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

Abstract

The kinetics of ethylene polymerization with a TiCl 4 /MgCl 2 -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.

Kinetics of short‐duration ethylene–propylene copolymerization with MgCl2‐supported Ziegler–Natta catalyst: Differentiation of active centers on the external and internal surfaces of the catalyst particles

“…10 In the homopolymerization system, [C*]/[Ti] increased to 5.5% after 300 s reaction, while [C*]/[Ti] of 7.9% was reached in the copolymerization. 10 In the homopolymerization system, [C*]/[Ti] increased to 5.5% after 300 s reaction, while [C*]/[Ti] of 7.9% was reached in the copolymerization.…”

“…10 This difference can be explained by faster increase of polymer yield with time in the copolymerization system. 10 This difference can be explained by faster increase of polymer yield with time in the copolymerization system.…”

“…10 This difference can be explained by faster increase of polymer yield with time in the copolymerization system. 10 It means that disintegration of the polymer/catalyst particles can take place only when their cracks and voids have been filled by enough amount of polymer. It was at this ratio the second stage of [C*]/[Ti] increasing began in the homopolymerization system.…”

“…Due to growing market demand for polyolefin based polymers, each year more than one hundred million tons of PE and PP resins are produced based on this kind of catalysts. 4,[8][9][10] In our previous work, 10 formation of active centers in the induction period of ethylene polymerization with MgCl 2 -supported ZN catalyst was experimentally studied. [1][2][3] However, there are still many unsolved mechanistic problems in this group of important industry catalysts, among them are the formation or initiation of active centers, distribution of multiple active centers, and correlations between polymer chain structure and active center structure.…”

“…Formation of the active centers in the initial stage of olefin polymerization has been studied by several research groups in the past, [4][5][6][7] but only a few literatures studied the active center formation process by tracing the change of active center concentration in the induction period. 4,[8][9][10] In our previous work, 10 formation of active centers in the induction period of ethylene polymerization with MgCl 2 -supported ZN catalyst was experimentally studied. The number of active centers was found to increase with the polymerization time in two stages, and the polymer-catalyst particles underwent evident disintegration during the second induction period.…”