Cooperative Helical Takeover Effects of Polysilane Copolymers

Michiya Fujiki
Materials Science Laboratory

Preparation of single-handed helical polymers is a long-standing issue covering broad chemistries and practical applications, such as biochemistry, polymer chemistry, supramolecular chemistry, analytical chemistry, and chiral separation column and chiral pharmaceutical technologies. The widely accepted way to selectively obtain a single helical polymer is to use an enantiopure chiral substance with dextro- (d-) or levo- (l-) forms in residues, repeating units, side chains, or initiators. Actually, for DNA, choice of the corresponding d- or l-deoxyribose gives ideal mirror-image helical motifs. Similarly, d- or l-amino acids provide the ideal mirror-images of protein. Here we found that even enantiomerically impure chiral side-chains induce a left- or right-handed screw-sense helical structure of polysilane copolymers consisting of d- and l- side-chains form.
Figure 1 plots the contents of right-handed-helix as a function of the contents of l-form chiral side-chains in the copolymers.  Even just 6 % enantiopurity (e. e.) of l-chiral side-chain induced an intense right-handed helical form, while 6 % e. e. of d-chiral side-chain determined its opposite-handed helical form.  Interestingly, screw-pitch in the racemic copolymers is independent of the purity of chiral side-chains because the UV ?max peak in the copolymer systems was unchanged with the contents of chiral side-chains in the copolymers.  Although this might raise the possibility of enantiomer-selective polymerization of racemic chiral silanes, this would be excluded because the temperature dependence of the UV and circular dichroism spectra of polymers with l-chiral / d-chiral ratios = 60 / 40 (and 40 / 60) are different from those of enantiopure l-chiral / d-chiral ratios =100 / 0 (and 0 / 100).
From the viewpoint of technological importance, the knowledge will open a new way to easily obtain single-handed helical polymers from commercially cheaper enantiomerically impure substances.
[1] M. Fujiki, Polym. Prepr. (Am. Chem. Soc. Polym. Sci. Div.) 37(1996) 454.
[2] Japanese Patent Pending.
[3] M. Fujiki, manuscript in preparation.

Fig. 1: Contents of right-handed-helix as a function of the contents of l-form chiral side-chains in the copolymers.