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)Cl] 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) × 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.