Books
in black and white
Main menu
Home About us Share a book
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
Previous << 1 .. 86 87 88 89 90 91 < 92 > 93 94 95 96 97 98 .. 240 >> Next

reversible fashion (Scheme 6). Addition of an epoxide (11) to the system
containing 34 leads to insertion of 11 into the aluminum-carbonate bond
in 34 and generates an aluminum alcoholate (35). If further amounts of
carbon dioxide and 11 reach with 35 in an alternating fashion, a
polycarbonate should be formed. However, the product here is not a linear
carbonate but a cyclic carbonate (36), indicating a facile intramolecular
cyclization of 35, that regenerates an aluminum alcoholate (33) with a
release of 36. Accordingly, in the presence of excess amount of carbon
dioxide and 11 with respect to 33, catalytic formation of 36 takes place.
Aluminum porphyrins (1) upon mixing with onium salts such as
quaternary ammonium or phosphonium halides or carboxylates have been
found to form similar six-coordinate aluminum porphyrin complexes (37,
Scheme 7),43 which are, in contrast with 33 (Scheme 6), capable of
causing alternating copolymerization of carbon dioxide and 11 in a highly
controlled fashion:44 When (TPP)A102CR (If) is mixed with an equimolar
amount of Et4N R'COj in CDCb, the solution turns from bright reddish
purple to bluish purple, characteristic of six-coordinate aluminum

42 / Metalloporhpyrins as Catalysts
147
Scheme 6
I
I
MeN.
/ \
Melm
Al-OR
MeNx^N">"AI-OR
1d
I
"Al-
I
C02
33
MeN. M........Al-
OR
34
Scheme 7
I
I
Al-X
R4N+V
(c)
r4n
I
....Al...X
I
C)
37

11
C02
I
R4N Y-polycarbonate' Al......polycarbonate-X

\ I /
()
R
-\A-
porphyrins.42 The 'H NMR spectrum at a low temperature, such as - 60 C,
shows the presence of three different six-coordinate complexes, 39, 40,
and 41, indicating a possible disproportionation of the carboxylate
ligands in 37.4 ( Such six-coordinate aluminum porphyrins, under a carbon
dioxide pressure of 50 kg cm 2, bring about living, alternating
eopolymerizalion of carbon dioxide and epoxides (11) to form narrow MWD
polycarbonates, without any concomitant formation of cyclic carbonate.4'1
Furthermore, the same initiator systems can also give rise to alternating
copolymerization of 11 and cyclic anhydrides such as phthalic anhydride,
affording polyesters with narrow MWD.4'' Extensive spectroscopic studies
have shown that the aluminum porphyrin (38) remains six coordinate
throughout the copolymerization, and the chain growth therefore occurs on
both sides, respectively, of an aluminum porphyrin moiety (Scheme 7).
This mechanism also shows a potential utility of aluminum porphyrins for
activation of onium salts.40
A notable effect of trans coordination has also been observed for the
anionic polymerization of methacryloni-trile (22) initiated from a living
polymer of methyl methacrylate (21, R = Me) with an aluminum enolate
reactive end (32"), in which the chain growth is promoted by the trans
coordination of pyridine, to afford a narrow MWD block copolymer (42)2i-
21 On the other hand, in the absence of axially coordinating pyridine
under otherwise identical conditions as described, no block
copolymerization of 22 from 32" takes place.
E. UTILIZATION OF STERIC
HINDRANCE FOR ACCELERATION OF POLYMERIZATION
It is well-known that nucleophilie reactions are promoted by Lewis acids
through coordination and activation of substrates in which, however, a
side reaction such as
148
Aida and Inoue
I
RC02..w...o2cr
(c)
0
RdN
39
I
RC02..j...o2cr
(c)
r4n
40
0
I
R'C02..w...o2cr
(c)
0
RjN
41
Me Me
C02R \ ,CN
42
degradative attack of nucleophiles to Lewis acids (neutralization) is
usually inevitable. On the other hand, a new concept, "Lewis acid-
assisted high-speed living anionic polymerization," has been proposed, in
which the degradative neutralization between nucleophiles and Lewis acids
is sterically suppressed by the combination of bulky Lewis acids (Table
6) with aluminum porphyrins as sterically protected nucleophiles.47
Figure 7 shows a schematic representation of the principle of Lewis acid
assisted highspeed living anionic polymerization, where the nucleophile
and the Lewis acid are both so large that they are unable to react
directly with each other, while the monomer is able to coordinate to the
Lewis acid center and then undergo a nucleophilic attack.9
An example of the Lewis acid assisted high-speed living anionic
polymerization is given by the polymerization of methyl methacrylate (21,
R = Me) initiated with (TPP)AlMe (la) [(MMA)0/(la)0 = 217] in CH2C12.47
The polymerization at 35C under irradiation with visible light (> 420
nm) proceeds to attain only 6.1 % monomer conversion in 2.5 hours. On the
other hand, upon addition of a
Previous << 1 .. 86 87 88 89 90 91 < 92 > 93 94 95 96 97 98 .. 240 >> Next