Helix-sense tunability induced by achiral diene ligands in the chiral catalytic system for the helix-sense-selective polymerization of achiral and bulky phenylacetylene monomers

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)C](2) Catalyst system gave red polymers whose CD spectra showed the stronger Cotton effect though the yield (2-3%) and molecular weight (M-n = (1.1 - 1.4) x 10(4)) 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.