Chemistry Reference
In-Depth Information
using a photodecomposition of either azo compounds or dithiocarbamates in the presence of SG1
6
[N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)
nitroxide] (also referred to as DEPN) were
reported.
70
O
N
O
N O
EtO P
OEt
SG1
TIPNO
7
6
An extensive amount of work has been performed on the use of alkoxyamines prepared with steri-
cally bulky nitroxides. The C-O bond dissociation energy of these alkoxyamines is significantly decreased
when compared to that of TEMPO derived alkoxyamines. A number of papers have shown how the
steric bulk of a nitroxide affects the rate of C-O bond dissociation of alkoxyamines prepared with
styrene.
71-74
An increase in the ring size of cyclic nitroxides also causes a significant decrease in the
C - O bond dissociation energy when compared to TEMPO.
75
Seven- and eight-membered ring nitroxides
have large k
d
values as compared to TEMPO and exhibit increased polymerization rates for styrene.
76,77
Interestingly, even with their larger k
d
values, these nitroxides do not allow controlled polymerizations
of
n
-butyl acrylate, giving instead polymerizations that stop at low polymer conversion or polymer-
izations that proceed to high conversion but yield polymers with broad molecular weight distributions
(M
w
/M
n
>
2).
A number of recent studies on the other factors that affect C-O bond homolysis of alkoxyamines
have been reported. Studies with aromatic alkoxyamines have shown that there is a competition between
N-O and O-C homolysis depending on the alkyl substituent on the oxygen of the nitroxide. Primary or
secondary alkyl groups result in N - O cleavage while tertiary or benzyl groups allow O-C cleavage.
78
The identity of the counterion of the carboxylate salts of SG1-derived alkoxyamines has been shown to
affect the C-O homolysis rate of the RS/SR diastereoisomers but appears to have little effect on the SS/RR
isomers.
79
Also, the effect of the penultimate unit in the polymer on the C-O bond homolysis rates in
SG1-based alkoxyamines
80
and alkoxyamines based on imidazoline and imidazole nitroxides
81
have also
been reported.
Other sterically bulky nitroxides, for example
8
-
11
, have been more successful in allowing the poly-
merization of acrylates.
82-85
The steric bulk of
11
, for example, reduces the bond dissociation energy
enough that polymerization of
n
-butyl acrylate occurs at relatively low temperatures (70 - 105
◦
C).
86,87
In
spite of the low reaction temperatures these reactions were quite fast, not due as suggested by the authors
to the higher k
d
of the N-O bond but instead due to the lower rate of recombination (k
c
) of the nitroxide
with the transient macroradical,
82
a conclusion supported with results obtained by Debuigne
et al
.using
the diadamantyl nitroxide
12
.
88
OTMS
OTBDMS
HO
N
O
N
O
O
N
O
N
O
OTMS
OTBDMS
8
10
11
9
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