Chemistry Reference
In-Depth Information
concentration is complicated and variable. Some of this complexity is due to the
association of growing polymer chains and their counterions into dimers and
larger aggregates. The detailed mechanisms of
these reactions still
require
clarification.
Initiation by soluble organolithium compounds in hydrocarbons is a slow pro-
cess and is related to the existence of aggregates of initiators alone as well as
aggregates which include both initiators and macromolecular species. The propa-
gation reactions are also complex and not well understood at this time.
11.2.4
Termination Reactions
Anionic polymerizations must be carried out in the absence of water, oxygen, car-
bon dioxide, or any other impurities that may react with the active polymerization
centers. Glass surfaces carry layers of adsorbed water that react with carbonions.
It is necessary to take special precautions, such as flaming under vacuum, to
remove this adsorbed water in laboratory polymerizations. Anionic reactions are
easier to carry out on a factory scale, however, because the surface-to-volume
ratio is much less in large reactors.
Polymers with specific end groups can be prepared by deliberately introducing
particular reagents that “kill” living polymers. Thus, in the anionic polymerization
of butadiene with bifunctional initiators, carboxyl end groups are produced by ter-
mination with CO
2
:
H
H
H
H
-
+
+
-
CH
2
Na
+ 2 CO
2
CH
2
C
C
Na
CH
2
C
C
2
H
H
H
H
+
-
-
+
CH
2
CH
2
C
C
CH
2
C OO
Na
Na
OOC
C
C
2
(11-21)
Hydroxyl end groups are provided by termination with ethylene oxide:
O
CH
3
OH
-
-
+
+
+
CH
2
Li
CH
2
CH
2
CH
2
CH
2
CH
2
O Li
(11-22)
CH
2
CH
2
CH
2
OH + Li OCH
3
10,000) versions of such elastomers are used to
produce liquid rubbers which can be shaped more easily than the conventional
high-viscosity elastomers. The liquid rubbers can be vulcanized by reactions of
their specific end groups. Hydroxyl-ended polybutadienes can be caused to grow
in molecular size and to cross-link, for example, by reaction with isocyanates
with functionalities
Low-molecular-weight (3000
2 (cf. reaction 1-12).
.
