Chemistry Reference
In-Depth Information
The alkoxy radicals are very reactive and initiate radical polymerizations readily On the other
hand, the borinate radicals are stabilized by the empty p-orbitals of boron through back-donating
electron density and are too stable to initiate polymerizations. During the polymerization, the borinate
radicals may form weak and reversible bonds with the growing chains.
Boroxyl mediated living radical polymerizations were subsequently described by Chung [
287
]ina
review article. The chemistry is centered on the living radical initiator, i.e., alkylperoxydialkylborane
(C—O—O—BR) species, similar to the one shown above, that are formed by mono oxidation of an
asymmetric trialkylborane with oxygen. In the presence of polar monomers (including acrylates and
methacrylates), the C—O—O—BR., undergoes a spontaneous hemolytic cleavage at ambient tem-
perature to form an active alkoxy radical and a stable boroxyl radical The alkoxyl radical is active in
initiating the polymerization of vinyl monomers, and the stable boroxyl radical forms a reversible
bond with the propagating radical site to prevent undesirable termination reactions. The living
polymerizations were characterized by predictable polymer molecular weight, narrow molecular
weight distributions, and by formation of telechelic polymers and block copolymers through sequen-
tial monomer addition. Furthermore, this living radical initiator system benefits from two unique
features of trialkylborane. These are (a) easy incorporation into polymer chains (chain ends or side
chains) and (b) in situ auto-transformation to living radical initiators.
Lacroix and coworkers reported a reverse iodine transfer polymerization (RITP), where elemental
iodine is used as a control agent in living radical polymerization [
288
]. Styrene, butyl acrylate, methyl
acrylate, and butyl
-fluoroacrylate were homopolymerized, using a radical catalyst and I
2
as a chain
transfer agent. Methyl acrylate was also copolymerized with vinylidene chloride using this process.
a
3.14.6 Kinetics of Controlled/Living Free-Radical Polymerizations
Several papers were published to describe the kinetics of controlled free radical polymerization. Goto
and Fukuda [
289
] postulate two activation processes for nitroxy/styrene polymerization systems:
k
d
k
c
X
*
process I: P
P
+X
process II: P
X
þ
P
0
k
ex
P
þ
P
0
X
K ¼ K
d
/
K
c
. Stationary-state concentrations of P and X are
The equilibrium constant,
1
=
2
½
P
¼ðR
i
=k
i
Þ
½
X
¼k½
P
X
=½
P
The stationary concentration of P and X are determined by different mechanisms. [P] is
determined by the balance of the initiation rate
k
t
[P] [
2
]. This is the
same as in conventional free radical polymerization systems. [X] is determined, however, from the
equilibrium equation shown for process I. It depends, therefore upon the equilibrium constant
R
I
and the termination rate
and
on the concentration of the adduct [P-X] and [P] [
289
]. The rate of polymerization during the
stationary state is
K
1
=
2
1
=
2
i
R
p
¼ k
p
½
P
½
M
¼ðk
p
=k
t
Þ
R
½
M
