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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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through possible coordination to the ester functionalities in the polymer
main chain. When the polymerization of <5-valerolactone (14) with an
aluminum porphyrin having an axial alcoholate ligand (Id) is carried out
in the presence of 45, the chain growth is considerably accelerated ( x
2000), but the MWD of the polymer becomes broader at a final stage,
indicating the occurrence of transesterification. In
contrast, use of 46 in place of 45 as a Lewis acid results in a notable
acceleration of the polymerization without any broadening of MWD. In
relation to these trends,l3C NMR studies on a mixture of 14 and poly-14
in the presence of 45 or 46 have shown that the Lewis acid 46 coordinates
with 14 exclusively, while 45 coordinates with both 14 and its polymer
without any structural preference. Therefore, the clean acceleration in
the polymerization of 14 with the 1 (X = OR)/46 system is due to the high
chemoselectivity in coordinative interaction of 46 with the monomer 14.
In addition to organoaluminum compounds, organoboron compounds can
also be used as Lewis-acid accelerators for the polymerization of 21
initiated with la, where triphe-nylboron and tris(perfluorophenyl)boron
are effective accelerators (See Table 6).56 Tributylboron (Bu,B) bearing
no particular steric hindrance neither terminates nor accelerates the
polymerization of 21, indicating a low transmetalation activity and a
rather poor Lewis acidity of Bu3B. This is in sharp contrast with the
polymerization upon addition of trimethylaluminum (Me3Al) as a monomer
activator48 where the chain growth is terminated by the transfer of the
enolate moiety from 32n to Me,Al, unless the temperature is lowered to -
40 C or below.
As already described, aluminum complexes of tetra-azaannulene (9) and
phthalocyanine (10) have much lower reactivities than aluminum porphyrins
for the polymerization of epoxides (11). However, in the presence of
appropriate Lewis acids such as 44 or 45, the polymerizations with these
initiators take place rapidly to give polyethers with fairly narrow
MWD.50
The Lewis acid assisted high-speed living anionic polymerization has
been utilized for controlled synthesis of a polyether from oxetane
(12).57 Oxetane is the most basic cyclic ether with a low susceptibility
towards nucleophiles and is viewed as a monomer which is only
cationically polymerizable. Due to the poor controllability of cationic
polymerization, a polyoxetane with narrow MWD has not been available.
However, aluminum porphyrins (lc and Id) in conjunction with a Lewis acid
such as 45 bring about anionic polymerization of 12 at room temperature,
affording a narrow MWD polyether. By virtue of this achievement, a novel
polyvinyl-polyether block copolymer with narrow MWD has been synthesized
by a sequential two-stage living polymerization of methyl methacrylate
(21, R = Me) and oxetane (12) with the la/45 system.57 As described in
section IIA, 12 is polymerizable with a chloroaluminum porphyrin (lc)
alone as initiator at an elevated temperature such as 60C.12 On the
other hand, at room temperature, the mixing of 12 with lc results in
exclusive formation of an aluminum 1-chloropropanolate, a one-to-one
adduct between 12 and lc (See Table 2). In contrast, in the presence of
45, 12 is activated and reacts with the alcoholate even at room
temperature, resulting in the formation of a polymer with narrow MWD.
F. LIGAND-EXCHANGE ACTIVITY (IMMORTAL POLYMERIZATION)
The activity of aluminum porphyrins for axial ligand exchange is not only
of fundamental interest but is also
150
Aida and Inoue
important for understanding the mechanism of polymerization. NMR studies
on mixtures of two different aluminum porphyrins with different axial
ligands (double-labelling experiments) have shown that the ligand-
exchange activity is highly dependent on the type of the axial ligand.40-
58 For example, methylaluminum tetraphenylporphyrin la and ethylaluminum
tetrakis(4'-chlorophenyl)porphyrin 2b (R'=H, R2 = C1) show characteristic
'FI NMR signals due to their axial ligands (See Table 2). If the axial
ligand exchange occurs between these two aluminum porphyrins, the mixing
of la and 2b (R1 =H, R2 = C1) should result in the appearance of new
axial ligand signals due to ligand-exchanged products (lb and 2a).
However, no such signals appear in the 'H NMR spectrum of an equimolar
mixture of la and 2b in C6Dft, even at elevated temperatures.58 Enolate-
aluniinum poiphyrins (32"), prepared by the polymerization of methacrylic
esters (21) with la, also show no sign of ligand exchange,58 whereas
alcoholate, phenolate, and carboxylate complexes undergo rapid exchange
of the axial ligands.40
In relation lo the ligand-exchange activity, it is interesting to know
whether the chain growth of a particular polymer molecule proceeds at the
active site of a particular aluminum porphyrin molecule. As for this
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