Despite decades of thorough mechanistic investigations, it is still hard to predict the activity of a novel olefin polymerisation catalyst, even when the precursor is a well-defined molecular entity. In the present study, we highlight the crucial importance of activation entropy on the polymerisation rate and how weak interactions of the catalytic species with electron donating species in the reaction pool can ultimately lower the activation free energy.

Quenched-flow studies of MgCl2 -supported Ziegler-Natta catalysts were combined for the first time with (13) C NMR fingerprinting of the nascent polymer and conclusively proved that, depending on the catalyst formulation, propene polymerization can be slowed down significantly by the occurrence of the few regiodefects (2,1 monomer insertions), changing active sites into dormant sites. Catalysts modified with ethylbenzoate show little dormancy. The more industrially relevant phthalate based catalysts, instead, are highly dormant and require the presence of H2 to counteract the deleterious effect of this self-poisoning on productivity and stereoselectivity.

Accurately measuring molecular kinetics of catalytic olefin polymerization has been a challenging objective. Many methods have been proposed in the literature, but all of them have drawback(s). In this paper, we introduce a labeling method to count active sites employing methyl propargyl ether (MPE) as the quench-labeling agent. It is commercially available, does not react with Al-alkyl species, and has a labeling efficiency close to 100%. The labeling reaction was evidenced by a mechanistic study on the reaction between the model system Cp 2 ZrMe 2 /MAO (Cp = cyclopentadienyl) and MPE that it may occur through a coordination−insertion mechanism without noticeable multiple insertions. The method was benchmarked by studying a MgCl 2 -supported Ziegler−Natta catalyst in 1-hexene polymerization. The fraction of the active transition metal χ* is found to be <1%. It rises in the very first few seconds and reaches a plateau within 10 s in the absence of precontact. With precontact, χ* remains constant over polymer yield from 0.3 up to 5.3 g(PH) g(cat) −1 , demonstrating that χ* does not necessarily grow during polymerization. Longer precontact resulted in decreased χ* and decreased average rate constant of propagation which is in the order of 10 2 M −1 s −1 .