Propylene polymerization Ziegler-Natta catalysts

Ziegler-Natta Polymerization. The polymeri2ation of propylene with Ziegler-Natta catalysts, ie, complexes of TiCl3—(C2H3)3A1 on MgCl2... 

Stereospecific Polymerization of Propylene with Ziegler-Natta-Catalysts in Organic Suspension... 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

The mechanism for polymerization of propylene using Ziegler-Natta catalysts is analogous to that discussed in section 3.7 with ethylene. However, unlike ethylene, propylene can be said to have "head" and "tail" portions and regiochem-istry can vary. More importantly, the orientation (stereochemistry) of the methyl group in the polymer has a dramatic effect on polymer properties. These factors make polymerization of propylene (and other a-olefins) more complex (17). 

It is known that both the isotactic and syndiotactic polymerizations of propylene with Ziegler-Natta catalysts occur by the cis opening of the double bond. Figure 4.1 shows the relationship between two kinds of copolymers obtained from deuterium-substituted styrene monomer and the conformation of their vicinal protons. 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

The three different stereochemical forms of polypropylene all have somewhat different properties, and all can be made by using the right polymerization catalyst. Propylene polymerization using radical initiators does not work well, but polymerization using Ziegler-Natta catalysts allows preparation of isotactic, syndiotactic, and atactic polypropylene. 

Conjugated dienes are among the most significant building blocks both in laboratories and in the chemical industry [1], Especially, 1,3-butadiene and isoprene are key feedstocks for the manufacture of polymers and fine chemicals. Since the discovery of the Ziegler-Natta catalyst for the polymerizations of ethylene and propylene, the powerful features of transition metal catalysis has been widely recognized, and studies in this field have been pursued very actively [2-7]. 

We have a choice of four major polymerization techniques by which to manufacture polypropylene using Ziegler-Natta catalysts slurry, liquid propylene, solution, and gas phase. Regardless of which technique is employed, all polymerization plants must accomplish the same basic goals they must... 

In heterogeneous polymerizations in bulk, the formed polymer is insoluble in its monomer and the polyreaction is performed below the softening point of the polymer. On an industrial scale, this type of process is especially utilized for chain polymerizations, for example, the radical polymerization of liquid vinyl chloride, the polymerization of liquid propylene with Ziegler-Natta or with metallocene catalysts, and the polymerization of molten trioxane. 

On the industrial scale, suspension polymerizations are not only carried out in the aqueous phase, but also in aliphatic hydrocarbons using Ziegler-Natta catalysts, as for example, in the polymerization of ethylene and propylene (see also Sect. 3.3.1). 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

H. Schnecko, K. A. Jung, and W. Kern, The Number of Active Sites for the Polymerization of Ethylene, Propylene, and Butene-1 by Ziegler-Natta Catalysts, in Ref. 9, p. 73. 

In 1961 Mienes (156) in a survey article reported that certain modified Ziegler-Natta catalysts can initiate 1,3 polymerizations. Allegedly propylene polymerizations yield more or less branched poly-ethylenes having a melting point of 130° C. By judicious choice of the catalyst the polymerization can be directed in such a manner that some of the propylene polymerizes partly 1,2 and partly in the 1,3... 

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