Chemistry Reference
In-Depth Information
dialkylpolysilanes [393 - 396] , alkyl - arylpolysilanes [397 - 401] , and diarylpolysilanes [402] ,
in which the use of (
S
) - 2 - methylbutyl group and (
S
) - or (
R
) - 3,7 - dimethyloctyl group as
a side group was found to be essential for successful helical induction.
13.3.2.8. Polymerization of Other Monomers
Optically active monomers bearing a
binaphthyl unit were found to give polymers with a single-handed helical or propeller
conformation [403,404]. Helical poly(biaryl carbonate)s have been synthesized by
anionic ring-opening polymerization of optically active cyclic carbonates such as
241
[405] and
242
[406,407] . Polymer
243
obtained from
241
seemed to have a single-handed
4
1
helical conformation.
The phenyleneethynylene-based polymers derived from L-glutamic acids were
reported to show intense CD signals due to the predominant single-handed helical con-
formation [408] .
O
O
O
O
O
O
O
O
O
n
241
242
243
13.4. ENANTIOMER - SELECTIVE POLYMERIZATION
Enantiomer-selective polymerization is a type of polymerization reaction in which one
enantiomer of racemic chiral monomers is preferentially polymerized to give an optically
active polymer. This is a kind of kinetic resolution of enantiomers. Thus, enantiomer-
selective polymerization has been mainly employed for racemic monomers in the pres-
ence of optically active catalysts and/or initiators. The degree of enantiomer selection is
generally estimated by measuring the enantiomeric excess of the starting monomer as
well as that of the resulting polymers. The ratio of the consumption rate of one enan-
tiomer to that of the other enantiomer,
k
rel
, is used as a useful indicator to evaluate the
catalytic performance. It should be noted that a racemic mixture of some chiral mono-
mers can be converted into a mixture of polymers or stereoblock copolymers, which
mainly consist of one enantiomer of the chiral monomer, with the aid of a chiral catalyst
or an initiator in racemic form. Although this type of polymerization could be also cat-
egorized as enantiomer-selective polymerization, it is omitted in this chapter since there
are already some comprehensive review articles [1,409].
13.4.1. Polymerization of Monosubstituted Ethenes
The polymerization of racemic monosubstituted ethenes,
215b
,
215c
,
244
, and
245
, using
achiral Ziegler-Natta catalysts proceeded stereoselectively to give a mixture of two
types of polymers: one consisting of (
R
)-enantiomer and the other consisting of (
S
) -
enantiomer, which can be separated by chromatographic resolution [410-413]. When the
polymerization was carried out in the presence of chiral catalysts such as TiCl
4
/Zn[(
S
) - 2 -
