Isotactic polypropylene produced by rac/meso mixtures of hafnocene complexes through selective rac activation

Abstract

Meso isomers of indenyl‐based group IV hafnocene complexes produce atactic polypropylene (aPP) and thus, have to be tediously separated from their racemic analogs for the production of isotactic polypropylene (iPP). Herein, we report a facile activation protocol to selectively convert only the racemic precatalysts into catalytically active species when isomeric precatalyst mixtures are employed. At the same time, the meso isomers are deactivated, thus yielding no polypropylene at all. The macromolecular characteristics of iPP, that is, melting transition, molecular weight, and tacticity, obtained by this protocol were analog to the characteristics of iPP produced by the corresponding pure racemic isomers, as proven for different complexes, isomeric ratios, and polymerization conditions. Nuclear magnetic resonance experiments revealed that the employed precatalyst activation protocol – that is, alkylation with triisobutylaluminum and subsequent generation of a free coordination site with [Ph3C][B(C6F5)4] – led to catalytically active, binuclear hydride‐bridged racemic species but inactive meso resting states. The separation of both precatalyst isomers, which was believed to be crucial to obtaining pure iPP since the 1980s, rendered hafnocene syntheses unnecessarily laborious and can be circumvented by the activation protocol established in this work.