Chemistry Reference
In-Depth Information
Triazolinyl radical
16
decomposes at elevated temperatures generating a phenyl radical that is capable
of initiating polymerization.
132
More importantly, this slow, high temperature decomposition prevents the
accumulation of excess triazolinyl radicals that occurs as a result of unavoidable chain termination reactions.
In comparison to living-radical polymerizations performed using nitroxides, triazolinyl-mediated polymer-
izations typically produce polymers with broader molecular weight distributions, a result of premature
chain termination that occurs in the early stages of the polymerizations.
11.4.2 Verdazyl radicals
Verdazyls are stable radicals that contain a constrained resonance-delocalized hydrazyl moiety in their
ring, similar to the above mentioned triazolinyl radicals. The most common verdazyls have the general
structures
18
and
19
, where X
oxygen or sulfur and R is typically methyl, ethyl or isopropyl. The R
group can be quite varied and includes hydrogen, alkyl, phenyl, substituted benzenes, imidazoles, pyridines
and the like.
137 - 144
=
X
R
R
R
R
N
NN
N
N
NN
N
R'
R'
19
18
Although verdazyl radicals were originally used as polymerization inhibitors
145
and are important in
the study of molecular magnets,
146,147
they also have also been employed as mediators in living-radical
polymerizations. 1,3,5-Triphenylverdazyl
18
was originally used for the polymerization of styrene and
methyl methacrylate at 60
◦
C, but reaction times were long and conversions were low.
148
Polymerizations
at 110
◦
C allowed a linear increase in molecular weight with conversion, although the molecular weight
distributions were typically broad (M
w
/M
n
>
2) and approximately 60 % of the chain ends were dead, a
result of bimolecular termination.
149
O
O
O
N
N
N
N
N
N
N
N
N
N
N
N
O
R
19
20
18
A series of 6-oxoverdazyl radicals, such as
19
, proved more successful for the polymerization of
styrene
150
and
n
-butyl acrylate.
151
Adducts of the verdazyl radical with a monomer unit (
20
,where
R
hydrogen, ethyl, phenyl) were used as initiators for the polymerizations. The success of
19
to moder-
ate the aforementioned polymerizations can be attributed in part to an inherent instability of the verdazyl
radical resulting in the formation of an azomethine imine.
152
As a result an excessive amount of verdazyl
does not accumulate in the polymerization solution as a consequence of unavoidable coupling reactions
between polymer chains.
=
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