Chemistry Reference
In-Depth Information
1.12.2
Stereoisomerism
Stereoisomerism occurs in vinyl polymers when one of the carbon atoms of the
monomer double bond carries two different substituents. It is formally similar to
the optical isomerism of organic chemistry in which the presence of an asymmet-
ric carbon atom produces two isomers which are not superimposable. Thus glyc-
eraldehyde exists as two stereoisomers with configurations shown in 1-66. (The
dotted lines denote bonds below and the wedge signifies bonds above the plane of
the page.) Similarly, polymerization of a monomer with structure
OHC
H
CHO
H
C
C
HOCH
2
OH
HO
CH
2
OH
1-66
1-67 (where X and Y are any substituents that are not identical) yields poly-
mers in which every other carbon atom in the chain is a site of steric isomerism.
Y
CH
2
C
X
1-67
Such a site, labeled C
x
in 1-68, is termed a pseudoasymmetric or chiral carbon
atom.
Y
Y
Y
C
x
C
x
C
x
CH
2
CH
2
CH
2
X
X
X
1-68
The two glyceraldehyde isomers of 1-66 are identical in all physical properties
except that they rotate the plane of polarized light in opposite directions and form
enantiomorphous crystals. When more than one asymmetric center is present in a
low-molecular-weight species, however, stereoisomers are formed which are not
mirror images of each other and which may differ in many physical properties.
An example of a compound with two asymmetric carbons (a diastereomer) is
tartaric acid, 1-69, which can exist
in two optically active forms (D and L,
