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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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terminals,64 which is useful as a prepolymer for the production of
segmented polyurethanes.
The principle of immortal polymerization indicates the possibility of
recycling the aluminum porphyrin initiator. In order to facilitate
isolation of the polymer and regeneration of the initiator, an aluminum
porphyrin bound to a cross-linked polystyrene (4) has been developed.65
For example, the immortal polymerization of 1,2-epoxypropane (11, R = Me)
initiated with 4 can be repeated 9 times without any loss of the
initiator activity and without any broadening of polymer MWD.
G. STEREOCHEMICAL ASPECTS
Stereochemical studies have been made on the polymerization of 1,2-
epoxypropane (11, R = Me) with aluminum porphyrins (1)25.66.67 sjnce (R
= Me) has an asymmetric carbon atom, the tacticity of the polymer is
informative of the stereochemical course of the polymerization. Figure 8
shows the 13C NMR spectrum of the polymer obtained by lc, which is
typical of a polymer consisting exclusively of head-to-tail linkages as a
result of selective ring-opening at the O-CH bond of 11 (R = Me).66 The
two resonances due to methylene carbon and the three resonances due to
methine carbon reflect the diad, meso (m) and racemo (r), and triad,
isotactic (/), syndiotactic (5), and heterotactic (H) tacticities of the
main chain, respectively. An integration of these signals gives the
following mole fractions of the corresponding sequences: m = 0.74, r =
0.26, / = 0.57, = 0.35 and 5 = 0.08, values of which are much different
from those of poly( 1,2-epoxypropane) obtained by alkali metal
alcoholates (m = r = 0.5, /=#=0.25, 5 = 0.5). Statistical considerations
have shown a relationship I = m2, indicating that the optical antipodes
of 11 (R = Me) are selected by the asymmetric center of the growing
CH
CH2
S (ppm)
S (ppm)
53
Figure 8. 13C NMR spectrum in CDCI3 of poly(1,2-epoxypropane) prepared by
the polymerization with chloroaluminum tetraphenyl-porphyrin (1c). m, .
I, S and H represent meso, racemo (diad) and isotactic, syndiotactic, and
heterotactic (triad) sequences, respectively.
42/Metalloporhpyrins as Catalysts
153
alcoholate unit (growing chain control mechanism). In the case of anionic
polymerization of substituted epoxides with alkali metal alcoholates,
only 3,3-dimethyl- 1,2-epoxybu-tane (11, R = tert-Bu) with a large steric
bulk has been reported to polymerize by this stereo-differentiation
mechanism.68 Therefore, the steric control observed in the polymerization
of 11 (R = Me) with aluminum porphyrins may indicate one of the
characteristic features of the polymerization at confined reaction sites.
III. Metalloporphyrins Containing Transition Metals
Compared to polymerization with metalloporphyrins of nontransition
metals, those with transition-metal complexes have not been well explored
to date. Nevertheless, some metalloporphyrins of transition metals have
been found to serve as initiators for controlled polymerization. Examples
include manganese porphyrins for controlled ring-opening polymerization69
and organo-cobalt and -rhodium porphyrins for controlled addition
polymerization.70-72
[MeOH]o/[6]o
Figure 9. Immortal polymerization of 1,2-epoxypropane (11,
R = Me) initiated with manganese porphyrin (6, X = OAc) at a feed mole
ratio [11]0/[6]0 of 400 in the presence of MeOH at 30 without solvent.
Effects of [MeOH]o/[6]o on molecular weight (Mn) and molecular weight
distribution (Mw/Mn).
A. RING-OPENING POLYMERIZATION OF HETEROCYCLIC MONOMERS
A metalloporphyrin containing manganese(HI) such as 6 (X = OAc) has been
found to initiate ring-opening polymerization of epoxides to give
polyethers with fairly narrow MWD.69 A typical example is given by the
polymerization of 1,2-epoxypropane (11, R = Me) with 6 (X = OAc) at a
feed mole ratio of 400 without solvent at 30 C, which starts with a
short induction period and is completed within 20 hours. The polymer
formed has a MWD consisting of a sharp main peak with a small shoulder at
a higher molecular-weight region. As the polymerization proceeds, the
molecular weight is increased in proportion to the monomer conversion.
Furthermore, when a fresh feed of 11 (R = Me) is added to the system
after 100% monomer conversion, the molecular weight of the polymer
increases uniformly from the prepolymer. Therefore, the polymerization
essentially is of a living character. 13C NMR spectroscopy has shown that
the resulting polymer has a very low stereo-regularity, in contrast to
the case where aluminum porphyrins are used as initiators.
The active species of the polymerization has been considered to be a
manganese alcoholate,69 that is formed by the insertion of the epoxide
into the Mn-OAc bond of the initiator. In the presence of methanol or
acetic acid, the polymerization of 11 (R = Me) has been found to proceed
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