Chemistry Reference
In-Depth Information
11.3.6 Nitroxides as mediating agents
The stability of nitroxides is attributed to the delocalization of three electrons in the N - O bond. The
high delocalization energy (
4 kcal/mol) provides a high thermodynamic stability to the radical center
and prevents the dimerization of two nitroxide radicals.
93
The overall thermodynamic stability of different
nitroxide radicals is influenced by the substituents on the carbons attached to the nitrogen. Thus, for
example, if all four groups attached to the carbons attached to the nitroxide nitrogen are alkyl, the nitroxide
is stable. Replacing one of the alkyl groups with a hydrogen or attaching a phenyl group directly to nitrogen
renders the nitroxide relatively unstable.
93
In the former case abstraction of the alpha hydrogen of one
nitroxide molecule by another results in a disproportionate reaction producing a hydroxylamine and a
nitrone. In the later case, the reaction follows the path depicted in Scheme 11.3.
∼
30
.
11.3.7 Nitroxides and their ability to moderate polymerizations
The cyclic nitroxide TEMPO was initially shown to be an efficient mediator for the homopolymerization of
styrene and the copolymerization of styrene and acrylates in which the acrylate concentration was 50 % or
less on a molar basis. Attempts at the homopolymerization of acrylates and methacrylates were uniformly
unsuccessful proceeding to about 5 % conversions and producing low molecular weight oligomers. This
inability of TEMPO to work efficiently with acrylate monomers has been well documented.
82,94 - 99
It has
been suggested that the failure of acrylate homopolymerizations in the presence of TEMPO is due to the
strong C - O bond formed between the acrylate chain end and nitroxide.
100
This causes an unfavorably low
value of the equilibrium constant (K
eq
), attributed to a very low dissociation rate constant (
k
d
) relative to
a very high recombination rate constant (
k
c
). As such, the cleavage of the TEMPO - acrylate bond is not
much faster than monomer conversion and the rate of recombination of TEMPO with the active acrylate
polymer chain end is fast. This prevents both a linear increases in molecular weight with conversion
and low M
w
/M
n
when TEMPO is the mediating nitroxide. With this in mind, a series of nitroxides
were developed that formed alkoxyamines with styrene and acrylate monomers with relatively large
k
d
values. Alkoxyamines, comprised of a benzyl radical with the acyclic nitroxides 2,2,5-trimethyl-4-phenyl-
3-azahexane-3-oxy (TIPNO)
7
101
or SG1
6
,
102,103
10
−
3
s
−
1
were shown to have
k
d
values of 3
.
6
×
and
10
−
3
s
−
1
for TEMPO.
51
Both TIPNO and SG1 were
shown early on to be very effective in mediating the polymerization of acrylates and other non-styrenic
monomers, specifically acrylamides, 1,3-dienes, and acrylonitriles.
104,105
A strong case has been made for
the importance of
k
d
in successful polymerizations and the need for k
p
/k
d
to be
10
−
4
s
−
1
, respectively, as compared to 1
3
.
3
×
.
1
×
10
5
lmol
−
1
,in
addition to the need to be careful with experimental conditions such as the purity of the monomers and
the amount of reagent in a given-sized reaction vessel.
106
It has also been proposed that the difficulty associated with TEMPO-mediated polymerizations of
acrylates is directly related to the excess free nitroxide that is generated throughout the course of the
≤
6
.
0
×
R
O
N
H
O
2
+
N
R
O
NH
N
R
R
O
N
R
O
Scheme 11.3
Decomposition pathway for N- phenyl containing nitroxide
Search WWH ::
Custom Search