Kinetics of Initiation, Propagation, and Termination for the [rac-(C₂H₄(1-indenyl)₂)ZrMe][MeB(C₆F₅)₃]-Catalyzed Polymerization of 1-Hexene

Abstract

Metallocene-catalyzed polymerization of 1-alkenes offers fine control of critical polymer attributes such as molecular weight, polydispersity, tacticity, and comonomer incorporation. Enormous effort has been expended on the synthesis and discovery of new catalysts and activators, but elementary aspects of the catalytic processes remain unclear. For example, it is unclear how 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 relationships between average molecular weights and monomer have been established for many systems, the underlying mechanisms of chain termination are unclear. Another area of intense interest concerns the role of ion-pairing in controlling the activity and termination mechanisms of metallocene-catalyzed polymerizations. Herein we report the application of quenched-flow kinetics, 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-hexene in toluene solution as catalyzed by the contact ion-pair, [rac-(C(2)H(4)(1-indenyl)(2))ZrMe][MeB(C(6)F(5))(3)] (1) over the temperature range of -10 to 50 degrees C. Highly isotactic (>99% mmmm) poly-1-hexene is produced with no apparent enchained regioerrors. Initiation and propagation processes are first order in the concentrations of 1-hexene and 1 but independent of excess borane or the addition of the contact ion-pair [PhNMe(3)][MeB(C(6)F(5))(3)]. Active site counting and the reaction kinetics provide no evidence of catalyst accumulation in dormant or inactive sites. Initiation is slower than propagation by a factor of 70. The principal termination process is the formation of unsaturates of two types: vinylidene end groups that arise from termination after a 1,2 insertion and vinylene end groups that follow 2,1 insertions. The rate law for the former termination process is independent of the 1-hexene concentration, whereas the latter is first order. Analysis of (13)C-labeled polymer provides support for a mechanism of vinylene end group formation that is not chain transfer to monomer. Deterministic modeling of the molecular weight distributions using the fundamental rate laws and kinetic constants demonstrates the robustness of the kinetic analysis. Comparisons of insertion frequencies with estimated limits on the rates of ion-pair symmetrization obtained by NMR suggest that ion-pair separation prior to insertion is not required, but the analysis requires assumptions that cannot be validated.

Alk‐1‐ene Polymerization in the Presence of a Monocyclopentadienyl Zirconium(IV) Acetamidinate Catalyst: Microstructural and Mechanistic Insights

“…This may be taken as an indication that, with the latter two monomers, the process is monomer-assisted, and possibly proceeds as shown in Figure 4B. An alternative interpretation is that, as proposed for metallocene catalysts by Landis et al, [24] the process is actually intramolecular, but its rate coincides with that of 2,1 monomer mis-insertion, because once a secondary chain is generated this is too sterically hindered to propagate and can only undergo b-H transfer to the metal. As a matter of fact, as noted before (Table 1), internal regioirregular units in poly(but-1-ene) and poly(hex-1-ene) are below 13 C NMR detectability (<0.1 mol-%, indicatively).…”

Metallocene-catalyzed alkene polymerization and the observation of Zr-allyls

“…Polymerization of 1-hexene catalyzed by [(EBI)ZrMe][MeB(C 6 F 5 ) 3 ] at Ϫ40°C enables direct characterization of growing Zr-polymeryl species, monitoring of monomer consumption kinetics, determination of instantaneous active site counts, and measurement of termination rates. Finer details of the polymerization mechanism, such as demonstration that chain growth occurs in a continuous rather than intermittent manner (8,12), and the mechanism of chain-end epimerization …”

“…2). The corresponding elementary rate laws and kinetic constants completely describe the catalytic kinetics and the distribution of polymer products (8). We have shown that a combination of active site counting methods, quenched flow reaction kinetics, and polymer end group analysis applied to 1-hexene polymerization catalyzed by B(C 6 F 5 ) 3 2 ], lead to rigorous characterization of the kinetic rate laws (8)(9)(10)(11).…”

Generalized treatment of NMR spectra for rapid chemical reactions

“…However, the fundamental mechanism underlying alkene polymerization with single-site catalysts is not fully understood, and kinetic studies are particularly challenging. Our research group has successfully studied polymerization of hexene catalyzed by [(rac-C 2 H 4 (indenyl) 2 )Zr(Me)][MeB(C 6 F 5 ) 3 ] using quenched-flow and in situ NMR methods (64)(65)(66)(67)(68). The quenched-flow method is an indirect method, whereas direct observation using NMR spectroscopy has been applicable only to slow catalyst systems at low temperatures.…”