Chemistry Reference
In-Depth Information
there is an inhibition period of considerable length. It is induced by either slow fragmentation of the
intermediate RAFT radicals appearing in the preequilibrium or is due to slow reinitiation of the leaving
group radicals from the initial RAFT agent. The absolute values of the rate coefficients governing the
core equilibrium of the RAFT process (at a fixed value of the equilibrium constant) are found to be
crucial in controlling the polydispersity of the resulting
M
n
values. Higher interchange frequency
effects narrower distributions. They also demonstrated that the size of the rate coefficient controlling the
addition reaction of propagating radicals to polymer-RAFT agent,
M
w
/
K
b
, is mainly responsible for
optimizing the control of the polymerization. The fragmentation rate coefficient,
K
b
, of the macro
RAFT intermediate radical, on the other hand, may be varied over orders of magnitude without affecting
the amount of control exerted over the polymerization. Based on the basic RAFT mechanism, shown
above, its value mainly governs the extent of rate retardation in RAFT polymerizations [
278
].
Calitz, Tonge, and Sanderson reported the results of a study of RAFT polymerization by means of
electron spin resonance spectroscopy [
276
]. They observed intermediate radical signals that were not
consistent with current RAFT theory [
276
].
Sawamoto and coworkers reported obtaining simultaneous control of molecular weight and steric
structure in RAFT polymerization of
N
-isopropylacrylamide by addition of rare earth metal, Y(
O
-
tetrafluoromethanesulfonate)
3
, Lewis acid. The
M
w
/
M
n
ratio of the products ranged between 1.4-1.9
and the isotactic content was 80-84% [
277
].
Goto et al. [
279
] developed a process that they describe as
reversible living chain transfer radical
polymerization
[
278
], where they us Ge, Sn, P, and N compounds iodides in the iodide mediated
polymerizations.
ref
In this process, a compound such as GeI
4
is a chain transferring agent and the
polymer-iodide is catalytically activated via a RFT process. They proposed that the new reversible
activation process be referred to as RTCP [
279
]. The process can be illustrated by them as follows [
279
]:
1. reversible activation:
k
act
k
deact
polymer
X
polymer
monomers
+
2. exchange or degenerative chain transfer with X=1
k
ex
k
ex
polymer
I
polymer
polymer
polymer
I
+
+
3. RT or reversible chain transfer with X=1
k
a
k
da
polymer
I
A
polymer
I
A
+
+
where,
I
A
is GeI
4
; SnI
4
; PI
3
; NIS; etc.
3.14.4.1 Combinations of Click Chemistry and ATP as Well as ATP
and RAFT Polymerizations
In the last few years, “click reactions,” as termed by Sharpless et al. [
280
] received attention due to
their high specificity, quantitative yields, and good fidelity in the presence of most functional groups.
The “click chemistry” reaction includes a copper-catalyzed Huisgen dipolar cycloaddition reaction
between an azide and an alkyne leading to 1,2,3-triazole. Recent publications on this “click reaction”
indicate that it is a useful method for preparation of functional polymers [
281
].
