Chemistry Reference
In-Depth Information
n-1
OO
O
O
The cobalt hydride in turn reacts with a new monomer molecules to regenerate the Co(II) [
235
].
One publication describes a chain transferring agents that can be used in controlled polymerization
of methacrylate monomers where reductive elimination of cobalt hydride from the neighboring
methyl group deflects further chain growth [
236
]. The agent was illustrated as follows:
N
N
Co
N
N
A described example is a reaction conducted at 60
C in deoxygenated benzene, using
neopentylcobalt with tetramesityl-porphyrin ligand and methyl acrylate monomer [
236
]. A slow
polymerization yields 66% conversion in 38 h. The product is a narrow molecular weight distribution
polymer of
144,000. The polymerization is even slower with less hindered phenyl substituents
on the porphyrin ligand. Both homopolymers and block copolymers can be formed.
Catalytic chain transfer by a cobalt(ll) porphyrin in radical polymerization of MAA in water was
studied by Wayland and coworkers [
237
]. Cobalt tetrasulfonatophenylporphyrin was found by them
to be exceptionally effective in the catalytic chain transfer for the radical polymerization of MAA in
water. A remarkable feature of this process is that the increase in the degree of polymerization, with
conversion requires that more monomer be consumed in chain growth of the existing
macromonomers than in initiation and propagation of new chains through chain transfer to monomer.
Reinitiation of oligomer olefins and chain growth are significant inherent reactivity features of the
cobalt
++
-porphyrin catalyzed chain transfer process.
M
n
ΒΌ
3.14.2 Atom Transfer Radical Polymerizations
Atom transfer radical polymerizations (ATRP) were reported simultaneously by two groups: (1)
Matyjaszewski et al. [
218
] and (2) Sawamoto and coworkers [
226
]. Matyjaszewski et al. utilized a
Cu/bipyridine complex as a
halogen transfer agent
that functions between dormant and active
polymer chains. Formation of polymers with predetermined molecular weight of up to
10 [
5
]
and polydispersity as narrow as 1.05 was reported [
238
,
239
]. This type of polymerization appears to
offer the possibility of preparing a broad range of polymeric materials [
240
-
242
]. The reactions
proceed under conditions that could make the process commercially attractive. Thus, for instance, by
using nonionic surfactants, such as poly(oxyethylene oleyl ethers) it is possible to prepare polymers
from butyl methacrylate, methyl methacrylate, styrene, and butyl acrylate in aqueous emulsions. In
addition, by using multidentate ligand such as tris[(2-dimethyl-amino)ethyl]amine the atom transfer
M
n
