Chemistry Reference
In-Depth Information
11.2.5
Temperature Effects
The direction of temperature effects in anionic polymerizations is conventional,
with increased temperature resulting in increased reaction rates. Observed activa-
tion energies are usually low and positive. This apparent simplicity disguises
complex effects, however, and the different ion pairs and free ions do not respond
equally to temperature changes. Overall activation energies for polymerization
will be influenced indirectly by the reaction medium because the choice of sol-
vent shifts the equilibria of
Eq. (11-1)
.
11.2.6
Anionic Copolymerization Reactions
11.2.6.1
Statistical Copolymerization
Copolymerizations analogous to free-radical reactions occur between mixtures of
monomers which have more or less the same
e
values [
Table 11.1
]. The copoly-
merizations of styrene and dienes have been particularly studied in this connec-
tion. The simple copolymer equation (Eq. 9-13) applies to most of these
systems, but the reactivity ratios will vary with the choice of solvent and posi-
tive counterion because these factors influence the nature of the propagating ion
pair.
Note that styrene and conjugated dienes can be copolymerized to yield statisti-
cal or block copolymers. The latter process, which involves additions of one
monomer to a living polymer of the other monomer, is described in the following
section.
The styrene/methyl methacrylate pair contains monomers with different rela-
tive reactivity levels in
Table 11.1
. Polystyryl anion will initiate the polymeriza-
tion of methyl methacrylate, but the anion of the latter monomer is not
sufficiently nucleophilic to cross-initiate the polymerization of styrene. Thus, the
anionic polymerization of a mixture of the two monomers yields poly(methyl
methacrylate) while addition of methyl methacrylate to living polystyrene pro-
duces a block copolymer of the two monomers.
11.2.6.2
Block Copolymers
Anionic polymerizations are particularly useful for synthesizing block copoly-
mers. These macromolecules contain long sequences of homopolymers joined
together by covalent bonds. The simplest vinyl-type block copolymer is a two-
segment molecule illustrated by structure (11-2). This species is called an AB
block copolymer, because it is composed of a poly-A block joined to a long
sequence of B units. Other common block copolymer structures are shown as
(11-3)
(11-6).
AB block copolymer
(M
1
M
1
M
1
M
2
M
2
M
2
)
m
n
11-2
