Chemistry Reference
In-Depth Information
The rearrangement that follows can result in either 1,2; 1,4; or 3,4 additions to the double bond:
C
4
H
9
Li
1,2
C
4
H
9
Li
+
1
3
4
2
1,4
3,4
C
4
H
9
CH
2
Li
C
4
H
9
CH
2
Li
4.4.1.2 One Electron Transfer Initiation
Alkali metals initiate anionic polymerizations of olefins in either homogeneous or heterogeneous
conditions. This depends upon the metal and upon the solvent. For instance, potassium is soluble
in ethers, like dimethoxyethane or tetrahydrofuran, and the initiation conditions are homogeneous.
On the other hand, sodium dispersions are insoluble in hydrocarbons and the initiations are heteroge-
neous. Liquid ammonia is a solvent for many alkali metals, though for some, like sodium, it can be a
reactant and form metal amides, provided that traces of iron are present. Initiation reactions in many
metal solutions take place by an electron transfer from the metal to the monomer to form anion
radicals. The resultant anion radicals may then undergo propagation reactions. These propagations
can proceed anionically, or by a free-radical mechanism, or by both simultaneously. If the radicals are
unstable the anion-radicals dimerize and the propagation proceeds by an anionic mechanism at both
ends of the chain.
When liquid ammonia is employed as the solvent, as stated earlier, the particular mechanism of
initiation will depend upon the metal used. Lithium metal forms solutions in liquid ammonia and
initiates polymerization of monomers like methacrylonitrile by an electron transfer [
169
]:
Li
+
CH
2
Li
N
These radical-anions couple and chain growth takes places from both ends of the chains:
Li
2
CH
2
Li
Li
N
N
N
Dimerization of radical-ions depends not only upon the radical's stability but also upon the
p
-energy changes that accompany the reaction [
170
,
171
].
