Chemistry Reference
In-Depth Information
anion can be formed from the monomer, which acts as an electron acceptor in
anionic polymerizations. The relatively weak base sodium methoxide (NaOCH
3
)
can polymerize acrylonitrile. This monomer contains the electronegative — CN
group which decreases the electron density on the C
C double bond and
enhances reactivity toward anions. Vinylidene cyanide carries two — CN groups
on the same carbon atom and can be polymerized by even weaker bases like
water and amines. Similarly, cyanoacrylates have two electron-withdrawing
groups on the same carbon [
Q
O(OCH
3
)] and are polymerized by
water; they can be used to glue human tissue together in oral surgery procedures.
By contrast, polymerization of nonpolar monomers such as conjugated diolefins
requires initiation by very strong bases like metal alkyls.
The two principal anionic initiation processes are nucleophilic attack on the
monomer and electron transfer. Nucleophilic attack is essentially addition of a
negatively charged entity to the monomer and involves mainly alkali metal alkyls,
“living polymers,” metal alkoxides, metal amides, and Grignard reagents. The
general initiation process is
a
CN and
a
C
Q
H
H
-
+
(11-2)
Z
Z
B + CH
2
C
B
CH
2
C
X
X
where the nucleophilic part of the initiator is denoted B and the counterion is
formed from the metal Z. Alkali metal alkyls and living polymers are the most
important initiators of this type for vinyl monomers.
The group of electron transfer initiators includes alkali metals and complexes
of these metals with hydrocarbons. The general reaction is
H
H
-
+
(11-3)
Z
Z + CH
2
C
CH
2
C
X
X
The more important anionic initiators are reviewed briefly in the following
paragraphs.
11.2.2.1
Alkali Metal Alkyls
These initiators produce anionic propagating species by attack on the C
C dou-
ble bond of vinyl and diene monomers. A common example of this type is
n
-butyl
lithium. The C
Q
Li bond is not ionic in hydrocarbon media where the initiator
molecules exist as aggregates. The unaggregated form is more active for initia-
tion. Butyl
a
lithium is usually available as a solution in hexane. Addition of
