Zirconocene-Catalyzed Polymerization of α-Olefins: When Intrinsic Higher Activity Is Flawed by Rapid Deactivation

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-tBu(2)-Flu)(2-Me-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-1), {Ph(H)C(3,6-tBu(2)-Flu)(2-Et-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-2), {Ph2C(2,7-tBu(2)-Flu)(2-Me-4-tBu-Cp)}ZrCl2 ({Cp/Flu}-3), {Ph(H)C-(2,7-tBu(2)-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 degrees C) to that of the {SBI}-type precatalysts (4-18%). Moreover, 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 beta-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-sec-alkyl species with the {Cp/Flu} ancillaries can be efficiently reactivated by introduction of small molecules (H-2 or ethylene).