Natta initiator polymerization

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

Natta, a consultant for the Montecatini company of Milan, Italy, applied the Zeigler catalysts to other vinyl monomers such as propylene and found that the polymers were of higher density, higher melting, and more linear than those produced by the then classical techniques such as free-radical-initiated polymerization. Ziegler and Natta shared the Nobel Prize in 1963 for their efforts in the production of vinyl polymers using what we know today as solid state stereoregulating catalysts. 

For polar monomers, heterogeneity is seldom a requirement for isoselective polymerization with traditional Ziegler-Natta initiators syndiotactic polymers are obtained only with the soluble initiators. Styrene and 1,3-dienes are intermediate in behavior between the polar and nonpolar monomer. These monomers undergo isoselective polymerization with both homogeneous and heterogeneous traditional Ziegler-Natta initiators. 

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Acetylene is polymerized to polyacetylene [IUPAC poly(ethene-l,2-diyl)] by Ziegler-Natta initiators such as titanium tetraisobutoxide with triethylaluminum [Ito et al., 1974 Shelburne and Baker, 1987 Shirakawa, 2001 Theophilou and Naarman, 1989]. Polymerization at... 

The absence of a second cyclopentadienyl ring coupled with the short bridge gives a very open environment at the metal site. This allows easier access for bulky monomers, including 1-alkenes and norbomene, compared to polymerization with metallocenes. CpA initiators yield ethylene copolymers not easily available with metallocenes. Copolymers containing significant amounts of comonomers such as styrene, norbomene, and a-olefins from 1-hexene to 1-octadecene are easily obtained with CpA, but not with metallocene or traditional Ziegler-Natta initiators. 

Both traditional Ziegler-Natta and metal oxide Phillips-type initiators are used in suspension polymerizations (Secs. 8-4a, 8-4j) [Kaminsky, 2001], Both types of initiators are used for ethylene, but only the traditional Ziegler-Natta initiators are used for propene since Phillips-type initiators do not yield stereoselective polymerizations. 

The polymerization cir-l-rf-propene by traditional Ziegler-Natta initiators in hydrocarbon solvents yields the erythrodiisotactic structure, while under similar solvent conditions anionic polymerization of cis- -d-methyl acrylate yields the threodiisotactic polymer. Explain the factor(s) responsible for this difference. 

If polymerization catalysts, for instance for alkenc polymerization (Zicglcr-Natta-type catalysts), are prepared by precipitation methods, they can be formed by precipitation from organic solvents, as claimed in several patents [22], In these patents the precipitation of titanium-magnesium compounds in THF with hexane as precipitating agent is used for the formation of the catalyst. Many important Zicgler-Natta initiators are solids, and heterogeneous initiator systems seem to be necessary for the production of isotactic polyalkenes [23]. However, not much information on the details of catalyst preparation is available in the open literature. 

It is evident that the tendency for free radical polymerization is a function of the monomer. Natta (287) stated that these radicals can initiate polymerization of styrene and diolefins to high molecular weight products but not that of aliphatic alpha olefins. North (339) showed that the decomposition of phenyl tri-isopropoxy titanium initiates radical polymerization of styrene but not of ethylene. 

Table 4 gives data which indicates that about 1—10% of a-TiCl3 becomes complexed with AlEt3 or AlEt2Cl. Not all of this necessarily initiates polymerization, since it may be in non-active positions on the main faces of the subhalide crystals. Natta [81] observed that when metal alkyl was adsorbed at high temperatures (100°C) it all appeared in the polymer, but at lower temperatures only a relatively small fraction became combined with polymer. 

The classic Ziegler-Natta heterogeneous catalysts initiate polymerization at diverse, somewhat randomly scattered active sites on modified TiCls crystals, with the result that polymer chain length is variable even though essentially complete isotacticity can be achieved. Furthermore, these catalysts offer little opportunity to manipulate the degree of stereoregularity and hence the mechanical properties of the product—the Ziegler-Natta polypropylenes are exclusively isotactic. 

The ability of early transition metals to initiate polymerizations has been known and utilized since the mid-20th century. [46] Such Ziegler-Natta catalyst systems with homoleptic and simple Ti and A1 compounds have paved the way to the development... 

Oxides of a variety of metals on finely divided inert support materials initiate polymerization of ethylene and other vinyl monomers by a mechanism that is assumed to be similar to that of heterogeneous Ziegler-Natta polymerization that is, initiation probably occurs at active sites on the catalyst surface [2j. Unlike the traditional Ziegler-Natta two-component catalyst systems, the supported metal-oxide catalysts are essentially one-component systems. Among the metals that have been investigated for these catalyst... 

The reactivity of this vinylcyclopropane compound toward cationic or Ziegler-Natta initiators is greater than that of purely cyclopropane-type monomers. This stems from the conjugation of the two unsaturated systems the results obtained agree with work carried out on the polymerization of vinyl cyclopropane itself. 

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