Anion influence in metallocene-based olefin polymerisation catalysts

The effects of perfluoroarylborate anions on the activities of metallocene-based propylene polymerisation catalysts on the one hand, and on the cationic polymerisation of isobutylene to butyl rubber are summarised. In both systems the anions follow the same activity sequence, [CN{B(C6F5)3}2]−  > [H2N{B(C6F5)3{2]−] ≈ [B(C6F5)4]− > [MeB(C6F5)3]−. In an effort to maximise the delocalisation of negative charge over large volumes, new anions based on the very bulky borane B(C6F4-2-C6F5)3 (PBB) with longer linking groups have been prepared, e.g [N(CN-PBB)2]−. The van der Waals volumes of representative anions and cations are reported. The influence of anions on the structure of catalytically active species is demonstrated using (SBI)Zr(CH2SiMe3)(µ-Me)B(C6F5)3 and [(SBI)Zr(CH2SiMe3)+ · · · B(C6F5)equation/tex2gif-stack-1.gif] as examples. The alkyl ligand in the latter is stabilised by agostic bonding, which relegates the anion into the outer coordination sphere. The differences in ion pair structure profoundly influence the nature of chain termination in 1-hexene polymerisations: whereas the MeB(C6F5)equation/tex2gif-stack-2.gif catalyst gives a mixture of vinylene and vinylidene terminations, with the latter predominanting at higher temperatures and conversions, the B(C6F5)equation/tex2gif-stack-3.gif system terminates exclusively after 2,1-misinsertion to give vinylene-terminated poly(1-hexene).