copolymerizations

Abstract

The zirconocene complexes Cp2ZrMe2 and Cp*2ZrMe2 activated with B(C6F5)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*2Zr(OH)2 and (Cp2ZrMe)2(μ-O) in the presence of B(C6F5)3 suggests that initiation by traces of protons is less efficient than initiation by cationic metallocene alkyl species, while oxo-bridged complexes (Cp‘2ZrMe)2(μ-O)/B(C6F5)3 (Cp‘ = C5H5 or C5H4SiMe3) are inactive.