Stereochemistry of Repeating Units

Early in the history of organic compounds, two substances were sometimes found that appeared to be chemically identical except that they rotated plane-polarized light equally, but in opposite directions. With the development of the tetrahedral carbon bond model, it gradually became clear that the two isomers were mirror images of each other. As an illustration, the two possible spatial configurations of the compound 

The polymerization of a monosubstituted ethylene, such as a vinyl compound, leads to polymers in which every other carbon atom is a chiral center. This is often marked with an asterisk for emphasis  

Such carbon atoms are referred to as pseudochiral centers in long-chain polymers because the polymers do not in fact exhibit optical activity (12). The reason for the lack of optical activity can be seen through a closer examination of the substituents on such a pseudochiral center  

The two chain segments are indicated by — and and in general will be of unequal length. The first few atoms of the two chain segments attached to C are responsible for the optical activity, not those farther away. These near atoms are seen to be the same, and hence the polymer is optically inactive. 

The dotted and triangular lines represent bonds to substitutents below and above the plane of the carbon-carbon backbone chain, respectively (12). 

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