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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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possibility, the polymerization of 1,2-epoxypropane (11, R = Me) using
two different aluminum porphyrins (1 and 3) with different reactivities
has been investigated.40 In such a case, if the growth of polymer
molecules proceeds in individual aluminum porphyrin complexes without
exchange (migration), a polymer with a bimodal MWD should be formed,
since the growing species with a higher reactivity provides a polymer
with a higher molecular weight, and vice versa. However, the
polymerization of 11 (R = Me) with an equimolar mixture of lc and 3c has
shown that a polymer with a unimodal, sharp MWD is formed at a rate
somewhere in between those with lc and 3c, respectively, as initiators.40
This observation indicates the occurrence of a rapid alcoholate-
alcoholate exchange during the polymerization. A similar investigation on
the polymerization of four-membered lactones (13) has shown that exchange
of the carboxylate growing species occurs much more rapidly than the
chain growth.
As described previously, aluminum porphyrins with axial bound alkyl
and enolate ligands (32n), in contrast with
aluminum complexes having axial alcoholates and carbox-ylates (Id and
le), do not exchange their ligands at room temperature.58 However, the
polymerization of methyl methacrylate (21, R = Me) with an equimolar
mixture of la and 3a again results in the formation of a polymer with a
sharp, unimodal MWD, although the reactivities of the corresponding
enolate complexes are much different from each other. Here, the rate of
chain growth with a mixture of la and 3a is again intermediate between
those with la and 3a, respectively, as initiators. These observations
indicate that the growing polymer molecules do migrate among different
aluminum porphyrin-active sites during the polymerization. In order to
explain this unexpected result, a linear transition state mechanism
(Scheme 8) has been proposed for the polymerization of methacrylic esters
(21) by aluminum porphyrins,58 namely, the growing enolate species (32n)
migrates from one aluminum porphyrin molecule to the other whenever it
adds to the monomer. This mechanism involves the coordination of 21 on
the back side of the aluminum porphyrin, which would lead to an
activation not only of 21 but also of the enolate species (32n) by trans
coordination.
In relation to Scheme 8, a kinetic study on the polymerization of
lactones such as (5-valerolactone (14) via an aluminum alcoholate growing
species (29, Scheme 3-
B) has indicated that the rate of polymerization is second-order with
respect to 29, but first-order with respect to the monomer,16 indicating
a simultaneous participation of two aluminum porphyrin molecules in the
chain growth. Of further interest is a clear acceleration effect of
chloroaluminum porphyrin (lc) on the polymerization of lactones. Compound
lc has no capability of initiating the polymerization of six- and seven-
membered lactones (14, 15). However, when lc is added to the
polymerization of 14 initiated with an alcoholate (Id), the chain growth
is considerably accelerated. A kinetic study on this accelerated
polymerization shows that the initial rate of polymerization is expressed
by k[ld]0 [lc]0 [14]0 or 9]0 [lc]" [14]".16 The acceleration
effect of lc can be interpreted in terms of the monomer activation
mechanism, where the high Lewis acidity of lc serves to provide an
activated monomer for the chain growth. This mechanism is similar to the
conception of Lewis acid-assisted high-speed living anionic
polymerization (Section II.E).
Scheme 8
42 / Metalloporhpyrins as Catalysts
151
Aluminum porphyrins with alcoholates (Id), phenolates (le), and
carboxylates (If) undergo axial ligand exchange with alcohols, phenols,
andcarboxylic acids, respectively, in a fully reversible fashion.59 On
the other hand, exchange of Id with phenols, carboxylic acids and other
stronger acids such as hydrogen chloride is irreversible. On the basis of
such exchange activities, a new concept-"immortal polymerization"-has
been introduced, in which a polymer with narrow MWD is formed with the
number of the molecules exceeding that of the initiator molecules.6-7
Immortal polymerization involves a "chain-transfer reaction." In a usual
sense, chain-transfer reaction is a side reaction where the growth of a
polymer molecule is irreversibly terminated while a new polymer chain is
formed and grows. On the other hand, in immortal polymerization, the
chain-transfer reaction is reversible, and takes place much more rapidly
than the chain growth. Consequently, a narrow MWD polymer is formed with
the number of the polymer molecules exceeding that of the initiator
molecules. In this sense, immortal polymerization is a "catalytic"
version of living polymerization, and is therefore of high practical
importance. This possibility has been noticed in the course of the
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