Stereospecific catalysts, structure

In the absence of any external influence such as a catalyst which is biased in favor of one configuration over the other, we might expect structures [VIII] and [IX] to occur at random with equal probability as if the configuration at each successive addition were determined by the toss of a coin. Such, indeed, is the ordinary case. However, in the early 1950s, stereospecific catalysts were discovered Ziegler and Natta received the Nobel Prize for this discovery in 1963. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

Consistent with the discussion on alkali metal alkyls, the least stereospecific catalysts for vinyl polymerizations should be those which are derived from the least electronegative metals having the weakest p or d bonding orbitals. On this basis, one expects increasing stereospecificity for making isotactic or cis-1,4 products in the order Ba < Sr < < Ca Mg Be, with some variations due to monomer structure. 

The chemical meaning did not appear until 1733. tacticity Gk. tax- (arrange), referring to the arrangement of atoms in the molecular structure of high polymers. This and related terms (syndiotactic, atactic, etc.) were introduced in the 1930s when stereospecific catalysts were developed by Ziegler and Natta. 

Staudinger, Hermann (1881-1965). Fundamental research on high-polymer structure, catalytic synthesis, polymerization mechanisms, resulting eventually in development of stereospecific catalysts by Ziegler and Natta (stereoregular polymers). Nobel Prize 1963. 

Synthetic polyisoprene, prepared by free-radical polymerization of isoprene monomer, is a copolymer of six structurally distinct kinds of isoprene chain units. Unlike natural rubber, which is a regularly repeating Class I structure (cis-1,4), such synthetic polyisoprene does not crystallize. On the other hand, by the use of the appropriate stereospecific catalyst, isoprene monomer can be converted to a regular Class I polymer with the same structure as natural rubber (. ... 

Tactic PS. Isotactic (iPS) and syndiotactic (sPS) PSs can be obtained by the polymerization of styrene with stereospecific catalysts of the Ziegler-Natta-type. Aluminum-activated TiCls yields iPS while soluble Ti complexes [eg, ( j -CsHslTiCls] in combination with a partially hydrolyzed alkylaluminum [eg, methylalumoxane] yield sPS. The discovery of the sPS catalyst system was first reported in 1986 (33). As a result of the regular tactic structure, both iPS (phenyl groups cis) and sPS (phenyl groups alternating trans) are highly crystalline. Samples of iPS quenched from the melt are amorphous, but become... 

When examining binary copolymers obtained with stereospecific catalysts, it is important to consider not only the effects of mixing the monomer units in the chain but also the stereoregulating effect of the catalytic system on the structure of the sequences. 

As a result of polymerization of 1,3-butadiene and its derivatives one obtains synthetic rubbers (elastomers) whose properties depend on the structure of the formed products. The stereospecific Ziegler-Natta catalysts offer new opportunities for the synthesis of rubbers with a defined structure. It turned out that by choosing an appropriate catalyst one can be obtain the following polymer structures cis-1.4, trans-1.4, isotactic 1.2, and syndiotactic 1.2, all in a relatively pure form. The influence of the catalyst structure on the stereospecific polymerization of butadiene is shown in Table 8.11. 

The first metallocene catalyst (CP2MCI2.MAO) was completely nonstereospecific for the polymerization of propylene and the polymer was atactic. It was then shown that when the catalyst structure was modified by replacing the halogen atoms with phenyl groups (Cp2MPh2.MAO) the catalyst became stereospecific at a temperature of -30°C. ... 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

Occasionally, however, stereospecific results are encountered in the literature which clearly implicate ligands about the transition metal in steric control. For example, when a typical catalyst system based on WCle was modified by the addition of triphenylphosphine, Dall Asta found (77) that the reaction of c/s-2-pentene led very selectively to the formation of tr n.s-olefinic products. On the other hand, Katz demonstrated (75) that when (CO)5W=C(Ph)2 was used, e/.v-2-pentene afforded butenes and hexenes having about 95% cis structure, and notably that this specificity persisted even for reactions carried to near-equilibrium. 

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