Chemistry Reference
In-Depth Information
According to a Tsuruta mechanism [
36
] the first step in propylene oxide polymerization, with
catalysts like zinc alcoholates, is the coordination of the ether oxygen onto a zinc atom. The second
step is a nucleophilic attack at the oxirane ring by the alkoxy ion. Almost all the bond cleavage takes
place at the CH
2
-O bond. This results in retention of the steric configuration of the carbon atom at the
C-H group. The next oxirane molecule repeats the process, coordinates with the same zinc atom and
then undergoes the ring-opening reaction to form a dimer. Repetition of this process many times
yields a high molecular weight polymer [
36
]:
(S)
(S)
O
O
O
Zn(OR)
2
(S)
O
(R)
(S) and (R)
O
O
O
(R)
(R)
The catalyst can also be ZnR
2
-CH
3
-OH.
Special catalyst complexes, like [Zn(OCH
3
)
2
(C
2
H
5
OCH
3
)
6
], form through carefully control of
reaction conditions by adding 16moles of methyl alcohol to 14moles of diethylzinc in heptane under an
argon atmosphere. X-ray analysis shows that two different structures [
36
]. One of them is a centrosym-
metric complex of two enantiomorphic distorted cubes that share a corner Zn atom. The two would be
equivalent if they were not distorted. Another structure, also centrosymmetric, consists of two
enantiomorphic distorted structures that resemble “chairs without legs,” where the surfaces share a
common seat. Both types of complexes are active initiators for polymerization of propylene oxide.
Each has two enantiomorphic sites for polymerization. Based on that knowledge, NMR spectra and
GPC curves, Tsuruta suggested the following mechanism of a monomer coordinating with the catalyst
[
36
] (see Fig.
5.2
). The bonds at the central zinc atom are loosened and coordination takes place with
methyl-oxirane molecule at the central atom. Cleavage at the O-CH
2
bond of the oxirane takes place by
a concertedmechanism. If the bond loosening takes place at the
cube and the nucleophilic attack takes
place at one of the methoxy groups on that cube then chirality around the central zinc will favor
L
d
monomer over the
D
monomer. This is the origin of the
l
* catalyst site. If the bond loosening takes
place in the
l
cube the catalyst site will have
d
* chirality. Because the probability of bond loosening in
the
d
cube is exactly the same as in
l
cube, an equal number of
l
* and
d
* sites should be expected to form.
These two cubes become a source of
d
* and
l
* chiral nature [
35
].
5.4 Polymerization of Oxetanes
Oxetanes (or oxacyclobutanes) are preferably polymerized in solution to maintain temperature and
stirring control. It is necessary to purify both the monomer and the solvent, because impurities
interfere with attainment of high molecular weight.
