Chemistry Reference
In-Depth Information
signifi cant contributions to the fi eld of asymmetric polymerization. In addition, the reac-
tions that give a racemic mixture of chiral polymers are not described even though each
polymer molecule is optically active.
13.2. ASYMMETRIC SYNTHESIS POLYMERIZATION
Asymmetric synthesis polymerization is a polymerization reaction of optically inactive,
achiral monomers in the presence of optically active catalysts or initiators to give poly-
mers with confi gurational chirality in the main chain. In general, chiral induction is
achieved by differentiation of the enantiofaces of an olefi nic unsaturated bond or by
desymmetrization of a
meso
-compound. Therefore, a variety of alkenes or
meso
-
compounds have been utilized as a monomer in this type of polymerization. Although
it is diffi cult to determine the extent of chiral induction in most cases, it can be estimated
by comparison with the corresponding oligomeric compounds prepared under the same
conditions, or by the results of model reactions. The relation between the optical activity
of polymers and the confi guration of the main chain has been discussed in several reports
[10 - 19] .
13.2.1. Polymerization of Achiral Monomers
13.2.1.1. Polymerization of Alkenes
Stereoregular polymers of 1 - substituted or
1,1-disubstituted ethenes cannot be optically active even if effective chiral induction
takes place through polymerization because the polymer chain has a mirror plane of
symmetry and the main - chain chiral centers are pseudoasymmetric [10 - 25] . Polystyrene
[21,22], polypropylene [23], polymethacrylate [24,25], and polyacrylonitrile [24] did not
show optical activity arising from the chirality of the main chain although some of them
exhibited optical activity, based on the asymmetric centers of chiral initiators attached
to the polymer chain in the initiation step. However, if some higher-order tacticities in
the main chain are realized for a vinyl polymer, it can be optically active. The chirality
of a polymer chain can be tested by assuming a stereotactic infi nite chain as a cyclic
compound and checking the symmetry of the compound (Fig. 13.1) [2,10,11,19]. All the
stereosequences of diad, triad, tetrad, and pentad are not chiral because they have a
plane of symmetry. On the other hand, a hexad sequence having triad tacticity of mm/
mr/rr = 0/67/33 is chiral (Fig. 13.1 (T) and (U)). The number of chiral sequences increases
as the stereosequence becomes longer. The chirality of copolymers can be tested in the
same manner (Fig. 13.2). Among the three A-A-B sequences, the one shown as (D) is
chiral. This method of chirality test can be applied to other vinyl polymers including
1,2 - disubstituted ethenes.
13.2.1.1.1. Monosubstituted Ethenes
Enantioselective oligomerization of monosubsti-
tuted ethenes has been investigated for propylene, 1-pentene, and 4-methyl-1-pentene
by using optically active zirconium catalysts such as
1
and
2
, with methylaluminoxane
(MAO) under H
2
atmosphere, producing optically active oligomers [26,27]. Chiral
induction was also achieved by the oligomerization of 1-butene by using (R*O)
2
TiCl
2
/
MgCl
2
(R* is an enantiomerically pure monosaccharide derivative) in the presence of
organoaluminum cocatalysts [28] .
