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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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studies on the polymerization of epoxides initiated with aluminum
porphyrins.60
A typical example is given by the polymerization of 1,2-epoxypropane
(11, R = Me) initiated with lc in the presence of alcohol as a chain-
transfer agent. In this case, the exchange between the growing alcoholate
species (51) and
the alcohol (Scheme 9A) leads to a chain-transfer reaction, since the
resulting aluminum alcoholate (Id') is able to reinitiate the
polymerization (Scheme 9B). Furthermore, this exchange reaction (Scheme
9C) takes place reversibly and much more rapidly than the reaction
between the alcoholate species (Id', 51, and 51') with epoxides, thereby
allowing catalytic formation of a narrow MWD polymer. Immortal
polymerization is so called because the polymerization of epoxides cannot
be terminated by a strong acid such as hydrogen chloride.59 Although
hydrogen chloride irreversibly reacts with the growing alcoholate species
(51) (Scheme 10A), the resulting chloroaluminum porphyrin (lc) is capable
of reinitiating the polymerization to generate an aluminum alcoholate
(52, Scheme 10B), which is exchangeable with the dead polymer (51dead)
and is similar to Scheme 9C. Therefore, the polymerization ensues from
all the polymer molecules (immortal character). This is in sharp contrast
with anionic polymerization of epoxides with sodium or potassium
alcoholates where the chain growth is terminated by hydrogen chloride,
because the resulting sodium or potassium chloride is unable to
reinitiate the polymerization. Thus, the immortal character of the
polymerization with aluminum porphyrins is due to the unusual reactivity
of the aluminum-axial ligand bonds.
By selecting appropriate transfer agents, immortal polymerizations
can be applied to controlled synthesis of polyesters from lactones (13-
15) and lactide (16).59-18 A thiolate complex of zinc jV-substituted
poiphyrin (5g) leads to an immortal polymerization of epoxides (ll)35
Scheme 9
51' + 51
dead
51
51
(9C)
Scheme 10
51 + HCI
Irreversible
-X-
I
Al-Cl + H-
X (10A)
1c
51d
R
I
1c + n C"C О
11
:
-Cl
(10B)
52
152
Aida and Inoue
and episulfides (18)34 using alcohol and thiol, respectively, as chain-
transfer agents. Interestingly, no chain-transfer reaction occurs in the
polymerization of 18 with 5g in the presence of methanol, because the
propagating thiolate species (31, Scheme 4B) is not exchangeable with
alcohols.34
In principle, the number of polymer molecules in immortal
polymerization can be increased by increasing the mole ratio of chain-
transfer agent to initiator, but unfortunately polymerization at a higher
concentration of chain-transfer agent is slower. However, when a
sterically crowded Lewis acid (See Table 6) is added to the system, the
immortal polymerization of epoxides (11) can be dramatically accelerated,
and a ratio of the number of the polymer molecules to that of the
initiator molecules exceeding
1,000 has been achieved under appropriate conditions.61 The formation of
a narrow MWD polymer here indicates that not only the chain growth but
also the alcoholate/alcohol exchange process is accelerated by Lewis
acids. In fact, the exchange between the aluminum 2-propanolate complex
(Id, R = iso-Pr) and 2-propanol ([2-ргорапо1]0/ [ld]0 = 13) in CD2C12 is
accelerated by 45 (3 equivalents with respect to Id) by a factor of at
least 50, as evaluated by 'H NMR saturation transfer method.61 The
accelerated alcoholate/ alcohol exchange in the presence of 45 is
considered to be a consequence of the interaction of alcohol with 45.
Immortal polymerization is an effective method for the synthesis of
end-functionalized polymers and oligomers with narrow MWD. For example,
the use of unsaturated protic chain-transfer agents such as acrylic acid,
methacrylic acid and 2-hydroxyethyl methacrylate for immortal
polymerization leads to polymers carrying polymerizable end groups
(macromonomers).62 Multifunctional protic chain-transfer agents such as
diols and triols enable synthesis of a,co-bifunctional and star-shaped
polymers. A novel amphiphilic porphyrin carrying four polyether side
chains of the same chain length (53) has been synthesized by using a
porphyrin containing four hydroxyphenyl functionalities as a chain-
transfer agent for the immortal polymerization of epoxyethane (11,R = H)
initiated with lc.63 Here, the length of the four poly(oxyethylene) side
chains can be controlled by changing the mole ratio of 11 (R - H) to the
precursor porphyrin. Due to its amphiphilic character, 53 spontaneously
forms cofacial aggregates in aqueous media depending on the length of the
water-soluble polyether side
, poly ether
chains. Immortal polymerization has also been applied to the synthesis of
poly( 1,2-epoxypropane) with primary hydroxyl functionalities at both
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