Chemistry Reference
In-Depth Information
Thus, vinyl acetate would be classed as an electron-donor-type monomer
(Section 9.10.2) but it cannot be polymerized cationically because the carbonyl
group complexes the active center. (It
is polymerized only by free radicals;
anionic initiators attack the ester linkage.)
Other monomers that are suitable for cationic polymerization include cyclic
ethers (like tetrahydrofuran), cyclic acetals (like trioxane), vinyl ethers, and
N-
vinyl carbazole. In these cases the hetero atom is bonded directly to the electron-
deficient carbon atom, and the respective carboxonium ion (11-13) and immonium
ion (11-14) are stabler than the corresponding carbocations.
H
H
H
CH
2
C
OR
C
C
C
C
vinyl ether
(11-28)
O
O
R
R
11-13
H
H
H
H
2
C
C
CH
2
C
CH
2
C
N
N
N
(11-29)
N
−vinyl
carbazole
11-14
Because of this difference in stability monomers that yield onium ions will not
copolymerize cationically with olefins like isobutene or styrene. (A similar differ-
ence in radical stabilities accounts for the reluctance of styrene to copolymerize
with vinyl chloride in free-radical reactions; Section 9.10.1.)
Cationic polymerizations of vinyl monomers differ from other chain-growth
polymerizations particularly as follows:
1.
Cationic reactions often involve complex initiation and preinitiation equilibria
that may include the rate-determining steps in the polymerization sequence.
2.
The reaction medium in cationic polymerizations is usually a moderately
polar chlorinated hydrocarbon like CH
3
Cl
(dielectric constant
12.6 at
5
20
C). A greater proportion of the macroions are free of their counterions
in cationic than in anionic polymerizations in the usual solvents for the
latter processes. Cationic polymerizations are characterized by extremely fast
propagation rates.
Chlorinated aliphatic solvents are useful only for cationic reactions, because
they would be attacked by the strong bases used to initiate anionic
2
