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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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Bulky Nucleophile (Initiator/Growing Species)
Bulky Lewis Acid
Nucleophilic Attack
to Lewis Acidic Center Prohibited
Figure 7. Schematic representation of the principle of Lewis acid-
assisted high-speed living anionic polymerization.7
sterically crowded methylaluminum diphenolate (43, 0.14 mol-% with
respect to MMA) to the system, the polymerization is dramatically
accelerated and reaches a complete monomer consumption within only 3
seconds. This corresponds to the acceleration of polymerization by a
factor of, at least, 45,300. The produced polymer has a narrow MWD (Mw/Mn
= 1.09), and the Mn value (25,500) is close to the theoretical one
(21,700), as expected from the assumption that every molecule of la as
initiator produces
Table 6. Lewis Acids for High-Speed Living Anionic Polymerization
Lewis acids serving as accelerators
Ph,Al
Me V
4 /
Ph
\
Ph / Ph
Me
i, Ph
' / - -
\'Ph
Ph
(CftHs)iB
(C6FS).,B
43: R' = R3 = R2 = H 44: R1 = R2 = R3 = ,e"Bu 45: R1 = R2 = lertBu,
R3 = Me 48: R' = R3 = Ph, R2 = H
Lewis acids serving as terminators
Me,Al
Et,Al
iso~Bu,Al

Me
\_^0/ X H
w
47
Ph"y

Ph
Me
I
Al

\
50
y'Ph
Ph
BF, OEb BCh
Initiator: Aluminum tetraphenylporphyrin (1).
42/Metalloporhpyrins as Catalysts
149
one polymer molecule. When the mole ratio of 43 to la is increased at a
constant mole ratio of 21 (R = Me) to la, the magnitude of the
acceleration becomes more pronounced, while the molecular weight of the
produced polymer remains unchanged.48 On the other hand, upon increment
of the mole ratio of 21 (R = Me) to la, the polymer molecular weight
increases proportionally, and a monodisperse, high-molecular-weight
polymer with Mn > 106 can be synthesized in a short polymerization
time.49 A thiolate complex of aluminum porphyrin such as (TPP)AlSPr (lg)
is a more convenient initiator than (TPP)AlMe (la) for the high-speed
living polymerization of 213fi since lg does not require irradiation for
initiating the polymerization.
Lewis acid-assisted high-speed living anionic polymerization can be
applied not only to the accelerated synthesis of naiTOw MWD
poly(methacrylic esters) but also to the synthesis of polyethers from
epoxides (ll)50 and polyesters from lactones (14 and 15)51 with aluminum
porphyrins as initiators. Furthermore, ring-opening polymerization of
episulfides (18)52 with zinc -substituted porphyrins (5) can also be
accelerated by Lewis acids.
The choice of Lewis acid is very important to realize high-speed
living polymerization (See Table 6). As for the Lewis acid component,
methylaluminum diphenolates having bulky ortho substituent(s) (43-46) are
representative effective candidates,48-53 while diphenolates without
ortho substituent(s) (49) are ineffective because of a lower steric bulk
around the aluminum atom, which allows concomitant degradation by attack
of the nucleophilic growing species towards the Lewis acidic center. An
aluminum triphenolate such as 47 is also effective, where a polymer with
very narrow MWD (Mw/Mn < 1.03) is formed.54 Steric hindrance of the
porphyrin ligand also plays an important role. A good example of this has
been reported for the polymerization of methyl methacrylate 21 (R = Me)
with three aluminum porphyrin initiators having different steric
environments, la, 2 (R1 = R2 = Me), 3.55 The polymerization initiated
with the least bulky 3 is terminated as soon as the Lewis acid
triisobutylaluminum (iso-Bu3Al) is added, while the polymerization
initiated with sterically crowded 2 (R'=R2 = Me) is not terminated by
iso-Bu3Al but is accelerated to attain 100% monomer conversion, affording
a narrow MWD polymer. On the other hand, the polymerization with la
having an intermediate steric bulk between 2 (R1 = R2 = Me) and 3 is
accelerated by iso-Bu-,Al at the initial stage, but is terminated before
complete consumption of 21 (R = Me).
The importance of the choice of Lewis acids has also been indicated
for the high-speed living anionic polymerization of lactones.51
Polymerization of lactones is often accompanied by trans esterification,
leading to reshuffling of the polymer chains once formed (broadening of
MWD). In general, transesterification can be promoted by Lewis acids
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