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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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fitted with a three-way stopcock, are added 200 equivalents of 21 (R =
Me) at room temperature under dry nitrogen.22 Then, the mixture is
exposed to a xenon arc light through UV- and thermal-cut off filters,
where the color of the solution gradually turns from dark greenish purple
characteristic of la to dark reddish purple due to the enolate species
(32n). Upon continuous stirring at room temperature for 24 hours, the
polymerization reaches 100% monomer conversion, to give a poly(methyl
methacrylate) with Mn and Mw/Mn values of 19,960 and 1.06, respectively.
Scheme 5
I
i
Al-Me +
1a
I
i
Me Me
Al-
(5A)
OR
32i
hv n-1 21
(5B)
H+
R02C Me
,Me
(5C)
146
Aida and Inoue
Time (hr)
Figure 6. Polymerization of butyl methacrylate (21, R = Bu) from a living
polymer of methyl methacrylate (32n, R = Me; Mn = 9500) in CH2CI2 at 15
(a) in the dark and (b) upon irradiation with visible light.
The polymer molecular weight can be controlled over a wide range by
changing the monomer-to-initiator mole ratio or the monomer conversion,
where the number of the monomer units in the polymer chain agrees well
with the mole ratio of 21 which has been reacted with la. Of interest to
note here is that the initiation step (Scheme 5A) requires excitation of
the porphyrin ligand with visible light.22 Without irradiation, no
polymerization occurs, even upon heating to 80 C. In sharp contrast, the
subsequent propagation step (Scheme 5B) takes place even in the dark
(Figure 6a), but can be accelerated by irradiation with visible light
(Figure 6b). Similar to the aluminum enolate species (32n), aluminum
thiolate complexes of porphyrins (lg) can initiate polymerization of
acrylates (20) and methacrylates (21), even in the dark, to give polymers
with narrow MWD. '6
The wide applicability of aluminum porphyrin initiators (1) leads to a
variety of tailored block copolymers such as polymethacrylate-polyether
and polymethacrylate-polye-ster, as well as polymethacrylate-
polymethacrylate and polymethacrylate-polyacrylate, that can be
synthesized by sequential living polymerization of the corresponding
monomers.-17 For example, when 1,2-epoxypropane (11, R = Me) is added to
a polymerization mixture of methyl methacrylate (21, R= Me) with la at
100% conversion of 21, the polymerization of 11 takes place from the
enolate growing end (32") to give a narrow MWD polymethacrylate-polyether
block copolymer having an alcoholate growing terminal (Table 4).
Likewise, the aluminum enolate species (32n) can also react with lactones
(14,15), thereby allowing the formation of a poly(methyl methacrylate)-
polyester block copolymer with narrow MWD.
C. STRUCTURE-REACTIVITY RELATIONSHIPS
Since the polymerization with aluminum porphyrins takes place at the
central aluminum atom, the rate of monomer consumption may be affected by
the structure of the
porphyrin ligand. For example, in the case of the polymerization of four-
membered lactones (13) via an aluminum carboxylate as the growing species
(Scheme 3-
A),3S an aluminum tetraphenylporphyrin carrying methoxy groups at the
or?/!o-positions of the phenyl rings (2, R1 = R2 = OMe) is much more
reactive than the nonsubstituted aluminum tetraphenylporphyrin (1), while
an aluminum tetraphenylporphyrin having methyl groups (2, R'=R2 = Me) in
place of the methoxy groups is much less reactive than l,39 indicating
the importance of a polar environment around the active site. On the
other hand, 2 bearing phenyl substituents at the or?/?o-positions (R1 =
Ph, R2 = H) is virtually inactive for the polymerization, possibly due to
a large steric hindrance around the active center. When compared to
aluminum tetraphenylporphyrins (1), an aluminum octaalkylporphyrin such
as 3 is also a well-behaved but less-reactive initiator, indicating a
possible electronic effect of the porphyrin ligand on the reactivity of
the growing species.40 Although aluminum complexes of tetraazaannulene
(9) and phthalocyanine (10) have similar structures to those of aluminum
porphyrins, they are much less reactive than 1, and require external
heating (>80C) for the polymerization of, for example, 1,2-epoxypropane
(11, R = Me).41
D. EFFECT OF AXIAL COORDINATION
Nucleophilic reactivities of some aluminum porphyrins are enhanced by
?ra".s--coordination of Lewis bases such as 1-methylimidazole (Melm) and
pyridine. A clear example indicating the "rrans effect" has been given by
the reaction of (TPP)AIOR (Id) with carbon dioxide.42 Id does not react
with carbon dioxide due to a rather low nucleophilic reactivity of the
Al-OR bond. On the other hand, when Melm is coordinated to the trans
position of Id, the alcoholate group in the resulting six-coordinate
aluminum porphyrin (MeIm)(TPP)A10R (33) reacts readily with carbon
dioxide to form an aluminum carbonate (MeIm)(TP-P)A102C0R (34) in a fully
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