PERSPECTIVE

Comments on “Une Nouvelle Classe de Polymeres d’a-Olefines ayant une Regularit6 de Structure Exceptionnelle,” by C. Natta, J. Polym. Sci., XVI, 143 (1955)

PAOLO CORRADlNl

Dipartimento di Chimica, Universiti Federico II, Via Mezzocannone 4, 1-801 34, Napoli, Italy

The discovery of how to synthesize stereoregular polymers of olefins was made by Natta and his co- workers in 1954l.’ (soon after that of Ziegler, for the preparation of linear ethylene polymers), using as catalysts combinations of aluminum alkyls and transition metal chlorides. In a few years after the discovery, an entire new chapter had been disclosed in the field of macromolecular chemistry. It was possible to obtain polymers from olefinic and various other hydrocarbon monomers, with an extraordinary regularity of structure from both their chemical constitution and the configuration of the successive monomeric units along the chain of each macro- molecule (“stereoregular” polymers).

At the end of the 1960s, new catalytic systems for the polymerization of ethylene were imple- mented, in which the titanium chloride is supported on a matrix, as, for instance, magnesium oxide or chloride. These new catalysts show a very high ac- tivity in the polymerization of ethylene, with yields of the order of 106 instead of lo4 grams of polymer per gram of titanium.

The new supported catalysts, however, were un- satisfactory for the polymerization of propylene, where control of the succession of the m versus the r configurations along the polymer chain is neces- sary.

Research to design a high-yield supported catalyst for the isotactic polymerization of propylene has led, with the help of the previous experience in the field, to the implementation in the industrial research

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 34,317-319 (1996) 0 1996 John Wiley & Sons, Inc. CCC 0%37-624X/96/030317-03

laboratories of new catalytic systems, which are ca- pable of such high yields (thousands of kilograms of polymer per gram of titanium) that the purification process can be eliminated in the production plants; on the other side, the isotacticity is so high that the heavy cost of the extraction of the amorphous frac- tion can be considerably reduced. The research ef- forts, started already by Professor Natta soon after the discovery, for a better comprehension of the ac- tion mechanism of the polymerization catalysts, of the nature of the catalyst surface, and of the influ- ence of various chemical agents are now paying back, in this way, also in terms of a simpler industrial process. The production of isotactic polypropylene, in the whole world, is presently of the order of 15 million tons per year!

The discovery of homogeneous stereospecific cat- alysts for the polymerization of a-olefins-a further big breakthrough-was achieved 10 years ago by E ~ e n , ~ . ~ on the basis of earlier research on metal- locenes in combination with alkyl-Al-oxanes by Ka- minsky and Sinn.5 It has opened up new prospects for research on stereospecific polymerization and on stereoregular polyolefins.

Depending on the specific metallocene a-ligands used, these systems present completely different stereospecific behaviors. For example, catalytic sys- tems containing the metallocene stereorigid ligand ethylene-bis(1-indenyl) or ethylene-bis(4,5,6,7-tetra- hydro-1-indenyl) polymerize a-olefins to isotactic polymers while catalytic systems comprising the metallocene stereorigid ligand isopropyl(cyc1open- tadienyl- 1-fluorenyl) instead polymerize a-olefins to syndiotactic polymers (Fig. 1). In the case of pro-

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318 CORRADINI

pylene, it is possible to obtain at will syndiotactic and isotactic (Fig. 2), hemitactic and atactic poly- mers, tuning the catalyst structure to the desired polymer properties.

One of the most exciting features of these ho- mogeneous catalysts is that the structure of the cat- alyst precursors can be accurately determined and the influence of the ligand geometry on the stereo- specificity of the polymerization reactions can be studied in detail.

Correlations of the polymer microstructure with the structure of the catalyst precursor have revealed an extraordinary amount of information on the po-

-ti Monorncr

Figure 1. Models of homogeneous catalytic sites for the polymerization of propylene. The structures of the ligands a-coordinated to the metal (Mt) in the catalytic inter- mediates, which precede the insertion of a monomer unit, are evidenced with a marked line (and black small circles for the carbon atoms). (A) rac-ethylene-bis-(1-indenyl); (B) isopropyl(cyclopentadieny1-1 -fluorenyl).

Isotactic Polymorphous forms polypropylene of syndiotactic polypropylene

Figure 2. Models of the chain of stereoregular propyl- ene polymers in the crystalline state. Projections along and perpendicular to the chain axis.

lymerization mechanism and the origin of stereo- differentiation for the polymerization reactions. It has been possible to show that, for the homogeneous as well as for the heterogeneous Ziegler-Natta cat- alysts of polymerization of olefins, nonbonded in- teractions, enforcing a chiral orientation of the first C - C bond of the growing chain, play the most im- portant role in determining their surprising and tunable specificities6

A main feature of the new homogeneous catalytic systems is that they can be single site, that is, they comprise all identical catalytic sites, thus allowing better control of the molecular mass distribution as well as, for copolymers, better control of the co- monomer composition and distribution. This can be a great advantage with respect to the heterogeneous catalytic systems, for which several sites with dif- ferent reactivities and regio- and stereospecificities are present. Relevant industrial applications may be foreseen in the future.

Hence, it has been possible to tune the structure of these catalysts to the preparation of a series of new stereoregular polymers, in particular of a series of new crystalline syndiotactic polymers.

PERSPECTIVE 3 19

The discovery of soluble catalysts has also allowed the preparation of new types of stereoregular hy- drocarbon polymers, giving rise to a reflourishing of structural studies in our and other laboratories, in particular on the newly synthesized syndiotactic polystyrene and on many new syndiotactic polyole- fins.7 For the hydrocarbon polymers, which can be obtained with Ziegler-Natta heterogeneous and ho- mogeneous catalysts, a new flowering of fundamen- tal studies and industrial utilizations is to be antic- ipated in the near future.

REFERENCES AND NOTES

1. (a) G. Natta, Acc. Naz. Lincei. Mem., 4(sez. 2) , 61 (1955); (b) G. Natta, J. Polym. Sci., 16, 143 (1955).

2. (a) G. Natta and P. Corradini, Acc. Naz. Lincei. Mem., 4(sez. 2) , 73 (1955); (b) G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, and G. Moraglio, J. Am. Chem. SOC., 77,1708 (1955).

3. J. A. Ewen, J. Am. Chem. SOC., 104,6355 (1984). 4. J. A. Ewen, R. L. Jones, A. Razavi, and J. D. Ferrara,

J. Am. Chem. Sac., 110,6255 (1988). 5. H. Sinn and W. Kaminsky, Adv. Organomet. Chem.,

1 8 , 9 9 (1980). 6. (a) P. Corradini, V. Barone, R. Fusco, and G. Guerra,

Eur. Polym. J., 15, 133 (1979); (b) P. Corradini, V. Barone, R. Fusco, and G. Guerra, J. Catal., 77, 32 (1982); (c) G. Guerra, L. Cavallo, G. Moscardi, M. Vacatello, andP. Corradini, J. Am. Chem. SOC., 116, 2988 (1994).

7. P. Corradini and G. Guerra, Adv. Polym. Sci., 100, 183 (1992).