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recombination rate constant. At any given time most of the polymer chains are in the dormant form,
reducing the overall concentration of propagating radicals, thereby dramatically reducing irreversible ter-
mination by radical chain combination. Although some termination continues to occur throughout the
course of the polymerization,
40
the rate at which termination occurs decreases as the polymerization pro-
ceeds since termination by coupling generally involves at least one short chain.
41
In the absence of other
side reactions that generate new chains, polymer growth from the initiating species occurs in a “living”
fashion. Ultimately, the SFRP process enables a linear increase in the molecular weight of the poly-
mer chain with conversion, while producing polymers with very low M
w
/M
n
. Since termination reactions
cannot be completely avoided in any free radical polymerization process, these are not truly “living” sys-
tems. Comprehensive kinetic analyses of the SFRP process have been reported
42-48
using a plethora of
methods, such as, HPLC,
49,50
ESR,
51-53
GPC/SEC,
54-56
alkoxyamine fluorescence quenching
57
31
P
58
and
1
HNMR.
59
and
11.3.3 Unimolecular initiators
While conventional radical initiators can be used in stable free radical polymerization systems, their use
often requires tweaking the system to obtain the correct ratio of initiator to mediator, since the initiation
efficiency of various primary initiators differs and any large excess of free nitroxides adversely affects the
rates of polymerization.
60
An alternative to primary initiators are alkoxyamines,
33-36
adducts of an initiator
fragment (optional), a monomer, and a nitroxide (for example
3-5
). The C - O bond of the alkoxyamine is
labile and upon heating dissociates to form an initiating species and a stable radical mediator in a 1 : 1 ratio,
although this is not necessarily the right ratio of the propagating chain to nitroxide for a well controlled
polymerization.
61
There are several methods to preparing alkoxyamine unimer initiators that involve the controlled gen-
eration and trapping of carbon-centered radicals by stable radicals. A commonly used alkoxyamine,
2-phenyl-2-(2,2,6,6-tetramethylpiperidine-1-oxy)ethyl benzoate (BST)
4
, has been synthesized by the reac-
tion of benzoyl peroxide with an excess of styrene in the presence of TEMPO at 80
◦
C.
47,61,76
Synthesis
of the acrylate derivative of
4
has been accomplished by oxymercuration of
t
-butyl acrylate to generate
a mercury(II) acetate intermediate, followed by an oxidative demercuration in the presence of TEMPO.
A subsequent benzoylation gives the desired alkoxyamine product
5
.
62
Other alkoxyamine derivatives have been synthesized that do not contain the benzoyloxy moiety. A
reaction of di-
tert
-butyl peroxide with ethyl benzene generates the corresponding benzylic radical, which
is trapped by TEMPO to form the alkoxyamine. This reaction can be performed either thermally
63
or
photolytically,
64
with the latter reaction allowing for an increase in product yield.
Another alkoxyamine synthesis uses Jacobsen's catalyst, a reagent typically used in the epoxidation of
olefins. The epoxidation mechanism proceeds via a radical intermediate that generally collapses to give
the corresponding epoxide. However, the radical intermediate can be trapped by a stable nitroxide radical
and then reduced to form the corresponding alkoxyamine.
65
Alkoxyamine synthesis has also been attained through atom transfer radical addition (ATRA)
in which halogen abstraction from an alkyl halide by a copper complex yields a radical that can
be trapped by a stable radical, such as a nitroxide.
66
Wang and Zu reported the synthesis of the
alkoxyamine composed of the adduct of the benzoyloxy radical, derived from BPO, and TEMPO,
simply by heating BPO in the presence of TEMPO.
67
Braslau and Hill described the synthesis of
arylethyl-functionalized N-alkoxyamines and highlighted their uses as initiators for polymerizations.
68
More recently, a method for preparing alkoxyamines starting with 4-hydroxyTEMPO dissolved in a
simple ketone in the presence of hydrogen peroxide, followed by the addition of copper(I) chloride
(CuCl), gave varying yields of the alkoxyamine,
69
while the synthesis of SG1-based alkoxyamines
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