Pseudoasymmetric carbon

In most cases with more than two chiral centers, the number of isomers can be calculated from the formula 2", where n is the number of chiral centers, although in some cases the actual number is less than this, owing to meso forms.An interesting case is that of 2,3,4-pentanetriol (or any similar molecule). The middle carbon is not asymmetric when the 2- and 4-carbon atoms are both (/ ) [or both (5)] but is asymmetric when one of them is (/ ) and the other (5). Such a carbon is called a pseudoasymmetric carbon. In these cases, there are four isomers two meso forms and one dl pair. The student should satisfy himself or herself, remembering the rules... 

If the configurations of successive pseudoasymmetric carbons differ, a given substituent will appear alternatively above and below the reference plane in this planar zigzag conformation (Fig. 4-1 b). Such polymers are called syndiotactic. 

It may also be u.seful at this point to reiterate that the stereoisomerism which is the topic of this section is confined to polymers of substituted ethylenic monomers. Polymers with structures like 1-5 or 1-6 do not have pseudoasymmetric carbons in their backbones. 

If the substituents, say X, are then all above or all below the plane [Fig. 2.11(a)], the configurations of successive pseudoasymmetric carbons are the same the polymer is termed isotactic and designated as dddd... or lllL... If, however, a given substituent appears alternately above and below the reference plane in the planar zigzag conformation [Fig. 2.11 (b)], the polymer is termed syndiotactic and... 

If the observation is made from outside the system, i.e., observation of the relative configurations or diads, then, with isotactic molecules, all configurations about the central carbon atoms are equivalent. There is no configuration reversal about the pseudoasymmetric carbon atom as is observed when absolute configurations are being considered. 

In extended vinyl polymer chains, only relative chiralities can be distinguished. In a meso diad (c), two consecutive pseudoasymmetric carbon atoms (C2, C4) have the same configuration, whereas in syndiotactic diads (d) the configuration is opposite... 

Fig. 9. A polymer segment between two pseudoasymmetric carbons is called a diad (a), and a segment centered around the (bi)substituted carbon is an interdiad (b). A sequence of two diads is called a triad (c) (because of including three pseudoasymmetric carbons), and a sequence of three a tetrad (d)...

Fig. 14. Structural formula of poly(propylene) (a), which has pseudoasymmetric carbons. Definition of dihedral angles adapted to the case of poly(propylene) (b) (cf. Fig. lO.b)...

For the conformational statistics we rely on a model developed by Williams, Flory, and Mark [116, 117, 118] which assumes three states (trans t (0°), gauche g (-+-120°) and g (—120°)). Note that in PVC, pseudoasymmetric carbons are present, and the special convention for the sense of rotation of the dihedral angles must be obeyed. A recent MD simulation [119] is in good agreement with the rotational isomeric state model (fraction of trans states, characteristic ratio). 

For a complete description of the monomer sequence distribution and relative stereochemical configuration of monomer sequences, at least in terms of N- and M-centred triads, it is necessary to take into consideration as many as 10 different triads with a central N unit, which may be magnetically distinguishable as is shown in the scheme of Figure 7.22. Since both monomeric repeating units have a quaternary pseudoasymmetric carbon, from a stereochemical point of view 10 triads with a central M unit sensitive to tacticity must also be considered. The statistical analysis of the monomer sequence distribution and of the stereochemical configuration of the copolymer sequences was carried under the following assumptions with respect to the chemical composition of copolymer sequences, it was assumed that... 

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