Synthesis of double-end-capped polyethylene by a cationic tris(pyrazolyl)borate zirconium benzyl complex

The cationic complexes Tp*Zr(CH₂Ph)₂⁺ (I, B(C₆F₅)₄⁻ salt; Tp* = HB(3,5-Me2-pyrazolyl)3) and {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)+ (II; Me₂pz = 3,5-Me₂-pyrazolyl) polymerize ethylene at −78 to −60 °C to linear polyethylene (PE) without significant chain transfer. For I, chain growth takes place at only one benzyl group. Quenching these polymerizations with MeOH yields benzyl-capped PE (PhCH₂(CH₂CH₂)_nCH₂CH₃). Quenching I-catalyzed ethylene polymerization with Br2 yields double-end-capped PE containing benzyl and bromo chain ends (PhCH₂(CH₂CH₂)_nCH₂CH₂Br). DFT calculations on model catalysts show that ethylene insertion into a Zr−η²-CH₂Ph bond requires greater structural distortion than insertion into a Zr−CH₂CH₂CH₂Ph bond. The calculations also show that the β-H transfer to monomer and β-H elimination chain transfer pathways are both strongly disfavored for I, but the β-H transfer to monomer path may be possible for II.