Chemistry Reference
In-Depth Information
Each monomer addition to the growing chain requires a transfer of the alkoxide anion to the
carbonyl group. This results in a formation of a new alkoxide anion. (A hydride transfer from the
alkoxide group to the carbon atom of the aldehyde can take place by the Meerwein-Ponndorf
reduction.) Chain growth takes place by repetition of the coordination of the aldehyde, and
subsequent transfer of the alkoxide anion.
In the Vogl and Bryant [
345
] mechanism, four oxygens are coordinated to the metal atom. These
oxygens are from the penultimate and ultimate units of the growing chains and from two monomers:
R
R
R
O
2 RCHO
R
Me
Me
O
O
O
O
O
O
O
Me
R
R
R
O
O
R
R
R
R
O
O
O
O
O
O
O
Me
O
R
Me
R
A simultaneous coordination of the two aldehydes prior to addition may explain the observed
sequence of isotactic dyads.
Yashida and Tani derived their mechanism from investigations of isotactic aldehyde
polymerizations by [R
2
AlOCR
0
NC
6
H
5
] catalysts [
346
-
348
]. The bulkiness of the substituent group
is considered to be the most important factor in steric control. The catalyst enhances the degree of
stereoregulation by controlling the mode of approach through coordination. Also, the Lewis acidity of
the catalyst must be confined to a narrow range for each particular aldehyde to yield isotactic
polymers [
347
,
348
]. If the acidity is too strong, an amorphous polymer forms. If it is too weak, no
polymerization takes place [
348
]. In addition, special techniques, like high pressures, for instance,
can result in formations of isotactic butyraldehyde and heptaldehyde [
349
].
4.6.3 Polymerization of Unsaturated Aldehydes
Unsaturated aldehydes are unique, because they polymerize in special ways. Much of the effort in
acrolein polymerizations is to form products with the aldehyde groups in tact. This means, the
reactions must be confined to the carbon to carbon double bond portions of the molecules
[
350
-
356
]. Organometallic catalysts like
n
-butylmagnesium bromide, however, are also initiators
for polymerizations through the carbonyl groups. In fact, unsaturated aldehyde monomers can be
polymerized in four ways similar to dienes. The placement can be through carbon to carbon double
bond, or through the carbonyl group, or it can be 1,4:
O
1 2 3 4
ladder structure
1,2 and 3.4
1,2
1,4
3,4
O
O
n
n
n
O
O
O
