Ziegler-Natta polymerization

What is Ziegler-Natta polymerization?

The Ziegler-Natta polymerization is a process that involves using a Lewis acid of an early transition metal, such as Ti, Zr, or V, to catalyze the polymerization of vinyl monomers in the presence of trialkylaluminum, resulting in the formation of stereoregulated or tactic polymers. This reaction was first reported by Ziegler in 1953 and subsequently extended by Natta in 1954 (both Nobel Prize in 1963).

Ziegler-Natta polymerization - general reaction scheme
Ziegler-Natta polymerization

R = H, Me, Et, Ph
Over the past half-century, the Ziegler-Natta polymerization has been extensively improved, leading to the development of several generations of Ziegler-Natta catalysts.

The first generation of Ziegler-Natta catalysts included chromium and zirconium, which were used at the initial development stage of the Ziegler-Natta polymerization. The second generation of Ziegler-Natta catalysts comprised TiCl4/Et3Al or TiCl3/Et2AlCl. However, due to their low activities, these first two generations of Ziegler-Natta catalysts have little use in industries.

The third generation of Ziegler-Natta catalysts, represented by magnesium chloride (MgCl2) supported TiCl4 (with electron donor) developed in the 1960s, increases the activities more than 50 times for propylene polymerization. This catalyst was implemented for production in the 1980s.

The fourth generation of the Ziegler-Natta catalysts are metallocene-based homogeneous single-site catalysts in combination with methylalumoxane (MAO) discovered by Kaminsky in 1976, although the combination of triethylaluminum and bis-(cyclopentadienyl)-titanium dichloride had been used for the polymerization reaction as early as 1958. A variety of homogeneous catalysts have been explored, which can be divided into three groups: two-component catalysts with group IV metallocene dihalides and a large excess of MAO co-catalyst, two-component catalysts containing group IV metallocene dialkyls and a stoichiometric (or near stoichiometric) amount of an activator, and single-component catalyst including a Lewis base adduct of cationic group IV metallocene alkyls or lanthanide metallocene alkyls.

Late transition metal (Ni, Pd) mediated polymerizations were reported in 1998 and 1999, affording highly branched polyethylenes, in which the amount of branches were controlled in large part by the pressure of ethylene. Due to the high efficiency of the Ziegler-Natta polymerization, almost 160 million tons of polymers are produced annually, and among these polymers,∼15 million tons of polyethylene and polypropylene are made from metallocene-based Ziegler-Natta catalysts.

Compared to the second generation of the Ziegler-Natta catalysts, the MgCl2-supported TiCl3 catalysts (i.e., the third generation catalysts) have advantages such as two to three orders of magnitude higher activities, significant enhancement of isotactic polymer, easier control of polymer morphology, and annihilation of de-ashing and pelletization processes. However, compared to the first homogeneous Ziegler-Natta catalyst (Cp2TiCl2/AlR2Cl), the replacement of AlR2Cl by MAO increases the catalytic activity by approximately 105 times, making it unnecessary to remove the catalyst from the polymers. When B(C6F5)3 is applied as the co-activator, only a stoichiometric amount of B(C6F5)3 in regard to the catalyst is needed. It has been reported that decreasing the electron density at the metal center in these catalysts may reduce the chain-transfer and propagation rate for the ethylene polymerization.

References

  • Ziegler, K.; Holzkamp, E.; Breil, H. andCorradini, P.; Guerra, G. and Cavallo, L., Acc. Chem. Res., 2004, 37, 231
  • Ziegler, K., Brennstoff-Chem., 1954, 35, 321
  • Martin, H., Angew. Chem., 1955, 67, 426
  • Ziegler, K.; Holzkamp, E.; Breil, H. and Martin, H., Angew. Chem., 1955, 67, 541
  • Ziegler, K.; Holzkamp, E.; Breil, H. and Martin, H., Chem. Ind. (Milan, Italy), 1955, 37, 881
  • Natta, G., J. Poly. Sci., 1955, 16, 143
  • Natta, G.; Pino, P.; Mantica, E.; Danusso, F.; Mazzanti, G. and Peraldo, M., Chim. Ind., 1956, 38, 124
  • Natta, G.; Pino, P.; Mazzanti, G.; Corradini, P. and Giannini, U., Atti Accad. Nazl. Lincei, Rend., 1955, 19, 397
  • Natta, G., Angew. Chem., 1956, 68, 393
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