Chemistry Reference
In-Depth Information
Various descriptions of different unimolecular initiators can be found in the literature. A presence
of
-methyl groups on the alkoxyamines appears to be essential [
266
]. These compounds yield, upon
dissociation, both stable radicals and initiating ones and can be shown as follows [
267
].
a
O
N
O
O
O
N
O
O
O
N
O
O
etc.
The optimal amount of the radical initiator depends on the efficiency of the initiation. Ideally the
concentration of the radicals generated from the initiator should be slightly higher than the concen-
tration of the scavenger.
At higher temperatures, such as 120
C, the polymerizations of styrene tend to exhibit ideal
behavior. Also, at higher temperatures narrower molecular weight distributions are obtained,
indicating sufficiently high exchange rates.
A low-temperature method for the preparation of unimolecular initiators was reported [
268
]. In
this method, oxidation is used to generate carbon radicals in the presence of nitroxide traps such as
TEMPO.
BuO
2
C
1. PbO
2
BuO
2
C
O
N
2. CuCl
2
, TEMPO, -78
o
C
