Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler⁻Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae.

Zhen Zhang,Zhisheng Fu,Feng He,Junting Xu,Baiyu Jiang,Zhiqiang Fan

doi:10.3390/polym11020358

Abstract

The kinetic behaviors of ethylene and propylene polymerizations with the same MgCl2-supported Ziegler–Natta (Z–N) catalyst containing an internal electron donor were compared. Changes of polymerization activity and active center concentration ([C*]) with time in the first 10 min were determined. Activity of ethylene polymerization was only 25% of that of propylene, and the polymerization rate (Rp) quickly decayed with time (tp) in the former system, in contrast to stable Rp in the latter. The ethylene system showed a very low [C*]/[Ti] ratio (<0.6%), in contrast to a much higher [C*]/[Ti] ratio (1.5%–4.9%) in propylene polymerization. The two systems showed noticeably different morphologies of the nascent polymer/catalyst particles, with the PP/catalyst particles being more compact and homogeneous than the PE/catalyst particles. The different kinetic behaviors of the two systems were explained by faster and more sufficient catalyst fragmentation in propylene polymerization than the ethylene system. The smaller lamellar thickness (<20 nm) in nascent polypropylene compared with the size of nanopores (15–25 nm) in the catalyst was considered the key factor for efficient catalyst fragmentation in propylene polymerization, as the PP lamellae may grow inside the nanopores and break up the catalyst particles.

Highlights

After rapid and continuous growth in the last sixty years, the industrial production of polyolefins has become one of the most important branches of modern chemical industry, and MgCl2 -supported Ziegler–Natta (Z–N) catalysts are playing dominant roles in polyolefin production
Activity of ethylene polymerization was about 75% lower than that of propylene, and the polymerization rate quickly decayed with time in the former system, in contrast to a stable Rp versus time profile in the latter
A large proportion of active sites was exposed through catalyst particle fragmentation in the polymerization process, but the degree of particle fragmentation was remarkably lower in the ethylene system

Summary

Introduction

After rapid and continuous growth in the last sixty years, the industrial production of polyolefins (polyethylene, polypropylene, and olefin copolymers) has become one of the most important branches of modern chemical industry, and MgCl2 -supported Ziegler–Natta (Z–N) catalysts are playing dominant roles in polyolefin production. The kinetics and mechanism of ethylene and propylene polymerizations with Z–N catalysts have been studied in a broad span of reaction durations ranging from less than 1 s to more than one hour [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. Y.V. Kissin reported that ethylene polymerization with a TiCl3 -based classical Z–N catalyst and MgCl2 -supported Z–N catalyst both presented build-up type rate curves with a rather long induction period (10–30 min), in which the reaction rate gradually grew to the stationary level. Propylene polymerization with the same catalyst presented decay type rate curves, in which the reaction rate quickly rose to the maximum and began descending over a Polymers 2019, 11, 358; doi:10.3390/polym11020358 www.mdpi.com/journal/polymers

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