This review gives an overview of approaches and techniques used for the assessment of active sites count in homogeneous group 4 metal single-site α-olefin polymerization. The main advantages and limitations of these methods, in particular their ability to selectively and quantitatively discern the catalytic sites effectively at work, leaving aside dormant sites and other metal species not directly involved in propagation, as well as the results of their application onto some important single-site olefin polymerization catalyst systems are exemplified. The associated mechanistic information is of interest for engineering more efficient catalytic systems reluctant towards side reactions.

Kinetic studies of homogeneous 1-hexene polymerization have been used for determining the propagation rates and active sites concentrations of industrially-relevant zirconocene catalytic systems incorporating {SBI}-and {Cp/Flu}-ancillaries: {rac-Me2Si(2-Me-Benz[e]Ind)2}ZrCl2 ({SBI}-1), {rac-Me2Si(2-Me-4-Ph-Ind)2}ZrCl2 ({SBI}-2), {Me2C(3,6-tBu2-Flu)(2-Me-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-1), {Ph(H)C(3,6-tBu2-Flu)(2-Et-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-2), {Ph2C(2,7-tBu2-Flu)(2-Me-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-3), {Ph(H)C(2,7-tBu2-Flu)(2-Me-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-4). The influence of different activation parameters, such as aging time and nature of coactivator (MAO vs boraluminoxane), has been investigated. It was found that the activation efficiency of {Cp/Flu}-type precatalysts by MAO is similar (112% at 30 °C) to that of the {SBI}-type precatalysts (418%). More, the propagation rates for the {Cp/Flu}-based systems appeared to be superior to those obtained with the {SBI}-type congeners. Deactivation processes, arising from monomer 2,1-misinsertions and resulting in dormant Msec-alkyl species, were demonstrated to occur for both types of catalytic systems. While the {SBI}-type systems are capable to undergo further regular 1,2-insertions of 1-hexene into the Msec-alkyl (or undergo regeneration of an active species through -H elimination), the {Cp/Flu}-type congeners are apparently reluctant to further enchainement; this is proposed to account for the observed overall lower productivity of the latter {Cp/Flu}-type systems. Dormant Msecalkyl species with the {Cp/Flu} ancillaries can be efficiently reactivated by introduction of small molecules (H2 or ethylene).

Industrially relevant single-site precatalysts used to produce isotactic polypropylene (iPP) include C2-symmetric {SBI} and C1-symmetric {Cp/Flu} complexes of group 4 metals. While the latter can produce iPPs with a higher degree of isotacticity, they also suffer from poor productivity compared to their {SBI} counterparts. Several causes for this trend have been suggested—2,1-Regioinsertions are frequently pointed out, as they are suspected to drive the catalyst into a dormant state. While this event does not seem to significantly impact the productivity of {SBI} systems, the influence of these regioerror is poorly documented for isoselective {Cp/Flu} precatalysts. To address this issue, new Ph2X(Cp)(Flu) (Ph2X = Ph2C, FluC, Ph2Si) proligands (2a–k) and some of the corresponding dichlorozirconocenes (3a–h,k) were synthesized. These new compounds were characterized and tested in homogeneous propylene polymerization at 60 °C and the amounts of regioerrors in the resulting polymers were examined by 13C NMR spectroscopy. A possible correlation between poor productivity and a high number of regioerrors was investigated and is discussed. Furthermore, a C-H activation process in the bulky nBu3C substituent upon activation of 4c (the dimethylated analog of 3c) by B(C6F5)3 has been evidenced by NMR; DFT calculations support this C-H activation as a deactivation mechanism.