Coordination polymerization isotactic polypropylene

The mechanism of the polymerization of propylene to produce isotactic structure has been studied extensively. Natta and his coworkers hav presented the generally accepted anionic-coordinate mechanism. However, there has been increasing evidence that the Natta anionic-coordinate mechanism does not operate to produce isotactic polypropylene. 

Polypropylene owes its current market success to the development of coordination polymerization. Before 1957 it was not produced commercially because radical polymerization gives an atactic polymer that is amorphous and has poor mechanical properties. Using a coordination catalyst, however, enables the production of an isotactic polymer that is semicrystalline. This material is stiff and hard and has a high tensile strength. Among its many useful products are rope, molded objects, and furniture. 

Coordination polymerization—A polymerization in which propagation occurs when monomer and a growing polymer chain both coordinate to a transition metal catalyst and then react. The final polymer often consists of one particular tacticity, e.g., isotactic polypropylene. 

In dichloromethane,the polymerization of propylene under higher pressure yields an isotactic polypropylene with a small number of stereodefects, while in toluene a mixture of isotactic and atactic products was obtained. This fact can testify about the presence of two different active catalytic species. The nature of the intermediate responsible for the formation of the atactic fraction has not been fully elucidated yet. On the basis of NMR experiments, it seems that in toluene one of the benzamidinate ligations opens to an rf coordination, and the solvent occupies the vacant site. It was shown that similar heteroallylic ligations (alkoxysilyl-imido) have exhibited dynamic behaviors [64,65]. 

When propylene and higher olefins are polymerized the configuration of the polymer is controlled by the catalyst structure. Catalyst 9-25 contains two equivalent Cl atoms, because the molecule is symmetrical about the Zr-indenyl bonds. Recall that in the active catalyst one Cl will be replaced by the growing polymer and the other by the incoming monomer. The polymer chain could occupy either of the two Cl positions, while the monomer could coordinate at the other position. As a result, both positions are equivalent and this metallocene produces isotactic polypropylene. In structure 9-26, the two Cl sites are not equivalent. At one position, the monomer would find the same environment as in 9-25, but the second... 

Polypropylene (PP) with the formula [-CH2CH(CH3)-] , CAS 9003-07-0, is a common polymer that is usually obtained by coordination catalyst polymerization. This polymer can be made in isotactic, syndiotactic or atactic forms, typically head to tail. Most practical uses are known for isotactic polypropylene, which is linear and highly crystalline. As indicated in Section 1.3, the linearity of the isotactic polymer implies in fact that the backbone forms a regular spiral that in case of polypropylene has three units per turn, as schematically shown below ... 

Complexes [Mn(X)(Tp Bu ipr)] (X = Cl, Br, NO3) have been reported and described to possess good activity for ethylene polymerization and ethylene-R-olefin copolymerization. The [Mn(Cl)(Tp,Bu,pr)]/Al(iBu)3/[Ph3C][B(C6F5)4] system is also active toward propylene and gives isotactic polypropylene.113 Spectral properties and electronic structure of the four-coordinate high-spin... 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

With respect to titanium catalysts for polymerization of ethylene or propylene, Ziegler synthesized the first high-density polyethylene and Natta prepared isotactic polypropylene by means of coordination catalysts about 50 years ago. The preparation methods of catalysts have been studied extensively. These TiCL3 catalysts have very high activity for homopolymerization of ethylene and propylene, whereas, they exhibit low activity for random copolymerization of ethylene with propylene when compared to vanadium catalysts. Refer to Ziegler-Nata Catalyst, Vanadium Catalysts, and EP Terpolymer, (Source Elastomer Technology Handbook, N. P. Cheremisinoff - editor, CRC Press, Boca Raton, Florida, 1993). 

Coordination polymerization also produces high sterospecificity in the polymerization of alkenes. Isotactic and syndiotactic polymers can be obtained by appropriate choice of the catalyst components although such polymers are not useful as elastomers. However, Ziegler-Natta catalysts are used to produce EPR and EPDM rubbers. (Coordination polymerization is important for the synthesis of linear polyethylene and isotactic polypropylene which find extensive utility as plastics.) The Symposium paper by Su and Shih describes the synthesis of propylene-l-hexene block copolymers using several catalysts based on titanium and aluminum components. 

Free-radical polymerization arranges functional groups, such as alkyls, in a random manner, whereas coordination polymerization can exercise stereochemical control over functional groups. With the proper choice of experimental conditions, such as temperature, solvent, and catalyst, monomers can polymerize to any of three arrangements isotactic, syndiotactic, and atactic. (For polyethylene, there are no such stereoisomers, since the monomeric units are identical, —CH2—.) Isotactic and syndiotactic polypropylenes are highly crystaUine atactic polypropylene is a soft, elastic, and rubbery material. Following are the three stereoisomers of polypropylene ... 

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