Abstract
Helix-sense-selective polymerization of {4-[(3,5-di-tert-butyl-4-hydroxyphenyl)(3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dienylidene)methyl]phenyl}acetylene (abbreviated as (4-ethynylphenyl)hydrogalvinoxyl) was promoted by [Rh(cod)Cl]2 and [Rh(nbd)Cl]2 catalyst in the presence of (R)-(+)-1-phenylethylamine or (S)-(−)-1-phenylethylamine (PEA). The [Rh(cod)Cl]2 catalyst system gave red polymers whose CD spectra showed the stronger Cotton effect though the yield (2−3%) and molecular weight (Mn = (1.1−1.4) × 104) were lower than those of polymers obtained by [Rh(nbd)Cl]2. Moreover, we investigated the effect of bulkiness of the catalyst, cocatalyst, and monomer on helix-sense-selective polymerization of (4-ethynylphenyl)hydrogalvinoxyl in the presence of (R)-PEA. The CD patterns of polymers obtained by [Rh(nbd)Cl]2 and [Rh(cod)Cl]2 were nearly mirror image of each other, except for the magnitudes of the signals in spite of the same chiral condition, i.e., in the presence of (R)-PEA. That is, [Rh(nbd)Cl]2 and [Rh(cod)Cl]2 catalysts generated P-helix and M-helix, respectively. This is a novel result, since the control of helix sense is usually achieved by enantiomeric moieties of catalysts or initiators for the helix-sense-selective polymerization.
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