Chemistry Reference
In-Depth Information
R
+
R'
O
R'Me
Me
R
O
Where, Me represents a metal group. The propagation reaction is a series of successive nucleo-
philic additions [
340
], with the alkoxide ion as the propagating species:
R
R
R
R
+
Me
Me
O
n
R'
O
R'
O
O
n
There are indications that many aldehyde polymerizations result in formations of “living”
polymers similarly to anionic polymerizations of vinyl compounds. Termination can occur through
hydride transfer via a form of a crossed Cannizzaro reaction:
R
R
R
R
H
R
O
RCHO
Me
O
O
OO
O
+
Me
Me
O
O
The alkyl substituent has a tendency to destabilize the propagating anion by increasing the charge
density on the oxygen:
H
C
O
R
As a result, weak bases, like amines, fail to initiate polymerizations of higher aldehydes.
A
stereospecific anionic polymerization
of acetaldehyde was originally reported in 1960
[
343
,
344
]. Two alkali metal compounds [
341
] and an organozinc [
342
] one were used as the
initiators. Trialkylaluminum and triarylaluminum in heptane also yield crystalline, isotactic polymers
from acetaldehyde, heptaldehyde, and propionaldehyde at
80
C[
343
]. Aluminum oxide, activated
by diethylzinc, yields stereoblock crystalline polymers from various aldehydes [
342
,
344
]. Lithium
alkoxide formed polyacetaldehyde is insoluble in common solvents. It melts at 165
C[
341
].
The mechanism of stereoregulation is still debated. Some concepts are presented in the rest of this
section. Natta [
343
] believed that polymerizations initiated by organoaluminum compounds proceed
by coordinated anionic mechanism. The aluminum atoms were seen as forming complexes with the
oxygen atoms on the penultimate units:
+
O
O
Al
Al
O
O
Al
O
O
The activated complexes, which form, have steric configurations that allow minimum amounts of non-
bonded interactions. Other mechanisms were proposed since. For instance, Furukawa et al. [
344
]
concluded that metal alkyl compounds must become metal alkoxides through reactions with the aldehydes:
R'
R'
O
+
MeR
O
Me
R
