Assignment of helical sense for poly(phenylacetylene) bearing achiral galvinoxyl chromophore synthesized by helix-sense-selective polymerization

{4-[(3,5-Di-tert-butyl-4-hydroxyphenyl)(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methyl] phenyl}acetylene (abbreviated as (4-ethynylphenyl)hydrogalvinoxyl) was polymerized using [Rh(norbornadiene)Cl](2) catalyst in the presence of (R)-1-phenylethylamine or (S)-1-phenylethylamine to give red polymers. The circular dichroism (CD) spectra of polymers indicated that an excess of one-handed helical polyacetylene backbone was induced by helix-sense-selective polymerization under the asymmetric condition despite the achiral monomer. Results show that the CD signal intensity depends on temperature and decreases reversibly with increasing temperature, although the energy difference between the right-handed and left-handed conformations of each other is almost zero. The sign of the CD signal in the absorption region (420 nm) of the hydrogalvinoxyl chromophore, which was calculated from the polymer geometry, suggests that the excess of right-handed helix was induced by polymerization in the presence of (R)-1-phenylethylamine.