Chemistry Reference
In-Depth Information
13.3.2.5. Polymerization of Isocyanates
A variety of polyisocyanates having a helical
conformation with excess screw sense have been successfully synthesized by polymeriza-
tion of optically active isocyanates [278]. In 1970, Goodman and Chen fi rst demonstrated
that the optically active 2-phenylpropyl isocyanate gave polymers showing a large optical
rotation of [
468.8°, which may be attributed to a single-handed helical conformation
[377,378]. After the pioneering work, Green and coworkers have extensively studied
asymmetric polymerization of optically active isocyanate monomers such as (
R
) -
233
[379 - 381] , (
R
) -
234
[382,383] , (
R
) -
235
[381,382,384] , and (
R
) -
236
[383] to afford the
helical polyisocyanates. Optically active aromatic isocyanates, (
S
) -
237
and (
S
) -
238
, were
found to be effective for helix-sense-selective polymerization in the presence of Li-
piperidide as an initiator [385,386]. The obtained poly-(
S
) -
237
showed a large levorota-
tion of [
α
]
D
−
1969°, which underwent only minimal changes upon heating. The results
suggested that the poly-(
S
) -
237
had an almost perfect single-handed helical conforma-
tion since the optical activity of polyisocyanates with chiral side chains is often depen-
dent on temperature. Polymerization of aryl isocyanates bearing optically active alkoxy
groups at the
meta
- or
para
-position at the phenyl ring was also investigated [387].
α
]
365
−
NCO
NCO
NCO
NCO
NCO
NCO
H
H
D
∗
Me
D
O
H
N Ph
H Me
N Et
H Me
O
O
O
233
2
235
2
6
2
7
238
13.3.2.6. Polymerization of Carbodiimides
Titanium - initiated polymerization of
optically active carbodiimide ((
R
) -
239
) produced optically active polyguanidine (
240
)
([
]
365
+7.6 ° )
(Scheme 13.26). When annealed, however, the optical rotation of
240
was increased to
a value of [
α
]
365
+7.5°), which showed essentially the same optical rotation as (
R
) -
239
([
α
157.5°, which suggested that kinetically controlled, disordered confor-
mation evolved into thermodynamically controlled, highly ordered one [388]. The induc-
tion of helical chirality of racemic poly(di-
n
-hexylcarbodiimide) can be achieved by
protonating the polymer with chiral camphorsulfonic acid [388] .
α
]
365
−
N
CpTiCl
2
(
i
-OPr)
rt
N
C
N
i
-PrO
n
N
n
-C
6
H
13
n
-C
6
H
13
239
240
Scheme 13.26.
13.3.2.7. Polymerization of Silanes
Optically active, helical polysilanes were obtained
by Wultz-type reductive polymerization of dichlorosilane monomers bearing a chiral
side group [389-392]. Thus, single-handed helical conformation has been observed for
