Chemistry Reference
In-Depth Information
Because acrolein polymerizes by free-radical and by ionic mechanisms, all of the above reactions
are possible, and the products are quite complex. The structures of the materials include linkages
from both vinyl and carbonyl groups. In addition, tetrahydropyran rings, as shown above, can also
form [
357
].
Coordination complexes, like CdI
2
(pyridine)
2
, also initiate polymerizations of acrolein. Propaga-
tion reactions precede through both, vinyl and carbonyl groups [
358
]:
O
O
n
O
The ratio of vinyl to carbonyl placement depends upon the nature of the complex. Polymers
formed by complexes of metallic salts with triphenylphosphine contain considerably less aldehyde
groups than those formed with triphenylphosphine alone [
358
].
Polymerization of propionaldehyde (CH
O) is also unique. In dimethylformamide at 0
C
C-CH
ΒΌ
with sodium cyanide or with tri-
-butyl phosphine catalysts the reactions yield polymers composed of
two different structural units. One is a polyaldehyde and the other one is a polyacetylene [
355
].
The reaction in tetrahydrofuran, however, at
n
78
C with sodium cyanide catalyst results in a
crystalline poly(ethynyl oxymethylene) [
359
]. Radical initiated polymerizations of this monomer at
60
C, on the other hand, proceed through the acetylenic group only.
Crotonaldehyde, like acrolein can be expected to yield polymers with structures derived from 1,2;
3,4; or 1,4 additions. Anionic catalysts, however, yield predominantly polyacetal structures [
360
].
4.6.4 Polymerizations of Di Aldehydes
As one might expect, di aldehydes can be polymerized to yield polymers with cyclic ether linkages.
This resembles polymerizations of non-conjugated dienes [
361
]:
O
O
O
O
O
O
+
etc
R
R
R
Cationic polymerizations can take place by either one of three paths: stepwise additions,
intermediate-type additions, and concerted additions. When phthalaldehyde is polymerized by
cationic mechanism or by
78
C, the products consist of dioxyphthalan units [
362
]:
g
-irradiation at
n phthalaldehyd
e
+
R
H
H
CCH
CCH
H
CC
R
O
O
R
O
O
H
O
O
n
Aromatic aldehydes usually polymerize with difficulty. The enhanced polymerizability of
phthalaldehyde is due to formation of intermediates from concerted propagation reactions [
362
].
In the above illustration, the anhydride rings can be either
cis
or
trans
in configuration. Cationic
polymerizations yield polymers with high
content. Anionic catalysts and particularly coordination
catalysts, like triethylaluminum-transition metal halides, yield high
cis
trans
polymers [
363
].
