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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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with "immortal" character, to give a narrow MWD polyether with the number
of the molecule exceeding that of the initiator molecule 6 (X = OAc). An
example is shown by the polymerization at a feed mole ratio of 11 (R =
Me) to 6 (X = OAc) of 400 at 30 C (Figure 9), where the molecular weight
of the polymer produced at 100% monomer conversion can be controlled by
the mole ratio of methanol to 6 (X = OAc), while the MWD is almost
constant and close to unity. Therefore, the manganese complex 6
(X = OAc) is a versatile initiator for the catalytic production of a
narrow MWD polyether with a desired molecular weight.
B. ADDITION POLYMERIZATION OF UNSATURATED MONOMERS
The control of free-radical polymerization has been one of the central
interests in the field of macromolecular synthesis because free-radical
polymerization is widely applicable to various vinyl monomers, is highly
tolerant of water in contrast to ionic polymerization, and is therefore
very important for the commerical production of polymeric materials.
However, bimolecular terminations such as radical coupling and
disproportionation, inherent to free-radical polymerization, prohibit
uniform growth of macromolecules. Therefore, examples of controlled
radical polymerization to give narrow MWD polymers are limited.
Organocobalt porphyrins such as 7a, X = fer/-BuCH2, Me(Me02C)CH, have
been found to initiate a controlled radical polymerization of acrylic
esters (20) such as methyl and butyl acrylates (R = Me, Bu) at 60 C,
affording atactic polymers with Mw/Mn values ranging from 1.1 to 1.2.70
An example is given by the polymerization of 20 (R = Me) in benzene at 60
C at a feed mole ratio of monomer to initiator of 2500. As shown in
Figure 10, the polymer molecular weight is increased in proportion to
monomer conversion, while the molecular weight distribution (Mw/Mn)
remains around 1.1. The number-average molecular weight (Mi) is in
excellent agreement with that from the assumption that every initiator
molecule produces one polymer molecule, a phenomenon typical of living
polymerization. Removal of unreacted 20 (R = Me) followed by addition of
20 (R = Bu) to the system results in the formation of a block copolymer
with narrow MWD.
154
Aida and Inoue
Conversion (%)
Figure 10. Polymerization of methyl acrylate (20, R = Me) initiated with
neopentylcobalt tetramesitylporphyrin (7a, X = tert-BuCH2) [(20]o/[7a]o =
2500) in benzene at 60 C. Relationship between molecular weight (Mn),
molecular-weight distribution (Mw/Mn), and monomer conversion.
On the basis of 'H NMR studies, the controlled polymerization of 20
with organocobalt porphyrins (7a) is considered to take place as shown in
Scheme 11. Homolysis of the cobalt-alkyl bond in 7a produces 54 and a
carbon-centered radical (R-) (Scheme 11 A) which initiates polymerization
by reacting with 20 to form 55 (Scheme 11B). Then, 55 either combines
reversibly with 54 to give
7ai (Scheme 11C) or reacts with additional monomer molecules to form an
oligomer radical that reversibly combines with 54 to give 7an (Scheme
11D). Therefore, organocobalt species themselves are considered to be
dormant, while the role of 54 can be interpreted in such a way that it
serves as "a reversible radical scavenger," which can protect carbon-
centered radicals (growing polymer radicals) from bimolecular reactions
such as radical coupling and disproportionation.
In relation to the mechanism of polymerization, the cobalt octabromo
derivative (7b) is also an effective initiator for controlled free-
radical polymerization of acrylic esters,71 where the apparent first-
order rate constant of propagation (?app) at 50 C is more than 30 times
larger than kapp using 7a under identical conditions. This is due to a
higher concentration of radicals resulting from greater dissociation of
the dormant organocobalt species.
Although the previously described polymerization with organocobalt
porphyrins (7) is the first example of controlled radical polymerization,
the applicability of this system is only limited to acrylic esters
(20).73 Use of 7 for free-radical polymerization of methacrylic esters
(21) results in a chain-transfer reaction with respect to the a-methyl
group, to give oligomers with terminal unsaturation.
Rhodium(II) porphyrins (8) have also been reported to be usable as
reversible radical scavengers for control free radical polymerization of
acrylic esters (20) under irradiation with a 300-W tungsten bulb.72 For
example, a benzene solution of 8 (X = Me) has been irradiated (> 350 nm)
for several hours to generate the corresponding rhodium(II) porphyrin
(56) (Scheme 12A). Then 20 is vacuum-distilled to the above reaction
mixture, whereupon a four-carbon-bridged dimer (57) is formed (Scheme
12B).72 Upon
Scheme 11
42 / Metalloporhpyrins as Catalysts
155
Scheme 12
III
Rh-R
56 + '>=0
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