Chemistry Reference
In-Depth Information
the products are not as narrow as obtained in anionic polymerization but, generally, the
polydispersities produced by this process are proportional to the molecular weights of the polymers
produced. In fact, a linear relationship between polydispersity and the molecular weight of the
polystyrene product was demonstrated [
263
].
After the initial nonstationary period, typical alkene polymerizations in the presence of
alkoxyamines proceed according to the first order kinetics with the molecular weights increasing
with conversion. The dispersity of the products and the contribution of the nonstationary periods
depend upon the temperature, the particular initiating system and on the nature of the monomers.
Styrene polymerizations can be carried out in the presence of stable nitroxyl radicals, such as the
2,2,6,6-tetramethylpiperydinyl-1-oxy radical, commonly referred to as
TEMPO
[
264
] or ditertiary
butyl nitroxide, referred to as
DTB N.
N
N
O
O
TEMPO
DTBN
Such radicals are incapable of initiating polymerizations by additions to the double bonds, but
react selectively with growing radicals to form reversibly covalent species [
265
]. In addition, the
reactions of growing radicals with dormant species occur via degenerative transfer:
P
n
O
NR
2
+
R'
P
n
+
R
2
N
O
R'
The position of the equilibrium constant in reactions with TEMPO depends on the nature of the
radical, the solvent and the temperature. These polymerizations can be initiated by either bimolecular
initiators or by unimolecular ones. The bimolecular initiators utilize common free radical sources
such as benzoyl peroxide or azobisisobutyronitrile to start the reaction. The carbon-centered initiating
radicals that form in turn react with TEMPO. This can be illustrated as follows:
N
N
O
O
R
R
+
R
N
O
etc
.
R
R
+
N
O
