Chemistry Reference
In-Depth Information
Epoxides (1,2 epoxides are also called
oxiranes
) can be polymerized by
anionic, cationic, and coordinated anionic processes. These are the only cyclic
ethers that polymerize through mechanisms that are not cationic. This propensity
for ring opening reflects the high ring strain, because the bond angles in three-
membered rings are distorted from the equilibrium tetrahedral angle of carbon.
Cationic polymerization is a complex chain-growth process which is difficult to
control, and anionic reactions are preferred for the production of polymers of eth-
ylene and propylene oxide. These processes, which are important in the synthesis
of nonionic detergents and polyols for urethane reactions, involve catalysis by
alkali metal alcoholates, in alcohol solvents.
11.4.8
Living Cationic Polymerizations
[6]
As illustrated earlier, the inherent problem in controlled cationic polymerizations
is the instability of the macrocarbocations. However, living polymerizations can
be realized by stabilizing the growing carbenium ions with a suitable nucleophilic
counterion (B
~
), 11-19, or an added Lewis base (X) containing a weakly nucleu-
cleophilic counteranion (B
~
), 11-20
[7]
. That is:
H
C
X
B
CH
2
C
B
or
CH
2
R
11-19
11-20
Both methods spread the charge on the growing macrocation and render the
β
-proton less likely to transfer to monomer (as in reaction 11-41). The first
method is typified by initiation with HI
/
I
2
in which the nucleophilic counterion is
the I
~
/I
2
anion. The second primarily involves combinations of a cation-genera-
tor, like a tertiary alkyl halide, with a Lewis acid, such as EtAlCl
2
. A number of
initiator combinations of the latter type have been reported for living cationic
polymerization of isobutene
[8]
.
Polymers may be made with functionalized end-groups, leading to block
copolymers with controlled structures, in parallel with the anionic systems
describe
d i
n more detail in
Section 11.2.6.2
. Also, as in living anionic polymeri-
zations,
M
n
of the polymer is directly proportional to the monomer conversion,
and the polymerization may be restarted by adding more monomer after the initial
monomer charge has been consumed.
11.5
Coordination Polymerization
In polymerizations of this type, each monomer is inserted between the growing
macromolecule and the initiator. Complexing of the monomer to the initiator
