Chemistry Reference
In-Depth Information
mp 170
C), an optically inactive form (meso, mp 140
C), and as an optically
inactive mixture (DL racemic, mp 206
C).
COOH
HOC
H
H
C
OH
COOH
1-69
Vinyl polymers contain many pseudoasymmetric sites, and their properties are
related to those of micromolecular compounds that contain more than one asym-
metric carbon. Most polymers of this type are not optically active. The reason for
this can be seen from structure 1-68. Any C
x
has four different substituents: X, Y,
and two sections of the main polymer chain that differ in length. Optical activity
is influenced, however, only by the first few atoms about such a center, and these
will be identical regardless of the length of the whole polymer chain. This is why
the carbons marked C
x
are not true asymmetric centers. Only those C
x
centers
near the ends of macromolecules will be truly asymmetric, and there are too few
chain ends in a high polymer to confer any significant optical activity on the mol-
ecule as a whole.
Each pseudoasymmetric carbon can exist in two distinguishable configura-
tions. To understand this, visualize Maxwell's demon walking along the polymer
backbone. When the demon comes to a particular carbon C
x
she will see three
substituents: the polymer chain, X, and Y. If these occur in a given clockwise
order (say, chain, X, and Y), C
x
has a particular configuration. The substituents
could also lie in the clockwise order: chain, Y, and X, however, and this is a dif-
ferent configuration. Thus, every C
x
may have one or another configuration. This
configuration is fixed when the polymer molecule is formed and is independent of
any rotations of the main chain carbons about the single bonds that connect them.
The configurational nature of a vinyl polymer has profound effects on its
physical properties when the configurations of the pseudoasymmetric carbons are
regular and the polymer is crystallizable. The usual way to picture this phenome-
non involves consideration of the polymer backbone stretched out so that the
bonds between the main chain carbons form a planar zigzag pattern. In this case
the X and Y substituents must lie above and below the plane of the backbone, as
shown in
Fig. 1.5
. If the configurations of successive pseudoasymmetric carbons
are regular, the polymer is said to be stereoregular or tactic. If all the configura-
tions are the same, the substituents X (and Y) will all lie either above or below
the plane when the polymer backbone is in a planar zigzag shape. Such a polymer
is termed isotactic. This configuration is depicted in
Fig. 1.5a
. Note that it is not
