Methyl abstraction of ZrMe{N(SiMe3)(2)}(3) with B(C6F5)(3) gives the cationic zirconium amide complex [Zr{N(SiMe3)(2)}(3)](+) which acts as an initiator for the carbocationic polymerisation of isobutene and for isobutene-isoprene copolymerisations. High molecular weight homo-and co-polymers are obtained at temperatures up to -30 degrees C. Molecular weights are less temperature dependent than in metal halide initiated systems.