Stereoregulation catalysts

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

Since the coordination almost certainly involves the transition metal atom, there is a resemblance here to anionic polymerization. The coordination is an important aspect of the present picture, since it is this feature which allows the catalyst to serve as a template for stereoregulation. 

The versatility of such stereoregulating systems is demonstrated in the polymerization of 1,3-butadiene where all four of the potential structures, isotactic-1,2-, syndiotactic-1,2-, trans-, A-, and cis-, A-, can be synthesized in relatively pure form using different catalyst systems. 

The new soluble stereoregulating polymerization catalysts require three features ... 

FIGURE 5.1 Proposed mechanism for soluble stereoregulating catalyst polymerizations. 

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. 

Ultralinear polyethylene (ULPE) has recently become available through the use of soluble stereoregulating catalysts. Along with a decreased amount of short-chained alkyl branching, ULPE has a narrower molecular weight spread. 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

Discuss advantages of the soluble stereoregulating catalysts in comparison with the Natta-Zeigler catalysts. 

How are the difficulties associated with the use of solid state catalysts overcome by the use of soluble stereoregulating catalysts ... 

By 1953, Karl Ziegler and Giuilo Natta discovered a family of catalysts that allowed the introduction of monomer units onto growing polymer chains in an ordered manner. This allowed the synthesis of rubberlike polymers with greater strength and chemical stability in comparison with similar polymers made without the use of these stereoregulating catalysts. 

With the advent of the soluble stereoregulating catalysts, so-called older polymers have been synthesized with additional control over the structure giving products with enhanced strength and dimensional stability. Amorphous PS is relatively brittle, requiring a plasticizer to allow it to be flexible. The use of soluble stereoregulating catalysts allowed the synthesis of sPS with a I m of about 270°C and a Tg of about 100°C with good solvent and chemical resistance. DOW commercialized sPS under the trade name Questra in 1997. It is used in specialty electrical and under-the-hood automotive applications. 

In 1997, Dow introduced sPS under the trade name Questra. The production of Questra is based on relatively new technology and science involving soluble stereoregulating catalysts that produce PS, which has a fixed and repeating geometry as each new styrene monomer unit is added to the growing PS chain. Targeted areas include medical, automotive, and electronic applications. 

Each of the fluorinated catalysts has an optimum temperature-range for stereoselective polymerization. At the lower temperatures, the rate of propagation and yield of polymer decrease dramatically. At higher temperatures, the molecular weight of the polymer produced becomes lower, presumably because chain transfer or termination processes increase in importance. At still higher temperatures, the stereoregulation is lost, and the low-d.p. polymer produced has a mixed, anomeric configuration. 

Results from polymerizations of various types of homologs of monomers indicated that, in general, the requirement imposed on the catalyst to realize the stereoregulation was severest for the monomer having the smallest side group, i.e., the lowest homologs. Therefore, acetaldehyde and propylene oxide were most extensively investigated. 

Fig. 1. The proposed model of stereoregulation in the propagation process of isotactic polymerization of acetaldehyde by organoaluminum catalyst [Natta...

We should like to consider a molecular mechanism of stereoregulation in the stereospecific polymerization of aliphatic monoaldehydes by carrying out a molecular model examination based on the structure, chemical behavior and catalytic behavior of the catalyst. 

In this mechanism, the stereoregulation in the propagation reaction is realized faithfully by the steric effect acting between the incoming monomer and the growing end monomeric unit, both of which have interactions with an aluminum atom. The penta-coordinate aluminum atom of the catalyst, which is similar to that shown in the dimeric acetaldehyde-catalyst complex, plays an important role as an intermediate compound or as a transition state. In principle, an identical mechanism may be applied to the polymerization by the dialkyl-aluminum monoalkoxide catalyst. 

Poly[o-phthalaldehyde] was reported to be soluble in organic solvents, in contrast with isotactic polyacetaldehyde. This solubility behavior afforded us a good chance to test the stereoregulating capacity of our catalyst in the polymerization process. 

We had studied mainly the propylene oxide polymerization, because the requirement for stereoregulation imposed on the catalyst was supposed to be severest for this monomer among alkylene oxides. The catalysts extensively studied are R2A10A1R2 and EtZnNBu ZnEt, which are composed of two metal atoms (59). The latter catalyst, being a crystalline compound, is characterized by giving an isotactic poly-propylen oxide in high yield. 

An example of the application of this method is the examination of stereoregulating capacity of polymerization catalyst. A typical example... 

Results of the polymerization of propylene oxide by these four kinds of organometallic compounds showed that the zinc compounds are superior to the aluminum compounds with respect to their stereoregulating power. It is noteworthy that the zinc catalyst is generally superior to the aluminum catalyst as the stereospedfic polymerization catalyst. Among the two organozinc compounds, EtZnNBu ZnEt was thought to be more suitable than EtZnNHBu for detailed studies, because the former was monomeric and had no reactive hydrogen atom. 

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. 

The strategy we adopted for attacking our problem, i.e., the complete understanding of the stereoregulation mechanism in the stereospecific polymerization reaction, has been successfully applied to the stereospecific polymerization of acetaldehyde and propylene oxide. The same strategy should be applicable also to other types of catalysts and monomers, even if the difficulty encountered in the experimental performance is greater. The fruitful harvest must await future investigation. 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

ZIEGLER-NATTA POLYMERIZATION. Polymerization of vinyl monomers under mild conditions using aluminum alkyls and TiCL lor other transition element halide) catalyst to give a stereoregulated, or tactic, polymer. These polymers, in which the stereochemistry of the chain is not random have very useful physical properties. 

Most Ziegler catalyst systems are heterogeneous, but some homogeneous systems are known, and it is not clear at present that stereoregulation and stereoselectivity are the results of heterogeneity. 

As already mentioned, nearly all experiments and theories agree that polymerization occurs by addition of an olefin to a catalyst center, followed by insertion of the (stereoregulated, sometimes stereospecific) complexed olefin into a metal-carbon bond at the catalyst center. Figure 9 shows how such an active center can be situated at the edge of a crystal-lattice. It will be seen that the environment of the coordinatively unsaturated, but alkylated, Ti atom demands the stereospecific coordination of the propylene (81). 

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