Kinetics of initiation, propagation, and termination for the [rac-(C2H4(1-indenyl)(2))ZrMe][MeB(C6F5)(3)]-catalyzed polymerization of 1-hexene

Metallocene-catalyzed polymerization of I-alkenes offers fine control of cr itical polymer attributes such as molecular weight, polydispersity, tactici ty, and comonomer incorporation. Enormous effort has been expended on the s ynthesis and discovery of new catalysts and activators, but elementary aspe cts of the catalytic processes remain unclear. For example, it is unclear h ow the catalyst is distributed among active and dormant sites and how this distribution influences the order in monomer for the propagation rates, for which widely varying values are reported. Similarly, although empirical re lationships between average molecular weights and monomer have been establi shed for many systems, the underlying mechanisms of chain termination are u nclear. Another area of intense interest concerns the role of ion-pairing i n controlling the activity and termination mechanisms of metallocene-cataly zed polymerizations. Herein we report the application of quenched-flow kine tics, active site counting, polymer microstructure analysis, and molecular weight distribution analysis to the determination of fundamental rate laws for initiation, propagation, and termination for the polymerization of 1-he xene in toluene solution as catalyzed by the contact ion-pair, [rac-(C2H4(1 -indenyl)(2))ZrMe][MeB(C6F5)(3)] (1) over the temperature range of -10 to 5 0 degreesC. Highly isotactic (> 99% mmmm) poly-l-hexene is produced with no apparent enchained regioerrors. Initiation and propagation processes are f irst order in the concentrations of 1-hexene and 1 but independent of exces s borane or the addition of the contact ion-pair [PhNMe3][MeB(C6F5)(3)]. Ac tive site counting and the reaction kinetics provide no evidence of catalys t accumulation in dormant or inactive sites. Initiation is slower than prop agation by a factor of 70. The principal termination process is the formati on of unsaturates of two types: vinylidene end groups that arise from termi nation after a 1,2 insertion and vinylene end groups that follow 2,1 insert ions. The rate law for the former termination process is independent of the I-hexene concentration, whereas the latter is first order. Analysis of C-1 3-labeled polymer provides support for a mechanism of vinylene end group fo rmation that is not chain transfer to monomer. Deterministic modeling of th e molecular weight distributions using the fundamental rate laws and kineti c constants demonstrates the robustness of the kinetic analysis. Comparison s of insertion frequencies with estimated limits on the rates of ion-pair s ymmetrization obtained by NMR suggest that ionpair separation prior to inse rtion is not required, but the analysis requires assumptions that cannot be validated.