Ziegler-Natta catalysts acetylene polymerization

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

When heated, polyvinyl chloride (PVC) and polyvinyl alcohol (PVA) lose HC1 and H20, respectively, to produce dark-colored conductive polyacetylene. Superior polymers of acetylene can be made by the polymerization of acetylene with Ziegler-Natta catalysts. The conductivity of polyacetylene is increased by the addition of dopants, such as arsenic pentafluoride or sodium naphthenide. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

The 2000 Nobel Prize in Chemistry was awarded for work on poly acetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cw-Polyacetylene has a copper color, and frawi-polyacetylene is silver. 

By carrying out experiments on the polymerization of acetylene by using a concentrated Ziegler-Natta catalyst solution, Shirakawa and Ikeda were the first to report on the preparation of partially crystalline polyacetylene (PA) films under well-defined conditions [2]. Some six years later, Shirakawa, McDiarmid, and Heeger and colleagues [3] presented unequivocal evidence that, after doping with iodine vapor, the room-temperature electronic conductivity of PA film was... 

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. 

EPDM, Epcar 346 and 585 from Polysar PAc, polymerization of acetylene with Ziegler-Natta catalyst contained in EPDM solution. 

Synthesis and properties of acetylenic polymers TABLE 2. Ziegler-Natta catalysts for acetylene polymerization... 

Combination of WCl6 and EtAlCb (or Et3Al) is also effective for acetylene polymerization . This catalyst shows high activity towards linear and cyclic alkene metathesis and a metathesis propagation step is to be invoked also in this case. However, this combination is a typical Ziegler-Natta catalyst system, and it may be probable that, in this particular case, the components are acting as a simple coordinated anionic catalyst system. 

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

As shown in Table 3, cis- and trans-coni nU of poly acetylene prepared by the Ziegler-Natta catalysts depends strongly upon the polymerization temperature ss x ere are two possible explanations for this observation One is that the fundamental mechanism is the formation of cis double bonds by the cis insertion of acetylene monomer into the Ti—C bond of the catalyst. This fits the orbital interaction consideration for the role of the catalyst by Fukui and Inagaki, according to which the initially formed configuration of the double bond is cis as a result of the favorable orbital interaction between the inserting acetylene monomer and the active site of the catalyst. Because the cis double bond is thermodynami-... 

Polymerization of substituted acetylenes has been carried out by a wide range of catalysts and condi-tions. Polymerization conditions include a homogeneous and heterogeneous Ziegler—Natta catalyst, transition metal complexes (Pd. Pt. Ru. W. Mo. Ni. etc.), free radical initiators such as 2.2 -azobis(isobu-tyronitrile) (AIBN). benzoyl peroxide (BPO). and di-tert-butylperoxide (DTBP). thermal polymerization, y-irradiation. cationic initiation with BF3. and anionic initiation by butyllithium. triethylamine. and sodium amide. 

S. Ikeda and his coworkers studied a mechanism of acetylene polymerization in connection with olefin polymerization by various Ziegler-Natta catalysts. They found that polymerization yields not only highly polymerized polyacetylene but also benzene, which is a cyclic trimer of acetylene, and that the ratio of these two products depends... 

Polyacetylene, which was originally prepared by a polymerization of acetylene by Shirak-awa with the aid of a Ziegler-Natta catalyst, is an insulator. It can be shaped into a silvery-looking film. Partial oxidation of the film, however, with iodine or other materials transforms it and increases its conductivity 10 -fold. The process of transforming a polymer to its conductive form through chemical oxidation or reduction is called doping. Polyacetylene, which can be cis or trans, is more thermodynamically stable in the trans form and converts from cis to trans when heated above 150 °C. 

FIGURE 3.3 Construction of asymmetric reaction field for acetylene polymerization by dissolving Ziegler-Natta catalyst, Ti(0-n-Bu)4-AlEt3, into the chiral nematic LC. The chiral nematic LC includes an axially chiral binaphthyl derivative or an asymmetric center containing chiral compound. 

---