Chemistry Reference
In-Depth Information
The ruthenium(II) complexes interact with CCl
4
and are oxidized in the process to become Ru(III)
and radicals CCl
3
that add to molecules of methyl methacrylate. The polymerization proceeds via
repetitive additions of methyl methacrylate molecules to the radical species that are repeatedly
generated from the covalent species with carbon-halogen terminal groups [
226
]. Suwamoto also
reported [
226
] that addition of a halogen donor, Ph
3
C-Cl aids the shift of the equilibrium balance to
dormant species. The reaction of polymerization can be illustrated as follows:
PPh
3
II
Ru
Cl
PPh
3
Cl
PPh
3
Cl
Cl
Ph
3
Cl
Br
III
R
Cl
Cl
Br
+
Ru
Br
PPh
3
Cl
Cl
Cl
Cl
Cl
Cl
Ph
3
PPh
3
R
monomer
Cl
Cl
III
Ru
Br
n
Cl
Cl
PPh
3
PPh
3
R
R
Cl
Ph
3
II
Ru
Cl
PPh
3
Cl
Ph
3
Klumperman and coworkers [
259
] observed that while it is lately quite common to treat living
radical copolymerization as being completely analogous to its radical counterpart, small deviations in
the copolymerization behavior do occur. They interpret the deviations on the basis of the reactions
being specific to controlled/living radical polymerization, such as activation—deactivation equilib-
rium in ATRP. They observed that reactivity ratios obtained from atom transfer radical copolymeri-
zation data, interpreted according to the conventional terminal model deviate from the true reactivity
ratios of the propagating radicals.
Velazquez and coworkers [
260
], developed a kinetic model incorporating effects of diffusion-
controlled reactions on atom-transfer radical polymerization. The reactions considered to be
diffusion-controlled are monomer propagation, bimolecular radical termination, chain transfer
between propagating radicals and catalyst, and transfer to small molecules. Model predictions indicate
that a diffusion-controlled propagation reduces the “living” behavior of the system, but a diffusion-
controlled termination enhances its livingness. Also, diffusion-controlled transfer between chains
and catalyst is the same in the forward and in the reverse directions. The “livingness” of the system is
enhanced, but if one of them is kept unchanged the other is increased, and the “livingness” of the
system is reduced. When diffusion-controlled termination is important, their simulations show that the
overall effect of diffusion-controlled phenomena in ATRP is to enhance the livingness of the system.
Preparation of gradient copolymer of styrene and
-butyl acrylate was reported by the use of
ATRP [
261
].
Gradient copolymers
are copolymers with sequence distributions varying in a well-
defined order as functions of chain lengths. It is suggested that gradient copolymers have the potential
of outperforming block and alternating copolymers in some instances [
261
].
n
3.14.3 Nitroxide-Mediated Radical Polymerizations
A nitroxide mediated polymerization of styrene was first reported in 1985 [
262
]. This reaction,
however, was studied extensively only since 1993. The monomer conversion rates vs. temperatures
are much slower than they are in conventional styrene polymerization. Also, the polydispersities of
