Kinetics of Initiation, Propagation, and Termination for the [rac-(C2H4(1-indenyl)2)ZrMe][MeB(C6F5)3]-Catalyzed Polymerization
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- 作者：Zhixian Liu ; Ekasith Somsook ; Curtis B. White ; Kimberly A. Rosaaen ; and Clark R. Landis
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：November 14, 2001
- 年：2001
- 卷：123
- 期：45
- 页码：11193 - 11207
- 全文大小：225K
- 年卷期：v.123,no.45(November 14, 2001)
- ISSN：1520-5126
文摘
Metallocene-catalyzed polymerization of 1-alkenes offers fine control of critical polymer attributessuch as molecular weight, polydispersity, tacticity, and comonomer incorporation. Enormous effort has beenexpended on the synthesis and discovery of new catalysts and activators, but elementary aspects of the catalyticprocesses remain unclear. For example, it is unclear how the catalyst is distributed among active and dormantsites and how this distribution influences the order in monomer for the propagation rates, for which widelyvarying values are reported. Similarly, although empirical relationships between average molecular weightsand monomer have been established for many systems, the underlying mechanisms of chain termination areunclear. Another area of intense interest concerns the role of ion-pairing in controlling the activity and terminationmechanisms of metallocene-catalyzed polymerizations. Herein we report the application of quenched-flowkinetics, active site counting, polymer microstructure analysis, and molecular weight distribution analysis tothe determination of fundamental rate laws for initiation, propagation, and termination for the polymerizationof 1-hexene 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 50 
C. Highly isotactic (>99% mmmm) poly-1-hexene isproduced with no apparent enchained regioerrors. Initiation and propagation processes are first order in theconcentrations of 1-hexene and 1 but independent of excess borane or the addition of the contact ion-pair[PhNMe3][MeB(C6F5)3]. Active site counting and the reaction kinetics provide no evidence of catalystaccumulation in dormant or inactive sites. Initiation is slower than propagation by a factor of 70. The principaltermination process is the formation of unsaturates of two types: vinylidene end groups that arise fromtermination after a 1,2 insertion and vinylene end groups that follow 2,1 insertions. The rate law for the formertermination process is independent of the 1-hexene concentration, whereas the latter is first order. Analysis of13C-labeled polymer provides support for a mechanism of vinylene end group formation that is not chaintransfer to monomer. Deterministic modeling of the molecular weight distributions using the fundamental ratelaws and kinetic constants demonstrates the robustness of the kinetic analysis. Comparisons of insertionfrequencies 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.
C. Highly isotactic (>99% mmmm) poly-1-hexene isproduced with no apparent enchained regioerrors. Initiation and propagation processes are first order in theconcentrations of 1-hexene and 1 but independent of excess borane or the addition of the contact ion-pair[PhNMe3][MeB(C6F5)3]. Active site counting and the reaction kinetics provide no evidence of catalystaccumulation in dormant or inactive sites. Initiation is slower than propagation by a factor of 70. The principaltermination process is the formation of unsaturates of two types: vinylidene end groups that arise fromtermination after a 1,2 insertion and vinylene end groups that follow 2,1 insertions. The rate law for the formertermination process is independent of the 1-hexene concentration, whereas the latter is first order. Analysis of13C-labeled polymer provides support for a mechanism of vinylene end group formation that is not chaintransfer to monomer. Deterministic modeling of the molecular weight distributions using the fundamental ratelaws and kinetic constants demonstrates the robustness of the kinetic analysis. Comparisons of insertionfrequencies 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.