Stereoisomeric centers

Figure 2.1 Definition of (—) and (+) bonds adjacent to tetrahedral stereoisomeric center.

When a monomeric unit contains more than one tetrahedral stereoisomeric center, the relative configuration of the centers has to be defined. In the case of two adjacent stereoisomeric centers, for instance, -CHA-CHB-, with A B, two configurational signs can be assigned to the bonds connecting the centers (Figure 2.3a). The pairs (+, +) or (—, —) define a relative threo configuration,... 

The conformation assumed by polymer chains in the crystalline state depends on the configuration of the stereoisomeric centers present along the chains and is defined by two basic principles1,2,14 16 ... 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

Our definitions of the stereoisomeric center, line, and plane all stipulate the existence of bonds between the ligating element and its ligands. The exclusive use of these elements limits our analysis to classical stereochemistry and thus does not encompass the so-called topological isomerism (47) of interlocked rings—catenanes (48)—or of knots. As there is no bond between the rings of the catenanes we cannot expect to handle such compounds with a system based on connectedness. At the present stage of development, this limitation in scope... 

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

A polymer is defined as isotactic when the bonds issuing from the successive stereoisomeric centers (conventionally indicated by a vertical line) always have the same disposition when observed in the same direction -F —F —f- —1- —h — or— -l—1+—13—13— + itis defined as syn-diotactic when the disposition of the signs is inverted at each stage -t- --I++I----- -t-+ —. An isotactic sequence is, therefore, represented by... 

Ditactic polymers possess two stereoisomeric centers per constitutional monomeric unit, and tritactic polymers possess three. Ditactic polymers may be formed by the polymerization of 1,2-disubstituted ethylene derivatives, as, for example, with pentene-2 ... 

By polymerizing the double bonds of unsaturated rings, it is possible to synthesize polymers with rings as stereoisomeric centers. The other ring atoms bonded directly onto the main chain atoms of the ring should be considered as... 

Recently, it has been shown that alternating ethylene-norbornene (EN) copolymers are crystalline [202-206] and that this crystallinity is not necessarily related to a regular succession of configurations of stereoisomeric centers in the norbornene units [121,207]. 

Polymers with one stereoisomeric center per base unit are called monotactic. Examples of monotactic polymers are poly(ethylidene), - CH(CH3)-3h, with one central asymmetric atom per chain link, poly-(propylene), - H2—CH(CH3>, with one asymmetric central atom per two chain links, and poly(propylene oxide), - CH2—CH(CH3)—O, with one asymmetric central atom per three chain links. 

Fischer projection formulas can be used to represent molecules with several stereoisomeric centers, and they are commonly used for carbohydrate molecules. For other types of structures, a common practice is to draw the molecule in an extended conformation with the main chain horizontal. In this arrangement, each tetrahedral carbon has two additional substituents, one facing out and one in. The orientation is specified with solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

Figure 2.1 Definition of (-) and (+) bonds adjacent to a tetrahedral stereoisomeric center [1]. The substituent Ri is bulkier than R2 [15].

A pair of consecutive, but not necessarily contiguous, tetrahedral stereoisomeric centers defines a diad [16]. Stereosequences terminating in tetrahedral stereoisomeric centers at both ends, and which comprise two, three, four, five, and so on, consecutive centers of that type, may be called diads, triads, tetrads, pentads, and so on, respectively. If the two units belonging to a diad have the same configuration, the diad is defined meso (m) and is characterized by a mirror plane of symmetry if the two units are enantiomorphous, the diad is defined racemo (r) and is characterized by a twofold rotation axis of symmetry (Fig. 2.2b). 

Figure 2.3 Threo and erythro relative configurations in monomeric units containing two adjacent tetrahedral stereoisomeric centers and two different substituents A B, and succession of (+) and (-) bonds in r/zr o-diisotactic, ryt/zro-diisotactic, and disyndiotactic polymers. When A = B the relative configurations are defined racemo and meso.

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