Zinc-Mediated Chain Transfer from Hafnium to Aluminum in the Hafnium-Pyridyl Amido-Catalyzed Polymerization of 1-Octene Revealed by Job Plot Analysis

Abstract

This report applies a Job plot analysis to demonstrate that dialkyl Zn reagents mediate chain transfer between Hf-pyridyl amido polymerization catalysts and AlR3 chain-transfer agents. Polyoctene produced in the presence of both ZnEt2 and AlEt3 is lower in molecular weight and has a narrower distribution than polyoctene produced in the presence of ZnEt2 alone. I2 labeling and NMR analysis of polymer chain ends enable quantification of the amount of Zn- and Al-polymeryls produced during polymerization. A Job plot of the number of Zn and Al polymer chains produced versus the mole fraction of ZnEt2 at constant moles of Zn + Al reveals a nonlinear relationship. The nonlinearity of the Job plot indicates cooperative exchange between Zn and Al. The chain-transfer agent, AlEt3, inhibits initiation, but not propagation, of polymerization as catalyzed by the Hf-pyridyl amido complex. This inhibition can be obviated by a two-step polymerization protocol. AliBu3 does not inhibit initiation or propagation and can be ...