Chemistry Reference
In-Depth Information
11.2.2.4
Alkali Metals
The initiation step in this case involves a one-electron transfer from a Group IA
metal to the monomer. A radical ion is formed:
H
H
-
+
(11-7)
C
Li
Li
+
CH
2
C
CH
2
CH
2
CH
CH
C
H
The radical ion may dimerize to give a dianion:
-
-
-
+
+
+
Li
CH
2
CH
CH
CH
2
CH
2
CH
CH
CH
2
Li
2 CH
2
CH
CH
C Li
H
(11-8)
Further electron transfer from another alkali metal atom can also produce a
dianion:
-
-
-
(11-9)
+
+
+
CH
2
CH
2
CH
2
Li
CH
2
CH
2
Li
Li +
CH
Li
CH
CH
The eventual result of the initiation process is a species capable of propagating
at both of its ends.
Historically, the most important application of this initiation step involved the
production of stereoregular diene rubbers by lithium metal initiation. Other alkali
metals are not as attractive for this purpose because polymers with the higher
overall 1,4 contents and molecular weights are provided by lithium initiation.
Direct attack of the monomer on the metal is a heterogeneous reaction. The
lithium was used as a fine dispersion with a large surface area to speed up the ini-
tiation reaction, and the process was carried out in hydrocarbon solvents because
polar solvents increase the generally undesired vinyl side chain content of the
product polymer (cf.
Section 11.2.7
).
11.2.2.5
Alkali Metal Complexes
Polycyclic aromatic compounds can react with alkali metals in ether solution to
produce monomeric radical ions with an extra electron in the lowest unoccupied
π
orbital of the hydrocarbon. For sodium and naphthalene, for example,
-
(11-10)
+
Na
+
Na
Tetrahydrofuran is a useful solvent for such reactions. This fairly polar solvent
(dielectric constant
7.6 at room temperature) promotes transfer of the 3
s
elec-
tron from the sodium to the aromatic compound and stabilizes the resultant com-
plex. The stability of such complexes depends on the solvent, alkali metal
counterion, and nature of the aromatic compound.
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