Ziegler-Natta catalysts catalyst composition

Ramazani, A., Tavakolzadeh, E, and Baniasadi, H. 2009. In situ polymerization of polyethylene/ clay nanocomposites using a novel clay-supported Ziegler-Natta catalyst. Polymer Composites 30 1388-1393. 

Table 9. Different Ziegler-Natta Catalyst Generations—Composition, Performance, Morphology and Process Requirements...

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Among one-component polymerization catalysts subhalides of the transition metals are most similar in composition to the traditional Ziegler-Natta catalysts. In this connection, the study of the simplest one-component catalyst of this type (especially TiCl2) is of great importance for the clarification of still disputable problems of the mechanism of polymerization by two-component catalysts. 

Although both Phillips and Ziegler Natta catalysts copolymerise ethylene and propylene, the latter catalysts are more widely used because they can be more readily tailored to produce a narrow distribution of compositions and molecular weight [43]. 

Copolymers of ethylene and propylene have come to stay as important materials with diverse practical applications. They span the full range of polymeric properties, from soft elastomers to hard thermoplastics depending on the relative composition of the two monomers and the manner of their enchainment. Ethylene-propylene copolymers are manufactured commercially using Ziegler-Natta catalysts [1]. For the purposes of this discussion, we will treat these copolymers in terms of three distinct classes of materials ... 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

Thus, the experimental data show that the composition of catalytic systems does not influence the stereoregularity of the corresponding polymer fractions but only their relative content. Hence, the stereospecificity of the active centers of these catalysts is the same including the one-component catalyst TiCl2. This confirms the concepts on the monometallic character of AQ in the Ziegler-Natta catalysts. The possible existence of chiral titanium atoms on the titanium chloride surface was studied by Cossee and Arlman ... 

If a copolymer such as VLDPE or LLDPE is the target resin, satisfactory comonomer incorporation must be achieved. This is manifested by the amount of comonomer incorporated (evidenced by density) and the distribution of comonomer in the polymer (evidenced by composition distribution). In general, supported chromium oxide catalysts incorporate comonomer more easily than Ziegler-Natta catalysts. 

Studies on Ziegler Natta Catalysts J. Polymer Sci. A-1 (1966). — Rodriguez, L. A. M., H. M. VAN Looy, and J. A. Gabant Part I. Reaction between Trimethylaluminium and a-Titanium Trichloride, p. 1905 — Part II. Reaction between a-or jS-TiClj and AlMe, AlMejCl, or AlEtj at Various Temperatures, p. 1917. — van Looy, H. M., L. A. M. Rodriguez, and J. A. Gabant Part III. Composition of the nonvolatile product of the reaction between titanium trichloride and trimethylaluminum or di-methylaluminum chloride, p. 1927. — Rodriguez, L. A. M., and H. M. VAN Looy Part IV. Chemical nature of the active site, p. 1951. Part V. Stereospecificity of the active center, p. 1971. 

A number of block copolymers prepared with Ziegler-Natta catalysts have been reported however, in most cases the compositions may include significant amounts of homopolymer. The Ziegler-Natta method appears to be inferior to anionic polymerization for synthesizing carefully tailored block copolymers. Nevertheless, bock copolymers of ethylene and propylene (Eastman Kodak s Pofyallomers) have been commercialized. Unlike the elastomeric random copolymers of ethylene and propylene, these are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene. They also contain homopolymers in addition to block copolymers. 

Composites. The (CH)X/LDPE composites were prepared using the Ti(0Bu)4./Et3Al Ziegler-Natta catalyst system as previously described (10). The amount of (CH)X incorporated was determined by monitoring the acetylene uptake during the polymerization. Electrically conductive derivatives were prepared by immersion of the composites in a saturated 12/pentane solution for 24-48 hours. Electrical conductivities were measured by standard four-probe techniques. 

Random copolymers of ethylene and a-olefins (l-aUcenes) can be obtained with Ziegler-Natta catalysts, the most important being those of ethylene and 1-butene (LLDPE) and of ethylene and propylene (EPM or EPR and EPDM). Some reactivity ratios are listed in Table 9.5. Tha ratios vary with the nature and physical state of the catalyst and in most instances, r r2 is close to unity. However, all these values show that ethylene is much more reactive than higher alkenes. Copolymers produced using Ziegler-Natta catalysts usually have a wide range of compositions. This may be due to the presence of different active sites in the catalyst giving rise... 

---