Natta initiator

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. 

Table 6 Examples of typical components of Ziegler-Natta initiators...

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. 

Modification of an initiator system to increase activity has often come at the expense of stereoselectivity. The great utility of the Ziegler-Natta initiator system is the ability to change one or another of the components, or to add additional components (usually electron donors) to achieve very high stereoselectivity with high activity. The choice of the initiator components evolved in an empirical manner because of a less-than-complete understanding of the detailed structure of these initiators and the mechanism of their stereoselectivity. 

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-... 

A variety of structures have been proposed for the active species in Ziegler-Natta initiator systems [Allegra, 1971 Arlman and Cossee, 1964 Corradini et al., 1989 Natta, 1960a,b Patat and Sinn, 1958 Rodriguez and van Looy, 1966 Tait and Watkins, 1989]. Structure... 

The Cossee-Arlman mechanism as originally proposed has a weakness—the back-flip is required to explain isoselective placement since the two active (coordination) sites are assumed to be enantiotopic. However, the structure of the traditional Ziegler-Natta heterogeneous initiators is not sufficently understood to either support or reject the assumption of enantiotopic sites. Further, even if the sites are enantiotopic, there is no overwhelming reason why the polymer chain is more stable at one site than the other—which is the rationale for the back-flip. The mechanism of isoselectivity with various metallocene initiators is much better understood since these are initiators whose molecular structures are well-established [Busico and Cipullo, 2001 Busico et al., 1997, 1999 Cavallo et al., 1998 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000], Considerable advancements in understanding heterogeneous Ziegler-Natta initiators occur if one assumes that the active sites in these initiators mimic those in metallocene initiators. Two types of metallocene initiators offer possible models... 

The stereoselectivity and activity of Ziegler-Natta initiators vary considerably depending on the identity and relative amounts of the initiator components [Boor, 1979], The interpretation... 

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... 

Propagation proceeds in a manner similar to that described for the traditional Ziegler-Natta initiators. The transition metal has two active sites—the polymer chain is held at one site (the one occupied by a methyl group in XXVII) and monomer at the other site (shown as the vacancy ). The reactivity of the active sites is high because the counteranion, which is either (ClMAO) or (OR MAO) or a mixture of the two, is a weakly coordinating anion. Reactivity is decreased when the counterion is strongly coordinating. 

Activation energies are generally in the range 40-60 kJ mol-1 some lower values have been observed, in a range similar to that for traditional Ziegler-Natta initiators. 

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. 

Wanless and Kennedy (S) pyrolyzed polymers synthesized using 4-methyl-1-pentene and 4-d-4-methyl-1 -pentene. The mass, infrared, and HNMR spectra of the pyrolyzates, as well as those of a polymer produced by Ziegler-Natta initiator indicated that the cationically produced polymer contained about 70% 13 and 1,4 units, with 1,4 units predominating, and that the 1,4 structure most probably resulted from two consecutive hydride shifts rather than a single hydride jump". 

---