Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

NITRONE APPLICATION IN RADICAL POLYMERIZATION 295
2.7. NITRONE APPLICATION IN RADICAL POLYMERIZATION
The tendency of nitrones to react with radicals has been widely used in new
synthetic routes to well-deÞned polymers with low polydispersity. The recent
progress in controlled radical polymerization (CRP), mainly nitroxide-mediated
polymerization (NMP) (695), is based on the direct transformation of nitrones
to nitroxides and alkoxyamines in the polymerization medium (696, 697). In
polymer chemistry, NMP has become popular as a method for preparing living
polymers (698) under mild, chemoselective conditions with good control over
both, the polydispersity and molecular weight.
SigniÞcant improvement in controlled polymerizations of a variety monomers,
including styrene, acrylates, acrylamide, acrylonitrile, 1,3-dienes, and maleic
anhydride has been achieved when alkoxyamines have been used as initiators
for living, free radical polymerization.(696c, 697) Alkoxyamines can be easily
synthesized in situ by the double addition of free radicals, generated by thermal
decomposition of an azo-initiator, such as 2,2’-azo-bis-iso-butyronitrile (AIBN),
to nitrones (Scheme 2.206).
The effect of the nitrone structure on the kinetics of the styrene polymerization
has been reported. Of all the nitrones tested, those of the C-PBN type
(Fig. 2.29, family 4) are the most efÞcient regarding polymerization rate, control
of molecular weight, and polydispersity. Electrophilic substitution of the phenyl
group of PBN by either an electrodonor or an electroacceptor group has only a
minor effect on the polymerization kinetics. The polymerization rate is not governed
by the thermal polymerization of styrene but by the alkoxyamine formed
in situ during the pre-reaction step. The initiation efÞciency is, however, very low,
consistent with a limited conversion of the nitrone into nitroxide or alkoxyamine.
A study of the polymerization kinetics of methyl methacrylate, in the presence
of PBN, and of molecular-mass properties of the obtained polymers shows that
the systems react by the “pseudoliving” mechanism (699). In the Þrst stages of
the polymerization process, PBN reacts with oligomeric radicals, forming stable
nitroxyl radical-spin adducts A·, see Scheme 2.207.
Nitroxyl spin adducts (A·) react with the growing radical chain leading to the
formation of labile end groups (Scheme 2.208).
A labile bond at the end of the polymeric chain is capable of cleavage with
regeneration of the active macroradical. The periods of “sleep” and “life” of
polymer chain alternate, and the molecular weight of the polymer increases
+
N
O
−
AIBN
Benzene
N
O
CN
Scheme 2.206
21%
N
O
CN
CN