Polymerizations stereospecific

For poly(/-butylace1ylene), the catalyst MoOCl4-SnBu4-EtOH (1 1 1) is both living and stereospecific [111, 125]. A comparative study between the poly(/-butylacetylene) made by M0OCI4 alone 

Ikble Vin. MefeCR- pe Monomers that are Pofymerizable with StQ5 and a Cocatalyst 

Thble IX. HC=CSiMe2R-TJ pe Monomers that are Polymerizable by WQs in Toluene at Either 30°C or 80°C 

Natta (around 1955) was able to show that the monosubstituted polyethylene (propylene, butene, styrene) produced by means of this method is stereoregular, idio- or syndio-tactic according to the case. This important discovery opened up new possibilities and won a shared Nobel prize for Natta and Ziegler in 1963. 

Polymerization occurs in heterogeneous phase, and the catalytic activity strongly depends on the structure and the quality of the microcrystals of titanium chloride which are used. In titanium chloride, the valence of titanium is six (see Fig. 1.10) and, on the edges of the crystal, there are empty chlorine sites which play the role of active centres. Titanium behaves like an electron attractor with respect to the double bond of the monomer. The polymerization process is rather complex we shall sum it up by saying simply that the end of a growing chain is attached to an empty site of a titanium atom and in this way may react upon monomers attached to other empty sites, with formation of a longer chain. 

The polymerization of a pure chiral monomer to a polymer is trivial. The relationships are more interesting when one starts with racemic mon- 

In the second case, both isomers polymerize independently of each other with the same rate A mixture of pure (R) chains and pure (S) chains is produced. Now, if there is initially an equal amount of each catalyst required to polymerize these two chains, then the same amount of (R) and (S) chains is always produced, and the resulting polymer racemic mixture is optically inactive. However, the mixture can, at least in principle, be separated into its components, for example, with chromatography. If, however, the two polymerization-inducing species are not initially present at the same concentration, then the (R) and (5) units are also not initially produced in the same amounts. The polymer mixture is optically active at low conversions. The optical activity, however, decreases with increasing conversion. An optically inactive polymer is obtained at 100% conversion. 

A series of other cases can occur between these two limiting cases. As a rule, they can be conceived in terms of one of the following two models (Sections 16.5.3.1 and 16.5.3.2). 

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Ziegler-Natta polymerization Stereospecific polymerization of olefines using a Ziegler catalyst. See titanium(IIl) chloride. 

In the mid-1950s, the Nobel Prize-winning work of K. Ziegler and G. Natta introduced anionic initiators which allowed the stereospecific polymerization of isoprene to yield high cis-1,4 stmcture (3,4). At almost the same time, another route to stereospecific polymer architecture by organometaHic compounds was aimounced (5). 

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

The occurrence of stereospecific polymerization in solution has been explained by the stetic restrictions of ligands bonded to the metal center. For example, the following stmcture has been postulated as an intermediate in solution catalysis (68) ... 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

The revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

The Kinetics of the Stereospecific Polymerization of a-Olefins G. Natta and I. Pasquon Surface Potentials and Adsorption Process on Metals... 

Yuki, H. and Hatada, K. Stereospecific Polymerization of Alpha-Substituted Acrylic Add Esters. Vol. 31, pp. 1-45. 

Many recent publications have described the stereospecific polymerization of dienes by ir-allyl compounds derived from Cr, Nb, Ni, etc. Of particular interest is the work of Durand, Dawans, Teyssie who have shown that ir-allyl nickel catalysts (XXI) in the presence of certain additives polymerize butadiene stereospecifically (87, 38). The active center results from reaction of acidic additives with the transition metal. 

The organolanthanide initiators allowed stereospecific polymerization of ethyl, isopropyl, and t-butyl methacrylates (Table 3). The rate of polymerization and the syndiotacticity decreased with increasing bulkiness of the alkyl group in... 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

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