Isomerism steric

Each of these double bonds provides a site for a steric isomerism, depending on whether the CH2 groups... 

The elements of stereoisomerism considered thus far consist of a point occupied by an atom and a line wholly occupied by one or more bonds. The ligands are joined directly to these elements. This emphasis on bonding relationships appears to be proper, as the distinction between constitutional and steric isomerism similarly depends on established patterns of connectedness. From this point of view it seems less than satisfactory if direct connectedness between specific atoms is assumed, when chemical theory envisions no such localized bond. This situation prevails in the description of ir complexes such as the metallocenes. Initially (44a), the 1,2- (22) or 1,3-heteroligated ferrocenes were considered to... 

The definitions given in Sects. IV and VI are based on a principle that still has to be stated and justified The process of factorization is terminated only after the smallest entities have been reached that allow a description of the changes that can result from any steric isomerization of a given structure. These entities are the elements of its stereoisomerism. Such a use of the term elements is akin to its meaning when chemists speak of elements of matter. Thus an element of stereoisomerism ought to be something that cannot be factored any further. 

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... 

We found that the stereoisomeric lithium (94) or sodium (95) enolates showed precisely the same stereoisomerism. Benzoylation of the enolates (XI) and (XII) gave the appropriate stereoisomeric benzoates. Hence, the enolates are not subject to steric isomerization under these conditions. Consequently, no C-metallic derivative of the ketone species is involved, otherwise it would function as an intermediate, allowing the stereoisomeric enolates to isomerize to an equilibrated mixture. Indeed, if a tetramethyl-ammonium salt whose cation cannot form any covalent bond with oxygen is... 

In addition to copolymer composition, geometrical and steric isomerism play important roles in the determination of chain stiffness and thus of Tg. Polydienes, for example, can exist as cis and trans geometrical isomers. In the case of poly(l, 5-butadiene), the cis isomer has a Tg of-102 °C while the stiffer trans isomer shows a Tg of -48 °C. The effect is not nearly so marked in the... 

Vinyl polymers of the type CH2CHX and vinylidene polymers of the type CH2CXY are capable of existing in different stereoregular forms. These are generally referred to as isotactic, syndiotactic, and atactic forms it is here assumed that the reader is familiar with this nomenclature and the specific structures associated with it. It has been shown that steric isomerism has little effect on the Tg of vinyl polymers but profoundly affects the Tg of vinylidene polymers.12 In the latter case, the isotactic form invariably is associated with the lowest Tg values. This result has also been rationalized on the basis of the Gibbs-DiMarzio theory, but again the details are beyond the scope of this book. 

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). 

Figure 2. Mislow s classification for constitutionai and steric isomerism.

Another aspect of the chemical composition of a polymer which affects crystallisation and hence physical properties is steric isomerism and stereo regularity. For simplicity, consider only vinyl polymers such as polypropylene or polystyrene where a substituent group (in these cases... 

Conventional base unit of a polymer n. Base unit, defined without regard to steric isomerism (lUPAC). 

Stereobase unit ster-e- 6- bas yii-n9t n. The base unit of a polymer, taking steric isomerism into account. 

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). 

Stereo- and Regiochemistry of Monomer Insertion. For substituted O -olefins a number of issues concerning monomer coordination/insertion must be considered. The way in which the monomer inserts itself into the pol5uner chain determines the microstructure of the polymer and subsequently the properties. Does the methyl group on the propylene end up towards the chain or away What is the position of the methyl group on the propylene molecule with respect to coordination site of the metal and the polymer chain, cis versus trans The way in which the propylene molecule may be inserted in the polymer chain by various mechanisms can give rise to complex phenomena of structural and steric isomerism. 

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. 

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