Catalyst Mileage in Olefin Polymerization: The Peculiar Role of Toluene

Ti–C homolysis
is generally regarded as a catalyst deactivation
step in olefin polymerization catalyzed by titanium complexes. The
present work demonstrates that for industrially relevant Ti catalysts
in toluene and related solvents a “detour” via radical
activation of the solvent can prolong catalyst lifetime and productivity,
leading to chain transfer to solvents (CTS). For differently substituted
phosphinimide half-titanocenes and other Ti catalyst classes, i.e.
constrained geometry (CGC) and amidinate catalysts, CTS leads to formation
of up to ∼60% benzylated chains. The efficiency of the reactivation
pathway depends mostly on steric factors and correlates well with
the percentage of buried volume, %VBur, as well with as DFT predictors. Thus, the solvent is far from innocent,
and catalyst behavior in such solvents may not be representative of
polymerizations in more innocent saturated hydrocarbons. On the other
hand, it might be the case that, in truly innocent solvents, the monomer
itself can play a less innocent role.