Chain Transfer with Dialkyl Zinc During Hafnium–Pyridyl Amido-Catalyzed Polymerization of 1-Octene: Relative Rates, Reversibility, and Kinetic Models
HeatherC. Johnson, Eric S. Cueny, Clark R. Landis
Index: 10.1021/acscatal.8b00524
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Abstract
Kinetic modeling is applied to the polymerization of 1-octene catalyzed by Hf–pyridyl amido complex 1/[CPh3][B(C6F5)4] in the presence of chain-transfer agent ZnEt2. The model reproduces monomer consumption, end group data, and the bimodal MWD (molecular weight distribution) of polyoctene. Consistent with experimental data, the model suggests chain transfer of Hf–polymeryls to ZnEt2 is an essentially irreversible process. However, to adequately fit the MWDs, slow exchange between two Zn(Et)Pol species to produce ZnEt2 and ZnPol2 is proposed. The model shows agreement with experimental data that approximately one polymer chain per ZnEt2 is produced. Changes in the MWDs are observed upon raising or lowering the polymerization temperature; we propose these changes arise due to increasing rates of Zn disproportionation reactions at elevated temperatures. ZnMe2 also is examined as a chain-transfer agent using chromophore quench-labeling techniques. Qualitative comparison of RI (“bulk” polymer) and UV (catalyst-bound polymer) MWDs show that chain transfer to ZnMe2 is faster than for ZnEt2, and the bulk polymer distributions are trimodal. Furthermore, evidence for >1 chain per ZnMe2 is observed by I2 labeling of polymer chains. These results highlight the dramatic effects of the sterics of the chain-transfer agent upon the MWD of polyoctene produced using catalyst 1.