Ziegler‐Natta Polymerization

Abstract

This reaction involves a Lewis acid of an early transition metal (e.g., Ti, Zr, V, etc.) to catalyze the polymerization of vinyl monomers in the presence of trialkylaluminum to form stereoregulated or tactic polymers, and is generally known as the Ziegler–Natta polymerization or Ziegler–Natta olefin polymerization. Over the past half century, the Ziegler–Natta polymerization has been extensively improved, and several generations of Ziegler–Natta catalysts have been developed. On the basis of such seminal work, a variety of homogeneous catalysts have been explored, which can be divided into three groups. Further important developments for the Ziegler–Natta polymerization include the application of silyl or amido‐containing metallocenes as the catalysts as well as using organosilanes as polymer chain transfer reagents. More importantly, late transition metal (Ni, Pd) mediated polymerizations have been reported to afford highly branched polyethylene. It has been found that the stereoregulation is probably due to the fixed chiral orientation of the terminal CC bond of the growing polymer chain attaching to the catalytic site. The study finds that the isotactic polymers are commercially important, owing to their various favourable properties. In addition, the correct choice of electron donor for the MgCl 2 ‐supported TiCl 4 can also enhance the isotactic index (II). It has been found that the metallocenes with a C 2‐symmetry give isotactic polymers and syndiotactic polymers as represented by the highly isotactic (ethylenebis(tetrahydroindenyl))ZrC1 2 /MAO and syndiotactic isopropyl(cyclopentadieny1‐1‐fluorenyl)‐ZrC1 2 /MAO catalytic systems. Besides the tacticity, the molecular weight and molecular weight distribution (MWD) of the polymers also affect the properties of the polymers. It should be pointed out that the Ziegler–Natta polymerization is also good for the production of copolymers arising from different comonomers. Nevertheless, the Ziegler–Natta polymerization has some limitations.