Structure stereoregular

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

This production of stereoregular structures has been known for sometime and is especially strong for vinyl ethers. Several general observations have been noted. First, the amount of stereoregularity is dependent on the nature of the initiator. Second, stereoregularity increases with a decrease in temperature. Third, the amount and type (isotactic or syndiotactic) are dependent on the polarity of the solvent. For instance, the isotactic form is preferred in nonpolar solvents, but the syndiotactic form is preferred in polar solvents. 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

The versatility of polymerization resides not only in the different types of reactants which can be polymerized but also in the variations allowed by copolymerization and stereoselective polymerization. Chain copolymerization is the most important kind of copolymerization and is considered separately in Chap. 6. Other copolymerizations are discussed in the appropriate chapters. Chapter 8 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of initiators and polymerization conditions. In the last chapter, there is a discussion of the reactions of polymers that are useful for modifying or synthesizing new polymer structures and the use of polymeric reagents, substrates, and catalysts. The literature has been covered through early 2003. 

There are four types of possible stereoregular structures for each frans-1,4 polymer of 1,4-disubstituted butadiene (Fig. 13). The stereochemistry of polymers is represented by two kinds of relationship as follows, one of which is the relative configuration between the two repeating monomer units. When all the... 

Polymers, with their highly stereoregular structures, are frequently of sufficiently high symmetry for infrared spectroscopy to give only an incomplete picture of the vibrational characteristics of the compounds. In some, as many as half of the fundamental modes are infrared inactive. These non-absorbing modes can frequently be observed in the Raman effect (e.g. polyethylene where mutual exclusion" applies and at least eight modes are Raman active and infrared silent ). 

Using such a-olefins as monomers, Natta (4,5) found that most of the product had a stereoregular structure. Thus, under conditions easy to realize, isotactic structures (A)... 

Scheme 8 [30] shows that a stereoregular structure entirely consisting of (1 -+5)-a-D-glucofuranosidic units can be obtained by the trialkyloxonium ion mechanism (path c). Probably the selective 1,5-ring opening of 22 occurs from the 1,5-linked... 

The enumeration and geometry of the possible stereoregular structures of polypropylene oxide illustrate the value of the above concepts of symmetry. Propylene oxide monomer has a truly asymmetric carbon, and the repeat unit in the polypropylene oxide chain can be either of the two optical isomeric... 

Figure 6. Stereoregular structures of polymer chains described by the four-parameter model...

High -cis polybutadiene has relatively high heat resistance, which is advantageous in the processing of HIPS. On the other hand, this type of polybutadiene crystallizes at about 0 °C, owing to its stereoregular structure, with the consequence that the low-temperature toughness of polystyrene, produced in this way, is reduced. 

Further in this review we will deal with fully synthetic analop of polynucleotides, (compare examples in Fig. 2.), that do not have stereoregular structures. Thus, the first important aspect of similarity to nucleic acids is lost. The ateence of stereoregularity renders polynucleotide analogs ineffective in biological syntheses where natural polynuc-... 

Cationic polymerization can produce polymers with stereoregular structures. It has been observed that in cationic polymerization processes ... 

Mechanism of stereoregulation on the basis of the data on polyolefin stereoregularity. The structure of a polymer chain is the recording of events proceeding in the insertion of olefin molecules into an active metal-carbon bond. To understand the stereochemistry of the propagation reaction, the data on the stereoregular structure of polymer chains are important. Recently, for this purpose, C-NMR spectroscopy has been extensively used... 

For the catalysts of different composition two polypropylene fractions can be singled out which differ greatly in their stereoregular structure (Table 6) a) a fraction insoluble in boiling n-heptane with an insignificant amount (1-2%) of steric defects of the type... 

Synthetic natural rubber, cw-polyisoprene, is an example of a stereospecific polymer made possible by this means. There are five types of stereo specific (or stereoregular) structures cis, trans, isotactic, syndi-otactic, and tritactic. 

The NMR method of identifying stereoregular structures can be applied also to polymers that are not predominantly stereoregular, yielding information about the probability Pm that a monomer adding on to the... 

The stereospecific polymerization of ot-olefins is one of the best examples which illustrate the possible applications of high resolution NMR to the determination of reaction mechanisms. As a matter of fact, the stereoregular structure of the reaction products and the sensitivity of chemical shifts to stereochemical environments make it possible to obtain considerable and important information about very subtle details of the reaction mechanism. This is especially true when NMR is used in conjunction with isotopic substitution. 

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