Chemistry Reference
In-Depth Information
3.14.4 Reversible Addition-Fragmentation Chain Transfer Polymerization
Another type of “living”/controlled radical polymerization involves
reversible addition-fragmentation
chain transfer
. It was named, therefore,
RAFT polymerization
. Great versatility and effectiveness
was shown for the process [
274
]. The process is said to be compatible with a very wide range of monomers
including functional monomers containing such functional groups as acids, acid salts, and hydroxyl or
tertiary amine groups. The conditions of polymerization are those used in conventional free-radical
polymerizations. They can be carried out in bulk, solution, emulsion and suspension (see Sect.
3.16
). The
usual azo or peroxide initiators are employed [
274
]. The reaction was originally illustrated as follows [
274
]:
polymerization
k
p
Y
Y
S
R
k
add
Y
J
S
S
R
S
m-1
+
J
X
m
X
Z
k
-add
Z
X
k
p
Y
Y
S
S
k
p
Y
J
m
R
+
J
p
Z
X
X
X
The RAFT process depends upon rapid addition—fragmentation equilibrium reaction between
propagating (P
n
) as well as intermediate radicals, and chain activity and dormancy, as shown below
in the reaction scheme. The concentrations of each of the species within the equilibrium is dependent
on the relative rate coefficients for addition of a propagating radical to the RAFT agent (
K
add
) and
fragmentation of the formed intermediate radical (
K
frag
). This equilibrium applies correctly only for
polymeric chains that are present in significant concentrations after an initialization period. During
the initiation period there are mainly shorter chains present. The important part of this equilibrium is
the relatively stable radical intermediates.
It was reported that RAFT-mediated polymerization reactions typically contain anomalies, such as
an “inhibition” period and rate retardation. The rate retardations or reductions in the polymerization
rates apparently occur in the presence of RAFT agents, and are not observed when RAFT agents are
absent. Examples are dithiobenzoate-mediated polymerization reactions [
275
].
Tonge and coworkers [
276
] investigated the reactions of short-chain species during the initial period
of cumyl dithiobenzoate mediated polymerization of styrene at 84
C. Using electron spin resonance and
hydrogen and carbon NMR spectroscopies they were able to demonstrate that the reactions are very
specific during the initial stages. There is a strong preference to add single monomer species. This is
followed by fragmentation and release of shorter radicals prior to formation of longer chains.
The effectiveness RAFT agents were investigated by Moad and coworkers [
277
]. These RAFT
agents, such as thiocarbonylthio compounds, depend in effectiveness on the nature of the group, Z
and R (shown below) that modify the reactivity of the thiocarbonyl group toward free radical
addition. R is the free radical leaving group [
277
]:
S
Z
S
R
