Chemistry Reference
In-Depth Information
observed enantioselectivity was low in the early stages of polymerization, it increased
as the reaction proceeded [189]. The enantiomer selection was governed by the helical
conformation of a growing polymeric anion rather than a chiral ligand, as demonstrated
by the polymerization using polymeric anions of various degrees of polymerization as
initiators [189] .
Enantiomer selection was achieved by free-radical polymerization of
250
[433]
or by radical cyclopolymerization of racemic 2,4-pentanediyl dimethacrylates
[434-436] although the selectivity was much lower than that observed in the anionic
polymerization.
O
Ph
O
O
O
250
13.4.3. Polymerization of Epoxides and Thiiranes
The fi rst enantiomer-selective polymerization of epoxides was performed using propyl-
ene oxide (
251
) as a monomer. The polymerization of
251
in the presence of Et
2
Zn/
(+) - borneol or Et
2
Zn/(
)-menthol produced optically active polymers. The recovered
monomer was found to be rich in (
S
)-isomer, suggesting that the (
R
) - isomer preferen-
tially polymerized [437,438]. A variety of catalytic systems based on chiral zinc com-
pounds [439-441] and aluminum compounds [442-444] have been developed for the
enantiomer-selective polymerization of
251
. Recently, Coates and coworkers reported
an extremely selective bimetallic cobalt catalyst for enantiomer-selective polymerization
of epoxides. The enantiomerically pure catalyst
252
exhibited
k
rel
of 370 for
251
[445] .
The catalyst was active and selective for 1-butene oxide, 1-hexene oxide, and styrene
oxide (
k
rel
= 63 - 330).
−
R
t
-Bu
t
-Bu
N
R
O
N
M
O
M
O
S
O
O
N
251
R
N
t
-Bu
t
-Bu
R
252
(M = CoCl)
Propylene sulfi de was polymerized enantiomer-selectively in the presence of optically
active zinc [446 - 449] , lithium [450] , or cadmium [451,452,455] catalysts. The ratio of
consumption rate of (
S
)-monomer to that of (
R
) - monomer,
k
S
/
k
R
, reached up to 20 in
the polymerization using Et
2
Zn/
40
system [448,449] . Enantiomer - selective polymeriza-
tion of racemic (phenoxymethyl)thiirane was also reported [453].
