Chemistry Reference
In-Depth Information
F
F
B
O
O
L
N
N
Co(III)
Co
+
CO
2
R
N
N
CO
2
R
CO
2
R
L
+
O
O
F
F
B
B
F
O
O
F
CO
2
R
(
Co(II))
L
N
N
CO
2
R
Co
Co(III)
propagating
polymer
chain
re-initiation
N
N
CO
2
R
L
O
O
B
F
F
Co(III)
H
Fig. 3.2
Cobalt mediated controlled/“living” polymerization
3.14.1 Cobalt Mediated Polymerizations
Catalytic chain transfer polymerizations can utilize metals such as low spin cobalt(II) compounds as
chain transferring agents. The mechanism is believed to involve repeated disturbing of each propagating
step by abstraction of hydrogen atoms from the propagating polymers. This yields chains with unsatu-
rated terminal units and hydrogen transfer agent adducts, Co(III)-H. Subsequent transfers of hydrogens
to the growing chains result in reinitiating the processes [
241
]. The
-hydrogen abstractions from the
growing radicals and the formations of metal hydrides [
229
] can be illustrated as follows:
b
R
R
Mt:Ligand
Ligand:Mt
+
H
where Mt
Co
This catalytic cycle for a cobalt mediated polymerization, using bis-dimethyl-glyoximate cobalt
boron fluoride catalyst was illustrated Haddleton et al. [
233
,
234
]. A similar illustration of the process
is shown in Fig.
3.2
. The affinities of metals for hydrogen abstractions, and/or their abilities to lose
electrons depend on their oxidation states and the nature of their ligands. Such reactions can, therefore, be
suppressed. This can be done by choosing high oxidation state metals, ligands that protect the metal from
the abstraction of the P-H atoms, and by controlling the position of the oxidation-reduction equilibrium.
¼
R
Co+++Ln
Ð
R
þ
Co + + Ln
RM
x
þ
Co + + Ln
Ð
RM
x
Co + + + Ln
The metal cobalt(II) is usually chelated. This can be cobalt porphyrin, cobalt phthalocyanin, or
cobalt oxime, as well as others. The polymer molecules that form have, as stated earlier, terminal
double bonds and can be illustrated as follows,
