Kinetics of short-duration ethylene polymerization with MgCl2-supported Ziegler-Natta catalyst: Two-stage initiation evidenced by changes in active center concentration

摘要

The kinetics of ethylene polymerization with a TiCl4/MgCl2-type Ziegler-Natta catalyst was studied. Changes in polymerization activity and concentration of active centers (C*) in the first 5 min were determined. Initiation of the active centers was found to proceed in two stages. In the first stage, C*/Ti quickly rose to about 1 in less than 30 s and then remained stable in the subsequent 60 s. Then the C*/Ti value started to increase again, forming the second buildup stage. The polymerization activity was found to change roughly in parallel with the change in C*/Ti. Changes in the polymer/catalyst particle morphology and polymer molecular weight distribution with polymerization time were studied. A mechanistic model was proposed to explain the two-stage kinetics: initiation of active sites on the outer surface of catalyst particles takes place in the first stage, and initiation of active sites buried inside the particles takes place in the second stage. These buried sites are released when the catalyst particles are fragmented by the expanding polymer phase. (C) 2017 Wiley Periodicals, Inc.