Ziegler‐Natta Catalysts

Abstract

Ziegler‐Natta catalysts, discovered in the years 1953–1954, account today for a production volume of ∼65 million tons of polyolefins, including mainly polyethylene and polypropylene products. Since their first discovery, the development of Ziegler‐Natta catalysts has been relentless, and their evolution is the result of the exploitation, starting from the mid‐1960s, of four major breakthroughs: the active form of MgCl 2 ; the stereoregulating effect of electron donors (isotactic polypropylene); the chemical route to the active form of MgCl 2 ; and, finally, the control of the morphology of the catalyst/polymer particles. Thus, the most advanced Ziegler‐Natta catalysts are today prepared starting from controlled morphology MgCl 2 , or its precursors, TiCl 4 , or other titanium derivatives and, especially for isotactic polypropylene, electron donors. With respect to the first‐generation of Ziegler‐Natta catalysts based on TiCl 3 , MgCl 2 ‐based catalysts are not only much more efficient in terms of productivity ( both increased number of active centers and value of their propagation constant), but they also offer an unprecedented level of customization to meet any process (slurry, bulk, or gas‐phase) or product requirements (polyethylene, polypropylene, and their copolymers). Actually, MgCl 2 ‐supported catalysts are complex but also versatile systems whose architecture (particle shape, size, size distribution, and porosity) and performances (activity, hydrogen response, stereocontrol, and capability to tune polymer MW and MWD) can be directed acting on both the morphology of the MgCl 2 precursors and the nature of the electron donors. Many mechanistic aspects of the Ziegler‐Natta catalysis have been elucidated, eg, the mechanism of polymer particle growth, the role of prepolymerization, the activating effect of hydrogen in propylene polymerization, and the stereoblock nature of isotactic polypropylene. On the other hand, many other aspects are still open to debate or, at least, still need clarification; eg, the exact nature and number of active centers and their mechanism of deactivation, the intimate mechanism of action of electron donors, the difference between the species that are active in ethylene or propylene polymerization, and the comonomer effect in ethylene copolymerization. Thus, there is still a lot of room for further investigation and improvements in Ziegler‐Natta catalysts. A big step forward in this field would be the generation of catalysts combining the economics and morphological features of MgCl 2 ‐based systems with the peculiarities of single‐site systems.

Structural Characterization of Electron Donors in Ziegler–Natta Catalysts

“…Activation entails reaction with an Al-trialkyl (e.g., AlEt 3 ), usually complexed with a second donor (in most cases, an alkoxysilane). 1 , 2 Ever since their discovery, improvements on the catalytic system have been made, giving different generations of catalysts with varying electron donors and increasing performance.…”

Effect of AlEt3and an External Donor on the Distribution of Active Sites According to Their Stereospecificity in Propylene Polymerization over TiCl4/MgCl2Catalysts with Different Titanium Content

“…Supported titanium–magnesium catalysts (TMCs) containing titanium chlorides on the magnesium chloride support as an active component and stereoregulating electron‐donating compounds are the main type of catalytic systems for poly(propylene) (PP) production. The composition and the structure of these catalysts, as well as the olefin polymerization mechanism, have been widely studied over the past years . However, many questions concerning the composition and structure of active sites of these catalysts and the stereoregulation mechanism still remain a subject of debate.…”

Catalytic synthesis of functionalized (polar and non-polar) polyolefin block copolymers

“…In step 1, the catalytic olefin polymerizations yield exclusively olefin terminated polymers through β-X elimination or chain transfer to monomer. 20 In step 2, the selectivity of the metathesis reaction between olefins and acrylates quantitatively converts the polyolefins into macroinitiators. In step 3, the living coordinative Ring Opening Polymerization (cROP) of lactide, 14 anionic Ring Opening Polymerization (aROP) of epoxide, 21 and Atom Transfer Radical Polymerization (ATRP) of styrene 22 are employed to quantitatively synthesize the block copolymers from the polyolefin macroinitiators ( Scheme 1 ).…”