Chemistry Reference
In-Depth Information
In these polymerizations, the propagating oxonium ions are probably further solvated [
226
]:
CH
2
O
O
CH
2
O
CH
2
O
Terminations of chain growths occur through recombinations, reactions with impurities, and by
chain transfers:
MeX
n
O
+
O
2
X
MX
n-1
+
RX
+
O
O
2
X
R
H
+
O H
2
OH
+
O
H
2
O
O
where, MnX
n
is a Lewis acid.
Formaldehyde can polymerize in an anhydrous form in all three physical states of matter: as a gas,
as a liquid, or as a solid [
322
]. It also polymerizes in water with acid catalysts. Many Lewis acids are
efficient catalysts for this reaction. In addition, some protonic acids are also effective. Among them
are perchloric and sulfuric acids and monoesters of sulfuric acid [
322
].
The activity of the initiators is independent of the solvents in cationic polymerizations of
formaldehyde [
322
]. There are, however, differences in the performance of the initiators. With BF
3
the reaction reaches equilibrium at 30% conversion and stops. Much higher conversions are achieved
with SnCl
4
and SnBr
4
[
322
]. Both Lewis acids yield polyoxymethylenes that are very similar to those
formed by anionic mechanism [
324
-
327
].
Higher aldehydes can be polymerized by Lewis acids, Bronsted acids, and acidic metal salts [
322
].
Initiation and propagation reactions consist of repeated nucleophilic attacks by the carbonyl oxygens
on the electrophiles and formations of carbon-oxonium ions [
328
]:
R'
R
R'
+
R
O
O
R'
R'
R'
R'
R'
R'
R'
R
R'
+
n
O
O
R
R
O
O
O
O
O
O
n
The counterions are not shown in the above equations. When the initiators are protonic acids the
reactions can be illustrated as follows (additional monomers are not shown):
R
R
R
H
R
A
H
A
+
H A
O
O
O
O
n
Many aldehydes trimerize in acidic conditions. Paraldehyde and many other trimers fail to
undergo further polymerizations to high molecular weight, linear polymers. The trimer of formalde-
hyde, trioxane, however, is unique. It polymerizes by a mechanism of ring opening polymerization.
This is discussed in Chap.
5
.
To avoid trimer formations, low dielectric solvents must be used at low
temperatures. In addition, the initiator must be carefully selected.
