Site of steric isomerism

Isomerism is observed in the polymerization of alkenes when one of the carbon atoms of the double bond is monosubstituted. The polymerization of a monosubstituted ethylene, CH2=CHR (where R is any substituent other than H), leads to polymers in which every tertiary carbon atom in the polymer chain is a stereocenter (or stereogenic center). The 

There are two constitutional repeat units (Sec. l-2c) from a stereochemical viewpoint, one with R configuration for the stereocenter and the other with S configuration for the stereocenter (corresponding to la and lb). These are referred to as the two configurational base units and have an enantiomeric relationship [IUPAC, 1966, 1981, 1996]. 

The terms stereocenter, chirotopic, and achirotopic will be used in this text in line with the most recent terminology. However, other terms are found in the older literature. C was previously referred to as a chiral or pseudochiral center. The term pseudochiral center is based on the same convention used to classify C as achirotopic instead of chirotopic. The terms asymmetric and pseudoasymmetric center are found in much older literature. 

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Polymerization leads to a polymer structure (VII) with a repeating alkene double bond in the polymer chain. The double bond in each repeating unit of the polymer chain is a site of steric isomerism since it can have either a cis or a trans configuration. The polymer chain segments on each carbon atom of the double bond are located on the same side of the double... 

Optical Activity in Polymers Stereoisomerism in polymers is formally similar to the optical isomerism of organic chemistry. In a vinyl polymer with the general structure shown in (XIH) every other carbon atom in the chain, labeled C, is a site of steric isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length (Rudin, 1982). 

Tactic polymer n. A polymer in which there is an ordered structure with respect to the configurations around at least one main-chain site of steric isomerism per conventional base unit. Note— The carbon (or other) atom in the chain at the site of the steric isomerism need not in a strict sense be an asymmetric atom, since in a chain of infinite length the two chain portions may be considered as equivalent however, for the purpose of this definition, such atoms are referred to as asymmetric atoms. 

Isotactic A stereoregular structure where the sites of steric isomerism in each repeating unit of the polymer has the same configuration. In other words in an isotactic polypropylene all the methyl groups will be located on one side of the plane of the polymer chain (Fig. 2.14). 

Syndiotactic. A stereoregular polymer where the sites of steric isomerism in each repeating unit are on the opposite side. The methyl groups in alternate repeat units are on the same side and in adjacent repeat units they are on the opposite side (Fig. 2.15). 

The objective of this paper is to provide a fundamental presentation of well established aspects of the synthesis of stereoregular macromolecules and related stereochemical features. Since the discovery of stereoregular polypropylene by Natta and coworkers 40 years ago the number of stereoregular polymers has increased enormously and similar concepts have been extended to natural macromolecules. Therefore, this presentation cannot be really exhaustive and we Umit ourselves to polymers of 1-olefins, which were the first studied systems and can be sites of steric isomerism in the main chain (formed during polymerization) and in the side chains (present in the monomer). 

An isotactic polymer structure occurs when the site of steric isomerism in each repeating unit in the polymer chain has the same configuration. This is illustrated in Chapter 4, p. 191. 

Polydisperse stereoregular polymers can be synthesized either from monomers having no stereoisomerism sites, the stereoisomerism sites being formed during the polymerization, or from monomers containing sites of steric isomerism. In the latter case, the synthesis of stereoregular polymers may or may not involve formation of new stereoisomerism sites, without affecting those already present in the monomers, or else it may affect these sites. 

Finally, stereoregular polymers with asymmetric carbon atoms in the side chains as stereoisomerism sites have been produced by polycondensation (125) and by polymerization of optically active isocyanates (X (126)). As stressed in section 2.2. we do not consider the amide groups in the main chain of polyisocyanates as sites of steric isomerism. 

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