Cationic and dicationic zirconocene compounds as initiators of carbocationic isobutenepolymerisation

Abstract

We report the polymerisation of isobutene with the intramolecular aryl stabilised zirconocene cations [η⁵-Cp-(η⁵-C₅H₄-CMe₂C₆H₄Me)ZrMe]⁺ (3), [(η⁵-C₅H₄-CMe₂C₆H₄Me)₂ZrMe]⁺ (4) and dication [(η⁵-C₅H₄-CMe₂C₆H₄Me)₂Zr]²⁺ (5) (Cp = C₅H₅) which were generated in situ by the reaction of the corresponding dimethyl compound with the cation generating agent (CGA) B(C₆F₅)₃. Whereas the cationic compounds gave polyisobutene in moderate yields, the dicationic compound gave polyisobutene in excellent yields and high molecular weight. Computational studies of the cationic and dicationic compounds were undertaken to ascertain the nature of the cationic initiator. Judging from natural charges and bond critical points, the coordinated monomer is more strongly polarised in the case of the dication 5, compared with the monocation 3. Furthermore, in the case of the cationic compound 3, decomposition into the allylic compound was computed. We found that the activation barrier of this process is 103.3 kJ mol⁻¹, which is too high for a direct C–H activation by the Zr–Me group.