Ziegler–Natta Catalysts

Abstract

First invented in 1953, Ziegler–Natta catalysts have been used widely in the industrial production of polyolefins and other important polymers since the 1960s. In recent years, polymers synthesized with these catalysts account for more than 30% of the total production of polymers. This article briefly introduces the history and main developments of Ziegler–Natta catalysts and explains their preparation, composition, and mechanism of the catalysis process. Particular emphasis is put on the currently more important MgCl 2 ‐supported Ziegler–Natta catalysts. Main features of the olefin polymerization with MgCl 2 ‐supported Ziegler–Natta catalysts are presented, including the formation and structure of active sites, main reactions with the active sites, origin of stereospecificity, reaction kinetics, and morphology of the nascent polymer particles. The main olefin polymerization processes with these catalysts are also briefly discussed.

Responses of a Supported Ziegler–Natta Catalyst to Comonomer Feed Ratios in Ethylene–Propylene Copolymerization: Differentiation of Active Centers with Different Catalytic Features

“…Copolymers of propylene with a small amount of ethylene, the so-called PPR resins, are widely used for making pipes and films, , and heterophasic reactor blends of PP with the ethylene–propylene copolymer (abbreviated as hiPP, ICP, or HECOs) are widely used as structural and engineering materials. , In the industrial production of propylene copolymers, MgCl 2 -supported Ziegler–Natta catalysts have been playing a dominant role since the 1980s, accounting for more than 95% of the annual output. The Z–N catalysts used for synthesizing PP copolymers have the same structure and composition as that for producing iPP, where the catalyst contains an organic electron donor introduced in its preparation (Di), and in most cases, another electron donor (De) is added in the polymerization system . The electron donors, mostly esters, ethers, and siloxanes, have been proven to be essential components for enhancing the catalyst’s stereoselectivity.…”

“…Industrial production of hiPP is realized by sequential propylene polymerization and ethylene–propylene copolymerization in two or more sequentially connected reactors catalyzed with the same catalyst. , Since the mechanical performances of hiPP are determined by chemical structures of both the iPP components (the continuous phase) and the copolymer components (the dispersed phase), it is necessary to know correlations between propylene homo and copolymerization behaviors of each type of active center and to understand possible changes in the catalytic behavior of the active center when the monomer changes from propylene to an ethylene/propylene mixture. Since it is difficult and inefficient to precisely differentiate each type of active center from the others and measure its number by, for example, fractionating the quench-labeled polymer into more than 10 fractions during propylene homopolymerization, a more efficient way to study active-center distribution of Z–N catalysts is fractionating the quench-labeled polymer into a few (e.g., 2–3) fractions and measuring the number of active centers in each fraction.…”

Assessing 1,9-Decadiene/Propylene Copolymerization with Ziegler-Natta Catalysts to Long-Chain-Branched Polypropylene

“…In regard of LCB-PP synthesis, however, Ziegler-Natta catalysts have largely stayed out of the spotlight despite their overwhelmingness in commercial PP resin production. − In modern PP industry, heterogeneous Ziegler-Natta catalysts predominantly in the form of MgCl 2 -supported TiCl 4 adduct (MgCl 2 /TiCl 4 catalysts) are greatly advantageous over metallocene/nonmetallocene homogeneous catalysts in the morphology control of polymer particles, for which they remain at the center of industrial PP production dominated by bulk and gas-phase polymerization processes. − Though grouped into the traditional catalyst category to underscore the advancement of metallocene/nonmetallocene catalysts for olefin polymerization, the state-of-the-art MgCl 2 /TiCl 4 catalysts are no-less able to produce PP with high isotacticity and controllable molecular weight and molecular weight distribution at high efficiency, thanks to the use of organic electron donors (Lewis bases) like esters, ethers, and alkoxysilanes in catalyst preparation (internal electron donor, Di) and polymerization (external electron donor, De). − The latest fifth-generation Ziegler-Natta catalysts using 1,3-diethers such as 9,9-bis­(methoxymethyl)fluorine (BMMF) as Di even produces high isotacticity (up to 97%) PP without the use of De. , The MgCl 2 /BMMF/TiCl 4 catalyst with triethylaluminum (TEA) as a cocatalyst also exhibits a fairly good copolymerization capability for propylene and higher α-olefins. It has been reported that the catalyst system incorporates over 20 mol % of 1-octene in copolymerization with propylene …”

Nonconjugated α,ω-Diolefin/Propylene Copolymerization to Long Chain-Branched Polypropylene by Ziegler–Natta Catalyst: Overcoming Steric Hindrance by Introducing an Extra Electronic Pulling Effect

“…In modern PP industry, heterogeneous Ziegler–Natta catalysts predominantly in the form of MgCl 2 -supported TiCl 4 adduct (MgCl 2 /TiCl 4 catalysts) are greatly advantageous over metallocene/nonmetallocene catalysts in polymer particle morphology control. , Thus, this family of catalysts takes most of the credit for the production of more than 50 million tons of PP worldwide annually. , With the use of organic electron donors (Lewis bases) like esters, ethers, and alkoxysilanes in the catalyst preparation (internal electron donor, Di) and polymerization processes (external electron donor, De), these adduct catalysts have been able to produce PP with high isotacticity and controllable molecular weight and molecular weight distribution at very high efficiency . However, Ziegler–Natta catalysts are known of their high sensitivity to steric bulkiness of olefin monomers, with which polymerization reactivity reduces quickly with substituent enlarging. ,, Heavy steric bulkiness prevents olefins from accessing to the coordination site of the Ti active center, which lacks the versatile ligand setups in organometallic metallocene/nonmetallocene catalysts to improve its openness.…”

“…7,15 With the use of organic electron donors (Lewis bases) like esters, ethers, and alkoxysilanes in the catalyst preparation (internal electron donor, Di) and polymerization processes (external electron donor, De), these adduct catalysts have been able to produce PP with high isotacticity and controllable molecular weight and molecular weight distribution at very high efficiency. 43 However, Ziegler−Natta catalysts are known of their high sensitivity to steric bulkiness of olefin monomers, with which polymerization reactivity reduces quickly with substituent enlarging. 1,2,44 Heavy steric bulkiness prevents olefins from accessing to the coordination site of the Ti active center, which lacks the versatile ligand setups in organometallic metallocene/nonmetallocene catalysts to improve its openness.…”