Chiral/Achiral Copolymers of Biphenylylacetylenes Bearing Various Substituents: Chiral Amplification through Copolymerization, Followed by Enhancement/Inversion and Memory of the Macromolecular Helicity

Abstract

A series of dynamic helical homo- and copolymers of chiral and/or achiral biphenylylacetylenes (PBPAs) bearing achiral methoxymethoxy or acetyloxy groups at the 2,2′-positions along with a chiral or achiral alkyl ether or alkoxy carbonyl group at the 4′-position of the biphenyl pendants were synthesized. The effects of the chiral/achiral substituents at different positions on their helical structures and amplification of the helicity through covalent and further noncovalent chiral interactions followed by the memory of the helicity were investigated. The chiral homopolymers formed a preferred-handed helical structure as revealed by their circular dichroism (CD) spectra, whose Cotton effect signs and intensities were significantly dependent on the chiral/achiral pendant groups introduced at the 2,2′,4′-positions of the biphenyl units as well as the solvents and temperatures, while the corresponding poly(phenylacetylene)s carrying the identical chiral substituents exhibited negligible CDs. Among the chiral homopolymers, a PBPA bearing the 2,2′-methoxymethoxy and 4′-chiral alkyl ether groups showed a unique helix-inversion/switching in aliphatic hydrocarbons according to whether they are cyclic or acyclic. The chiral/achiral copolymers displayed a moderate amplification of the helicity due to the chiral units covalently bonded to the pendants (the sergeants-and-soldiers effect). The macromolecular helicity of the copolymers with imperfect or no helical sense excesses was, however, significantly enhanced to the almost completely one-handed helices or inverted to the opposite ones through noncovalent chiral interactions with enantiomeric alcohols, which could be further memorized after the complete removal of the chiral alcohols.