Chemistry Reference
In-Depth Information
block and a vinyl block, were synthesised using cerium ammonium
nitrate/PEO as redox initiator system.
Polymerisations of vinyl compounds, initiated by ceric salts in combina-
tion with reducing agents such as alcohols, aldehydes and ketones, have
been described by several authors
63-70
. Ce(IV) salts are also well-known ini-
tiators for grafting vinyl monomers on naturally occurring compounds such
as cellulose
71
and starch
72
and for the synthesis of block co-polymers con-
taining a polyethyleneoxide
73,74
or polypropyleneoxide block
75
.The most
commonly used ceric salts are cerium ammonium sulphate, cerium per-
chlorate, cerium sulphate and cerium ammonium nitrate
76
. Other frequently
used metal ions in redox polymerisations are Mn(VII), V(V), Co(III),
Cr(VI) and Fe(III). Redox polymerisations can be carried out both in
aqueous and non-aqueous media, depending on the solubility of the
monomers used. The advantage of all the above-mentioned redox poly-
merisations, besides the small induction period, is the possibility of operat-
ing under very mild conditions compared with thermal polymerisations
76
.
In this way, the possibility of side-chain reactions is limited.
It is now generally believed that the initiation step for a Ce(IV)/PEO
system proceeds in accordance with the following general reaction scheme:
Ce (IV)
PEO
CH
2
CH
2
OH
PEO
CH
2
CH
OH
∑
CH
3
CH
3
PEO
CH
2
CH
OH + H
2
CC
PEO
CH
2
CH
CH
2
C
∑
∑
C
O
O
OH
C
O
O
R
R
[12.18]
In the first step, a redox reaction occurs between Ce(IV) and the -CH
2
OH
end group of PEO, generating a free radical in a-position of the -OH group
of PEO. In a consequent step, the radical is transferred from the PEO chain
to the vinyl monomer. The radicals formed initiate the actual polymerisa-
tion reaction (propagation):
CH
3
CH
3
CH
2
C
∑
PEO
CH
2
CH
+ H
2
CC
...
OH
C
O
C
O
[12.19]
O
O
R
R
