Chemistry Reference
In-Depth Information
tetrahydrofuran to this solvent increases the concentration of the unaggregated ini-
tiator by forming a 1:1 complex with this compound. This accelerates the rate of
initiation of styrene:
H
H
+
-
C
4
H
9
CH
2
C
4
H
9
Li + CH
2
C
C i
(11-4)
which is fairly slow in hydrocarbon media. Alkyl lithium initiators yield stereo-
regular polymers of conjugated dienes if the polymerization is carried out in
hydrocarbon solvents (
Section 11.2.7
). Addition of tetrahydrofuran or other more
polar solvents changes the microstructure of the polymers which are produced.
There is no inherent termination step in organolithium polymerizations of
hydrocarbon monomers, and this method of initiation yields living polymers.
Living polymerizations are defined as those in which there is no inherent ter-
mination reaction (as described in Section 8.3.3 for free-radical polymeriza-
tions) and in which the macrospecies continue to grow as long as monomer is
supplied.
Other metal alkyls, like triphenylmethyl sodium and potassium benzyl, are
sometimes used. They have lower solubilities in organic solvents than lithium
alkyls because of the greater ionic character of the Na
a
C and K
a
C bonds.
11.2.2.2
Living Polymers
A polymer with an active carbanionic end will initiate the polymerization of a dif-
ferent monomer if the initial macroion is more nucleophilic than the anion formed
from the second monomer in the particular solvent/counterion environment. Thus,
alpha-methylstyryl anion (11-1) will initiate the polymerization of methyl methac-
rylate (Fig. 1.4) but the poly(methyl methacrylate) carbanion will not initiate
polymerization of alpha-methylstyrene.
CH
3
-
+
CH
2
C
Z
11-1
The relative ease of anionic polymerization can be correlated with the base
strengths of the respective anions or crudely with the
e
values of the
Q
e
scheme
(Section 9.11) as shown in
Table 11.1
. Polymerization of a given monomer can
generally be initiated by a carbanion from any monomer higher in the list, but the
reverse reaction does not occur.
