Chemistry Reference
In-Depth Information
X
HX
HX
HX
H X
HX
HX
C
C
C
C
C
C
C
(a)
C
C
C
C
C
C
Y HYHYYHHYHY HY
XHYHXHYHXHYH
X
H
C
C
C
C
C
C
C
(b)
C
C
C
C
C
C
C
Y
HX
H Y
H X
H Y
H X
H
Y
H
FIGURE 1.5
(a) Isotactic polymer in a planar zigzag conformation. (b) Syndiotactic polymer in a planar
zigzag conformation.
possible to distinguish between all-D and all-L configurations in polymers
because the two ends of the polymer chain cannot be identified. The structure in
Fig. 1.5a
is thus identical to its mirror image in which all the Y substituents are
above the plane.
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. 1.5b
). Such polymers are called syndiotactic.
When the configurations at the C
x
centers are more or less random, the poly-
mer is not stereoregular and is said to be atactic. Polymerizations that yield tactic
polymers are called stereospecific. Some of the more important stereospecific
polymerizations of vinyl polymers are described briefly in Chapter 11.
The reader should note that stereoisomerism does not exist if the substituents
X and Y in the monomer 1-67 are identical. Thus there are no configurational iso-
mers of polyethylene, polyisobutene, or poly(vinylidene chloride). It should also
be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of
polyisoprene can exist as isotactic, syndiotactic, and atactic configurational iso-
mers. The number of possible structures of polymers of conjugated dienes can be
seen to be quite large when the possibility of head-to-head and head-to-tail isom-
erism is also taken into account.
It may also be useful at this point to reiterate that the stereoisomerism, which
is the topic of this section, is confined to polymers of substituted ethylenic mono-
mers. Polymers with structures like 1-5 or 1-6 do not have pseudoasymmetric car-
bons in their backbones.
The importance of stereoregularity in vinyl polymers lies in its effects on the
crystallizability of the material. The polymer chains must be able to pack together
in a regular array if they are to crystallize. The macromolecules must have fairly
