Ziegler-Natta coordination catalysts

Polypropylene. One of the most important appHcations of propylene is as a monomer for the production of polypropylene. Propylene is polymerized by Ziegler-Natta coordination catalysts (92,93). Polymerization is carried out either in the Hquid phase where the polymer forms a slurry of particles, or in the gas phase where the polymer forms dry soHd particles. Propylene polymerization is an exothermic reaction (94). 

A Iky nes have been polymerized using ionic and radical initiators, but the polymer molecular weights are low. High molecular weights are obtained by using Ziegler-Natta coordination catalysts (Sec. 8-4d-2) [Chien et al., 1980]. The polymers are of considerable interest in terms of their potential as (semi)conducting materials. 

One of the most effective forms of the Ziegler-Natta coordination catalysts for PE synthesis involves Et,Al with TiCl., supported on MgClj. These catalysts are believed to contain nearly 100 %oftitanium as active catalyst sites [22]. 

Table 13 Microstructure of polybutadienes produced using Ziegler-Natta coordination catalysts...

Si02 Ziegler-Natta (coordination catalysts) i-C4H9 bulk... 

Olefins played the principal role in the unprecedented, explosive burst of creativity, invention, and successful development that occurred in the latter half of the 20th century and gave the world a new class of polymers, a family of new plastics, a family of new synthetic rubbers a new class of catalysts (Ziegler-Natta coordination catalysts), and a new nomenclature (isotactic, syndiotactic, and atactic) [1], This period of discovery also spawned two Nobel prizes, thousands of patents, thousands of scientific papers, not to mention dozens of lawsuits [1],... 

With such structural information available complex mechanistic proposals can be tested and rather subtle questions can be answered. An illustration is furnished by the investigation of the mechanism of the isotactic polymerization of propylene with Ziegler-Natta coordination catalysts of the type TiCl3 Al(C2Hs)2Cl by determining the type of error occasionally introduced. ... 

We have reviewed experiments on two classes of systems, namely small metal particles and atoms on oxide surfaces, and Ziegler-Natta model catalysts. We have shown that metal carbonyls prepared in situ by reaction of deposited metal atoms with CO from the gas phase are suitable probes for the environment of the adsorbed metal atoms and thus for the properties of the nucleation site. In addition, examples of the distinct chemical and physical properties of low coordinated metal atoms as compared to regular metal adsorption sites were demonstrated. For the Ziegler-Natta model catalysts it was demonstrated how combination of different surface science methods can help to gain insight into a variety of microscopic properties of surface sites involved in the polymerization reaction. 

The field of coordinated cationic polymerizations using Ziegler-Natta type catalysts will be discussed in detail in section V/5/c. 

A second method of production utilizes the Ziegler-Natta TiCl4 catalyst with liquid cocatalysts such as an alkyl aluminum halide. This is a reactive catalyst that must be prepared at the exclusion of air and water. The alkyl group of the co-catalyst coordinates with the Ti+3 site. The polymer grows by insertion of the ethylene into the double bond of the adsorbed polymer on another site. 

Some hesitation still persists over the coordination catalysts . We know that they are not real catalysts but nevertheless the term is used [e. g. Ziegler-Natta (ZN) catalysts]. The nomenclature in the field of initiation is far from being unified. In descriptions of the start of coordination polymerizations, the term catalyst is so familiar that to change it would result in considerable difficulty. Therefore, since many examples of the acceptance of incorrect terms can be found (even the theory of equilibria is called thermodynamics), the usual custom will be observed in this volume. 

TiC14, the active component of a Ziegler-Natta PP catalyst system, has to be coordinated on the MgC12 carrier material together with an internal electron donor like di-isobutyl phthalate (DIBP). The close presence of TiC14 and DIBP might also result, however, in the formation of a TiC14.DIBP complex, which would influence directly the catalyst activity. 

What metals form catalysts in Ziegler-Natta coordinated anionic polymerizations. 

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta-type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic //-oxoalkoxides, such as [(RO)2A102]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 4.1). 

High molecular weight polybutadiene homopolymers are prepared conunercially with anionic catalysts and with coordinated anionic ones. Polybutadiene formed with sodium dispersions was prepared industrially in the former USSR, and perhaps might still be produced in that area today. This sodium-catalyzed polybutadiene contains 65% of 1,2-adducts. Most of the preparations by others, however, utilize either alkyllithium or Ziegler-Natta-type catalysts prepared with titanium tetraiodide or preferably containing cobalt. 

The mechanism of polymerization of alkenes using Ziegler-Natta-type catalysts is described as a coordination [239] or insertion [240] polymerization process. The coordination terminology assumes that the growing polymer chain is bonded to a transition metal atom and that insertion of the monomer into the carbon-metal bond is preceded by, and presumably activated by, the coordination of the monomer with the transition metal center. Since coordination of the monomer may or may not be a specific feature of these polymerizations, the insertion terminology focuses on the proposal that these reactions involve a stepwise insertion of the monomer into the bond between the transition metal atom and the last carbon atom of the growing chain. It is important to note that the bonding of carbon atoms and transition metals is... 

Silylene 1 is an unusually versatile catalyst for alkene and alkyne polymerization. The list of compounds polymerized by 1 includes ethene, propene, 1-hexene, styrene, dimethylbutadiene, vinylidene chloride, vinyl ethyl ether, methyl methacrylate, and phenylacetylene. The polymerization does not seem to take place by any of the usual mechanisms, anionic, cationic or free-radical. Instead it somewhat resembles coordination polymerization, as observed for Ziegler-Natta type catalysts. Silylene 2 also catalyzes the polymerization of 1-hexene, but the polymerization is 10 to 100 times slower than with 1. 

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