Chemistry Reference
In-Depth Information
11.2.7
Stereochemistry of Anionic Diene Polymerization
Anionic polymerization proceeds through only one of the two available double
bonds in dienes like butadiene or isoprene (Fig. 1.4). Possible structures that
could be produced from enchainment of isoprene have been listed earlier (in
Section 1.11.1). In addition, the polymer segments attached to internal C
a
C dou-
ble bonds may be in
cis
or
trans
configurations. The number of possible isomers
of a polymer of isoprene is clearly very large. The preferred structure for elasto-
meric applications is that in natural rubber produced by the
Hevea brasiliensis
tree. This polymer, which is sometimes called “synthetic natural rubber,” can be
made by anionic and Ziegler
Natta polymerizations.
The character of the counterion and the solvent both affect the microstructure
of polymers made anionically from dienes. In general, the proportion of 1,4
chains is highest for Li and decreases with decreasing electronegativity and
increasing size of the alkali metals in the order Li
Na
K
Rb
Cs. A very
.
.
.
.
high (
90%) 1,4 content is achieved only with lithium alkyl or lithium metal ini-
tiation in hydrocarbon solvents. The properties of polymers of conjugated diole-
fins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or
3,4 [reactions (1-16) and (1-17)]. Elastomeric behavior is realized from 1,4 poly-
merization and particularly if the polymer structure is
cis
about the residual dou-
ble bond.
Addition of polar solvents like ethers and amines causes an increase in side
chain vinyl content resulting from 1,2 or 3,4 polymerization. This effect is partic-
ularly marked in polymerizations with lithium alkyls, which are the only alkali
metal alkyls that are soluble in bulk monomer or hydrocarbon solutions. Polar
media also tend to increase the proportion of 1,4 units with
trans
configurations.
The molecular structures produced are influenced by (1) the relative stabilities
of the two conformations about the double bond in the terminal unit of the macro-
ion; (2) the conformation of the diene monomer when it adds to the macroion;
and (3) in the case of 1,4 addition, the relative rates of isomerization between
cis
and
trans
terminal units compared to the rates at which new monomers are added.
The double bond between carbons 3 and 4 of isoprene (11-8) is the more reac-
tive in anionic polymerizations because the electron-donating methyl group makes
carbon 1 more electron rich than carbon 4. The general reaction for anionic attack
on isoprene is thus given by
.
Z
4
3
2
1
4
3
2
1
CH
2
+
H
2
CC
C
CH
2
CH
2
CC
CH
2
Z
2
H
H
CH
3
3
11-8
11-9
(11-24)
