Chemistry Reference
In-Depth Information
distributions, and can be used to form copolymers. Some examples of such polymerizations include
nitroxyl radical-mediated polymerizations of styrene [
222
-
225
], atom transfer polymerizations
controlled by ruthenium-(II)/aluminum [
226
,
227
] or by copper/bipyridine complexes [
228
], Co
(II)-mediated polymerizations of methacrylates and acrylates [
229
], and polymerization of styrene
using a degenerative transfer method [
230
], as well as others. Some features are unusual for radical
processes and the radical nature of some of these reactions might be questioned, as for instance,
polymerizations catalyzed by transition metals. Evidence has been presented, however, that strongly
indicates radical nature in at least in atom transfer polymerizations [
231
]. The evidence, however, is
not unambiguous.
Some initial attempts at producing “living” polymerizations made use of
iniferters.
This term
appears to come from the word
inifer
, a bifunctional compound that brings about both initiation and
chain transfer. “Living” cationic polymerizations make use of inifers to form block copolymers. This
is discussed in Chap.
9
.
The term iniferter was proposed by Otsu and Yoshida in 1982 [
232
]. Iniferters
used in controlled/living free-radical polymerizations are sulfur-centered free radicals that can be
generated from sulfur-containing molecules such as dithiocarbamates. The radicals react reversibly
with growing polymeric chain ends, thereby controlling the concentration of the radical species.
Many of these sulfur centered radicals, however, can also initiate new polymer chains. This can lead
to uncontrolled growth. To overcome these difficulties, other approaches were also developed [
233
].
Deactivation of growing radicals with stable radicals can be carried out with the aid of various
nitroxyl radicals, protected phenoxy radicals, dithiocarbamate, trityl, and benzhydryl derivatives.
Growing radicals can also be deactivated with nonradicals in the presence of organometallic
compounds that form stabilized hyper-coordinated radicals. The polymerizations with the aid of
reversible degradative chain transferring are unique in that they requires very rapid and “clean” chain
transfers without side reactions. The enhanced control of polymerization process relies on reduction
in the ratio of the rate of termination to that of propagation, due to low instantaneous concentration of
growing radicals. This means that initiation and propagation reactions must proceed at similar rates
due to application of the initiators resembling polymer end groups in their dormant state. Also, in
these polymerization reactions, there must be a low proportion of chains marked by uncontrolled
termination and/or transfer due to relatively low molecular weights.
Homogenous controlled/“living” free radical polymerizations are based, therefore, on the revers-
ible deactivations of growing radicals. Early, Matyjaszewski divided such polymerizations into three
classes [
240
,
241
]. These were:
1. Deactivations of growing radicals with stable radicals by reversible formations of dormant
covalent species, followed by homolytic cleavages:
:
act
:
deact
R
!
P
þ
P ---------- R
2. Reversible deactivations of growing radicals with “nonradical” species by formation of dormant
persistent radicals:
deact
:
act
X
!
P
þ
ð
P --------- X )
:
3. Reversible degenerative transfers based on thermodynamically neutral exchange reactions
between growing radicals and transfer agents:
k
tr
P
1
-------- R
!
P
n
þ
P
1
þ
P
n
-------- R
