Page 786 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Appendix J

Stereogeometry of Polymers

The precise stereogeometry of molecules is important in determining the physical properties of a
material and it is critical in determining the biological properties of materials. Most synthetic and
nonspecific natural polymers are a mix of stereoshapes with numerous stereocenters along the poly-

mer chain. For polypropylene, every other backbone carbon is most likely a stereocenter. Even poly-
ethylene has stereochemical sites whenever there is branching. The imprecise structures of most

natural nonspecific polymers such as the polyisoprenes and polysaccharides have stereocenters at
each branch.

For stereospecific natural polymers the entire geometry is critical to the proper behavior and
activity of the material.
Thus, stereogeometry is important and a brief review is in order.
We have two general types of isomers—constitutional isomers that have the same number and
kind of atoms but connected in a different order, such as cis and trans arrangements, and stereoiso-
mers. We have two types of steroisomers—diastereomers that are not mirror images of one another
and enantiomers that are mirror image stereoisomers. Just as our hands cannot be superimposed
on its mirror image, enantiomers are nonsuperimposible on one another. Chiral objects or sites are
enantiomeric. In fact, the term “chiral” comes from the Greek word for hand, kheir.
By compairson, an achiral object, like a simple nail, ball, basket, white pocket-less T-shirt, are
all superimposible on their mirror image.
A collection containing only one enantiomeric form of a chiral molecule is referred to by several
names, including enantiopure, enantiomerically pure, or optically pure. A sample containing pre-
dominantly one enantiomer is called enantiomerically enriched or enantioenriched. A collection
containing equal amounts of two enantiomeric forms of a chiral molecule is called a racemic mix-
ture or racemate.
Unlike other stereoisomers, enantiomers have essentially identical physical properties and con-

sequently are difficult to separate. A process where enantiomers are separated is called resolution.
Diastereomers are nonenantiomeric isomers that result when more than one stereocenter is pre-
sent in a molecule. The distinction between diastereomers and enantiomers is not always clear but
in general, enantiomers have mirror images whereas diastereomers are not mirror images of one
another. As such diastereomers have different physical properties, different boiling and melting
points, solubilities, and so on.
n
The total number of stereoisomers due to tetrahedral stereocenters does not exceed 2 , where n is
the number of tetrahedral sterocenters. For a compound with two stereocenters n = 2, giving a total
of four (maximum) steroisomers.
Following are four formula isomers first drawn in the more conventional straight-chain manner

and below in more conformationally correct forms drawn for a compound, 2,3-dichlorohexane, with
two stereocenters.

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