Stereo-specific catalysts

Polymerised isoprene. Naturally-occurring polyisoprenes are natural rubber (cA-form) and gutta percha (trans- form). The use of stereo specific catalysts has made possible the manufacture of synthetic cA-polyisoprene and fraws-polyisoprene both of which are now available commercially. 

Stereoregular polymers are made by reactions involving the use of stereo specific catalysts. Stereospecific Catalysts... 

The first synthetic rubbers to be commercially available in United States were Thiokol (1930) and Neoprene (1931). Both of these are still being produced commercially because they have special properties that are not matched by natural rubber. Various types of synthetic rubbers were introduced during (1939-43) World War II. After world war, stereo rubbers have been made using stereo specific catalysts. 

Mitsui Chemicals, Inc. Polypropylene Propylene and ethylene High yield and high stereo specific catalyst in a simple process scheme 23 2000... 

The presence of a comonomer in the polymerization process, when a stereo-specific catalyst is used, results in the production of a rather linear polymer with very short branch-like pendant groups. This polymer is called linear low density polyethylene (LLDPE) or ultra low density polyethylene (ULDPE), depending on the density achieved by the addition of the comonomer. The larger the amount of comonomer added, the lower is the density of the copolymer. For example, if hexene is used, the pendant groups are as follows ... 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

Living polymerization and stereo-specific catalysts led to novel polymer structures such as dendrimers, star and comb (Figure 1.10). Living polymerization techniques are used to produce. These polymers are finding applications as catalysis and light amplifiers. 

The ability of a polymeric material to crystallise is determined by its molecular structure, that is its structural regularity and flexibility. A regular structure has the potential to exhibit crystallinity, while an irregular structure will tend to be amorphous. The general structure of polyester resin is very complex as these resins are obtained from a mixture of fatty acids with different structures and compositions. In addition, no stereo-specific catalyst is used in the resinification reaction, so the product obtained is random in nature. Thus crystallinity in polyester resins is rarely obtained, most being rather amorphous and highly flexible. 

The polymer is generally prepared in the presence of a stereo-specific catalyst system that can produce a polymer that contains in excess of 92 percent cis-1,4 structure. The cis and trans are shown here ... 

Polypropylene was initially produced commercially about 45 years ago after the successful development of a suitable stereo-specific catalyst, which enabled the polymer to have the kind of structural characteristics useful for rigid items. Tables 1.1 and 1.2 describe the historical and anticipated consumption of polypropylene. In Table 1.3 you can find the expected capacity growth by region for the future. The continuous growth of polypropylene is expected to continue into the next millenium as raw materials in an expanding number of end-use products for the automotive and film industries. 

Polypropylene (= polypropene) generates from polymerisation of propene. With stereo specific catalysts three types of polypropylene can be produced ... 

After World War 11, enormous contributions were made toward the xmderstanding of polymers and the development of new polymers. In that period. Professors Karl Ziegler of Germany and Giulio Natta of Italy developed the stereo-specific catalysts that have led chemists to order the molecular structure of polymers. Consequently, polymers can now be tailored for specific applications. Acetal and polycarbonate families were also developed in the 1950s together with nylon, polyimide, phenoxy, polysulfone, and... 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

A stereo specific polymer produced by the copolymerisation of ethylene and propylene with Ziegler-type catalysts. 

A catalyst made from the metal lithium and used in the preparation of stereo specific polymers. Lithopone... 

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. 

Metalloporphyrins and some related metal complexes are effective catalysts in IOB oxidations, as already discussed for alkenes, and acids (Sections 5.1.1.1 and 5.2.4). Also, sulphides have been oxidized to sulphoxides [58]. Some other substrates of various types underwent such catalysed oxidations, because these systems mimic the natural oxidant cytochrome P-450 [2]. From a synthetic point of view, only a few reactions are of importance alkanes were mainly used which underwent regio- and stereo-specific hydroxylation, for instance the methyl group of a pyrrole derivative was converted into hydroxymethyl, leading to one-pot preparation of dipyrro-methanes [59], The preparation of elaborated catalysts is, however, very demanding and precludes a wider use. 

Butler, Thomas, and Tyler (9) have reported on the stereo-specific polymerization of N,N-disubstituted acrylamides by alkyl lithiums. Overberger and Schiller (51) reported on the preparation of crystalline poly-/-butyl vinyl ketone by anionic catalysts in toluene at room temperature. 

This is a reaction that seems veiy attractive for synthesis but, in the absence of a transition met catalyst, the yields are very low. We showed in the last chapter how vinyl silanes can be made wit control over stereochemistry and converted into lithium derivatives with retention. Neither of thes vinyl metals couple with vinyl halides alone. But in the presence of a transition metal—Cu (I) for I and Pd(0) for Sn—coupling occurs stereo specifically and in good yield. 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

Using the asymmetric catalyst, (III), prepared by reacting (S,S)-l-amino-2-hydroxyindane and 2-hydroxyl-3-(diphenylmethyl)-benzaldehyde, illustrated in Eq. 2, a stereo specific aziridine cis-opening was described by the author in the current invention and depicted in Eq. 3 ... 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

In TiCl3, on the other hand, AlEt2Cl activates only the predominantly stereo-specific surface sites, while AlEt3 can disrupt the crystalline lattice of the catalyst thus forming non-stereospecific centers. 

Figure 7.10 Stereo specific Diels-Alder reaction using an organocatalyst and fluorous silica for catalyst recovery.

More highly functionalized products can be obtained, often in better yield, by the ene reaction of a-substituted acrylate esters. EtAlCb is a more effective catalyst than AlCb for these reactions because it is a Brpnsted base as well as a Lewis acid. The EtAlCb catdyzed reactions of methyl a-bromo- and a-chloro-acrylate with trans-1,2-disubstituted and trisubstituted alkenes are both regio- and stereo-specific. - The major product (13a 85-95%) is formed via transition state (12a) in which the methoxycar-bonyl group is endo. The stereochemistry of the major adduct was established by the stereospecific conversion of (13a) to both diastereomers of ( )-2-amino-4-methyl-5-hexenoic acid and confirmed by X-ray crystallographic analysis in a related system. ... 

Reactions with phosphine- or phosphite-modified catalysts, e.g. Ni(COD)2 + PPhs, show lower stereo-specificity and require higher reaction temperature. In the case of fumarate and maleate this catalyst system promotes only distal ring-opening cycloaddition (equation 85). ... 

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