Chemistry Reference
In-Depth Information
double bond in aldehydes and ketones. In such polymerizations the active species
A adds to one atom of the double bond and produces a new active species on the
other atom:
H
H
(8-1)
A* + CH
2
C
A
CH
2
C*
X
X
or
R
R
(8-2)
A* +
CO
A
C
O*
H
H
The nature of the new active site on the residue of the monomer deter-
mines whether the polymerization mechanism is radical or ionic. Reaction (8-
2) is not possible when A
is a free radical because of the difference in
electronegativity of the C and O atoms. Aldehydes and ketones are not poly-
merized by radicals; they are enchained only by ionic or heterogeneous cata-
lytic processes.
The alkene double bond can be polymerized in chain-growth reactions in
which the active site is a radical, ion, or carbon
metal bond. The processes
whereby a given alkene reacts depend on the inductive and resonance characteris-
tics of the substituent
X
in the vinyl monomer shown in reaction (8-1).
Electron-releasing substituents
H
H
(R, RO
, R
C
C
and
)
increase the electron density of the double bond and facilitate addition of a cation.
Thus, monomers like isobutene, styrene, and vinyl ethers all undergo cationic
polymerization. Some of these vinyl monomers can also delocalize the positive
charge, and this facilitates reactions with cations:
H
H
H
+
+
A + CH
2
C
A
CH
2
C
A
CH
2
C
(8-3)
OR
OR
OR
+
8-2
Styrene will act similarly but the effect of alkyl substituents is almost entirely
inductive.
Electron-withdrawing substituents
