Chromophore Quench-Labeling for Active Sites Counting in Ti-Based Ziegler–Natta Catalysts

Chromophore quench-labeling (CQL) is an elegant and effective method to count the fraction of active metal (<i>x</i>*) in olefin polymerizations mediated by molecular transition metal catalysts. In this study, the method was successfully applied for the first time to a heterogeneous Ti-based Ziegler-Natta catalyst of industrial relevance. CQL experiments using 1-hexene as the monomer ended up with a value of <i>x</i>* = 0.49 ± 0.09%, close to that measured for the same catalyst in the polymerization of propene under otherwise identical conditions using an alternative quenched flow (QF) approach. We ascribe such a low <i>x</i>* value to the fact that the catalytically active species are transient metastable surface Ti adducts, as proposed in the recent literature.