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 °C were investigated via 1H 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 an 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. β-Hydride elimination of 1b in the presence of additives such as 1,2-dichloroethane and 2 results in catalyst decomposition.

Accurate active-site counts are necessary for the establishment of olefin polymerization kinetics, yet current techniques are often tedious and limited in sensitivity. Herein, we describe the development of a novel method for determining the fraction of initiated catalyst using standard gel-permeation chromatography (GPC). The first insertion of monomer into the chromophore-bearing zirconocene 1 generates a single equivalent of a labeled polymer chain. The polymer-bound label can be quantified as a function of polymer molecular weight using a GPC with a UV detector; simultaneous RI detection allows both the extent of monomer conversion and the molecular-weight distribution to also be established. We estimate that monomer to catalyst ratios of as high as 10,000:1 can be measured using this technique.

DNA polymerase β (Pol β) repairs single-nucleotide gapped DNA (sngDNA) by enzymatic incorporation of the Watson-Crick partner nucleotide at the gapped position opposite the templating nucleotide. The process by which the matching nucleotide is incorporated into a sngDNA sequence has been relatively well-characterized, but the process of discrimination from nucleotide misincorporation remains unclear. We report here NMR spectroscopic characterization of full-length, uniformly labeled Pol β in apo, sngDNA-bound binary, and ternary complexes containing matching and mismatching nucleotide. Our data indicate that, while binding of the correct nucleotide to the binary complex induces chemical shift changes consistent with the process of enzyme closure, the ternary Pol β complex containing a mismatching nucleotide exhibits no such changes and appears to remain in an open, unstable, binary-like conformation. Our findings support an induced-fit mechanism for polymerases in which a closed ternary complex can only be achieved in the presence of matching nucleotide.

Polyethylene molecular weight distributions observed by ESI-MS following the quench of a Cp(2)ZrCl(2)/MAO-catalyzed ethylene polymerization with N,N'-dicyclohexylcarbodiimide do not agree with GPC or NMR analysis of methanol-d(1) quenched samples; factors contributing to the discrepant results include high sample polydispersities, polymer insolubility, and the incomplete quench of metal-polymer species. Kinetic analyses of molecular weight distributions obtained by ESI-MS are not representative of the predominate monomer-consuming pathways.