Chiroptical and Thermotropic Properties of Helical Styrenic Polymers: Effect of Achiral Group

Abstract

Six novel chiral bulky styrenic monomers, (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-phenylstyrene (A-1), (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-(4′-fluorophenyl)styrene (A-2), (+)-2-[4′-((S)-2″-methylbutyloxy)phenyl]-5-(4′-tert-butylphenyl)styrene (A-3), (+)-2-phenyl-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-1), (+)-2-(4′-fluorophenyl)-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-2), and (+)-2-(4′-tert-butylphenyl)-5-[4′-((S)-2″-methylbutyloxy)phenyl]styrene (B-3), were synthesized and radically polymerized to yield the corresponding polymers, PA-1–PA-3 and PB-1–PB-3. All of them consisted of laterally attached p-terphenyl pendants terminated by an identical (+)-(S)-2-methylbutyloxy end and an achiral end with various size. The first three differed the others by the position of vinyl group relative to chiral motifs. Evidenced by the results of NMR, polarimetry, circular dichroism spectroscopy, computer simulation, thermal properties, and X-ray diffractions, the chiral p-alkoxyphenyl group ortho to the vinyl group induced the helical conformation of polymer backbone with an excess screw sense as in PA-1–PA-3, whereas that meta to the vinyl group failed to dictate the growth of polymer backbone. The achiral end of the side group had a great effect on the optical rotation of polymer. The specific optical rotation of PA-3 that bore tertiary butyl groups was over 3 times larger than PA-1 and PA-2 terminated with hydrogen and fluorine atoms. Accompanied by the existence of helical structure with a predominant screw sense, stable liquid crystalline phases were generated by PA-1–PA-3 at above glass transition temperatures but not by PB-1–PB-3. An unusual glass transition temperature and structure relationship was also revealed.