Polymerization Using Ziegler-Natta Catalysts

In all cases the systems were heterogeneous and the active initiators are now known by the genraal name Ziegler-Natta catalysts. This encompasses a vast number of substances prepared from different combinations of organometaUic compounds where the metal comes from the main Groups I, II, or III and is combined with the halide or esto of a transition metal (Groups IV to VIII). Table 7.1 contains a number of the common components of the Ziegler-Natta catalysts, but this list is far from exhaustive. 

These catalysts traid to control two feamres, (1) the rate and (2) the specificity of the reaction, but this varies from reaction to reaction, and only a judicious choice of catalyst can effect control over both of these aspects. 

TiCl4 TiBrj TiCl3 VCI3 VCI4 (C5H5)jTiCl3 

Unfortunately, the insolubility of the catalyst poses the problems that the kinetics are hard to reproduce and the reaction mechanisms are difficult to formulate with real confidence. This means that the choice of a suitable catalyst for a system is somewhat empirical and based very much on trial and error, until optimum conditions 

A wide range of nonpolar monom can be polymerized nsing Ziegler-Natta catalysts, and monomer reactivity decreases with increasing steric hindrance abont the donble bond. 

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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. 

Polymerization using Ziegler-Natta catalysts yields a mostly saturated polymer with 2,3 linkages (17). The polymerization with heterogeneous Ziegler-Natta catalysts is accompanied by ROMP, whereas homogeneous metallocene, Ni, and Pd catalysts promote addition polymerization (18). 

Common PS is atactic and amorphous. It has good optical clarity, low dielectric loss factor, modulus E = 3.2 GPa, strength o = 45-65 MPa, density p = 1050 kg/m and CUT = 50-70°C. Because of brittleness and low chemical resistance the demand for neat PS has decreased, and except for foaming, PS is rarely used. PS can also be polymerized into crystalline forms isotactic (iPS) or syndiotactic (sPS) with T = 230 or 272°C, respectively. The former was polymerized using Ziegler-Natta catalyst [Ishihara et al., 1986], while the latter using a single-site metallocene titanium-based catalyst [Imabayashi et ah, 1994]. 

Polyisoprene is produced by solution polymerization using Ziegler-Natta catalysts. The cis-1,4-polyisoprene is a synthetic equivalent of natural rubber. However, the synthetic polyisoprenes have cis contents of only about 92-96% consequently, these rubbers differ from natural rubber in several ways. The raw synthetic polyisoprene is softer than raw natural rubber (due to a reduced tendency for a stress-induced crystallization be cause of the lower cis content) and is therefore more difficult to mill. On the other hand, the unvulcanized synthetic material flows more readily this feature makes it easier to injection mold. The synthetic product is somewhat more expensive than natural rubber. 

EPM and EPDM can be produced by solution polymerization, while suspension and slurry polymerization are viable options. EPDM can be gas-phase 1,-4 hexadiene polymerized using Ziegler-Natta catalysts. Union Carbide produces ethylene propylene rubber (EPR) using modified Unipol low-pressure gas-phase technology. 

Scheme 2.1 Acetylene polymerization using Ziegler-Natta catalyst.

The method just outlined was used, at the earlier stages of these kinds of study [101], for the elucidation of the mechanism of polymerization which was, at that time, an unsolved puzzle. The chemical problem can be stated briefly in the following way when ethylene is polymerized using Ziegler-Natta catalysts a linear polymer is formed. It was known that the catalysts acts on the double bond of the ethylene unit and the polymethylene chain can grow on both sides of the ethylene unit once the double bond has been opened. The opening, however, can occur in cis or trans configuration. 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

Olefin polymerization by catalysts based on transition metal halogenides is usually designated as coordinated anionic, after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+ - C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds ( polar-type inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminum-polymer bonds. 

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... 

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). 

The low-pressure polymerization of olefins using Ziegler-Natta catalysts, i.e., mixtures of compounds of transition groups IV to VI of the periodic table of the elements together with organometallic compounds of groups I to III is widely applied. Such catalysts, consist of titanium alkyl compounds and aluminum alkyl compounds or alkylaluminum halides. 

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

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 ... 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

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). 

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