Chemistry Reference
In-Depth Information
Although some block copolymers can be made by other techniques, anionic
polymerizations are particularly useful in this application. This is mainly because
of the absence of an inherent termination step in some anionic systems and
because anions with terminal monomer units of one type can be used to initiate
the polymerization of other selected monomers. The different anionic reaction
sequences that are employed include sequential monomer addition, coupling reac-
tions, and termination with reactive groups.
11.2.6.3
Sequential Monomer Addition
Monofunctional Initiators
. AB, ABA, and multiblock copolymers can be synthe-
sized by initiation of one monomer with a monofunctional initiator like
n
-butyl
lithium. When the first monomer has been reacted, a second monomer can be
added and polymerized. This monomer addition sequence can be reversed and
repeated if the anion of each monomer sequence can initiate polymerization of
the other monomer. The length of each block is determined by the amount of the
corresponding monomer
styrene block
copolymers can be made by this method by polymerizing in benzene solution and
adding the styrene first. Addition of a small amount of ether accelerates the slow
attack of dienyl lithium on styrene.
Bifunctional Initiation
. The bifunctional initiators like alkali metal complexes
of polycyclic aromatic compounds can be used to produce ABA triblock copoly-
mers even when the A anion is not sufficiently basic to initiate polymerization of
B monomers. In these cases polymerization would be started with monomer B to
produce a polymeric dianion that could initiate polymerization of the A monomer
which is added later. These initiators can be prepared only in aliphatic ethers,
however. This precludes their use for the synthesis of useful styrene-diene ABA
copolymers because polydienes made anionically in such solvents have low 1,4
contents and are not good rubbers.
Coupling Reactions
. In this technique, a living AB block copolymer is made by
monofunctional initiation and is then terminated with a bifunctional coupling agent
like a dihaloalkane. ABA copolymers can be made by joining AB polymeric anions:
that was provided. Styrene
isoprene
-
+
(M
1
M
1
M
2
M
2
M
2
M
2
Li
+
Br (CH
2
Br
2
i
j
k
(11-23)
CH
2
)
K
(M
2
M
2
M
2
M
1
M
1
M
1
+ 2 Li Br
(M
1
M
1
M
1
M
2
M
2
M
2
i
j+1
j+1
i
Reaction (11-23) could be used to produce an ABA copolymer of styrene and
methyl methacrylate even though the latter anion is too weakly basic to initiate the
polymerization of styrene. If the presence of a linking group, like the —(
a
CH
2
a
)
k
in the above reaction, is undesirable a coupling agent like I
2
can be used.
Star-shaped block copolymers can be made by using polyfunctional linking
agents, like methyltrichlorosilane or silicon tetrachloride, to produce tribranch or
tetrabranch polymers.
