Chemistry Reference
In-Depth Information
be suitable monomers for asymmetric polymerization. The CD analyses suggested that
the chirality of these polymers was attributed not only to the chirality of monomer units
but also to the stereogenic centers in the main chain.
O
O
O
O
O
O
O
O
O
O
N
N
N
N
N
OMe
O
O
Ph
Ph
O
O
O
R
(
S
)-
102
(
S
)-
104
(
S
)-
106
(
S
)-
103
(
R
)-
105
13.2.2.3. Polymerization of Dienes
The radical polymerization of (
) - 3 -
p
- menthyl
sorbates produced an optically active polysorbate, which was converted to a poly(sorbic
acid) by hydrolysis under alkaline conditions. The chirality induction seemed to take
place during the polymerization process since the poly(sorbic acid) was still optically
active [160]. Asymmetric copolymerization of (
−
)-menthyl sorbates and styrene is also
feasible [161]. A chiral monomer salt prepared from 1,3-butadiene-1-carboxylic acid and
optically active 1-phenylethylamine was found to be a good monomer for copolymeriza-
tion with styrene. The optical activity was maintained even after the removal of the chiral
ammonium moiety [162,163] .
−
13.3. HELIX - SENSE - SELECTIVE POLYMERIZATION
The helix is one of the most important and fundamental secondary structures found in
many biomacromolecules such as DNA and polypeptides. Since right- (
P
- helix) and
left - handed (
M
-helix) helices are a pair of enantiomers (atropisomers), helical polymers
can be optically active even in the absence of stereogenic centers in the main chain.
Thus, helix - sense - selective polymerization is defi ned as “the reaction which gives a
polymer having a preferential single-handed helical conformation” [2,5]. It should be
noted that there are two types of helical polymers: dynamic helical polymer and static
helical polymers. Dynamic helical polymers have low helix inversion barriers that result
in the frequent fl ipping of helical conformations. Since the dynamics of the polymer
chain is generally extremely fast, excess of screw sense in a helical conformation is
usually lost in solution. For example, isotactic polystyrene [21,22] and polypropylene
[23] prepared in the presence of chiral catalysts do not show any optical activity arising
from a helical conformation. If the polymers are static helical polymers, which have
higher inversion barriers due to a rigid main chain or bulky side groups, the helical
conformation with excess screw sense can be maintained even in solution.
13.3.1. Polymerization of Achiral Monomers
13.3.1.1. Polymerization of Alkenes
13.3.1.1.1. Propylene and 1 - Butene
Isotactic polypropylene and poly(1 - butene) obtained
by polymerization using an optically active zirconocene catalyst (
107
) have been reported
to show large optical rotations in suspension ([
]
D
− 123 ° , − 250 ° for polypropylene;
[α ]
D
+130° for poly(1-butene)). The optical activity was ascribed to a preferential single-
α
