Key intermediates in metallocene-and post-metallocene-catalyzed polymerization

The structures of intermediates formed upon the activation by methylaluminoxane (MAO) of a wide range of metallocene and post-metallocene catalysts of olefin polymerization were studied by ¹³C, ¹H, and ¹⁹F NMR. For all metallocenes considered (L₂ZrCl₂ and L₂TiCl₂), under conditions similar to real polymerization conditions (Al/Zr > 200), two types of intermediates were identified in the reaction solution, namely, heterodinuclear ion pairs L₂ M(μ-Me)₂AlMe₂]⁺Me-MAO]^? (III) and zwitterionic intermediates L₂ MMe⁺←Me-Al^?≡MAO (IV (M = Zr, Ti). The relative concentration of III increases with an increase in the Al/Zr ratio. In the post-metallocene/MAO catalytic systems, the reaction solution can be dominated either by heterodinuclear pairs of type III (bis(imino)pyridyl iron complexes) or by zwitterionic intermediates of type IV (half-titanocenes, complexes with restricted geometry). Both species III and species IV catalyze olefin polymerization. Both the species initiating polymerization, L ₂ ^′ TiMe(S)]⁺Me-MAO]^?, and the species responsible for chain growth, L L ₂ ^′ TiP]⁺Me-MAO]^? (P is the polymer chain, and S is a solvent molecule), were characterized in the bis(phenoxyimine) titanium complex/MAO system.