Mechanistic Investigations into the Behavior of a Labeled Zirconocene Polymerization Catalyst

Kinetics associated with the [(SBI)Zr(CH2SiMe2(C6H4)NMe2)][MeB(C6F5)(3)] (1a)-catalyzed polymerization of 1-hexene in a mixed toluene-d(8)/chlorobenzene-d(5) solvent at -33 degrees C were investigated via H-1 NMR and compared to the kinetics associated with the (SBI)ZrMe(MeB(C6F5)(3)) (1c)-catalyzed polymerization of 1-hexene under identical conditions. In the presence of 1-hexene, both catalysts form an identical propagating species, (SBI)Zr(poly-1-hexyl)(MeB(C6F5)(3)) (1b), but the concentration of 1b during 1a-catalyzed polymerization is only ca. 40% of the anticipated value. Under reaction conditions, 1b reacts reversibly with the model complex p-TMS-C6H4-NMe2 (2) to yield the outer-sphere ion pair tentatively identified as [(SBI)Zr(poly-1-hexyl)(2)][MeB(C6F5)(3)] (1e), which acts as art essentially dormant site during 1-hexene polymerization. Warming of 1b in the absence of additives generates the well-defined hydridoborate complex (SBI)ZrMe(HB(C6F5)(3)) (1d), which does not reinitiate in the presence of 1-hexene. beta-Hydride elimination of 1b in the presence of additives such as 1,2-dichloroethane and 2 results in catalyst decomposition.